-
19 May 1972, Volume 176, Number 4036
The Pressure VariableMaterials Resear4
Experiments at high pressure reveal trends in cry.structure,
superconductivity, and magnetiL
D. B. McW
The variation of physical propertieswith pressure provides new
insight intomaterials at 1 atmosphere, and the pres-sure variable
adds a new dimension tothe synthesis of materials. Building onthe
pioneering work of P. W. Bridgmanatnd sparked by the successful
synthesisof diamonds in the laboratory, the fieldof research at
high pressure has ex-panded at a rapid rate during the lastdecade.
It is now possible to carry outa wide variety of sophisticated
experi-ments at ever increasing ranges of pres-sure and
temperature. In order to con-vey the flavor of this field, I will
con-centrate in this article on the tip of theiceberg, namely, the
elements of theperiodic table and will illustrate theeffect of
pressure on only a few prop-erties. Clearly, many more
fascinatingthings can occur in the vast number ofcompounds that can
be made.
It has been found that the applica-tion of pressure tends to
smooth outdifferences in crystal structure, density,and
compressibility in the elements andthat, with increasing pressure,
many in-sulators are transformed into metalswhich are often
superconducting at lowtemperatures. At high pressure the sim-ple
metals such as the alkali metalshave properties similar to those of
typi-cal transition metals, and the onset ofmagnetic ordering in
the 3d transitionmetals is very sensitive to pressure.
The author is a member of the technical staffat the Bell
Telephone Laboratories, Mtirray Hill.New Jersey 07974.
19 MAY 1972
These are only a fewmore detailed informattopics of research at
hibe found in other revii8). I will not discuss t]aspects of
research at hcept to comment that vtechnology it is possipressures
of several hi(I kilobar = 987 atltemperatures ranging frabout
1000K. In manysible to design small, inement, thus making
lartinstallations Linnecessar
Trends in the Periodic
Many physical propements exhibit a markedincreasing atomic
numb(the cohesive energy, demodulus are shown in Ifact that the
least denmalso the most compressthe speculation that thdensity will
be smoothsures of about 10 melnow appears that the pments become
discontinipressible at high pressuperiodicity in compressiduced.
This discontinucits implications are disci
In the absence of p1the properties plottedsmooth functions of
pr
SOlE:NCES
available compression data can be rep-resented by a
two-parameter equationif we assume a linear dependence ofthe bulk
modulus B on pressure P:
in B=-(UP/O ln V)r-B.+B.' P (1)
ch where V is the volume, T is the tem-perature, and B(, and B,'
are the bulkmodulus and its first pressure deriva-
stal tive in the limit of P -- 0 [see the curvefor tungsten in
Fig. 2 (11) ] (12).
smi. The curve (Fig. 2) for cesium ismarkedly different from
that for tung-sten. Between the phase transitions at
han 42.5 and 120 kilobars the bulk modulusincreases dramatically
(13). At a pres-sure of 120 kilobars the ratio of thebulk modulus
of tungsten to that of
v samples, and cesium has been reduced from the valuetion on
specific of 200 at 1 atmosphere to 3. This typegh pressure can of
anomaly is not limited to cesium;Dew articles (1- almost all of the
pretransition elementshe experimental and many of the d and f
transition ele-igh pressure ex- ments near the beginning of each
seriesvith the present also have abrupt increases in bulkible to
achieve moduli. For example, in the 4d transi-undred kilobars tion
series the anomalies occur at 330mospheres) at kilobars
(strontium), 460 kilobarsom about 1 to (yttrium), 530 kilobars
(zirconium),y cases it is pos- and 860 kilobars (niobium)
(14).-xpensive equip- These anomalies oCCur in elements withge
high-pressure partially filled d bands, and they mayy. reflect
electronic transitions in which
electrons are transferred from sp bandsto d bands (14, 15).
Table This abrupt increase in bulk modulusmay be contrasted to
the larger bulk
rties of the ele- moduli of the d transition metals
com-periodicity with pared with those of the sp-bonded ele-er; for
example, ments at I atmosphere (see Fig. 1). Innsity, and bulk both
cases the larger bulk moduli coin-Fig. 1 (9). The cide with the
higher densities, a resultse elements are which suggests that their
origin maysible has led to lie in the interaction of the ion corese
periodicity in with the bonding electrons.ed out at pres- The
present theory of cohesion ingabars (10). It metals does not
account for these largeIretransition ele- bulk moduli. In the
simple metals theuously less com- kinetic energy of the conduction
elec-ire and thus the trons is lower than that of the
isolatedbility is also re- atoms because the electrons can moveDus
change and throughout the metal. This freedom ofussed below. motion
allows the electrons to take bet-hase transitions, ter advantage of
the potential energyin Fig. 1 are of the ions and results in a
lower energyessure, and the in the metal than in the isolated
atoms,
751
on
Feb
ruar
y 26
, 201
5w
ww
.sci
ence
mag
.org
Dow
nloa
ded
from
o
n F
ebru
ary
26, 2
015
ww
w.s
cien
cem
ag.o
rgD
ownl
oade
d fr
om
on
Feb
ruar
y 26
, 201
5w
ww
.sci
ence
mag
.org
Dow
nloa
ded
from
o
n F
ebru
ary
26, 2
015
ww
w.s
cien
cem
ag.o
rgD
ownl
oade
d fr
om
on
Feb
ruar
y 26
, 201
5w
ww
.sci
ence
mag
.org
Dow
nloa
ded
from
o
n F
ebru
ary
26, 2
015
ww
w.s
cien
cem
ag.o
rgD
ownl
oade
d fr
om
on
Feb
ruar
y 26
, 201
5w
ww
.sci
ence
mag
.org
Dow
nloa
ded
from
o
n F
ebru
ary
26, 2
015
ww
w.s
cien
cem
ag.o
rgD
ownl
oade
d fr
om
http://www.sciencemag.org/http://www.sciencemag.org/http://www.sciencemag.org/http://www.sciencemag.org/http://www.sciencemag.org/http://www.sciencemag.org/http://www.sciencemag.org/http://www.sciencemag.org/
-
that is, a cohesive energy. This effectis balanced by the Pauli
exclusion prin-ciple which says that electrons of iden-tical spin
and momentum may notoccupy the same volume (16). The in-creased
cohesion in the transitionmetals is attributed to the broadeningof
the atomic d levels into a band asthe atoms are brought together.
Theaverage energy of the total d band is
LI. 2.
M 3g21
ci 2
Fr
50 2
RO
the same as that of the atomic levels;however, for an unfilled d
band theaverage energy is lower, and this leadsto additional
cohesive energy. This sim-ple picture gives approximately
theobserved energy and its parabolic vari-ation with the number of
4d or Sdelectrons (17). It does not include arepulsive term which
can account forthe observed bulk moduli, however. If
C 94
25p 17
7L2S9
F
Ct
HE
NO
Pt 1IAU 1 io . 1 L i9241 1 13L29Nk30
Fig. 1. Periodic table showing the occurrence of phase
transitions (heavy borders) atroom temperature and pressures up to
several hundred kilobars. The occurrence ofsuperconductivity at low
temperatures is indicated by diagonal lines (1 atmosphere)or solid
triangles (only at high pressure). The curves below the periodic
table showthe periodic variation in (A) the cohesive energy (in
kilocalories per gram atom),(B) density (in gram atoms per cubic
centimeter), and (C) bulk modulus (in kilobars)with atomic number:
(solid lines) potassium through krypton; (dotted lines)
rubidiumthrough xenon; (dashed lines) cesium through radon. The
vertical step in the curvesfor cesium through radon is the
lanthanide contraction from lanthanum to lutetium.The crystal
structurse are designated by numbers. The Structurbericht notation
is usedfor structures 1 through 20, that is, 1 _= Al (face-centered
cubic), 2 - A2 (body-centered cubic), 3 A3 (hexagonal
close-packed), and so forth; 21, double hexagonalclose-packed; 22,
samarium structure; 23, w phase related to body-centered cubic;
24,tetragonal; 25, cubic-intermediate high pressure phase; 26,
tetragonal-intermediate highpressure phase; 27, tetragonal
distortion of body-centered cubic; 28, simple cubic; 29,monoclinic
related to A5; 30, similar but unknown structure; 31, simple
rhombohedral;32, antiferromagnetic, orthorhombic distortion of
body-centered cubic. The data arefrom (9, 18-23, 30).
752
the origin of this term is the interactionbetween the conduction
electrons andthe ion cores, then it may be moreprofitable to study
the simple metals athigh pressure. In this way the inter-action
could be increased in a con-trolled manner.The band-broadening
model for co-
hesion in the transition metals does notwork even at 1
atmosphere for the 3dseries. The elements in the series whichare
magnetic have lower cohesion ener-gies than one would expect on the
basisof a scaling with the 4d and Sd series.The very low value of
cohesive energyfor manganese implies that it is muchmore
atomic-like and that it does notgain the band energy which
contributesto the cohesive energy of the solid.Perhaps under
pressure manganese be-comes more band-like, and experimentsat very
high pressures would be animportant way to study the
delocaliza-tion of the 3d electrons.
Trends in Crystal Structure
Physical properties are often struc-ture-sensitive, and trends
in structureas a function of pressure are a usefulguide for the
synthesis of new materialswith a given structure or property.
Overone-third of the elements have phasetransitions at room
temperature andpressures of a few hundred kilobars.The available
data are summarized inFig. 1. Evidence for the occurrence ofa
transition is taken from x-ray dif-fraction studies (18),
resistivity mea-surements (19), and the discontinuousappearance of
superconductivity at lowtemperatures (20). In favorable casesthe
high-pressure phase may be re-tained in a metastable state at 1
atmo-sphere (21). The elements with phasetransitions are indicated
by the darkborders, and the structures are desig-nated by numbers
with the highestpressure phase on the bottom. Thenumbers in
parentheses indicate thatthe structure is not known unambigu-ously
(22).
There is a clear trend for elementsin any given group of the
periodic tableto adopt similar structures at high pres-sure. In the
transition metals only amanganese is not isostructural with therest
of its group, but it is anomalousin most of its properties, as
indicatedabove (23). The data in Fig. 1 (whichadmittedly are
incomplete and cover alimited pressure range) lead to
thespeculation that, if enough pressure
SCIENCE, VOL. 176
X --~- - -
K 2 Ca ISC XV r32 MntF . 2 m I Nl I Cu I Zn 3 GO GI4 A a or Kr?
? 23 2 3 (21) 6 26 ?I
RbSr iY 3b2 M Tc 3 Ru 3Rh i(Pd ) m 28 3q- --A s----..--~ v-ao VA
-a .. L&TwD A.t RnHf 3 TO 2 2 E 3I I I x Lx I x I I
La 2i 2i Pr zi Nd 2i Pm EU 2 Od a. Th 3 Dy 3 Ho 3 Er 3 TM3 b i
LU 31 1 i 21 ? 22 2,2 22 22 (21) (21) 2r . (21 (2i
Ac Po U Np Pu Am Cm Sk Cf Es Fm d No LW
I I I __j
1. 1. ntiruimit51 1
11
-
were applied to the elements in eachgroup of the periodic table,
the ele-ments would become isostructural. Thisidea implies that it
is possible to con-struct generalized phase diagrams foreach group
as shown for the group Velements phosphorus, arsenic, anti-mony,
and bismuth (Fig. 3) (24) andthe rare-earth metals (Fig. 4)
(25).The diagram for the group V ele-
ments is speculative and probablywould not be valid for alloys
madefrom other than adjacent elements be-cause of size differences.
However,there is a clear trend both with increas-ing pressure and
with atomic numberfrom a semiconductor to a semimetalwith a
distorted simple cubic structureto a metallic, simple cubic phase.
Thismetallic phase and the higher pressurephases all become
superconducting atlow temperatures.A series of structures involving
only
different stacking sequences of hexag-onal close-packed layers
of atoms isfound in the rare-earth metals (26).Here there is a
relation between thevolume at which a certain phase existsand the
atomic number (or number of4f electrons).The theoretical analysis
of phase
stability is very complicated because ofthe small energy changes
involved inmost phase transitions (27). There areroughly two
classes of transitions:those involving major changes in bond-ing
character, as exemplified by thegroup V elements (Fig. 3), and
thoseinvolving slight distortions from simplestructures or small
changes in volume,as in the rare-earth metals (Fig. 4).Little real
progress has been made inunderstanding the former class of
tran-sitions, but perhaps some of the recentextensions of Pauling's
concept of"ionicity" will lead to new insight intothis area (28).
In the latter class oftransitions it may be possible 'to sepa-rate
a structure-determining factor be-cause many of the contributions
to thetotal energy of each phase are mainlyvolume-dependent. In one
reasonablysuccessful model the structure-deter-mining factor is
assumed to be the con-tribution to the total energy arisingfrom the
electronic band structure(29). The energy spectrum of theelectron
gas will be perturbed by thelattice. In many cases a distortion
orchange in the stacking sequence ofsimple structures can lower the
bandstructure energy, and in this way manyof the observed
structures and phasetransitions have been rationalized (29).19 MAY
1972
A Dozen New Superconductors
The occurrence of numerous phasetransitions at high pressure has
led tothe discovery of many new supercon-ductors. At the present
time it is knownthat one-third of the elements that be-come
superconducting are supercon-ducting only at high pressure. This
in-formation is summarized in the upperpart of Fig. 1 where the
diagonal linesindicate the superconducting elementsand the solid
triangles designate thosethat are superconducting only at
highpressure (30). In some cases it is pos-sible to quench the high
pressure phases[for example, antimony (31)] and tostudy the
superconducting properties atI atmosphere, but this possibility is
morecommon in compounds than it is in theelements. The compounds
containingelements from groups III and V, suchas InSb (32), were
among the first tobe quenched, and recently reasonablyhigh
transition temperatures (15 to17K) have been observed in
somecarbides that were synthesized at highpressure and then
quenched to ambientconditions (33). In the case of theelements it
has been necessary to carryout experiments under the difficult
con-ditions of simultaneous ultrahigh pres-sures and ultralow
temperatures. These
Pressure (kbar)Fig. 2. A plot of bulk modulus versuspressure for
tungsten and cesium showinga linear increase in tungsten and
anoma-lous stiffening in cesium between phasetransitions at 42.5
and about 120 kilobars.Below and above these transitions cesiumhas
a linear increase in bulk modulus withpressure. Circles and
triangle are fromstatic compression measurements, andsquares and
diamonds are from shock-wave experiments, as discussed in thetext
and (11, 13).
studies have led to a much broader viewof superconductivity than
one mighthave had a decade ago. Not only doessuperconductivity
occur in some transi-tion elements and in a few elementslike tin
and lead, but it has been foundat high pressure even in the
alkali[cesium (34)] and alkaline-earth [bari-um (35)] eiements and
in many of theelements on the right side of the peri-odic table
[selenium (36) and tellurium(37), for example]. There appears tobe
a general trend in which the super-conducting transition
temperatures im-crease in successive high pressurephases; for
example, the sequence inbismuth is 3.9, 7.10, and 8.3K (38).In
essentially all cases the appearanceof superconductivity is
accompanied bya change in crystal structure. This re-sult further
emphasizes the sensitivityof many phenomena to structure. [Inthe
case of yttrium there is no evidencefor a phase transition, but
high pres-sure x-ray studies have not been made(34).] Studies at
high pressure havenot yet led to a definitive answer to thequestion
of how or whether supercon-ductivity is destroyed by pressure in
agiven structure. In many materials thesuperconducting transition
temperaturedecreases with pressure. Whether it ap-proaches zero
exponentially or linearlyas a function of volume is difficult
todecide experimentally. This investiga-tion would be an
interesting test ofsome aspects of the theory of
super-conductivity.
Onset of Magnetism in TransitionMetals
Although superconductivity is a verycommon phenomenon among the
ele-ments, magnetism is limited to the 3dmetals chromium,
manganese, iron, co-balt, and nickel and to the rare-earthmetals
with partially filled 4f states. Inan effort to understand the
origins ofmagnetism, the onset at chromium hasbeen studied
extensively. The magneticproperties of chromium are very sensi-tive
to the detailed shape of the Fermisurface which proves, in turn, to
bevery pressure-sensitive.Chromium is probably the unique
example of itinerant antiferromagne-tism; that is, above a Neel
temperatureTN of 312K there is no evidence forthe existence of
localized magnetic mo-ments such as are found, for example,in the
rare-earth metals. The Fermisurface is such that there are
pockets
753
-
of electrons and holes of similar sizeand shape. It is believed
that the at-tractive coulomb interaction betweenthe electrons and
holes in the matchingpockets causes them to condense
intoelectron-hole pairs with parallel spin(triplet excitons) at low
temperatures.This condensation is a cooperative ef-fect which
results in a spin density wavewhose periodicity may or may not
becommensurate with the lattice (39).The fact that the
electron-hole pairshave a lower energy at low tempera-tures results
in the formation of atemperature-dependent energy gap inthe
electronic spectrum. The density ofstates at the Fermi surface is
reducedas the excitons condense. This causesthe electrical
resistivity to increase be-low TN. By studying the electrical
re-sistivity as a function of temperatureat different pressures,
the dependence
of TN on pressure may be determinedas shown in Fig. 5 (19, 40).
In addi-tion, by scaling the resistivity in theparamagnetic region
of two curves atwidely different pressures, the
magneticcontribution to the resistivity below TNmay be obtained.
This contribution maybe interpreted in terms of the
tempera-ture-dependent energy gap and thefraction of the Fermi
surface that isdestroyed by the magnetic ordering(40). This is
shown in the inset in Fig.5. The energy gap at T = 0K, ob-tained
from fitting the data to the theo-retical curve (solid line in the
inset),is close to that observed directly inoptical measurements
(41).The Neel temperature of chromium
is much more strongly pressure-depen-dent than that of any of
the other ele-ments which order magnetically, with(d ln TN)/(d ln
V) = 26.5 for chro-
mium and 0 to 2 for other elements(4). Furthermore, the pressure
depen-dence is not linear but follows the rela-tion
TN = TN(O) exp(-26.5AV/Vo)where TN(O) and V0 refer to
quantitiesin the limit of P--0. This exponentialvariation is
predicted by the theory forhigh temperatures (40). In the
availa-ble pressure range (P < 80 kilobars)the
antiferromagnetism in chromiumcannot be completely suppressed,
but,by alloying chromium with smallamounts of vanadium or
molybdenum,it is possible to lower the critical pres-sure into the
experimentally observablerange. This result is shown in Fig. 6for
three alloys for which the magneticcontribution to the resistivity
in thelimit of T-*0K is plotted versus pres-sure (41, 42).
Theoretically it has been
E0-W
a
E
E0
E-a>
0 2 4 6 8 10 12 14La Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er Tm Yb
Lu,Y
0 l I \II I 1] Fig. 3 (left). Tentative generalized phase
diagram for theP As Sb Bi group V elements showing the trend with
increasingpressure or atomic number from semiconductor to
metal.
Squares and diamonds are from resistivity measurements of McWhan
and Kolobyanina et al. (24), respectively; open and solidsymbols
designate different phase transitions; triangles are the pressures
at which discontinuous changes in the superconducting transi-tion
temperature occur; circles are from x-ray measurements; the
inverted triangle is from Bridgman's resistivity measurements
[see(13)]. Fig. 4 (right). Tentative generalized phase diagram for
the rare-earth metals showing the sequence of close-packedphases as
a function of volume and of the number of 4f electrons or atomic
number (25); (circles) face-centered cubic to doublehexagonal
close-packed transition; (diamonds) samarium type to double
hexagonal close-packed transition; (squares and triangles)hexagonal
close-packed to samarium type transition in elements and alloys,
respectively.
-
30)
cncn0)0-
SCIENCED VOL. 176754
-
shown that at low temperatures theband structure of the group VI
ele-ments (chromium, molybdenum, andtungsten) leads to anomalies in
the one-electron band susceptibility (43). Asa result of these
so-called "Kohn cusp-type" anomalies, it has been predictedthat the
Neel temperature as TN-Oshould be a linear function of (P -PC)where
P, is the critical pressure for thesuppression of
antiferromagnetism (41).In alloys there are depairing effects(that
is, breaking up of electron-holepairs by impurities) which tend
tosmear out the anomalies and to changethe pressure dependence. The
availableresults on a series of alloys stronglysuggest that in pure
chromium TN willgo to zero linearly in agreement withthe theory.
The shape of the magneticresistivity curves shown in Fig. 6 ismuch
more difficult to calculate. Alarge change in slope is expected
be-tween the low- and high-temperatureregions because of the
impurity depair-
1.0, . .
4-
0.60.7 0.8 0.9 1.0
T/TN00Coo0')
0.4
100 200
ing effects, but the detailed shape ofthe curve is not
understood (42).
Pure chromium has a periodicity thatis incommensurate with the
lattice, butthe addition of manganese to chromi-um results in a
discontinuous jump toa commensurate structure. A similartransition
as a function of an externalvariable such as pressure was
predictedtheoretically on quite general grounds(44). Recent
experiments show a sharpchange in the pressure dependence ofTN as a
function of pressure in a seriesof chromium-manganese and
chromi-um-ruthenium alloys, in agreement withthe theoretical
prediction (42). Bymapping out the temperature-pressurephase
diagram for these alloys, it hasbeen possible to determine
experimen-tally the change in TN resulting fromthe depairing
effects of the impurities.Thus significant contributions to
theunderstanding of magnetism in chromi-um have been made by the
use of thepressure variable.
0
t
I-
'1
Semimetal-Semiconductor Transitions:The Group V Elements
The magnetism in chromium is de-pendent on a subtle matching of
partsof the Fermi surface, and the magnet-ism is very
pressure-sensitive. It is alsopossible to have electronic
transitionsthat involve not cooperative effects butthe crossing or
uncrossing of the con-duction and valence bands as a functionof
pressure at temperatures above 0K.This type of
semimetal-semiconductortransition seems to occur in the
face-centered cubic, divalent metals calcium(19), strontium (45),
and ytterbium(45, 46) and also in some of the groupV elements. In
an effort to illustrate thevariety of phenomena that can occur,
Iwill discuss the group V elements inmore detail.The crystal
structures of arsenic,
antimony, and bismuth at 1 atmospheremay be viewed as a
rhombohedral dis-tortion of a simple cubic structure in
20 40 60Pressure (kbar)
300
Temperature (OK) Fig. 5 (left). A plot of resistance R versus
temperature forchromium at pressures of 26.5, 45.7, and 64.9
kilobars (from
top to bottom) showing the strong dependence of the Neel
temperature on pressure (from 40). The inset shows the
temperaturedependence of the magnetic contribution to the
resistivity; points in the inset are the difference between the
solid and scaled dashedcurves in the main figure (see text). The
solid, theoretical curve in the inset is calculated as described in
the text and (40). Fig. 6(right). Variation of the magnetic
contribution to the resistivity with pressure at 4.2K for several
chromium alloys showing the sup-pression of the itinerant
antiferromagnetic state with pressure [from (42)]. Distinct symbols
represent separate experiments.19 MAY 1972 755
-
which there are two atoms per unit cellat the positions (u,u,u).
If -the rhom-bohedral angle, a, equals 600 and thepositional
parameter, u, equals 1/4, thenthe structure is simple cubic. The
param-eters for the group V elements are inthe range a = 540 to 570
and u = 0.23to 0.24 with the distortion from thesimple cubic
structure decreasing as onegoes from arsenic to bismuth (47).Both a
and u approach the cubic valueswith increasing pressure (48). This
in-crease in a toward 600 with decreasingvolume, ( V0- V) / V0, is
illustrated forantimony and arsenic in Fig. 7 (49).
Although the structural properties ofthe group V elements show a
markedsimilarity with increasing pressure, theirelectrical
properties vary quite differ-ently. The temperature coefficient
ofelectrical resistivity of bismuth becomesnegative at high
pressure (see Fig. 8),thus suggesting that bismuth has a
con-tinuous transition from a semimetalwith a small overlap of the
valence andconduction bands to a semiconductor(50, 51). Measurement
of the pressuredependence of both the frequency ofthe de
Haas-Shubnikov oscillations (52)and the residual resistivity (51)
con-firms that the Fermi surface shrinksto zero at P = 25
kilobars.The Fermi surface of bismuth con-
sists of separated pockets of electronsand holes in momentum
space, where-as that of arsenic consists of holes andpockets that
are connected (53). If thepockets become disconnected,
therebychanging the topology of the Fermi sur-face, then the
resulting electronic transi-tion will cause anomalies in many ofthe
physical properties (54). Frommeasurements of the pressure
derivativeof the de Haas-van Alfen frequenciesand from band
structure calculations, ithas been shown that this transition
oc-curs in arsenic as a function of pressureat 1.8 kilobars and
1.1K (55). Theelastic properties of arsenic are muchmore
anisotropic than those of anti-mony and bismuth as illustrated in
Fig.7 (56). Initially the rhombohedral angleincreases rapidly and
then approachesthe slope observed for antimony (57).It is possible
that this result reflects theelectronic effects observed at low
tem-peratures, and further work on arsenicmay lead to new insight
into this typeof electronic transition.
In both bismuth and arsenic theFermi surface shrinks with
increasingpressure, making them less metallic. Inantimony, however,
which lies betweenarsenic and bismuth in the periodictable, the
number of carriers increases
756
-o0 -
); 5970oA
8-5
co@57
0
-0
Xi 54t f0.05 0.10 0.15 0.20
(VO v)/vOFig. 7. Variation of the rhombohedralangle of the unit
cells of antimony andarsenic with volume, showing the ap-proach
toward an undistorted simple cubiccell (a = 600). Triangles are
from linearcompression measurements, and other sym-bols are from
x-ray measurements (49).The initial slopes for antimony and
arsenicare calculated from elastic constant andx-ray measurements,
respectively (56). Thedotted curve for antimony is from
Kolo-byanina et al. (18).
with increasing pressure, making itmore metallic (58). This is
illustratedin Fig. 8 where the temperature depen-dence and
magnitude of the resistivityof antimony are those corresponding
tometallic behavior even at 45 and 60kilobars (59). Using band
structurecalculations based on the pseudopo-
o-2
Bismuth
10~ 25 kbar
E 1 0 \4
E 1 atm0
U) 10-5f 60kbar
-6 Antimony
10-l
O 100 . 200
Temperature (K)
Fig. 8. Resistivity versus temperature atdifferent pressures for
bismuth (51) andantimony (59) showing how bismuth be-comes less
metallic and antimony becomesmore metallic with increasing
pressure.Curves for bismuth are for 25 kilobarsand after releasing
pressure.
tential method, one can fit the experi-mental Fermi surface data
at 1 atmo-sphere, but one cannot at presentexplain the differences
in the pressuredependence of the Fermi surfaces ofarsenic,
antimony, and bismuth (60).
New Transition Metals?
A more fundamental change in bandstructure as a function of
pressure isthought -to occur in some of the pre-transition elements
and the light rare-earth metals. It appears that underpressure
these elements become similarto d transition metals. The most
com-plete data are available for cesium andcerium for whic;h it is
believed that6s and 4f electrons, respectively, arepromoted to Sd
states. Recent experi-ments on both metals suggest that asingle
transition to d states does nottake place but that an
intermediatephase exists.
Cesium is the most anomalous ofthe alkali metals with respect to
theeffects of pressure. It has a maximumin its melting point as a
function ofpressure and a subsequent minimumat the 42.5-kilobar
transition (61). Ata pressure of 42.5 kilobars there areactually
two closely spaced transitions.At the first there is no change in
struc-ture, but there is a 12 percent changein volume and an
increase of a factorof 2 in the electrical resistivity (62).When
the pressure has increased by lessthan 1 kilobar, there is another
transi-tion accompanied by a phase change toan unknown structure
and a drop inthe resistivity to the value observed forthe first
phase (62). Both the first andthird phases have an anomalously
largeT2 term in the resistivity which peaksat the transition in
each phase (63).There are further anomalies in the be-havior of
cesium near 120 kilobars. Asshown above, the bulk modulus
risesanomalously between 42.5 and 120kilobars. Superconductivity
has beenobserved only above 120 kilobars (34).Finally, there is a
large rise in resistivityat 120 kilobars. A similar rise in
resis-tivity has been observed at higher pres-sures in the other
alkali metals withavailable d states (for example, rubidi-um and
potassium) (64). These resultssuggest that the conversion to a d
tran-sition metal starts at 42.5 kilobars, butthat it is not
complete until 120 kilo-bars.The transition from rare-earth
metal
to d transition metal in cerium appearsto take place in stages.
At a pressure
SCIENCE, VOL. 176
-
of I atmosphere cerium has one local-ized 4f electron, and it
orders antiferro-magnetically at low temperatures. At 7kilobars
there is a transition with nochange in structure and an 11
percentchange in volume (18). However, ce-rium becomes
superconducting only at50 kilobars where there is another
phaNetransition (65). The structure of thehigher pressure phase
resembles thatfound in the group IV elements tita-nium, zirconium,
and hafnium, whichis the structure cerium would have if ithad
completely lost its 4f electron (18.22). The intermediate phase
appears tobe a transitional one between localized4f states and Sd
states, as it is neithermagnetically ordered nor superconduct-ing
(66).
Summary
In this article I have touched on onlya few of the areas of
research in whichthe pressure variable has led to new in-sight into
materials at 1 atmosphere.Clearly in compounds other trends
instructure and physical properties aresignificant, and many of
them havebeen studied extensively. One examipleis in the transition
from metal to in-sulator which occurs in several transi-tion metal
oxides. The combination ofdoping experiments and experiments athigh
pressure has shown that V.Oa0must be considered as part of a
more(eneral phase diagram for metal-insula-tor transitions (67). It
has been arguedthat the transition involves a funda-mental change
from band states in themetal to localized states in the
insulator(67) and that it is a "Mott" transition(68). As
theoretical interest in thepressure variable increases, it is
reason-able to look forward to the continuedgrowth of research at
high pressure.
References and Notes
f. Electrical properties: H. G. Drickamci,G. K. Lewis, Jr., S.
C. Fung, Science 163.885 (1969); H. G. Drickamer, ibid. 156,
1183(1967); ibidl. 142, 1429 (1963).
2. Semicondtictors: W. Paul and D. M.Warschauer, in Solids
utnder Pressutre, W.Paul and D. M. WarschauLer, Eds. (McGraw-Hill,
New York, 1963).
a. SUperconductivity: R. 1. Boughton, J. L.Olsen, C. Palmy,
Progr. Low Temp. Ph/'s. 6,169 (1970): N. B. Brandt and N. I.
Ginzburg,Sci. Aster. 224, 33 (April 1971); Usp. Fiz.Nauik 98, 95
(1969) [Sos'. Phys. Usp. 12, 344(1969)].
4. Magnetism: D. Bloch and A. S. Pavlovic,Ad/san. High Pressure
Res. 3, 41 (1969); D.Bloch, High Temp. High Pressutre 1, 1
(1969).
5. Ferroelectricity: G. A. Samara, Adi'ani. HighPressure Rei. 3,
155 (1969).
6. Geophysics: F. R. Boyd, Science 145, 13(1964); R. C. Newton,
Ads an. High Pre.ssureRes. 1, 195 (1966).
7. Chemistry: E. Whalley, AnIill. Rev'. Phy,s.Chein. 18, 205
(1967).
8. StrLicture: D. B. McWhan, Ed. "Proceedings19 MAY 1972
of the Sympositlill on X-ray Diffraction atHigh Pressure,"
Tranis. Amer. Crystallogr.Ass. 5, 1 (1969); M. D. BantLis, High
Tenip.Higli Pressu(re 1, 483 (1969); W. Klement, Jr.,and A.
Jayaraman, Progr. Solid State Chem.3, 289 (1966).
9. Data taken from K. A. Gschneidner, Jr.,Solid State Phys. 16,
275 (1964): L. Brewer.Scienice 161, 115 (1968).
10. L. V. Al'tshuler, Usp. Fiz. Nauik 85, 197(1965) [Sos'. Phys.
Usp. 8, 52 (1965)].
11. Data f1 om (9) and from R. G. McQueen andS. P. Marsh. J.
Appl. Phys. 31, 1253 (1960).
12. See, for example, 0. L. Anderson, J. Phys.Chein. Solids 27,
547 (1966). The linearcompressicn in a hexagonal crystal can
beobtained by using the more drastic assump-tion of constant
contributions to Eq. 1 fromthe different directions [D. B. McWhan,
J.A ppl. Ph 's. 38, 347 (1967) and R. N.Thurston, J. Acoiist. Soc.
Amner. 41, 1092(1967)]. Other empirical equations based onin
expansion of the frce energy in powersof strain are equally useful
in representingcompression data and for extrapolating topressures
above the measured range. ForpressuLres in excess of a few kilobars
an ex-pansion of the volume as a power series inpressure with only
two or three terms isinadeqtiate.
3. The curve for cesium in Fig. 2 was ob-tained from a
presstire-volume curve con-striscted from available static, bulk,
and x-raycompression meastirements [M. S. Anderson,E. J. GtLitman,
J. R. Packard, C. A. Swenson,J. P/hys. Cheuu. Solids 30, 1587
(1969) andH. T. fll, L. Merrill, J. D. Barnett, Science146, 1297
(1964)]. The 100-kilobar data ofBridgman were scaled to agree with
therevised pressuLre and VolIme of the 42.5-kilobar transition [P.
W. Bridgman, Proc.Assser. Acad. Arts Sci. 76, 55 (1948)]. Thebulk
moduli wese then determined graph-ically. The higher pressure
points were cal-CUlated from stLiccessive data points obtainedfrom
shock-wave experiments [M. H. Rice,J. Phi's. Chem. Solids 26, 483
(1965)]. It isnot strictly correct to compare the adiabatic,dynamic
data with the isothermal, static data,but the necessary
cos-rections will not changethe qualitative ti-cnd shown in Fig.
2.
14. L. V. Al'tshuler and A. A. Bakanova, Usp.FiZ. Nassk 96, 193
(1968) [Sos. Phys. Usp.11, 678 (1969)].
15. R. Sternheimer, P/hys. Res. 78, 235 (1950);E. S. Alekseev
and R. G. Arkhipov, Fiz.Ts'erd. Tela 4, 1077 (1962) [Sos'. Phys.
SolidState 4, 795 (1962)].
16. R. E. Peeitls, Quantumn T/heory' of Solids(Oxford Univ.
Press, London, 1955).
17. J. Friedcl, Trasis. Met. Soc. A,sser. Inst. Miti.ELtg. 230,
616 (1964); in P/l 'sics of Metals,J. Zimnan, Ed. (Cambridge Univ.
Press, Lon-don, 1969). vol. 1; R. E. Watson and H.Ehrenteich,
Committ7etits Solid State Phys. 3,116 (1970).
18. Ccsium: H. T. Hall ,et al. (13); magnesium:H. G. Drickamer,
R. W. Lynch, R. L.Clendenen, E. A. Perez-Albuerne, Solid
StateP/lirs. 19, 135 (1966); strontium: D. B.McWhan and A.
Jayaraman, Appl. P/hy's.Lett. 3, 129 (1963); barium: J. D.
Barnett,R. B. Bennion, H. T. Hall, Science 141, 534(1963);
titaniumi and zirconium: J. C.Jamieson, ibid. 140, 72 (1963);
chromium:NM. 0. Steinitz, L. H. Schwartz, J. A.Marcus, E. F;awcett,
W. A. Reed, Phys. Re'.Le'tt. 23, 979 (1969); iron: T. Takahashi
andW. A. Bassett, Scienice 145, 483 (1964); mer-cuiry: M. Atoji, J.
E. Schirber, C. A. Swen-son, J. C/seiis. P/hYs. 31, 1628 (1969);
gallium:L. F. Vereslclagin, S. S. Kabalkina, Z. V.Troitskaya, Dokl.
Akad. Nauk SSSR 158,11561 (1964) [Sos. P/si's. Dokl. 9, 894
(1965)];C. E. Weir, G. J. Piermarini, S. Block, J.Chem. Phys. 54,
2768 (1971); thallium: G. J.Piermarini and C. E. Weir, J. Res. Nat.
Bur.Stand. Ser. A 66, 325 (1962); silicon andgermanium: J. C.
Jamieson, Scienice 139, 762(1963); J. S. Kasper and S. M.
Richards,Acta Cr.stallogr. 17, 752 (1964); tin: J. D.Bas-nett, R.
B. Bennion, H. T. Hall, Science141, 1041 (1963); lead: T.
Takahashi, H. K.Mao, W. A. Bassett, ibid. 165, 1352
(1969);phosphorus: J. C. Jamieson, ibid. 139, 1291(1963); arsenic:
M. J. Duggin, personal com-mtinication: antimony: T. N.
Kolobyanina,S. S. Kabalkina, L. F. Vereshchagin, L. V.Fedina, Zl/.
Eksp. Teor. Fiz. 55, 164 (1968);Sos. P/hsi. J. Exp. T/leor. Phys.
28, 88 (1969);T. R. R. McDonald, E. Gregory, G. S.Barberich, D. B.
McWhan. T. H. Geballe.
Ci. W. Hull, Phls. Lett. 14, 16 (1965); D. B.McWhan, unpublished
results; bismuth: R. M.Brugger, R. B. Bennion, T. G. Worlton,Phys.
Lett. A 24, 714 (1967); J. S. Kasper,Tranis. Amner. Crystallogr.
Ass. 5, 16 (1969);R. M. Brugger, R. B. Bennion, T. G.Worlton, W. R.
Myers, ibid., p. 152; tellur-iuLm: J. C. Jamieson and D. B.
McWhan,J. Chemii. Phys. 43, 1149 (1965); S. S.Kabalkina, L. F.
Vereshchagin, B. M.Shuleniin, Zh. Eksp. Teor. FiZ. 45, 2073(1963);
Sov. Phys. J. Exp. Theor. Phi's. 18,1422 (1964); lanthanum: D. B.
McWhanand W. L. Bond, Re'. Sci. Itnstrissm. 35, 626(1964); ceriuLm:
A. Lawson and T. Y. Tang,Phys. Rer. 76, 301 (1949); D. B.
McWhan,Phys. Rev. B 1, 2826 (1970); E. Franceschiand G. L. Olcese,
Phlys. Rev. Lett. 22, 1299(1969); lanthanum, praseodymium,
neodym-ium: G. J. Piermarini and C. E. Weir,Scienc e 144, 69
(1964); T. R. R. McDonald,G. S. Barberich, E. Gregory, in 1964
Symn-pusi/um on High-Pressuire Techns.ology, E. C.Lloyd and A. A.
Giardini, Eds. (AmericanSociety of Mechanical Engineers, New
York,1965); samarium: see A. Jayaraman andR. C. Sherwood (26);
gadolinitim: A. Jayara-nman and R. C. Sherwood, Phlvs. Res'.
Lett.12, 22 (1964); terbium, dysprosium, holmium,erbium, thulium:
D. B. McWhan and A. L.Stevens, Ph/is. RePi. 139, A682 (1965);
ibid.154, 438 (1967); D. R. Stephens and Q.Johnson, J. Less Commoon
Metals 17, 243(1969); see under magnesium, H. G. Drick-amer et al.
(18); ytterbium: H. T. Hall, J. D.Barnett, L. Merrill, Scienice
139, 111 (1963).
19. Lithium, potassium, rtibiditim, calcium, cad-mium, zinc,
selenitim, europium: H. G.Drickamcr, Solid State Phi's. 17, 1
(1965):chromium: P. W. Bridgman, Proc. Amler.Acad. Arts Sci. 68, 27
(1933); T. Mitsui andC. T. Tomizuka, Plh s. Rev. 137, A564
(1965).
21). Arsenic: see N. B. Brandt and N. I. Ginz-burg (U):
scleniUm: sec .1. Wittig, Phys.Rei. Lett. 15, 159 (1965).
21. Phascs of titaniuLm, zirconitim, silicon, ger-mnanium,
antimony, praseodymium, neo-dymiilll, samnaritLm, gadolinitim, and
tcrbitimhaive been quenched from high pressures. Insome cascs
(silicoti, gcrmanitLim, antimony)the retained phase does not always
corre-spond to the equilibriuLm phase at the pres-sure and
temperature fiom which the samplewas quenched.
22. For example, structure changes involvingdifTerent stacking
arrangcments of hexagonalclose-packed laycrs (magnesium, zinc,
cad-tlliLm, gadoliniuLm throtigh thullium). Similaranomalies in
resistivity wes-e observed inmnagnesitim, zinc, and cadmium but
only ininagnesitLim was there evidence fromr x-raydiffraction.
There are conflicting reports on/inc and cadnmium [D. B. McWhan, J.
Appl.P/ii's. 36, 664 (1965)]. BismuLth Ill and anti-mony Ill are
isostrLucttLIal, but the structureis tinknown; a recent suLggested
structureq S. S. Kabalkina, T. N. Kolobyanina, L. F.Vereshchagin,
Zli. Eksp. Teor. Fiz. 58, 486(1970) [Soi. P/Ys. J. ELp. T/ieor.
Phys. 31,259 (1970)]1 does not fit the data of R. M.Brtigger, R. B.
Bennion, T. G. Worlton,W. R. Mlyeris [Tsanis. Amner. C'ri s,
allogr.Ass. 5, 141 (1969)] or satisfy the deWolffreliability test
[P. M. deWolff, ActaCrystallogr. 14, 579 (1961)]. Early reportsof a
phase of tellurium with the arsenicstructure are apparently
incorrect. The datafor cei-ium above 50 kilobars do not fit
ahexagonal close-packed struLcture as well asexpected, but the
earlier suggestion of yet an-other face-centered cubic phase is
notconfirmed.
23. X-raly measurements ulp to pressuLres in excessof 100
kilobars showed no evidence for aphase transition in a
inanganese.
24. Figtire 3 is based on the structural datagiven in Fig. 1 and
transition pressures de-i-ived from resistivity (Bi1 rSb,),
superconduc-tivity measurements g phosphoius: J. Wittigand B. T.
Matthias, Scienice 160, 994 (1968);see also (3); for arsenic, see
(3) and phasediagram studies of alloys of neighboring ele-mients at
I atmospheIc [M. Hansen and K. P.Anderko, Cotnstitutionl of Binary
Alloys (Mc-Graw-Hill, New York, ed. 2, 1958) and firstsupplement by
R. P. Elliott of M. Hansen,Coisitit/itioti of Binary Alloys
(McGraw-Hill,New York, cd. 2. 1965)11. The Bil-.rSb,data are from
the work of McWhan andfrom T. N. Kolobyanina, S. S. Kabalkina, L.F.
Vereshchagin, A. Yamichkov, M. F.Kachan, Z/i. Ek-sp. Teor. Fiz. 59,
1146 (1970)
757
-
[Sov. Phys. J. Exp. Theoret. Phys. 32, 624(1971)]. McWhan made
measurements of re-sistivity versus pressure in a 0.5-inch
(1.2-centimeter) Bridgman anvil geometry. Singlecrystal samples of
bismuth and a bismuthalloy were measured simultaneously.
X-raymeasurements on various samples up toBi0. Sbo06 confirmed that
they are isostructuralabove the transition. The simple cubic
phaseof antimony was not observed in McWhan'smeasurements, and so
it is presumably stableonly over a narrow range of pressure.
25. Figure 4 was constructed from the data inFig. 1 and bulk
compression data (D. B.McWhan, paper presented at the 6th RareEarth
Research Conference, Gatlinburg, Tenn.,1967). Included are points
for the pure ele-ments and for alloys of the heavy
rare-earthelements with each other and with yttriumwhich is assumed
to be similar to lutetium.
26. A. Jayaraman and R. C. Sherwood, Phys.Rev. 134, A691
(1964).
27. P. S. Rudman, J. Stringer, R. I. Jaffee, Eds.,Phase
Stability in Metals and Alloys (Mc-Graw-Hill, New York, 1967).
28. J. C. Phillips, Science 169, 1035 (1970);and J. A. Van
Vechten, Phys. Rev. B 2, 2147(1970).
29. See review by V. Heine and D. Weaire[Solid State Phys. 24,
249 (1970)].
30. Data obtained from (3, 31, 34-38).31. See under antimony, T.
R. R. McDonald
et al. (18).32. H. E. Bommel, A. J. Darnell, W. F. Libby,
B. R. Tittmann, Science 139, 1301 (1963);S. Geller, D. B.
McWhan, G. W. Hull, Jr.,ibid. 140, 62 (1963).
33. M. C. Krupka, A. L. Giorgi, N. H. Krikorian,E. G. Szklarz,
J. Less Common Metals 19,113 (1969).
34. J. Wittig, Phys. Rev. Lett. 24, 812 (1970).35. - and B. T.
Matthias, ibid. 22, 634
(1969).36. J. Wittig, ibid. 15, 159 (1965).37. B. T. Matthias
and J. L. Olsen, Phys. Lett.
13, 202 (1964).38. J. Wittig, Z. Phys. 195, 228 (1966).39. W. M.
Lomer, Proc. Phys. Soc. London 80,
489 (1962); A. W. Overhauser, Phys. Rev.128, 1437 (1962).
40. D. B. McWhan and T. M. Rice, Phys. Rev.Lett. 19, 846
(1967).
41. T. M. Rice, A. S. Barker, Jr., B. I. Halperin,D. B. McWhan,
J. Appl. Phys. 40, 1337(1969).
42. T. M. Rice, A. Jayaraman, D. B. McWhan,J. Phys. Paris 32,
C39 (1970).
43. T. M. Rice and B. I. Halperin, Phys. Rev.B 1, 509
(1970).
44. C. Herring, in Magnetism, G. T. Rado andH. Suhl, Eds.
(Academic Press, New York,1966).
45. D. B. McWhan, T. M. Rice, P. H. Schmidt,Phys. Rev. 177, 1063
(1969), and referencestherein.
46. D. Jerome and M. Rieux, Solid State Com-mun. 7, 957
(1969).
47. R. W. G. Wyckoff, Crystal Structures (Inter-science, New
York, ed. 2, 1963).
48. B. Morosin and J. E. Schirber, Phys. Lett. A30, 512
(1969).
49. The linear compression measurements arefrom P. W. Bridgman
(Proc. Amer. Acad.Arts Sci. 77, 189 (1949)]. The x-ray
measure-ments are from C. W. Huddle, unpublishedresults (inverted
triangles) and McWhan, un-published results (other symbols).
50. P. C. Souers and G. Jura, Science 143, 467(1964); R. Jaggi,
in Proceedings of the In-ternational Conference on the Physics
ofSemiconductors, 7th, Paris, 1964, M. Hulin,Ed. (Academic Press,
New York, 1965), vol.1, p. 413.
51. D. Balla and N. B. Brandt, Zh. Eksp. Teor.Fiz. 47, 1653
(1964) [Sov. Phys. J. Exp.Theor. Phys. 20, 1111 (1965)].
52. E. L. Itskevich and L. M. Fisher, Zh. Eksp.Teor. Fiz. 53, 98
(1967) [Sov. Phys. J. Exp.Theor. Phys. 26, 66 (1968)].
53. P. J. Lin and L. M. Falicov, Phys. Rev. 142,441 (1966).
54. I. M. Lifshitz, Zh. Eksp. Teor. Fiz. 38, 1569(1960) tSov.
Phys. J. Exp. Theor. Phys. 11,1130 (1960)].
55. J. E. Schirber aind J. P. Van Dyke, Phys.Rev. Lett. 26, 246
(1971).
56. The initial slope for arsenic is from x-raymeasurements of
B. Morosin and J. E.Schirber [Solid State Commun. 10, 249
(1972)],and that for antimony is from the elastic con-stants [S.
Epstein and A. P. deBretteville,Phys. Rev. 138, A771 (1965)].
57. An even larger initial slope was calculatedfrom elastic
constants [N. G. Pace, G. A.Saunders, Z. Stimengen, J. Phys. Chem.
Solids31, 1467 (1970)] and linear compression mea-surements [see
Bridgman (19)]. However, theformer is within the experimental error
ofthe x-ray measurement and the latter was notreproducible from
sample to sample.
58. N. B. Brandt, N. YaMinina, Yu. A. Pospelov,Zh. Eksp. Teor.
Fiz. 55, 1656 (1968) [Sov.Phys. J. Exp. Theor. Phys. 28, 869
(1969)];J. E. Schirber and W. J. O'Sullivan, SolidState Commun. 7,
709 (1969).
59. Measurements were made as described in(45).
60. L. M. Falicov, in Physics of Solids at HighPressures, C. T.
Tomizuka and R. M. Emrick,Eds. (Academic Press, New York,
1965).
61. A. Jayaraman, R. C. Newton, J. M. Mc-Donough, Phys. Rev.
159, 527 (1967).
62. See H. T. Hall et al. (13).63. D. B. McWhan and A. L.
Stevens, Solid
State Commun. 7, 301 (1969).64. H. G. Drickamer, Rev. Sci.
Instrum. 41, 1667
(1970); Solid State Phys. 17, 1 (1965).65. J. Wittig, Phys. Rev.
Lett. 21, 1250 (1968).66. M. R. MacPherson, G. E. Everett, D.
Wohlle-
ben, M. B. Maple, ibid. 26, 20 (1971).67. D. B. McWhan, T. M.
Rice, J. P. Remeika,
ibid. 23, 1384 (1969); D. B. McWhan andJ. P. Remeika, Phys. Rev.
B 2, 3734 (1970),and references therein.
68. N. F. Mott, Proc. Phys. Soc. London 62,416 (1949); Phil.
Mag. 6, 287 (1961); J.Phys. Paris 32, Cl (1970).
69. I thank Dr. J. E. Schirber for helpful dis-cussions on the
effect of pressure on arsenic.I thank my colleagues at Bell
Laboratories,in particular T. M. Rice, W. F. Brinkman,M. Marezio,
and A. Jayaraman for manystimulating discussions, and A. L.
Stevensfor technical assistance.
Ever since the introduction of anti-biotics as a means of
controlling infec-tious disease, bacterial strains haveemerged that
are resistant to a varietyof chemotherapeutic agents. The
exam-ination of the mechanism of resistancein resistant strains
which have been iso-lated from nature or derived in thelaboratory
has revealed a variety ofways in which microorganisms can
758
survive in the presence of antibiotics.According to the tenets
of microbialgenetics, mutant strains should acquireresistance to
only one antibiotic at atime, and organisms resistant to
severaldrugs should arise in nature only by theaccumulation of
successive mutations.It is therefore quite extraordinary thatthere
has been a dramatic increase inthe frequency of occurrence of
Entero-
bacteriaceae with multiple drug resis-tance in essentially all
countries inwhich the problem has been examined.
In general, the multiple resistance ofthese microorganisms does
not seem tohave arisen in a series of discrete steps;but rather,
resistance to all of the drugsappears to have been acquired
simul-taneously. Genetic analysis has revealedthat multiple drug
resistance is speci-fied by an extrachromosomal element,which is
referred to as a drug-resistancefactor or R factor. Over the past
decade,R factors have been studied extensivelythroughout the world
because of theirtheoretical and practical consequences.We will
review a number of their prop-erties, particularly those that are
relatedto the biochemical mechanism of mul-tiple drug resistance
and to the waysin which the level of drug resistance ofhost
bacteria may be varied by the rep-lication and the dissociation and
re-association of the components of Rfactors.
The authors are affiliated with the Departmentof Biochemistry
and the Laboratory of MolecularBiology at the University of
Wisconsin, Madison53706.
SCIENCE, VOL. 176
Transmissible Multiple DrugResistance in Enterobacteriaceae
The structure, replication, and mode of action ofdrug-resistance
episomes are discussed.
J. E. Davies and R. Rownd