Superspace symmetry and superspace groups

Superspace symmetry and superspace groups

Sander van Smaalen

Laboratory of Crystallography

University of Bayreuth, Germany

Disclaimer and copyright notice

Copyright 2010 Sander van Smaalen for this compilation.

This compilation is the collection of sheets of a presentation atthe “International School on Aperiodic Crystals,“ 26 September – 2 October 2010 in Carqueiranne, France. Reproduction or redistribution ofthis compilation or parts of it are not allowed.

This compilation may contain copyrighted material. The compilation may not contain complete references to sources of materialsused in it. It is the responsibility of the reader to provide proper citations, ifhe or she refers to material in this compilation.

Symmetry of matter is required for

Determination of crystal structures (avoiding dependent parameters)

Understanding physical properties

Thermal expansion

Elasticity

Non-linear crystals (inversion center)

Neumann's Principle:

Symmetries of a physical property of a material include the crystal point group, but may include more symmetry

Symmetry of aperiodic crystals

Aperiodic crystals lack 3D translational symmetry

Therefore, they cannot have rotational symmetry

Aperiodic crystals are an ordered state of matter:

we call them crystalline

Diffraction gives Bragg reflections

The diffraction pattern possesses 3D point symmetry

Eventually assign this symmetry to the aperiodic crystal structure (superspace groups)

Point group symmetries in 3D space

icosahedron 53m

http://www.SnowCrystals.com

Snow crystal 6/mmm Crystallographic point groups Modulated and composite crystals

7-fold protein n/mmm groups for n = 5,7,8,...

QuasicrystalsPDB: 1TZO

Quasicrystals

Diffraction by a modulated crystal

H = h1 a1* + h2 a2* + h3 a3* + h4 a4* = h1 a1* + h2 a2* + h3 a3* + m q

q = a4* = σ1 a1* + σ2 a2* + σ3 a3* Modulation wave vectorH = (h1 + mσ1) a1* + (h2 + mσ2) a2* + (h3 + mσ3) a3*

S. van Smaalen: Incommensurate Crystallography, Oxford University Press (2007)

Diffraction symmetry of an incommensurately modulated crystal

Main reflections possess point symmetry according to one of the 32 crystal classes Rotational operator R transforms

main reflection into main reflection satellite reflection of order m into satellite of order m

1D modulation: R q → ε q with ε = ±1 q = σ1 a1* + σ2 a2* + σ3 a3* → (σ1, σ2, σ3)Condition for possible modulation wave vectors: (σ1, σ2, σ3) R-1 − ε-1 (σ1, σ2, σ3) = (0, 0, 0)

Implications of mirror symmetry for q

( ) ( ) ( )00032111

321 =− −− σσσεσσσ R

=== −

100010001

1zmRR

( ) ( ) ( ) ( )000200 3321321 ≡=−− σσσσσσσmz with ε = 1 :

mz with ε = -1 :

)0,,( 21 σσ=q

( ) ( ) ( ) ( )000022 21321321 ≡=+− σσσσσσσσ

),0,0( 3σ=qS. van Smaalen: Incommensurate Crystallography, Oxford University Press (2007)

Admissible incommensurate wave vectors for 1D modulations

Triclinic (σ1, σ2, σ3)Monoclinic (σ1, σ2, 0)

(0, 0, σ3)

Orthorhombic (σ1, 0, 0)

(0, σ2, 0)

(0, 0, σ3)

Tetragonal (0, 0, σ3)Trigonal (0, 0, σ3)Hexagonal (0, 0, σ3)Cubic none

Umklapp terms

( ) ( ) ( )*3

*2

*1321

11321 ,,,,,, mmmR =− −− σσσεσσσ

( ) structurebasictheofvectorlatticereciprocal,,* *3

*2

*1 mmm=m

( ) ( ) ( ) ( )*3

*2

*133 001021021 mmm≡=+− σσ

),0,21( 3σ=q),0,0( 3σ=q


Admissible incommensurate wave vectors with non-zero rational components (1D)

Monoclinic—P (σ1, σ2, 1/2) (1/2, 0, σ3) (0, 1/2, σ3)

Monoclinic—B (1/2, 0, σ3)

Monoclinic—A (0, 1/2, σ3)

Orthorhombic—P (1/2, 0, σ3) (0, 1/2, σ3) (1/2, 1/2, σ3)

Orthorhombic—A (1/2, 0, σ3)

Orthorhombic—B (0, 1/2, σ3)

Orthorhombic—C (1, 0, σ3) (0, 1, σ3)

Orthorhombic—F (1, 0, σ3) (0, 1, σ3)

Tetragonal—P (1/2, 1/2, σ3)

Trigonal—P (1/3, 1/3, σ3)

Conclusions—point symmetry

Diffraction symmetry is a 3D point groupPoint symmetry restricts admissible modulation wave vectors q = qr + qi

Combination of 3D point group and q vectors leads to Bravais classes of superspace groups

What about symmetry of the crystal structure?

Symmetry operators and coordinates

=

333231

232221

131211

RRRRRRRRR

R}|{ vR

=

3

2

1

vvv

v

scoordinatespacephysical

3

2

1

=

xxx

x

vxxv +→ RR :}|{ 1:}|{ −→ RR SSv

( )vectorspace

reciprocal

321 SSS=S

}|{}|{ 111 vv −−− −= RRR

ntranslatiolattice}|{ TE

Symmetry operator in direct and reciprocal space

+

=

3

2

1

3

2

1

333231

232221

131211

3

2

1

vvv

xxx

RRRRRRRRR

x'x'x'

=

*3

*2

*1

333231

232221

131211

*3

*2

*1

aaa

aaa

RRRRRRRRR

'''

=

−

3

2

11,

333231

232221

131211

3

2

1

SSS

RRRRRRRRR

S'S'S' t

;

3

2

11,

333231

232221

131211

3

2

1

=

−

aaa

aaa t

RRRRRRRRR

'''

}|{ vR

Lack of translational symmetry in physical space

}2|{}|{}|{ 2aTv EER ==

Modulation wave parallel to a2* : q = σ2 a2*

)( 02224 xltxs ++= σ

)( 420222 sxuxlx ++=

Required phase shift ( )1mod224 nxs σ−=⋅−=∆ Tq

Translations in superspace

⇒= }|{}|{ 2ssss EE aT

}|{}|{ 2aT EE =

24plus σ−=⋅−=∆ Tqsx

Relations between symmetry in physical space and superspace

R is point symmetry in 3D space implies symmetry operators Rs in superspace

H = h1 a1* + h2 a2* + h3 a3* + h4 a4*

a4* = q = σ1 a1* + σ2 a2* + σ3 a3* Modulation wave vector

( )ε

εε

,

*000

000

*3

*2

*1

333231

232221

131211

RR

nnnRRRRRRRRR

Rs =

=

=

n

( ) ( ) ( ) 1with*3

*2

*1321321 ±==− εσσσεσσσ nnnR

Transformation of reflection indices in superspace

H = h1 a1* + h2 a2* + h3 a3* + h4 a4*

H' = h'1 a1* + h'2 a2* + h'3 a3* + h'4 a4*

H and H' describe equivalent reflections: F(H) = F(H')

( )

=

−

=

−

−

−−−

−−

1

1

111

11

*000

*000

εεε mn

R

R

RRs

( ) ( ) ( ) 143214321

−= sRhhhhh'h'h'h'

( ) ( ) ( )*3

*2

*1321

11321 mmmR =− −− σσσεσσσ

Transformation of coordinates by a symmetry operator of superspace

+

=

4

3

2

1

4

3

2

1

*3

*2

*1

333231

232221

131211

4

3

2

1

000

s

s

s

s

s

s

s

s

s

s

s

s

vvvv

xxxx

nnnRRRRRRRRR

x'x'x'x'

ε

x = x1 a1 + x2 a2 + x3 a3

xs = xs1 as1 + xs2 as2 + xs3 as3 + xs4 as4

xi = xsi for i = 1, 2, 3

}|{ ssR v

Transformation of atoms in superspace

;

1000010000100001

4

3

2

1

4

3

2

1

=

s

s

s

s

s

s

s

s

xxxx

x'x'x'x'

inversion center at the origin

( ) ( ) Identity,,,:1,),( tzyxERRs == ε

( ) ( ) inversion,,,:1, tzyxi −−−−−

=

0000

04

03

02

01

s

s

s

s

vvvv

Origin-dependent translational components

)1,1( −=sR

+

=

044

3

2

1

4

3

2

1

000

1000010000100001

ss

s

s

s

s

s

s

s

vxxxx

x'x'x'x'

( ) ( )tvzyxi s −−−−− 04,,,:1,

( ) inversion1,),( −== iRRs ε

404 along

21atcenterinversion ss xv

Intrinsic translations

}|{}|{}|{ 1ssss

ns

ns

nss ERRR Lvvv =++= −

sn

s ER =for

nstranslatiointrinsicgiveSolutions

4

3

2

1

4

3

2

1

≠

l

l

l

l

v

v

v

v

s

s

s

s

Translational components of a superspace mirror plane

( )

=−

1000010000100001

1,zm

( )3,0,0 σ=⇒ q

},,,|1,{}|{ 4321 sssszss vvvvmR −=v

ssssRn Lvv =+⇒= 2

( ) ( )4421 ,,,:1, sssszs xxxxmR −−−=

===

componentsdependent -origin4,3:componentsnaltranslatiointrinsic2,1)1(mod21,0

knsrestrictionovkv

sk

sk

Notation of intrinsic translations

3D-part of translation by the usual symbols 21 screw axis, a-glide, b-glide, c-glide, n-glide and d-glide operatorsIntrinsic translation along the additional axes by symbol:

vs4 0 1/2 1/3 -1/3 1/4 -1/4 1/6 -1/6symbol 0 s t q h

(m,-1) (0, 0, 0, 0) (x, y, -z, -t) mirror(a,-1) (1/2, 0, 0, 0) (1/2+x, y, -z, -t) a-glide(b,-1) (0, 1/2, 0, 0) (x, 1/2+y, -z, -t) b-glide(n,-1) (1/2, 1/2, 0, 0) (1/2+x, 1/2+y, -z, -t) n-glide

qt h

Exercise: translational components for a twofold axis in superspace

( )

=

1000010000100001

12 ,z

( ) ( )4421 ,,,:1,2 ssssz

s xxxxR −−=

( )3,0,0 σ=⇒ q

},,,|1,2{}|{ 4321 ssssz

ss vvvvR =v

}|{}|{}|{ 1ssss

ns

ns

nss ERRR Lvvv =++= −

Solution: translational components for a twofold axis in superspace

},,,|1,2{}|{ 4321 ssssz

ss vvvvR =v

ssssRn Lvv =+= :2 ( ) ( )432143 ,,,2,2,0,0 llllvv ss =

( ) ( )4421 ,,,:1,2 ssssz xxxx −−

l1 = l2 = 0 & vs1, vs2 : no restrictions

origin-dependent components

l3, l4 = 0,1,... ⇒ vs3, vs4 = 0, 1/2 (mod 1)

intrinsic translational components

Twofold screw axes in superspace groups

Point-symmetry operator symbol: (2,1)Superspace group symmetry operator symbol:

(2, 0) (0, 0, 0, 0) (-x, -y, z, t) twofold rotation(21, 0) (0, 0, 1/2, 0) (-x, -y, 1/2+z, t) screw(2, s) (0, 0.5, 0, 1/2) (-x, -y, z, 1/2+t) screw(21, s) (0, 0, 1/2, 1/2) (-x, -y, 1/2+z, 1/2+t) screw

( ) ( )432211 21,,,:,2 ssssssz xxxvxvs +−−

( ) ( )43211 21,21,5.0,:,2 ssssz xxxxs ++−−

Equivalence of superspace groups

=

*4

*3

*2

*1

44*3

*2

*1

333231

232221

131211

*4

*3

*2

*1

000

s

s

s

s

s

s

s

s

QnnnQQQQQQQQQ

''''

aaaa

aaaa

Coordinate transformation Qs provides an alternative unit cellin superspace

Qs is unimodular (3+d)x(3+d) matrix ⇒ space groups

Qs is (3,d)-reduced (of the same type as symmetry operators) ⇒ superspace groups

Example of equivalence in 4D and (3+1)D spaces

=

0010000110000100

1000010000100001

0010000110000100

1000010000100001

(mz, -1) ⇔ (2z, 1) as 4D space group

(mz, -1) ≠ (2z, 1) as (3+1)D superspace group, because Qs is unimodular but not in (3,1)-reduced form

Sources of superspace group information

(3+1)D superspace groups De Wolff, Janssen & Janner (1981) Acta Cryst. A 37, 625; IT-Vol. C Orlov & Chapuis (2005) at http://superspace.epfl.ch

(3+d)D Bravais classes (d = 1, 2, 3)Janner, Janssen & De Wolff (1983) Acta Cryst A 39, 658; IT-Vol. C

(3+d)D superspace groups (d = 1, 2, 3)Yamamoto (2005) at http://quasi.nims.go.jp/yamamoto/spgr.htmlNEW: Harold Stokes, Branton Campbell & S. van Smaalen (2010) submitted to Acta Crystallogr. A

Tables and WEB tool "SSG(3+d)D" Extended information and numerous corrections for d = 2, 3

The number of (super-)space groups

ε*3

*2

*1

333231

232221

131211

000

nnnRRRRRRRRR

Classification Dimension of space or superspace1 2 3 4 3+1 3+2 3+3

Bravais lattices 1 5 14 64 24 83 215Crystal classes 2 10 32 227 31Space groups 2 17 219 4783 755 3338 12584

compare 28 927 922 space groups of dimension six

Stokes, Campbell & van Smaalen, submitted to Acta Cryst. A (2010): SSG(3+d)D

Numbers and symbols for superspace groups

Bravais classes as d.sequence number Examples: 1.24 2.83 3.215

Superspace group 51.3.122.769 is the 769th superspace group with basic space group No. 51 it belongs to Bravais class 3.122

Pcmm(α1, β1, 0)000(- α1, β1, 0)00s(0, 0, γ2)0s0


Symbol ambiguity and symbol degeneracy

Symbols for superspace groups specify generators The symbol is not unique – already so for 3D space groups No. 23 I222 and No. 24 I212121 both contain 2 and 21 axes Eight valid symbols for either group: I222 I2221 I2212 I22121

I2122 I21221 I21212 and I212121

I222 "Origin at intersection of 222" I212121 "Origin at midpoint of three non-intersecting pairs of parallel 2 axes"

Choice: simplest symbol for symmorphic space group Problem much more profound for superspace groups


Symbols for superspace groups

SSG(3+d)D has formulated a series of conventions and rules leading to a unique symbol for superspace groups of dimension (3+d), d = 1, 2, 3. It is advised to always specify the symmetry operators rather than to rely on symbols. Even more so, because often a non-standard setting of the SSG is used. Symbol of SSG depends on a mixture of the BSG setting and SCG setting of the superspace group.

54.2.29.32 Pbcb(0, β1, 0)000(0, 0, γ2)s00Intrinsic translations from reflection conditions in SCG setting.


SSG(3+d)D: 11.1.6.4 P21/m(1/2,0,γ)00

Superspace group: 11.1.6.4 P2_1/m(1/2,0,g)00 [Y:1.37]Bravais class: 1.6 P2/m(1/2,0,g) [JJdW:1.6]Transformation to supercentered setting: A1=2a1+a4, A2=a2, A3=a3, A4=a4

BASIC SPACE GROUP SETTINGModulation vectors: q1=(1/2,0,g)Centering: (0,0,0,0)Non-lattice generators: (-x,-y,z+1/2,-x+t); (x,y,-z+1/2,x-t)Non-lattice operators: (x,y,z,t); (-x,-y,z+1/2,-x+t); (-x,-y,-z,-t); (x,y,-z+1/2,x-t)

SUPERCENTERED SETTINGModulation vectors: Q1=(0,0,G), where G=gCentering: (0,0,0,0); (1/2,0,0,1/2)Non-lattice generators: (-X,-Y,Z+1/2,T); (X,Y,-Z+1/2,-T)Non-lattice operators: (X,Y,Z,T); (-X,-Y,Z+1/2,T); (-X,-Y,-Z,-T); (X,Y,-Z+1/2,-T)Reflection conditions: HKLM:H+M=2n; 00LM:L=2n


35.2.24.5 Cmm2(1,0,γ1)000(0,0,γ2)000

Superspace group: 35.2.24.5 Cmm2(1,0,g1)000(0,0,g2)000 [Y:2.764]Bravais class: 2.24 Cmmm(1,0,g1)(0,0,g2) [JJdW:2.24]Transformation to supercentered setting: A1=a1+a4, A2=a2, A3=a3, A4=a4, A5=a5

BASIC SPACE GROUP SETTINGModulation vectors: q1=(1,0,g1), q2=(0,0,g2)Centering: (0,0,0,0,0); (1/2,1/2,0,0,0)Non-lattice generators: (-x,y,z,-2x+t,u); (x,-y,z,t,u); (-x,-y,z,-2x+t,u)Non-lattice operators: (x,y,z,t,u); (-x,-y,z,-2x+t,u); (-x,y,z,-2x+t,u); (x,-y,z,t,u)

SUPERCENTERED SETTINGModulation vectors: Q1=(0,0,G1), Q2=(0,0,G2), where G1=g1, G2=g2Centering: (0,0,0,0,0); (1/2,1/2,0,1/2,0)Non-lattice generators: (-X,Y,Z,T,U); (X,-Y,Z,T,U); (-X,-Y,Z,T,U)Non-lattice operators: (X,Y,Z,T,U); (-X,-Y,Z,T,U); (-X,Y,Z,T,U); (X,-Y,Z,T,U)

Reflection conditions: HKLMN:H+K+M=2nStokes, Campbell & van Smaalen, submitted to Acta Cryst. A (2010): SSG(3+d)D

221.3.210.7 Pm-3m(0,β,β)000(β,0,β)000(β, β,0)000

Superspace group: 221.3.210.7 Pm-3m(0,b,b)000(b,0,b)000(b,b,0)000 [Y:3.11160]Bravais class: 3.210 Pm-3m(0,b,b)(b,0,b)(b,b,0) [JJdW:3.212]Transformation to supercentered setting: A1=a1, A2=a2, A3=a3, A4=a5+a6, A5=a4+a6, A6=a4+a5

BASIC SPACE GROUP SETTINGModulation vectors: q1=(0,b,b), q2=(b,0,b), q3=(b,b,0)Centering: (0,0,0,0,0,0)Non-lattice generators: (x,y,-z,-u+v,-t+v,v); (-z,-x,-y,-v,-t,-u); (y,x,z,u,t,v)Non-lattice operators: (x,y,z,t,u,v); (x,-y,-z,-t,-t+v,-t+u); (-x,y,-z,-u+v,-u,t-u)... (48)

SUPERCENTERED SETTINGModulation vectors: Q1=(B,0,0), Q2=(0,B,0), Q3=(0,0,B), where B=bCentering: (0,0,0,0,0,0); (0,0,0,1/2,1/2,1/2)Non-lattice generators: (X,Y,-Z,T,U,-V); (-Z,-X,-Y,-V,-T,-U); (Y,X,Z,U,T,V)Non-lattice operators: (X,Y,Z,T,U,V); (X,-Y,-Z,T,-U,-V); (-X,Y,-Z,-T,U,-V);... (48)Reflection conditions: HKLMNP:M+N+P=2n


P21(0,0,γ)s ⇔ P21(0,0,γ)0

Input setting Centering noneOperators (x,y,z,t); (-x,-y,z+1/2,t+1/2)

Standard settingsSuperspace group: 4.1.5.2 P2_1(0,0,g)0 [Y:1.5]Bravais class: 1.5 P2/m(0,0,g) [JJdW:1.5]Transformation to supercentered setting: noneModulation vectors: q1=(0,0,g)Centering: (0,0,0,0)Non-lattice generators: (-x,-y,z+1/2,t)Non-lattice operators: (x,y,z,t); (-x,-y,z+1/2,t)Reflection conditions: 00lm:l=2nTransformation matrix to standard supercentered setting <deleted>d = 1: (0, 0, γ) transformed into c* - (0, 0, γ) = (0, 0, 1- γ)d = 2, 3: mixing of q vectors


Conclusions

Symmetry of aperiodic crystals is based on point symmetry in physical (3D) space

(3+d)D Superspace groups are a (3,d)-reducible subset of (3+d)D space groups

Equivalence of superspace groups is non-intuitive Preferably employ the supercentered group (SCG) setting SSG(3+d)D: WEB tool for d = 1,2,3 superspace groups.

See Harold Stokes, Branton Campbell & S. van Smaalen (2010) submitted to Acta Crystallogr. A.

Symmetry restrictions by superspace groups

Sander van Smaalen

Laboratory of Crystallography

University of Bayreuth, Germany

Symmetry of the generalised electron density

{ }4321 ,,,|, sssss vvvvRR ε=

+

=

4

3

2

1

4

3

2

1

*3

*2

*1

333231

232221

131211

4

3

2

1

000

s

s

s

s

s

s

s

s

s

s

s

s

vvvv

xxxx

nnnRRRRRRRRR

x'x'x'x'

ε

and are in different sections t.

4sx4sx'

One atom of the generalised electron density

)( xq⋅++= tuxx isisi

)(44 xquq ⋅+⋅+= txx ss

xq ⋅+= txs40xLx +=

Atomic string:

),,,( 4321 ssss xxxx

),,()( 332211µµµ

µµ ρρ ssssssss xxxxxx −−−=x

'Line' atoms instead of point atoms:

variation of t from 0 to 1

Structural parameters for a modulated structure

Each independent atom µ = 1,...,N of the basic structure has parameters:

( ))()( 00 xLq +⋅+++= tuxlx iiiiµµ

( ) ( )∑∞

=

+=1

444 2cos2sin)(n

sμn,is

μn,isi xnπBxnπAxuµ

position in the unit cell (3)

temperature parameters (6)

modulation parameters (6nmax)μn,i

μn,i B,A

μi,jU

( )][][][][ 03

02

01

0 µµµµ x,x,x=x

{R | v} is symmetry of the basic structure

{ }4321 ,,,|, sssss vvvvRR ε=

+

=

3

2

1

3

2

1

333231

232221

131211

3

2

1

)1()1()1(

)2()2()2(

s

s

s

s

s

s

s

s

s

vvv

xxx

RRRRRRRRR

xxx

+

=

4

3

2

1

4

3

2

1

*3

*2

*1

333231

232221

131211

4

3

2

1

000

s

s

s

s

s

s

s

s

s

s

s

s

vvvv

xxxx

nnnRRRRRRRRR

x'x'x'x'

ε

Transformation of modulation functions

Modulation functions are functions of the basic structure coordinates.

The transformation of a function of coordinates is

in case of zero rational components (supercentered setting)

Rotation of the modulation functions (not for occupancy)

Change of their arguments

)( 4siiisi xuxxx +==

( ) ( )][]}|{[][ 4411

411

42

sssssss RRRx vxuxvuu −== −− ε

Example of mirror symmetry

( )

=−

1000010000100001

1,zm

( )1, −= zs mR ( )γ,0,0=q

}0,0,0,0|1,{}|{ −= zss mR v

−=

)1()1()1(

)2()2()2(

3

2

1

3

2

1

s

s

s

s

s

s

xxx

xxx

−−−−

=

)()()(

)()()(

413

412

411

423

422

421

s

s

s

s

s

s

xuxuxu

xuxuxu

( ) ( )tzyxm −− ,,,:1,

Special positions

A special position is a position in the unit cell that is left invariant by the symmetry operator

An atom at a special position is mapped onto itself by the symmetry operator

As a consequence restrictions apply to the structural parameters of this atom

But

In superspace 'atoms' are lines instead of points

This gives additional possibilities and degrees of freedom

Symmetry of a structure in superspace


Restrictions on the modulation functions


Tc1 = 145 K q = (0, 0.241, 0)

|u(Nb3)| = 0.05 Å

Modulation of Se through elastic coupling toward Nb3

NbSe3 SSG 11.1.5.3 P21/m(0,β,0)s0

CDW along b*

Atomic modulation functions

All atoms in mirror planes

S. van Smaalen et al., Phys. Rev. B 45, 3103-3106 (1992)

Mirror plane of P21/m(0,β,0)s0

( )

=−

1000010000100001

1,ym

}0,0,2/1,0|1,{}|{ yss mR =v

−=

)1()1(2/1)1(

)2()2()2(

3

2

1

3

2

1

s

s

s

s

s

s

xxx

xxx

−−−−

=

)()()(

)()()(

413

412

411

423

422

421

s

s

s

s

s

s

xuxuxu

xuxuxu

{ } ( )tzyxmy −− ,,2/1,:0,0,2/1,0|1,

( ) ( )tzyxmy −− ,,,:1,

Restrictions on basic-structure coordinates

{ } { } ( )tzyxmvR yss −−= ,,2/1,:0,0,2/1,0|1,|

−=

)1()1(2/1)1(

)1()1()1(

3

2

1

3

2

1

xxx

xxx )1(2/1)1( 22 xx −=⇒

)1(mod2/1)1(2 =⇔ sx

43or412 =⇔ x

=

3

1

3

1

3

2

1

43;41)1()1()1(

s

s

s

s

x

x

x

x

xxx

Atoms in mirror planes at x2 = 1/4 and 3/4

Modulation functions for atom µ on (x1,1/4,x3)

{ } { } ( )tzyxmvR yss −−= ,,2/1,:0,0,2/1,0|1,|

−−−−

=

)()()(

)()()(

43

42

41

43

42

41

s

s

s

s

s

s

xuxuxu

xuxuxu

µ

µ

µ

µ

µ

µ

even]2cos[)()()(1

43,434343 ∑∞

=

=⇒−=n

snsss xnBxuxuxu πµµ

even]2cos[)()()(1

41,414141 ∑∞

=

=⇒−=n


odd]2sin[)()()(1

42,424242 ∑∞

=

=⇒−−=n

snsss xnAxuxuxu πµµ

Crystal structure of TiOCl at room temperature

H. Schäfer et al., Z Anorg. Allg. Chem. 295, 268 (1958)

• Pmmn a = 3.78 b = 3.34 c = 8.03 Å

• Chains of Ti along a and along b• Isostructural compounds: TiOCl, TiOBr, VOCl, FeOCl

Monoclinic twinned incommensurate structure of TiOCl

Incommensurately modulated below Tc2 = 90 K

Modulation wavevector q = (0.07, 0.511, 0)

Superspace group P2/n(α β 0)-10 (c unique)

13.1.2.1 P2/b(α,β,0)00

Modulation functions (i=1,2,3) ui [t + q·x0]

Structure refinement R(main) = 0.018 R(sat) = 0.080

Lock-in transition toward q = (0 1/2 0) below Tc1 = 67 K

Atoms on twofold axesA. Schönleber et al., Phys. Rev. B 73, 214410 (2006)S. van Smaalen et al., PRB 72, 020105(R) (2005)

Superspace group P2/n(α β 0)-10

Origin-dependent translational components cannot be avoided.

( ) ( )( ) ( )( ) ( )( ) ( )4321

4321

4321

4321

2121:1,2121:1,

:1,2:1,

ssss

ssss

ssss

ssss

xxxxmxxxxi

xxxxxxxxE

−++−−−−

−−−

SSG(3+d)D 13.1.2.1 P2/b(α,β,0)00

Exercise: twofold rotation (2, -1) at the origin

( ) ( )tzyxz −−− ,,,:1,2

( )0,, βα=q}0,0,0,0|1,2{}|{ −= zssR v

−−

=

)1()1()1(

)2()2()2(

3

2

1

3

2

1

s

s

s

s

s

s

xxx

xxx

−−−−−

=

)()()(

)()()(

413

412

411

423

422

421

s

s

s

s

s

s

xuxuxu

xuxuxu

( )

=

1000010000100001

1,2z

{ } ( )tzyxz −−− ,,,:0,0,0,0|1,2

Restrictions on basic-structure coordinates by (2, -1)

)1()1( 11 ss xx −=⇒

)1(mod0)1(2 1 =⇔ sx

21or01 =⇔ sx

=

33

2

1

00

)1()1()1(

ss

s

s

xxxx

−−

=

)1()1()1(

)1()1()1(

3

2

1

3

2

1

s

s

s

s

s

s

xxx

xxx

3

210

sx

3

2121

sx

3

021

sx

Four twofold axes in the unit cell

Modulation functions for an atom on (0, 0, x3)

even]2cos[)()()(1

43,434343 ∑∞

=

=⇒−=n


odd]2sin[)()()(1

41,414141 ∑∞

=

=⇒−−=n


−−−−−

=

)()()(

)()()(

413

412

411

413

412

411

s

s

s

s

s

s

xuxuxu

xuxuxu

odd]2sin[)()()(1

42,424242 ∑∞

=

=⇒−−=n


Structural parameters for an atom on (0, 0, x3)

even]2cos[)(1

43,43 ∑∞

=

=n

sns xnBxu πµµ

odd]2sin[)(1

41,41 ∑∞

=

=n

sns xnAxu πµµ

odd]2sin[)(1

42,42 ∑∞

=

=n

sns xnAxu πµµ

=

03

03

02

01

00

xxxx

0231312332211 == UUUUUU

03,2,1, === µµµnnn ABB

The twofold rotation (2, -1) at (0, 0, 0, 1/4)

[ ][ ][ ]

−−−−−−−−

=

)21()21()21(

)()()(

413

412

411

423

422

421

s

s

s

s

s

s

xuxuxu

xuxuxu

( )

=

1000010000100001

1,2z( ) ( )tzyxz −−− ,,,:1,2

( )0,, βα=q}5.0,0,0,0|1,2{}|{ −= zssR v

−−

=

)1()1()1(

)2()2()2(

3

2

1

3

2

1

s

s

s

s

s

s

xxx

xxx

{ } ( )tzyxz −−− 5.0,,,:5.0,0,0,0|1,2

Restrictions on basic-structure coordinates by (2, -1) at (0, 0, 0, 1/4)

Restrictions on the basic-structure coordinates are the same as before

)1()1( 11 ss xx −=⇒

)1(mod0)1(2 1 =⇔ sx

21or01 =⇔ sx

=

33

2

1

00

)1()1()1(

ss

s

s

xxxx

−−

=

)1()1()1(

)1()1()1(

3

2

1

3

2

1

s

s

s

s

s

s

xxx

xxx

3

210

sx

3

2121

sx

3

021

sx

Modulation functions for an atom on (2,-1) at (0,0,0,1/4)

−−−−−

=

)21()21()21(

)()()(

413

412

411

413

412

411

s

s

s

s

s

s

xuxuxu

xuxuxu

⇒−−=⇒−=

⇒functionoddharmonicseven)()(functionevenharmonicsodd)()(

4141

4141

ss

ss

xuxuxuxu

⇒−=⇒−−=

⇒functionevenharmonicseven)()(

functionoddharmonicsodd)()(

4343

4343

ss

ss

xuxuxuxu

Symmetry restrictions i = 1 for odd harmonics

]2sin[]2sin[ 44 ss xAxA ππ ≡−=

)21()( 4141 ss xuxu −−= µµ

)odd(01, ==⇒ nAnµ

]2cos[]2cos[ 44 ss xBxB ππ ≡=

)odd(restrictednot1, =⇒ nBnµ

( ) ( )]212sin[]212sin[ 44 −=−− ss xAxA ππ

( ) ( )]212cos[]212cos[ 44 −−=−− ss xBxB ππ

]2sin[)( 41,141 ss xAxu πµµ =

Symmetry restrictions i = 3 for odd harmonics

]2sin[]2sin[ 44 ss xAxA ππ ≡=

)21()( 4343 ss xuxu −= µµ

)odd(restrictednot3, =⇒ nAnµ

]2cos[]2cos[ 44 ss xBxB ππ ≡=

)odd(03, ==⇒ nBnµ

( ) ( )]212sin[]212sin[ 44 −−=− ss xAxA ππ

( ) ( )]212cos[]212cos[ 44 −=− ss xBxB ππ


Symmetry restrictions i = 1 for even harmonics

( ) ]22sin[]22sin[]122sin[ 444 sss xAxAxA πππ ≡=−=

)21()( 4141 ss xuxu −−= µµ

)even(01, ==⇒ nBnµ

)even(restrictednot1, =⇒ nAnµ

( ) ( )]2122sin[]2122sin[ 44 −=−− ss xAxA ππ

( ) ( )]2122cos[]2122cos[ 44 −−=−− ss xBxB ππ

( ) ]22cos[]22cos[]122cos[ 444 sss xBxBxB πππ ≡−=−−=


Symmetry restrictions i = 3 for even harmonics

( ) ]22sin[]22sin[]122sin[ 444 sss xAxAxA πππ ≡−=−−=

)21()( 4343 ss xuxu −= µµ

)even(restrictednot3, =⇒ nBnµ

)even(03, ==⇒ nAnµ

( ) ( )]2122sin[]2122sin[ 44 −−=− ss xAxA ππ

( ) ( )]2122cos[]2122cos[ 44 −=− ss xBxB ππ

( ) ]22cos[]22cos[]122cos[ 444 sss xBxBxB πππ ≡=−=


Special positions on (2, -1)—two origins

=

03

03

02

01

00

xxxx

02313 == UU

03,2,1, === µµµnnn ABB

=

03

03

02

01

00

xxxx

µµµ3,2,1, nnn BAA

12332211 UUUU 03,2,1, === µµµnnn ABB

µµµ3,2,1,:even nnn BAAn =

03,2,1, === µµµnnn BAA

µµµ3,2,1,:odd nnn ABBn =

( )tzyx −−− 5.0,,,( )tzyx −−− ,,,

Conclusions

(3+d)D Superspace groups provide

Restrictions on the basic-structure coordinates

Restrictions on the shapes and phases of the modulation functions

Mathematical form of functions depends on origin

Reduction of the independent parameters makes structurerefinements possible

Superspace symmetry and superspace groups

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