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High-pressure-high-temperature synthesis, characterization and quantum-chemical
calculations of metal nitrides
Joint Project:Kai Guo, Ulrich Schwarz, MPI CPfS
Rainer Niewa, Dieter Rau, Univ. Stuttgart
Richard Dronskowski, RWTH Univ. Aachen
28. 09. 2012
Outline
1. High-pressure behaviors and single-crystal growth of ε-Fe3Nx under high-pressure, high-
temperature (HPHT).
2. Phase transition from γʹ -Fe4N and ζ-Fe2N to ε-Fe3Nx and subsequent recrystalization under HPHT.
3. Synthesis and characterization of ε-Fe2TMN (TM = Co, Ni), ε-Fe2IrNx and ε-Fe3(N, C).
4. Theoretical prediction of new pernitrides 2La3+(N2)2- (N2)4-.
TM
γʹ-Fe4N, cubic
ε-Fe3N, hexagonal/trigonal
ζ-Fe2N, orthorhomic
Fe
Fe
K.H. Jack, Proc. Roy. Soc. A 1951, 208, 200.2
N
Phase diagram of the binary system Fe-N.
③①
② ②
3
1. ε-Fe3Nx: high-pressure behaviors
Fe3N1.05±3O0.017±1
B0 = 172(4) GPa, B‘ = 5.7
Upon pressureincrease, thec/aratioincreasestowardthe ideal value (0.943 = 1.633/).
R. Niewa et al. Chem. Mater. 2009, 21, 392.
Pressure-volume data of ε-Fe3N.
Experimental data
Theoretical simulation
c/a ratio of the hexagonal unit-cell parameters of ε-Fe3N as a function of pressure.
No phase transition occurs under high pressure.
Theoretical simulation
4
The composition refined from P312 is much colser to the expected composition.
Theoretical analysis reveals that P312 is more energetically favored for Fe3N1.1.
p = 15(2) GPa, T = 1600(200) K
Two-stage multianvil device with a walker-type module
Starting material: Fe3N1.05±3O0.017±1
1. ε-Fe3Nx: HPHT single-crystal growth
MgO/Cr2O3
ZirconiaMolybdenum
GraphiteBoron NitideSample
MgO
Formation enthalpies and average magnetic moments on Fe atoms for ε-Fe3N and ε-Fe3N1.1.
Refined fomula for ε-Fe3Nx in sapce group P312 and P6322.
5
TM
②
2. Phase transition from γʹ-Fe4N to ε-Fe3N0.75
Herein, a phase trantion from γʹ-Fe4N to ε-Fe4N (Fe3N0.75) at 7 GPa is predicted based on
density-functional theory!
0 K
R. Niewa et al., J. Alloys Compd. 2009, 480, 76.
0 K
Energy–volume diagram for the system ε-Fe3N+Fe, γʹ-Fe4N and ε-Fe4N as calculated by
density-functional theory.
Endothermic
γʹ-Fe4N
ε-Fe4N
Induced by pressure!
Enthalpy-difference–pressure diagram for Fe4N as calculated by density-functional theory.
6
Starting material: γʹ -Fe4N0.995(5)
Conditions: p = 8.5 GPa, T = 1373 K
Fe3N0.77(4)
CA: Fe3N0.760(6)O0.018(2)
K. Guo, R. Niewa, D. Rau, Y. Prots, W, Schnelle, U. Schwarz, in preparation.
2. Phase transition from γʹ-Fe4N to ε-Fe3N0.75
XRPD patterns of the precursor γ’-Fe4N and the product ε-Fe3N0.75 after HPHT treatments.
Lattice parameters vs nitrogen content in Fe3Nx.
Phase transition from γʹ-Fe4N to ε-Fe3N0.75
under HPHT is observed
The nitrogen content deduced from the eqations is reasonablly agreement with results by CA.
γʹ-Fe4N
ε-Fe3N0.75
7
2. Crystal structure of ε-Fe3N0.75
Refined fomula for ε-Fe3Nx in space group P312 and P6322.
P312
P6322
CA: Fe3N0.760(6)O0.018(2)
Landau theory indicates that a change in space group within a homogeneity range is not possible!
Both descriptions for the crystal structure in space group P312 and P6322 look like reasonable results.
Combined the earlier results, space group P312 is suggested.
8
2. Thermal properties of ε-Fe3N0.75
ε-Fe3N0.75 remains metastable up to Tonset = 516 K before transforming into thermodynamically stable γ’-Fe4N at ambient pressure.
ε-Fe3N0.76 γʹ-Fe4N+ ε-Fe3Nx (x > 0.75)
9
FM-Fe3N0.75
NM-Fe3N0.75
ε-Fe3N0.75
γʹ-Fe4N
FM-Fe4N
NM-Fe4N
2 K: 183 emu/g = 5.83 μB
2. Magnetic properties of ε-Fe3N0.75
10
2. Magnetic moments in ε-Fe3N and ε-Fe3N0.75
Density-functional theory!
(□-FeΙ-N)
(N-FeΠ-N)
11
2. Phase transition from ζ-Fe2N to ε-Fe3N1.5
U. Schwarz, et al., Eur. J. Inorg. Chem. 2009, 12, 1634.
②
12This phase transition canʹt be induced only by the pressure!
2. High-pressure behaviors of ζ-Fe2N
XRPD taken on ζ-Fe2N at different pressures in a DAC.
Starting material: ζ –Fe2N0.986(6)O0.0252(8)
No phase transition occurs under high pressure
Pressure–volume data of ζ-Fe2N.
Bulk modulus: B0 = 172.1(8) GPa B0
ʹ = 5.24(8)
Theoretical simulation
Enthalpy-difference for ε-Fe3N1.5 in space group P312 and P6322, as well as 2Fe+α-N compared to ζ-Fe2N.
13
2. Phase transition from ζ-Fe2N to ε-Fe3N1.5
Conditions: p = 15(2) GPa, T = 1600(200) K
XRPD diagrams of ζ-Fe2N and the product of the HPHT treatment.
Refined fomula for ε-Fe3Nx in sapce group P6322.
Refinements with P312 lead to unreasonable results althoulh it is energetically favored baesd on quantum theoretical omputations
The phase transition is probably induced by the temperature
Enthalpy-difference for ε-Fe3N1.5 in space group P312 and P6322, as well as 2Fe+α-N
compared to ζ-Fe2N.
14
3. Synthesis of ε-Fe2TMN (TM = Co, Ni)
Starting material: ζ –Fe2N0.986(6)O0.0252(8) and
TM powders
Conditions: p = 15(2) GPa, T = 1473(150) K
XRPD results reveal pure phases for ε-Fe2TMN (TM = Co, Ni)!
TM
TMNFeTMNFe 22
K. Guo, R. Niewa, D. Rau, U. Burkhardt, W. Schnelle, U. Schwarz, submitted.
Si
SiSiSi
Si
BN
XRPD for the starting material ζ-Fe2N, the products -Fe2CoN and -Fe2NiN.
15
ε-Fe2CoN ε- Fe2NiN
3. Characterization of ε-Fe2TMN (TM = Co, Ni)
Typical optical micrographs of (a) -Fe2CoN and (b) -Fe2NiN.