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.

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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.

Nominal composition EDX

CA (N, wt

%)

Real composition

Fe2CoNFe1.931Co1.069Nx

6.92±0.32 Fe1.99(6)Co1.01(6)N0.91(4)Fe2.020Co0.980Nx

Fe2.019Co0.981Nx

Fe2NiN

Fe1.976Ni1.024Nx

8.08±0.45 Fe1.97(2)Ni1.03(2)N1.07(6)O0.03(1)Fe1.952Ni1.048Nx

Fe1.973Ni1.027Nx

Homogeneous composition

Metal ration: Fe : Co = 1.99(6) : 1.01(6)

Fe : Ni = 1.97(2) : 1.03(2)

The compositions detected by EDXS and CA.

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3. Thermal properties of ε-Fe2TMN (TM = Co, Ni)ε-Fe2CoN ε-Fe2NiN

Based on DFT, both ε-Fe2CoN and ε-Fe2NiN are metalstable

The reactions are triggered by the temperature but the pressure play an important role in the preservation of nitrogen content

Enthalpy-difference for ε-Fe2TMN and thier competitive phases under varing pressure

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3. Thermal properties of ε-Fe2TMN (TM = Co, Ni)

N: 6.92±0.32%

N: 8.08±0.45%

ε-Fe2CoN

ε-Fe2NiN

TG-DSC for ε-Fe2TMN.

ε-Fe2TMN decompose above 750 K involving the loss of nitrogen

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3. Magnetic properties of ε-Fe2TMN (TM = Co, Ni)

Fe2CoN: 4.3μB/f.u.

Fe2NiN: 234(3) KFe2NiN: 3.1μB/f.u.

Fe2CoN: 488(5) K

Fe3N: Ms = 6 μB; Tc = 575(3) K

A. Leineweber et al., J. Alloys Compd., 1999, 288, 79.

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Fe

1.26

–2

Co

1.25

7

Ni

1.24

–8

Ru

1.34

64

Rh

1.34

–23

Pd

1.37

–47

Os

1.35

108

Ir

1.36

2

Pt

1.39

–74

DRHth (kJ mol–1) M rM

DRHt

h

• M 1a

• Fe 3c

• N 1b

3. Synthesis of ε-Fe2IrNx

J. von Appen, R. Dronskowski, Angew. Chem. Int. Ed. 2005, 44, 2

Enthalp-pressure diagram for γʹ-IrFe3N and thier competing phases

γʹ -IrFe3N: high-pressure

phase, stable beyond 37 GPa,ferromagnetic

No experimental evidence!

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3. Synthesis of ε-Fe2IrNx

Changing synthetic pressure

Changing synthetic temperature

21

3. Synthesis of ε-Fe2IrNx

Fe3N, a = 4.6982(3) Ǻ, c = 4.3789(4) Ǻ

22

3. Synthesis of ε-Fe2IrNx

0 Gpa

12 Gpa, 1100 oC

0 Gpa

5 Gpa, 1300 oC

12 Gpa, 1100 oC

Characterization of composition and physical properties are needed to be done.

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3. Synthesis of bulk ε-Fe3(N,C)

The nitrogen content in ε-Fe3(N,C) can be tuned to a certain extent.

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4. Prediction of new pernitrides 2La3+(N2)2- (N2)4-

DHR = –11 kJ mol–1 at absolute zero T

B0 = 86 GPa

N–N = 1.30 Å

M. Wessel, R. Dronskowski, J. Am. Chem. Soc. 2010, 132, 2421.

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4. Prediction of new pernitrides 2La3+(N2)2- (N2)4-

Density-functional Gibbs free energy-pressure diagram for the synthesis of LaN2 in the [ThC2] type at a projected synthetic temperature of T = 300 K.

300 K

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Conclusions

1. No phase transition but recrystallization occurs for ε-Fe3N1.05±3O0.017±1 under HPHT.

2. Phase transitions from γʹ-Fe4N and ζ-Fe2N to ε-phase are studied.

3. Ternary metastable nitrides ε-Fe2TMN (TM = Co, Ni) are obtained under HTHP. Both ε-

Fe2CoN and ε-Fe2NiN are ferromagnetic (ε-Fe2CoN: Ms = 4.3 μB/f.u., Tc = 488(5) K; ε-

Fe2NiN: Ms = 3.1 μB/f.u. Tc = 234(3) K).

4. ε-Fe2TMNx is obtained by modified HPHT treatments.

5. New binary pernitrides Fe2+(N2)2- and 2La3+(N2)2- (N2)4- are predicted. In parallel, potential

synthetic conditions are given.

Further works

1. Synthesis of ε-Fe2TMNx (TM = Ir, Cr, Mn, etc.) under HTHP.

2. Synthesis and characterization of ε-Fe3(N,C) as bulk materials under HTHP.

…

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Acknowledgement

Philipp Marasas and Susann Leipe: HPHT experimental support

Yurii Prots and Horst Borrmann: collection of powder and single-crystal diffraction data

Ulrich Burkhardt: EDX and EXAFS measurements

Gudrun Auffermann and Anja Völzke: chenmical analysis

Susann Scharsach, Stefan Hoffmann and Marcus Peter Schmidt: Thermal analysis

Walter Schnelle: characterization of magnetic properties

Ralf Riedel and Dmytro Dzivenko: measurements of hardness

Michael Hanfland: beamtime of synthrotron radiation

Financial support from SPP 1236!

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Thanks for your attention!