The high-pressure behavior of the hard material ε-Fe 3 N 1+x was studied up to 33 GPa with in situ X-ray diffraction experiments using diamond anvil cells in combination with synchrotron radiation as well as by ex situ high-temperature, high-pressure treatment at 1600(200) K in a two-stage multianvil device with a Walker-type module. Evaluation of the pressure-volume data up to 10 GPa by fitting a Murnaghan-type equation reveals a bulk modulus of B 0 ) 172( 4) GPa (B′ ) 5.7, fixed). The calculated bulk modulus (220 GPa) on the basis of density-functional theory (GGA-PAW-PBE) is in satisfying agreement with the experimental one. Single crystals of ε-Fe 3 N 1+x as obtained by ex situ high-temperature, high-pressure experiments reveal in X-ray diffraction data refinements a structural model of iron atoms in the motif of a hexagonal close packing with occupation of octahedral voids by nitrogen atoms exhibiting long-range order. The preferred structural model is described in space group P312 (a ) 4.7241(2) Å, c ) 4.3862(2) Å, V ) 84.773(6) Å 3 , Z ) 2, R(F) ) 0.0339, wR(F 2 ) ) 0.0556) and compared to a second model in P6 3 22. This choice of structural description is corroborated by the results of density-functional calculations. These yield a total energy at 0 K, which is 5 kJ/mol lower for the model in space group P312 compared to the second best alternative arrangement. Using micro-and nanoindentation techniques, a Vickers hardness of H V ) 7.4(10) GPa, a nanoindentation hardness of H ) 10.1(8) GPa, as well as a reduced elastic modulus in the amount of E r ) 178(11) GPa were measured for ε-Fe 3 N 1+x single crystals.
In-situ X-ray and neutron diffraction investigations on Cu 3 N indicate the onset of a high-pressure phase transition at about 5 GPa. The tetragonal cell parameters of the high-pressure phase reveal a discontinuous volume decrease of about 20 %. The phase transition is reversible, with a hysteresis of about 2 GPa. Subsequent ex-situ investigations in a multi-anvil press evidence a reversible re-formation of ambient pressure Cu 3 N from XRD patterns. The structure refinement with nitrogen atoms disordered in distorted octahedral voids of a tetragonal body-centered copper substructure leads to an occupation of approximately 1 / 3 and thus to a composition of Cu 3 N 1.0(1) . Optical absorption measurements (IR-VIS) up to 10 GPa indicate a semicon-* Prof. Dr. R. Niewa
Structure D 2000High-Pressure-High-Temperature Behavior of ζ-Fe2N and Phase Transition to ε-Fe 3 N 1.5 . -High-pressure/high-temperature treatment of ζ-Fe 2 N (obtained by reaction of Fe powder with flowing NH3 at 435°C) at 1600 K and 15 GPa leads to the formation of ε-Fe3N1.5. The crystal structure of this phase is refined by single crystal XRD (space group P6322, Z = 2) yielding a composition of Fe3N1.47. High pressure experiments at ambient temperature and DFT calculations indicate that temperature is the driving force for the phase transition. -(SCHWARZ, U.; WOSYLUS, A.; WESSEL, M.; DRONSKOWSKI, R.; HANFLAND, M.; RAU, D.; NIEWA*, R.; Eur.
Abstract.A high-pressure high-temperature synthesis involving initial reaction, nitrogen depletion and annealing yields a new ε-type iron iridium nitride. Energy dispersive X-ray spectroscopy of metallographic samples evidence a main phase having a molar metal ratio of Fe:Ir = 2:1 within experimental error. Chemical analyses and thermogravimetric measurements quantify the nitrogen content to
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