High-Pressure-High-Temperature Behavior of ζ-Fe2N and Phase Transition to ε-Fe3N1.5

Under the high-pressure-high-temperature conditions of a resistivity-heated Walker-type two-stage multi-anvil device [1600(200) K and 15(2) GPa] zeta-Fe2N is transformed to single-crystalline epsilon-Fe3N1+x (x = 0.5). The crystal structure was refined in space group P6(3)22 [a = 4.8016(2) angstrom, c = 4.4269(2) angstrom, Z = 2, R(F) = 1.27%] resulting in a composition of Fe3N1.47(1) i.e., under the selected preparation conditions the structural change takes place under conservation of the nitrogen content within experimental error. The applied pressure is necessary to prevent decomposition and formation of elemental nitrogen at elevated temperatures. Application of pressure in a diamond anvil cell at ambient temperature without external heating gives no indication of a phase transition of the starting material. A least-squares fit of a Murnaghan-type equation of state to the experimental pressure-volume data up to 25 GPa results in a compressibility of B-0 = 162.1(8) GPa, B-0' = 5.24(8), with V-0 = 118.09(1) angstrom(3). Density-functional electronic-structure calculations on zeta-Fe2N and epsilon-Fe2N for 0 K indicate that no pressure-induced phase transition can occur such that the phase transition must be driven by temperature, as is observed in the experiments. The bulk module for zeta-Fe2N (B-0 = 219 GPa, B-0' = 4.46 GPa) is in acceptable agreement with the experimental results. Similar to the case of epsilon-Fe3N1.1, epsilon-Fe3N1.5 may be described both in space group P312 and P6322, and again space group P312 is favored by 12.1 kJ/mol. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)