High-pressure high-temperature phase transition of γ′-Fe4N

High-temperature high-pressure treatment of γ′-Fe4N at 1600 K and 15 GPa in a two-step multi-anvil module integrated in a uniaxial press leads to a phase transition concomitant to a re-crystallization of the nitride material in a hexagonal ɛ-type arrangement. Single crystals of ɛ-Fe3N0.95(2) with a = 4.6828(2) Å, c = 4.3705(2) Å, V = 83.000(6) Å3, Z = 2) were obtained. A structural model of iron atoms in the motif of a hexagonal close packing with occupation of octahedral voids by nitrogen exhibiting long-range order was constructed and refined on the basis of X-ray diffraction data revealing a nitrogen deficiency x = 0.05(2). Two likely structural order models of nitrogen in the space groups P312 (R(F) = 0.024, wR(F2)=0.030wR(F2)=0.030) and P6322 (R(F) = 0.021, wR(F2)=0.023wR(F2)=0.023) are discussed. By means of density-functional electronic-structure calculations a pressure-induced phase transition is predicted for Fe4N from Pm3¯m to P312 at about 6 GPa at 0 K, which also offers an interpretation for the reaction mechanism on the basis of the experimental data.