Journal of Alloys and Compounds 480 (2009) 76–80 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom High-pressure high-temperature phase transition of  ′ -Fe 4 N R. Niewa a,∗ , D. Rau a , A. Wosylus b , K. Meier b , M. Wessel c , M. Hanfland d , R. Dronskowski c , U. Schwarz b a Department Chemie, Technische Universität München, Lichtenbergstraße 4, 85474 Garching, Germany b Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Straße 40, 01187 Dresden, Germany c Institut für Anorganische Chemie, Landoltweg 1, RWTH Aachen University, 52056 Aachen, Germany d European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP220, 38043 Grenoble, France article info Article history: Received 16 July 2008 Received in revised form 17 September 2008 Accepted 19 September 2008 Available online 20 November 2008 PACS: 61.05.cp 61.66.Fn 61.50.Ks Keywords: Nitride materials Crystal structure High-pressure abstract High-temperature high-pressure treatment of  ′ -Fe 4 N 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 -type arrangement. Single crystals of -Fe 3 N 0.95(2) with a = 4.6828(2) Å, c = 4.3705(2) Å, V = 83.000(6) Å 3 , 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(F 2 ) = 0.030) and P6 3 22 (R(F) = 0.021, wR(F 2 ) = 0.023) are discussed. By means of density-functional electronic-structure calculations a pressure-induced phase transition is predicted for Fe 4 N from Pm ¯ 3m 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. © 2008 Elsevier B.V. All rights reserved. 1. Introduction  ′ -Fe 4 N is the iron-richest stable phase in the binary system iron–nitrogen. This and other binary iron nitride phases such as - Fe 3 N and -Fe 2 N have particular impact as hard, corrosion and wear resistant surface layers of iron and steel workpieces. Not surpris- ingly, a large number of scientific research projects are devoted to the determination of the mechanical properties of iron nitrides and their dependence on both the nitrogen content and properties like microstructure or chemical composition of the initial steel mate- rial. However, all previous investigations were carried out on either microcrystalline powders of nitridated pure iron or hard microcrys- talline layers of steel workpieces. High-pressure investigations on binary iron nitrides are rare at best. Fig. 1 shows a representation of the generally accepted phase diagram Fe–N [1,2]. Next to solution phases of nitrogen in Fe it contains the nitrides  ′ -Fe 4 N, -Fe 3 N and -Fe 2 N.  ′ -Fe 4 N and -Fe 2 N exhibit only narrow homogene- ity ranges, whereas -Fe 3 N covers a large range of compositions at ∗ Corresponding author. E-mail address: rainer.niewa@mytum.de (R. Niewa). higher temperatures. It should be noted that the depicted phase diagram is a non-equilibrium diagram, i.e., at elevated tempera- tures, nitrogen loss and thus changes in the composition occur, as the equilibrium nitrogen pressure above the nitride phase is usually not reached under normal conditions. For  ′ -Fe 4 N in earlier in situ high-pressure X-ray diffraction stud- ies performed on a two-phase sample consisting of  ′ -Fe 4 N and -Fe 3 N 1+x , it was observed that the diffraction intensities vanish near 30 GPa [3]. The authors interpreted this observation as either due to the occurrence of a phase transition or a pressure-induced disorder in the crystal structure. However, they noted that a tran- sition to an -type crystal structure would not be obvious, since the reflections were covered by the second initial phase. An in situ X-ray magnetic circular dichroism study of  ′ -Fe 4 N up to 11GPa [4] and two in situ Mößbauer spectroscopy investigations [5,6] up to 12GPa all revealed a pressure-induced demagnetization which also may indicate a phase transition. However, no structural change was observed up to 8GPa in energy-dispersive X-ray diffraction experiments [4]. Here we present, for the first time, a successful crystal growth of a pure binary iron nitride. Starting with  ′ -Fe 4 N we have obtained single crystals of -Fe 3 N as a result of a phase-transition reaction followed by crystallization at high pressures. 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.178