Crystal structures and compressibility of novel iron borides Fe2B7 and FexB50 synthesized at high pressure and high temperature

Abstract

We present here a detailed description of the crystal structures of novel iron borides, Fe2B7 and FexB50 with various iron content (x=1.01(1), 1.04(1), 1.32(1)), synthesized at high pressures and high temperatures. As revealed by high-pressure single-crystal X-ray diffraction, the structure of Fe2B7 possesses short incompressible B–B bonds, which make it as stiff as diamond in one crystallographic direction. The volume compressibility of Fe2B7 (the bulk modulus K0= 259(1.8) GPa, K0′= 4 (fixed)) is even lower than that of FeB4 and comparable with that of MnB4, known for high bulk moduli among 3d metal borides. FexB50 adopts the structure of the tetragonal δ-B, in which Fe atoms occupy an interstitial position. FexB50 does not show considerable anisotropy in the elastic behavior.