Crystal structures and constitution of the binary system iridium-boron

Abstract

The constitution of the binary system Ir-B has been established between 10 and 70 at.% boron for temperatures above 700°C based on differential scanning calorimetry, electron probe microanalysis, and isothermal low temperature annealing experiments (≤1000°C). Four binary phases have been found, namely Ir4B5+x , Ir5B4+x and the high and low temperature modification of Ir4B3-x . X-ray structure analyses were performed on single crystals of Ir4B5+x (x = 0, Ir4B5 type; space group C2/m; a = 1.05200(2), b = 0.289564(6) and c = 0.60958(1) nm, β = 91.156(2)°), Ir5B4+x (x=0, Ir5B4 type; space group I41/a; a = 0.62777(1) and b = 1.02599(2) nm) and on the low temperature modification of Ir4B3-x (x=0, IrB0.9 type; space group Cmc21; a = 0.27728(1), b = 0.75742(2) and c = 0.73152(2) nm). The high temperature modification of Ir4B3-x (WC type; space group $$P\overline 6 m2$$ ; a = 0.28137(5) and c = 0.2828(1) nm) has been confirmed by X-ray powder diffraction. By means of the first-principle calculations, in combination with the evolutionary structural search algorithm, the compositions, structures and enthalpies of the Ir-B system have been investigated theoretically. Confirming the experimental observations on Ir4B5, Ir5B4 and Ir4B3, we have investigated several metastable phases at other stoichiometries, such as IrB, IrB2 and Ir3B2. We also proposed three thermodynamically and dynamically stable new structures of oF28-Ir4B3, oC8-IrB and mC10-Ir3B2, which may be synthesized under certain conditions.