Crystal structure, compressibility and possible phase transitions in -FeSi studied by first-principles pseudopotential calculations

An investigation of the relative stability of the FeSi structure and of some hypothetical polymorphs of FeSi has been made by first-principles pseudopotential calculations. It has been shown that the observed distortion from ideal sevenfold coordination is essential in stabilizing the FeSi structure relative to one of the CsCl type. Application of high pressure to FeSi is predicted to produce a structure having nearly perfect sevenfold coordination. However, it appears that FeSi having a CsCl-type structure will be the thermodynamically most stable phase for pressures greater than 13 GPa. Fitting of the calculated internal energy vs volume for the FeSi structure to a third-order Birch-Murnaghan equation of state led to values, at T = 0 K, for the bulk modulus, K₀, and for its first derivative with respect to pressure, K₀', of 227 GPa and 3.9, respectively.