Structures and physical properties of epsilon-FeSi-type and CsCl-type RuSistudied by first-principles pseudopotential calculations

An investigation of the relative stability of the two known polymorphs of R uSi, having the epsilon-FeSi and CsCl structures, has been made by first-pr inciples pseudopotential calculations. The resulting cell volumes and fract ional coordinates at P = 0 are in good agreement with experiment. Applicati on of high pressure to the epsilon-FeSi phase of RuSi is predicted to produ ce a structure having almost perfect sevenfold coordination. However, it ap pears that RuSi having the CsCl-type structure will be the thermodynamicall y most stable phase for pressures greater than 3.6 GPa. Fitting of the calc ulated internal energy versus volume to a fourth-order logarithmic equation of state led to values (at T = 0 K) for the bulk modulus, K-0, of 202 and 244 GPa for the epsilon-FeSi and CsCl phases, respectively, in excellent ag reement with experiment. Band-structure calculations for both phases are al so presented.