THE CRYSTAL-STRUCTURES OF 2 TERNARY MX(GA,AS)Y PHASES (M = RH,PD) WITH RH5GE3-TYPE AND CR12P7-TYPE DERIVATIVE STRUCTURES

The pseudobinary compounds M(x)(Ga,As)y were synthesized in the Rh-Ga-As and Pd-Ga-As systems by reacting the constituent elements. The X-ray structures studied using three-dimensional single-crystal counter data were refined down to R values of 0.060 (space group Pbam, 376 independent reflections) and 0.034 (space group P6(3)/m, 607 independent reflections) leading to the formulae Rh5Ga2As and Pd12Ga4.5As2.5 respectively. The structure of Rh5Ga2As is derived from the Rh5Ge3 type with ordered distribution of gallium and arsenic atoms while the Pd12Ga4.5As2.5 structure is of Cr12P7 type with a fully disordered occupancy for the metalloid atoms. Strong correlation between the Rh-Ga-As ternaries and Rh-Ge binaries as well as structural relationship between MnP-, Co2P- and Rh5Ge3-type structures are evidenced. Stability of the M5X3 and M12X7 phases (M = Rh, Pd) in terms of dimensional and electronic effects is discussed. Finally, the importance of phase diagrams for rationalizing M-GaAs reactions is emphasized.