FROM A 3-CONNECTED TO A 4-CONNECTED NET IN TRIMORPHIC CAPTSI AT HIGH-PRESSURE

CaPtSi, midway between the Zintl phase CaSi2 and the Laves phase CaPt2, crystallizes at normal pressure in the LaIrSi-type structure, a derivative of the SrSi2-type structure. Two quenchable polymorphs have been obtained from this phase in a belt apparatus at high pressures (3-4 GPa) and high temperatures (800-1100°C). The first high pressure polymorph (EuNiGe-type structure, mP12, P21/n, a = 588.9(1), b = 581.0(1), c = 724.3(1) pm, β = 109.25(1)°, Rietveld technique) is 7.3%, the second (TiNiSi-type structure, oP12, Pnma, a = 711.5(1), b = 433.6(1), c = 723.2(1), single crystal investigation) 11.6% more densely packed than the normal pressure phase. The denser packing is correlated with increases of coordination, bond distances, and metallic properties. The transformation from the LaIrSi- into the TiNiSi-type structure results in a change from a three-dimensional three-connected (Pt-Si: 230 pm) into a three-dimensional four-connected net (averaged Pt-Si: 254 pm). The phases of trimorphic CaPtSi show metallic behavior of the electrical resistivity and their magnetic susceptibility extends from diamagnetic (LaIrSi-type structure) to weakly paramagnetic (TiNiSi-type structure). A possible structural path from Zintl to Laves phases is discussed. © 1993 Academic Press, Inc.