TRANSFORMATION OF 3-CONNECTED SILICON NETS IN CASI2

Up to 40 kbar and 1100°C, CaSi2 is dimorphic. Trigonal/rhombohedral CaSi2I (CaSi2-type structure) with corrugated layers of three-connected Si atoms can be transformed by a high pressure-high temperature treatment into tetragonal CaSi2II (α-ThSi2-type structure) with a three-dimensional net of three-connected Si atoms. The silicon net of CaSi2II is slightly distorted from the topologically simplest tetragonal three-dimensional three-connected net derived on a geometrical basis. In order to correlate crystal chemical with thermochemical data the transformation between both polymorphs of CaSi2 has been studied at equilibrium and nonequilibrium conditions. The pressure-temperature phase diagram of CaSi2 has been investigated by X-ray technique in quenched samples. From the slope of the equilibrium line and the change in molar volume the approximate values of the entropy and energy of transformation CaSi2(I-II) have been determined ΔS = 3.2 e.u., ΔU = 4.9 kcal/mole. Under nonequilibrium conditions the transformation CaSi2(II-I) yielded ΔH = -4.2 kcal/mole at 500°C and ambient pressure in a DTA apparatus. Complete transformation of metastable CaSi2II can be achieved within 5 min at a heating rate of 20°C/min. Due to the relatively high speed of transformation simple structural relations between both polymorphs of CaSi2 are discussed. © 1979.