THE POLYTYPISM AND PHASE WIDTH OF THE YTT RIUM CARBIDE BROMIDES 1T-Y2BR2C1-X AND 3R-Y2BR2C1-X

Y2Br2C crystallizes in two modifications, 1T-Y2Br2C and 3R-Y2Br2C. The se are temperature polytypes, and 1T-Y2Br2C is the high temperature fo rm. The transition temperature is 1 400 K. Both modifications show a r ange of homogeneity with Y2Br2Cl1-x(0.0 less than or equal to x less t han or equal to 0.3). For decreasing carbon content the c-lattice para meter increases, the colour of the compound changes from brown-black t o bright-silver and the electrical conductivity changes from semicondu cting to metallic behaviour. Extended Hueckel calculations indicate me tallic conductivity for x > 0 to arise from the occupation of Y-Y meta l bonding states above the gap.