Crystal structures and thermal and electrical properties of the new silver (poly)chalcogenide halides Ag23Te12Cl and Ag23Te12Br.

Two new silver (poly)chalcogenide halides, Ag23Te12Cl and Ag23Te12Br, were characterized by powder X-ray phase analysis, energy dispersive X-ray analysis, and crystal structure determinations at various temperatures. Thermal analyses of both compounds and electrochemical measurements for the bromide completed the investigation. The compounds Ag23Te12X (X = Cl, Br) are isostructural and crystallize orthorhombically (space group Pnnm, Z = 4) as systematic twins. The lattice parameter values derived from X-ray powder data were a = 21.214(2) A, b = 21.218(2) A, c = 7.7086(7) A, and V = 3469.8(6) A (3) for Ag23Te12Cl at 293 K and a = 21.170(1) A, b = 21.170(1) A, c = 7.7458(5) A, and V = 3471.4(4) A (3) for Ag23Te12Br at 298 K. An enhanced silver ion mobility was revealed by impedance spectroscopy investigations. No phase transitions were observed in the temperature range 100-750 K. These two silver(I) (poly)chalcogenide halides are the second set of representatives of a new class of coinage-metal (poly)chalcogenide halides in which both covalently bonded [Te2](2-) dumbbells and ionically bonded Te(2-) anions appear.