CRYSTAL-STRUCTURES AND IONIC CONDUCTIVITIES OF NEW SILVER HAFNIUM SULFIDES

The phase relations of the Ag-Hf-S system were examined at elevated temperatures (600-1000 degrees C). The new compounds Ag2HfS3 and Ag4Hf3S8 were found. Phase transformation of Ag2HfS3 was observed to occur at around 700 degrees C. The low-temperature Ag2HfS3 was obtained as single crystals from a stoichiometric mixture of Ag2S and HfS2 at 650 degrees C in evacuated silica tubes. It crystallizes in space group Cmca (No. 64) of the orthorhombic system with eight formula units of cell dimensions a = 11.458(2) Angstrom, b = 6.628(1) Angstrom, c = 12.774(2) Angstrom, V = 970.0(2) Angstrom(3). The structure of Ag2HfS3 was determined using single-crystal X-ray diffraction with a CAD4 four-circle diffractometer, refined to a reliability factor R = 4.1% from 700 independent reflections and 32 variables. Hf and Ag atoms are coordinated by six and four S atoms to form HfS, octahedra and AgS4 tetrahedra respectively. The structural feature of Ag2HfS3 is the alternative packing of layers of HfS, octahedra and AgS4, tetrahedra along the c direction. This compound exhibits silver ionic conductivity log sigma(Ag+) = -3.0 at 22 degrees C.