STRUCTURE-ANALYSIS OF A CA-NB-GA GARNET

Neutron powder-diffraction intensities were measured in the range from room temperature to 800 degrees C for a Ca-Nb-Ga garnet (CNGG) grown from a non-stoichiometric melt. A single crystal of CNGG was also studied at room temperature by means of X-ray diffraction. The space group I2(1)3 (No. 199) was tentatively assigned to CNGG based both on Rietveld analysis and on X-ray single-crystal structure analysis. There are some vacancies at one of the Ca2+ sites and partial substitutions of Ca2+ and Ga3+ for Nb5+. CNGG exhibited appreciably different thermal-expansion coefficients at room temperature and above 400 degrees C. The dielectric constant measured parallel to the [001] direction for CNGG starts to increase non-linearly at 400 degrees C, but exhibits no peak up to 600 degrees C.