X-RAY-DIFFRACTION STUDY ON PHASE RELATION OF BAZNGEO4

A single-crystal diffraction study has been carried out at 145 K, 195 K and 300 K to investigate the phase relations of BaZnGeO4. The space group of phase VI has been found to be P6(1)22 or P6(5)22 with the lat tice parameters a(VI) = 5.32 angstrom and c(VI) = 26.1 angstrom at 145 K. The symmetry is higher than that of the room-temperature phase III . A new phase, VII, has been found in the sample larger than 0.2 mm an nealed in the temperature range of 100-160 K for about 1 month. This p hase is thought to have a hexagonal symmetry (P6(3) or P6(3)/m) with t he lattice parameters a(VII) = 10.68 angstrom and c(VII) = 8.69 angstr om. Since phase VI did not appear in the case of crystals smaller than 0.2 mm, the particle size effect on the V-VI phase change has been co nfirmed. The relationships among phases I, II, III, III', IV, V, VI an d VII of BaZnGeO4 are discussed.