A single-crystal diffraction study has been carried out at 145 K, 195 K and 300 K to investigate the phase relations of BaZnGeC&gt
4. The space group of phase VI has been found to be P6i22 or P6s22 with the lattice parameters ayi=5.32 Å and cyi = 26.1 Å 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 annealed in the temperature range of 100-160 K for about 1 month. This phase is thought to have a hexagonal symmetry (P63 or P6s/m) with the lattice parameters ayn —10.68 Å and Cyn = 8.69 Å. 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 confirmed. The relationships among phases I, II, III, IIP, IV, V, VI and VII of BaZnGeO4 are discussed. © 1993 The Japan Society of Applied Physics.