Single crystal X-ray diffraction study of the vanadate garnet Ca 2NaZn 2V 3O 12

Abstract

Single crystals of the vanadate garnet Ca 2NaZn 2V 3O 12 were synthesized by the floating zone (FZ) method, and its crystal structure was investigated using single-crystal X-ray diffraction. The bonding character of the present vanadate garnet estimated from bond valence is similar to that of silicate garnets, which suggests that the crystal structure is subjected to the geometric constraints similar to that of silicate garnets. The present vanadate garnet has a notable structural feature that the dodecahedral–dodecahedral shared edge length is considerably longer than the unshared dodecahedral edge length and the octahedral–dodecahedral shared edge length is approximately equal to the unshared octahedral edge length. These unusually long shared edges brought about by the large sizes of the dodecahedral cations (Ca 2+ and Na +) are allowed because of the weak repulsions between the dodecahedral cations and between the dodecahedral and octahedral cations. In contrast, the geometric constraints of garnet structure force the tetrahedral–dodecahedral shared edge to become considerably shorter than the unshared tetrahedral edge, and thereby the strong repulsion between the dodecahedral and tetrahedral cations, observed from the atomic thermal vibrations, is shielded. In the difference-Fourier map, the residual electron density peaks that are considered to contribute to the shielding for the repulsion are observed in the field between the dodecahedral and tetrahedral cations.