Enantiomorphism of the Ca3Ga2Ge4O14 compound and comparison of the Ca3Ga2Ge4O14 and Sr3Ga2Ge4O14 structures

The absolute crystal structures of two enantiomorphic forms of the Ca3Ga2Ge4O14 crystals (a = 8.075(1) Å, c = 4.9723(6) Å, space group P321) with the positive and negative senses of the optical activity are determined using X-ray diffraction analysis. The final R factors are as follows: R = 1.75% and Rw = 2.57% for the crystal with the positive sense of the optical activity and R = 1.86% and Rw = 2.78% for the crystal with the negative sense of the optical activity. The replacement of the Ca2+ ions by larger Sr2+ ions (with the formation of the Sr3Ga2Ge4O14 compound) leads to an anisotropic expansion of the crystal lattice (with a more considerable increase in the lattice parameter a as compared to the lattice parameter c), a change in the occupation of the 1a and 3f positions by the Ga3+ and Ge4+ ions, and symmetrization of the octahedra and tetrahedra forming the structural framework. The shape of the dodecahedron changes so that its size along the polar electric axis 2 increases significantly. This change is the main factor responsible for the increase in the piezoelectric activity of the Sr3Ga2Ge4O14 compound as compared to the piezoelectric activity of the Ca3Ga2Ge4O14 compound.