Growth, structure, and electrophysical properties of single crystals of A2TiGeO5 (A = Li and Na)

Crystals of Li2TiGeO5 were obtained by solution melt crystallization, and those of Na2TiGeO5 were grown from a melt by pulling. The crystals art: isostructural with the natisite mineral Na2TiSiO5. The crystal structure of Li-2{TiOGeO4} was refined by X-ray diffraction analysis (a four-circle diffractometer, 2 theta/theta scan mode, MoKalpha radiation, theta (max) = 50 degrees). The unit cell parameters are a = 6.614(4) Angstrom and c = 4.435(4) Angstrom; space group P4/nmm, Z = 2, rho (calcd) = 3.67 g/cm(3), R = 0.031, s = 1.128, wR(F-2) = 0.071 (548 reflections with I greater than or equal to 2 sigmaI. The ionic conduction in both crystals was found to be anisotropic in the temperature range 250-600 degreesC. At 400 degreesC, the conductivity values are 10(-4) to 10(-5) S/cm along the a axis and 10(-6) to 10(-8) (for Na2TiGeO5) and 10(-7) to 10(-9) S/cm (for Li2TiGeO5) along the c axis.