Thermal expansion and ionic conductivity of Na3Sc(BO3)2 and Na3Sc2(BO3)3

In this study, two known double borates (Na3Sc2(BO)3, Na3Sc(BO3)2) were synthesized by solid state reactions, and their crystal structures were confirmed using the Rietveld method. The melting points of Na3Sc2(BO)3 and Na3Sc(BO3)2 at 1003 degrees C and 1069 degrees C, respectively, were measured through thermal analysis. The presence of BO3 groups in the crystal structures of the both borates has been confirmed by IR spectroscopy. The thermal expansion was characterized by HTXRD. It was found, that the thermal expansion of Na3Sc(BO3)2 is anisotropic: the maximum expansion is observed along the c-axis (alpha 33 = 179 x 10-6 degrees C- 1), while the minimum - in the bdirection (alpha 22 = 35 x 10-6 degrees C- 1) and in the tensor alpha 11-axis direction (alpha 11 = 31 x 10-6 degrees C- 1), which is caused by the orientation of BO3 groups. The results of shear strain calculations are also presented. In the entire investigated temperature interval, the unit cell of Na3Sc2(BO3)3 demonstrate expansion in the (ab) plane (alpha a = 10 x 10- 6 degrees C- 1) while exhibiting compression in the c-direction (alpha c = -6 x 10-6 degrees C- 1). In this work, we have successfully performed theoretical estimations of the activation energy and ion transport pathways of the studied borates. This analysis was conducted using valence-based energy maps (BVE), which predict predominantly three-dimensional conductivity. The theoretical calculations were confirmed by ionic conductivity experimental measurements. The conductivity of Na3Sc2B3O9 and Na3ScB2O6 at 973 K reached 1.3 x 10-3 Sm/cm (Ea = 0.6 eV) and 1.1 x 10-3 Sm/cm (Ea = 0.7 eV), respectively.