Ionic conductivity studies and phase analysis of the Na 2BeSiO 4Na 2BeSi 2O 6 system

Abstract

Compositions along the Na 2BeSiO 4Na 2BeSi 2O 6 join in the Na 2OBeOSiO 2 system were prepared by solid state reaction and investigated by means of X-ray powder diffraction and ionic conductivity measurements. Phase stabilities were studied by investigation of quenched samples from 975 to 1335 K. Two phases were observed: an orthorhombic solid solution phase, Na x Be x 2 Si 2− x 2 O 4 , e.g., 1.80 ≤ x ≤ 2.00, apparently isotypic with KGaO 2 and the high-temperature modification of Na 2ZnSiO 4. The cell parameters for the Na 2BeSiO 4 obtained are a = 9.861 ± 2Å, b = 13.875 ± 2Å, and c = 4.911 ± 1Å. The powder pattern of Na 2BeSi 2O 6 was indexed with an orthorhombic cell, a = 21.096 ± 3Å, b = 6.870 ± 1Å, and c = 21.142 ± 4Å, and the phase found to be isotypic with the mineral beryllium-chkalovite. The ionic conductivity for the Na x Be x 2 Si 2− x 2 O 4 composites, 1.33 ≤ x ≤ 2.00, were determined using ac impedance measurements. The conductivity at 600 K increases with decreasing Na content and shows a maximum at x = 1.80 (the compositional limit for the solid solution phase); 4.9 × 10 −3 [Ω cm] −1 with E a = 0.48 eV. The data are compared with the results obtained in previous studies of the analogous systems Na 2ZnSiO 4Na 2ZnSi 2O 6 and Na 2ZnGeO 4Na 2ZnGe 2O 6.