Preparation and structure of NaSr0.5Al2B2O7 and NaCa0.5Al2B2O7

Two compounds NaSr0.5Al2B2O7 and NaCa0.5Al2B2O7, have been found to crystallize into a new structure type by Rietveld refinement from X-ray powder diffraction data. Their structure belongs to hexagonal space group image, with lattice parameters of image, image for NaSr0.5Al2B2O7 and image, image for NaCa0.5Al2B2O7, respectively. The structure is built up by [Al2B2O7]2− double layer and Na+/Ca2+ or Na+/Sr2+ ions alternatively stacking along the c-axis. The sites in the inter-double layer are fully occupied jointly by Na and Ca or Sr, but the intra-double layer sites are only half occupied solely by Na. A mechanism of the transition of the structure from CaAl2B2O7 to present structure type by replacing only 1% Ca by Na (2%) as observed by Chang and Keszler (Mater. Res. Bull. 33 (1998) 299) is also proposed.