Ca1-xNa2xAl2B2O7: A Structure with Tunable Density of Na+ Vacancies

A series of samples with the composition Ca1-xNa2xAl2B2O7 (0 < x <=1) was investigated and a hexagonal structure with unusually large range of homogeneity (at least from x = 0.01 to 0.95) was revealed. The hexagonal phase consists of [Al2B2O7](infinity)^2- lamellae stacked along the c axis, as in CaAl2B2O7 and Na2Al2B2O7. Nevertheless, the configuration and stacking sequence of the [Al2B2O7](infinity)^2- lamellae are different in these three structures. In the hexagonal structure of Ca1-xNa2xAl2B2O7, Ca and half Na cations (Na1) statistically occupy the same crystallographic site which is located between the [Al2B2O7](infinity)^2lamellae, the other half Na cations (Na2) distribute in the planes bisecting the [Al2B2O7](infinity)^2- lamellae. Depending on the composition, the site occupation factor of Na2 site can vary in the same range as x, leading to a tunable density of Na+ vacancies in the structure. The AlO4 tetrahedra and BO3 triangles in the structure tilt in appropriate ways to improve the bond valence sum of Na2 cations which are not sufficiently bonded to the anions.