X-ray powder diffraction studies and thermal behaviour of NaK2B9O15, Na(Na.17K.83)2B9O15, and (Na.80K.20)K2B9O15

The crystal structures of NaK2B9O15 (image, image, image, image, image, image, image), Na(Na.17K.83)2B9O15 (image, image, image, image, image, image, image), and (Na.80K.20)K2B9O15 (image, image, image, image, image, image, image, image) were refined in the monoclinic space groups P21/c image using X-ray powder diffraction data and the Rietveld method. These nonaborates are isostructural to K3B9O15. Their crystal structure consists of a three-dimensional open framework built up from three crystallographically independent triborate groups. The alkali metal cations are located on three different sites in the voids of the framework. High-temperature X-ray diffraction studies show that NaK2B9O15 decomposes at about 700 °C in accordance with the peritectic reaction NaK2B9O15↔K5B19O31+liquid. The thermal expansion of NaK2B9O15 and Na(Na.17K.83)2B9O15 is highly anisotropic. A similarity of the thermal and compositional (Na–K substitution) deformations of NaK2B9O15 is revealed: heating of NaK2B9O15 by 1 °C leads to the same deformations of the crystal structure as increasing the amount of K atoms in (Na1−xKx)3B9O15 by 0.04 at% K.