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 (a=9.1158(1)A, b=6.6246(1)A, c=20.6812(3)A, β=94.080(1)o, Rp=0.047, Rwp=0.059, RB=0.026), Na(Na.17K.83)2B9O15 (a=9.0559(1)A, b=6.6170(1)A, c=20.5833(3)A, β=94.228(2)o, Rp=0.053, Rwp=0.068, RB=0.026), and (Na.80K.20)K2B9O15 (a=9.1534(1)A, b=6.6296(1)A, c=20.7422(3)A, β=94.071(1)o, Z=4, Rp=0.041, Rwp=0.052, RB=0.023) were refined in the monoclinic space groups P21/c(Z=4) 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 700deg. C in accordance with the peritectic reaction NaK2B9O15-bar 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 deg. C leads to the same deformations of the crystal structure as increasing the amount of K atoms in (Na1-xKx)3B9O15 by 0.04at% K