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

摘要

The crystal structures of NaK2B9O15 (a = 9.1158(1)A, b = 6.6246(1) A, c = 20.6812(3) A, beta = 94.080(1)degrees, R-p = 0.047, R-wp = 0.059, R-B = 0.026), Na(Na.17K.83)(2)B9O15 (a=9.0559(1)A, b=6.6170(1)A, c=20.5833(3)A, beta = 94.228(2)degrees, R-p = 0.053, R-wp = 0.068, R-B = 0.026), and (Na.80K.20)K2B9O15 (a=9.1534(1)A, b=6.6296(1)A, c=20.7422(3)A, beta = 94.071(1)degrees, Z = 4, R-p =0.041, R-wp = 0.052, R-B = 0.023) were refined in the monoclinic space groups P2(1)/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 700 degrees C in accordance with the peritectic reaction NaK2B9O15 <-> K5B19O31+liquid. The thermal expansion of NaK2B9O15 and Na(Na.17K.83)(2) B9O15 is highly anisotropic. A similarity of the thermal and compositional (Na-K substitution) deformations of NaK2B9O15 is revealed: heating of NaK2B9O15 by 1 degrees C leads to the same deformations of the crystal structure as increasing the amount of K atoms in (Na1-xKx)(3)B9O15 by 0.04at% K. (c) 2006 Elsevier Inc. All rights reserved.