X-ray powder diffraction studies and thermal behaviour of NaK2B9O15, Na(Na.17K.83)2B9O15, and (Na.80K.20)K2B9O15 |
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Authors: | R Bubnova S Filatov |
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Institution: | a Grebenshchikov Institute of Silicate Chemistry, RAS, St. Petersburg, 199155, Russia b Eduard-Zintl-Institut fur Anorganische und Physikalische Chemie, Technische Universität Darmstadt, 64287 Darmstadt, Germany c Department of Crystallography, St. Petersburg State University, St. Petersburg 199034, Russia |
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Abstract: | The crystal structures of NaK2B9O15 ( , , , β=94.080(1)°, Rp=0.047, Rwp=0.059, RB=0.026), Na(Na.17K.83)2B9O15 ( , , , β=94.228(2)°, Rp=0.053, Rwp=0.068, RB=0.026), and (Na.80K.20)K2B9O15 ( , , , β=94.071(1)°, 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 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. |
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Keywords: | Borate Solid solution Rietveld structure refinement Compositional deformation Thermal deformation |
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