Growth and X-ray diffraction study of Rb 2 Na(NO 3 ) 3 crystals
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Growth and X-ray Diffraction Study of Rb2Na(NO3)3 Crystals V. I. Nasirov, N. M. Namazova, G. G. Guseinov, and A. S. Amirov Institute of Physics, Azerbaijan National Academy of Sciences, Baku, Azerbaijan e-mail: [email protected] Received November 18, 2014
Abstract—Rb2Na(NO3)3 single crystals have been investigated by X-ray diffraction. It is established that the orthorhombic structure of the crystal is retained in the temperature range of 100–450 K. Polymorphic transformations, typical of pure rubidium nitrate, have not been observed in this temperature range. DOI: 10.1134/S106377451506022X
INTRODUCTION The study of polymorphic transformations in nitrate compounds of alkaline metals is of not only scientific but also practical interest, because, on the one hand, this process is closely related to the technology of growing polymorphic single crystals of mixed composition and, on the other hand, sodium nitrate is also used as a component of ignition mixtures in aluminothermy, in the production of nitrogen acid and fertilizers, and in the food industry [1]. Rubidium nitrate is known to have four different modifications in the temperature range from room to melting temperature: trigonal (phase IV), sp. gr. P31 [2], stable up to 437 K; cubic (phase III), sp. gr. Pm3 m [3], stable in the temperature range of 437–492 K; rhombohedral (phase II), sp. gr. R 3 m [4], stable in the temperature range of 492–564 K; and cubic (phase I), sp. gr. Fm3 m [5], existing at temperatures above 564 K. As was shown in [6], rubidium nitrate high-temperature phases I and II disappear at sufficiently high pressures, and only phase III remains as a high-temperature phase. In addition, phases I and II disappear in case of solid solutions of cesium and rubidium nitrates at a cesium salt concentration of ~25 mol % [7]. Studies [13–16] were devoted to the mechanism of polymorphic transformations in single crystals nitrates of alkali metals, including potassium and rubidium nitrates [8–12], as well as in solid solutions of silver nitrate in potassium nitrate and cesium nitrate in rubidium nitrate. It was demonstrated in these studies that polymorphic transformations in the aforementioned crystals occur mainly during the nucleation and growth of a daughter modification nucleus in the matrix. According to [17], sodium nitrate has a rhombohedral lattice in the hexagonal system with parameters
а = 5.0696 Å, с = 6.829 Å, sp. gr. R 3c , and Z = 6 at a temperature Т = 298 K. According to [18], at Т = 563 K, it also has a rhombohedral lattice (with parameters а = 5.0889 Å, с = 8.868 Å, Z = 3, and sp. gr. R 3 m) in the hexagonal system. According to the data of [19], RbNO3 and NaNO3 form solid solutions. In this paper we report the results of X-ray diffraction study of a solid solution of the RbNO3–NaNO3 system at different temperatures. EXPERIMENTAL Well-faceted Rb2Na(NO3)3 single crystals of lowtemperature modification IV (Fig. 1) with an average size of 1 × 0.5 × 10 mm (Fig. 2) were obtained from an aqueous solution
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