Morphology and kinetics of polymorphic transformations in K 0.965 Rb 0.035 NO 3 single crystals
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RYSTAL GROWTH
Morphology and Kinetics of Polymorphic Transformations in K0.965Rb0.035NO3 Single Crystals V. I. Nasirov, R. B. Bairamov, and E. V. Nasirov Institute of Physics, Academy of Sciences of Azerbaijan, Baku, Azerbaijan e-mail: [email protected] Received November 17, 2014
Abstract—Polymorphic transformations in K0.965Rb0.035NO3 single crystals have been investigated by optical microscopy and X-ray diffraction. The equilibrium temperature between modifications II and III of the crystal studied is determined to be Т = 452 ± 0.5 K. It is found that polymorphic transformations are of the single crystal ↔ single crystal type and occur with the formation and growth of nuclei of new crystals in matrix ones at II ↔ III transformations. An empirical dependence of the growth rate of modification III on temperature is established in the form ϑ = ϑ = (–0.478ΔТ + 0.712ΔТ2 – 0.00041ΔТ3) × 10–2 cm/s (ΔТ = Тtr – Т0, where Тtr is the transformation temperature and Т0 is the equilibrium temperature). DOI: 10.1134/S1063774515070184
INTRODUCTION A study of the regularities of polymorphic transformations in nitrate compounds of alkali metals is not only of scientific but also practical importance, because this process is closely related to the growth technology of single crystals of mixed composition that exhibit polymorphism. Potassium nitrate is used to synthesize gun powder (sulfur, coal, KNO3) and in food conservation. In addition, nitrate compounds of alkali metals are widely applied in the production of pyrotechnic compositions. It is well known that potassium nitrate has two modifications in the temperature range from room to melting temperature [1, 2], while rubidium nitrate has four modifications [4–7]. When potassium nitrate is cooled, another modification (rhombohedral modification I) arises between modifications III and II [3].
The structural parameters of these modifications and temperature ranges of their existence are listed in Table 1. The purpose of studies [8–12] was to reveal the mechanism of polymorphic transformations in single crystals of nitrate compounds of alkali metals, including potassium and rubidium nitrates. The phase diagrams of mixtures of potassium and silver nitrates [13] and silver and rubidium nitrates [14] were determined, phase transitions in K1 – xAgxNO3 were established in [15], and phase transitions in K1 – xRbxNO3 were investigated by Raman spectroscopy in [16]. The mechanism of polymorphic transformations of solid solutions of silver and cesium nitrates in potassium nitrate was studied in [17–20]. A series of microscopy and X-ray diffraction studies is planned to reveal regularities of structural phase transformations of the solid solution of rubidium nitrate in potassium nitrate.
Table 1. Structural data for different modifications of KNO3 and RbNO3 and the temperature ranges of their existence Composition KNO3
RbNO3
Lattice parameters System Orthorhombic Rhombohedral Cubic Rhombohedral Cubic Trigonal
a, Å
b, Å
c, Å
4.41 7.41 4.35 7.32 5.48 4.30 10.48
9.17
6.42 10.71
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