The influence of vacuum on polymorphic transformations in rubidium nitrate
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ATTICE DYNAMICS AND PHASE TRANSITIONS
The Influence of Vacuum on Polymorphic Transformations in Rubidium Nitrate V. I. Nasirov, N. M. Namazova, R. B. Bairamov, G. G. Guseinov, and A. S. Amirov Institute of Physics, Academy of Sciences of Azerbaijan, pr. Dzhavida 33, Baku, Az1143 Azerbaijan email: [email protected] Received March 21, 2014
Abstract—Structural phase transitions in rubidium nitrate under normal pressure and in a vacuum (under residual pressures of 8 × 10–1, 8 × 10–2, and 8 × 10–3 Torr) were investigated using hightemperature Xray diffraction. It is found that three transformations occur in RbNO3 crystals in the range between room tem perature and the melting point. Only at a pressure of 8 × 10–3 Torr does the temperature range of existence of individual modification narrow and the transformation point shift to higher temperatures. DOI: 10.1134/S1063774514070153
INTRODUCTION Study of the regularities of polymorphic transfor mation in nitrates of alkaline metals is interesting from not only a scientific but also a practical point of view, because this process is closely related to the growth technology of single crystals characterized by poly morphism. Moreover, rubidium nitrate is widely used as a component of IR light sources. Four different modifications have been found for rubidium nitrate in the range from room temperature to the melting point (Tm = 587 K). At room temperature modification IV has a trigonal symmetry with the following parame ters: a = 10.48 Å, c = 7.45 Å, sp. gr. P31 (P32) [1]. At temperatures T > T0 = 437 K) the trigonal modifica tion is transformed into cubic modification III with a lattice parameter a = 4.35 Å, sp. gr. Fm3 m [2]. At tem peratures T > T0= 492 K, cubic modification III is transformed into rhombohedral modification II with lattice parameters a = 5.48 Å, c = 10.71 Å, sp. gr. Pm 3 m [3], and, at T > T0= 564 K, this modification is transformed into cubic modification I with the parameter a = 7.32 Å, sp. gr. Fm3 m [4]. Numerous studies of different researchers have been devoted to polymorphic transformations in rubidium nitrate. The existence of a new x modifica tion between modifications III and IV was revealed based on data on the morphology and kinetics of poly morphic transformations in rubidium nitrate [5]. Lin et al. [6] investigated the structural phase transitions in rubidium nitrate by molecular dynamics simulation. Kallimaki and Meisado [7] analyzed the influence of pressure on the structural transformations in RbNO3 and revealed an orthorhombic modification V (at a pressure of 2.8 GPa) with parameters a = 5.66 Å, b =
5.181 Å, c = 4.798 Å, sp. gr. Pmmn. According to the data of [8], modifications I and II disappear in rubid ium nitrate at a fairly high pressure (103 atm), and only cubic modification III remains. A phase diagram of rubidium nitrate was reported in [9]. It was shown in [10] and [11] that polymorphic transformations IV III II I in rubidium nitrate occur with the formation and growth of nuclei of daughtermod ification crystal
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