Phase transitions in ammonium pyroselenite crystals
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ICE DYNAMICS AND PHASE TRANSITIONS
Phase Transitions in Ammonium Pyroselenite Crystals A. G. Lundina, Yu. N. Ivanovb, A. A. Sukhovskyb, and O. I. Nasluzovaa a
b
Siberian State Technological University, Krasnoyarsk, 660049 Russia Kirensky Institute of Physics, Siberian Branch, Russian Academy of Sciences, Krasnoyarsk, 660036 Russia email: [email protected] Received April 24, 2009
Abstract—The character of the phase transitions in ammonium pyroselenite crystals, (NH4)2Se2O5 and (ND4)2Se2O5, in the hightemperature phase has been investigated. The temperature dependences of the per mittivity and the angular dependences of the highresolution 77Se spectra in the phasetransition range have been measured. The parameters of the chemical shift tensor have been determined for 77Se nuclei in the Se2O5 pyroselenite ion in the hightemperature phase and compared with the known parameters of this tensor in the lowtemperature phase. Some conclusions are drawn on the character of transitions and the structure of the (Se2O5)2– ion. DOI: 10.1134/S1063774510040188
INTRODUCTION Recently, much attention has been paid to the properties of selenium and its compounds. It is well known that high doses of selenium and many of its salts are toxic for humans and animals. On the other hand, it is now clear that selenium is a necessary microelement for humans which reduces the proba bility of hypertension and eases the pains of this dis ease, suppresses sclerosis, and even serves as a pro tector in oncology. Therefore, many Russian and for eign corporations include selenium in the composition of complex polyvitaminic preparations. Selenium and its compounds are widespread in nature; many selenium compounds are concentrated in plants. They are washed down by water and can penetrate the human organism. Despite the large number of publications, the structure and dynamics of atoms and molecules in these compounds have not been investigated in detail. This paper is part of a sys tematic study of the physicochemical properties of the salts of selenious and selenic acids (selenites and selenates) [1–9]. Among selenites, ammonium salts are of great interest. They are generally characterized by a branched system of hydrogen bonds and exhibit a sig nificant dynamics of internal motion; many of them undergo phase transitions of different types. In [8] we studied the structure of pyroselenite ammonium (NH4)2Se2O5 by the 77Se nuclear magneticresonance (NMR) spectroscopy. According to the Xray data of [10], ammonium pyroselenite crystals belong to the orthorhombic sys tem (sp. gr. Р212121) and have the unitcell parameters a = 6.9971 Å, b = 7.4879 Å, c = 13.4318 Å, and Z = 4.
The structure consists of Se 2O52− and NH 4+ ions, which form a 3D framework through hydrogen bonds. The unit cell contains two nonequivalent ammonium groups and one nonequivalent pyroselenite ion. This ion consists of two pyramidal SeO3 groups which share a bridge oxygen atom. The Se–O bond lengths for this atom are 1.814 and 1.843 Å, whereas the other Se–O bond lengths a
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