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  Troyanov, S. I.; Morozov, I. V.; Reehuis, M.; Kemnitz, E.: Synthesis of two modifications of (NH₄)₂SeO₄(H₃PO₄) and their crystal structure as probed by x-ray and neutron diffraction. Russian Journal of Inorganic Chemistry 44 (1999), p. 1924-1930

Abstract:
The alpha and beta modifications of (NH(4))(2)SeO(4)(H(3)PO(4)) are obtained by crystallization from aqueous solution of (NH(4))(2)SeO(4) and H(3)PO(4) at room temperature and 60 degrees C, respectively. X-ray and neutron diffraction analysis of alpha-(NH(4))(2)SeO(4)(H(3)PO(4)) shows that the hydrogen atom distribution in the crystal structure corresponds to the occurrence of SeO(4)(2-) bicharged anions and H(3)PO(4) molecules. A similar distribution, but with a less intricate system of hydrogen bonds, is also found by X-ray crystallography for beta-(NH(4))(2)SeO(4)(H(3)PO(4)). The crystal structures of alkali metal and ammonium biselenophosphates and bisulfophosphates are compared.