Abstract
Abstract Large (NH4)H(SeO4)2 crystals and (NH4)4H22(SeO4)3 crystals grow usually from non-stoichiometric solutins with an excess of selenate acid. Complex dielectric anomalies at crystal phase transitions and suggest some kind of impurity in the crystal lattice. X-band ESR measurements of γ-irradiated single crystals identified SeO-3 - redicals with an electronic structure which depends on a crystal non-stoichiometry. Measurements show that in (NH4)3H(SeO4)2 crystal a some part of NH4-groups is replaced by H when the crystal grows with excess of H2SeO4 in growing solution. As a result the SeO4-trimers characteristic for (NH4)4H2(SeO4)2 are formed in the crystal lattice instead of SeO4-dimers existing in stoichiometric (NH4)3H(SeO4)2 structure. We conclude that non-stoichimetric crystals are rather solid solutions with a formula (NH4)3-xH1+x(SeO4)2. It is reflected also in differences in the unit cell dimensions published by different authors.
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