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Non‐stoichiometry, a Structural Aspect of Holmium Polysulfides
Author(s) -
Belaya S.,
Podberezskaya N.,
Naumov D.,
Vasilyeva I.
Publication year - 2002
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/1521-3749(200209)628:9/10<2161::aid-zaac2161>3.0.co;2-g
Subject(s) - chemistry , physics
Non-stoichiometry of rare-earth (RE) polysulfides is known to be caused by ordering of vacancies and sulfur atoms with different valences. The ordering is established very slowly, even at 500 to 800 °C. Therefore, the formation of metastable phases with varying degree of structural order is quite probable. Effects of this kind are aspected for the HoS1.5 HoS2 system, which is presented here. The experiments are based on structural study and analysis of the equilibrium p-T-x diagram of the system. The following procedures were performed: Growing the HoS2 x crystals under different conditions (T, ∆T, p(S)total). Classification of the crystals from the same bath on morphological identity. Random selection of 8 crystals for X-ray single crystal study. Calculation of theoretical powder patterns, based on X-ray single crystal data. Comparison of experimental powder diffraction patterns, using representative weight of the crystals from different bathes. Measuring of p(S2)-T dependence for the HoS2 x crystals. From these experiments the presence of two phases in the HoS1.5 HoS2 system was deduced. Phase (I) forms crystals with rectangular developed facet: HoS1.863(8), monoclinic, P21/m, a 10.961(2), b 11.465(2), c 10.984(2) Å, β 91.27(3)°, Z 24. Phase (II) was identified with crystals showing a square developed facet: HoS1.885(5), tetragonal, P4/nmm, a 3.820(1), c 7.840(3) Å, Z 2 [1]. The two phases were grown under the same conditions. Thus one of them might be considered a non-equilibrium phase or, alternatively, as the other boundary composition if the phase has a twosided homogeneity range. To clarify the origin of these phases the semi-quantitative phase analysis of experimental patterns and consideration of p-T-x diagram was done. The assumption that phase (II), which does not contribute more than 5 mass-% to the product, is metastable was supported. The appearance of the metastable phase (II) in the stability field of phase (I) is discussed as a common phenomenon in polysulfide systems.