Calorimetric study of the stability of spinelloids in the system NiAl2O4-Ni2SiO4 |
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| Authors: | Masaki Akaogi Alexandra Navrotsky |
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| Institution: | 1. Department of Chemistry, Arizona State University, 85287, Tempe, Arizona, USA
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| Abstract: | Enthalpies of solution in molten 2 PbO · B2O3 at 974 K were measured for four spinelloids, phases I (0.75 NiAl2O4 · 0.25 Ni2SiO4), II (0.60 NiAl2O4 · 0.40 Ni2SiO4), III and IV (0.50 NiAl2O4 · 0.50 Ni2SiO4) in the system NiAl2O4 · Ni2SiO4. The enthalpies (in cal per 4-oxygen mol) of formation from NiAl2O4 and Ni2SiO4 spinels are: phase I, 945±366; phase II, 1072±360; phase III, 2253±390; phase IV, 3565±544. Using these enthalpy data in combination with phase relations at high pressure at 1373 K, positive entropies of formation of the spinelloids from NiAl2O4 and Ni2SiO4 spinels were estimated (in cal mol?1 K?1): phase I, 1.2; phase II, 1.5; phase III, 2.0–2.3; phase IV, 3.0–3.1. The thermochemical data obtained above suggest that the spinelloids are “entropy-stabilized” phases with partially disordered cation distributions. The configurational entropies of the spinelloids were calculated based on the observed cation distribution in each spinelloid phase. The positive entropies of formation of the spinelloids from the spinel endmembers are due primarily to the configurational entropies although small positive vibrational entropy changes may also exist. |
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