Departmental Bulletin Paper 水和アルカリ金属モリブデンブロンズの常温固相転換反応の熱化学的研究

鈴木, 隆  ,  田中, 明子  ,  西田, 隆政  ,  枝, 和男

In previous studies, it was reported that hydrated alkaline molybdenum bronze changed slowly from a layer structure to a hexagonal structure at room temperature. In studies of conformational transition reaction, XRD revealed an intermediate of hydrated alkaline molybdenum bronze. The intermediate contains hydroxyl groups for charge compensation of residual cations, is a layer structure, and has an unstable energy state. It is reasonable to infer that the unstable energy state is the driving force of the conformational transition reaction; however, reactivity of the conformational transition reaction depends on inserted cations. In this study, in order to discuss these reactivities, standard enthalpies of formation, ΔfH, of intermediate were estimated and compared. Three kinds of intermediates–hydrated potassium, hydrated cesium, and hydrated potassium and cesium mixed–were synthesized from hydrated sodium molybdenum bronze by using methods of ion exchange, and oxidation by nitric acid vapour at room temperature. XRD confirmed that these intermediates were single phase. The composition was determined by using an atomic absorption spectrometer and a TG-DTA. These intermediates were dissolved in a 3.00 M KOH + K3[Fe(CN)6] solution and heats of dissolution were measured simultaneously by using a twin conduction calorimeter. By using of Hess’s law of the summation, ΔfH were estimated from heats of dissolution and other data, It was determined that ΔfH of the K+ + Cs+ mixed bronze sample had the largest negative value of the three intermediates. Consequently, it was obvious that the structure of the hydrated K+ + Cs+ mixed bronze sample was the most unstable, and that the conformational transition reaction occurred easily. These results will be reported in detail.

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