The hydrothermal syntheses of the alkali metal molybdenum bronzes from starting solids (H x MoO 3) with structural affinities to the desired products were investigated. Single-phase potassium blue and red bronzes were prepared by the hydrothermal treatments at around 430 K, and characterized by powder X-ray diffraction, IR spectroscopy, and SEM. The formation processes of these two bronzes during the hydrothermal treatments were found to differ. The blue bronze was formed by a structure-inheriting solid-state route from H x MoO 3 with x<0.3, whereas the red bronze was formed for x>0.3 through a solution dissolution/deposition route via the formation of MoO 3 + MoO 2 .

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We found some novel synthesis routes for alkali-metal molybdenum bronzes. The synthesis techniques are composed of simple hydrothermal treatments of mixed-valence compounds (or mixtures) with ACl or ACl + AOH aqueous solutions (A = alkali metal). Using these routes blue bronze (K0.30MoO3) and red bronze (K0.33MoO3) were obtained from hydrogen molybdenum bronze HxMoO3 or HxMoO3 + MoO3 even at 410 K.

The hydrothermal heating of H0.28MoO3 in a KCl solution at 431 K led to the formation of a single phase of K0.28MoO3. Hydrated molybdenum bronze was formed as a mid-product during conversion from hydrogen molybdenum bronze to blue bronze. The formation enthalpy of K0.28MoO3 was determined by solution calorimetry to be ΔfH° (K0.28MoO3, s) = −840.2 kJ mol−1 at 298.15 K. The enthalpies of the transformation reaction from hydrogen bronze to hydrated bronze and of the formation reaction of blue K0.28MoO3 from hydrated bronze were calculated to be +33 kJ mol−1 and −48 kJ mol−1 at 298.15 K, respectively. The calculated enthalpies of the reactions were concordant with the suggested reaction mechanism for a transformation from hydrogen molybdenum bronze to potassium bronze.