Report of the Double-Molybdate Phase Cs₂Ba(MoO₄)₂ with a Palmierite Structure and Its Thermodynamic Characterization

The existence of a novel double-molybdate phase with a palmierite-type structure, Cs₂Ba(MoO₄)₂, is revealed in this work, and its structural properties at room temperature have been characterized in detail using X-ray and neutron diffraction measurements. In addition, its thermal stability and thermal expansion are investigated in the temperature range 298-673 K using high-temperature X-ray diffraction, leading to the volumetric thermal expansion coefficient α_V ≈ 43.0 × 10^-6 K^-1. The compound's standard enthalpy of formation at 298.15 K has been obtained using solution calorimetry, which yielded Δ_fH_m°(Cs₂Ba(MoO₄)₂, cr, 298.15 K) = -3066.6 ± 3.1 kJ· mol^-1, and its standard entropy at 298.15 K has been derived from low-temperature (2.1-294.3 K) thermal-relaxation calorimetry as S_m°(Cs₂Ba(MoO₄)₂, cr, 298.15 K) = 381.2 ± 11.8 J K^-1 mol^-1.