Gibbs energy of formation of Ca7V4O17

• First measurement of thermodynamic properties of the ternary oxide Ca7V4O17.
• Measured chemical potential of CaO in the two-phase mixture (Ca3V2O8 + Ca7V4O17).
• Employed an electrochemical cell based on single-crystal CaF2 as electrolyte.
• A mixture of CaO and CaF2 under pure oxygen gas as the reference electrode.
• ΔfHo and So of Ca7V4O17 at T = 298.15 K are estimated using Neumann–Kopp rule.

Thermodynamic properties of Ca7V4O17 are measured for the first time using a solid-state electrochemical cell incorporating single crystal of CaF2 as the electrolyte over the temperature range from (900 to 1175) K. An equimolar mixture of CaO and CaF2 is used as the reference electrode and a mixture of Ca3V2O8, Ca7V4O17 and CaF2 as the measuring electrode. Both the electrodes are placed under flowing oxygen gas at ambient pressure. The standard Gibbs energy change for the reaction:2Ca3V2O8+CaO→Ca7V4O17,2Ca3V2O8+CaO→Ca7V4O17,which is related to the chemical potential of CaO in the two-phase region (Ca3V2O8 + Ca7V4O17) of the pseudo-binary system CaO + V2O5, is obtained from the electromotive force of the cell as:ΔrGo±127/(J·mol-1)=ΔμCaO=-11453+8.273(T/K).The derived standard enthalpy of formation of Ca7V4O17 from elements in their normal standard states is – (8208.97 ± 8) kJ ⋅ mol−1and its standard entropy is (560.05 ± 7.5) J ⋅ K−1 ⋅ mol−1, both at T = 298.15 K. The results indicate that Ca7V4O17 decomposes into Ca3V2O8 and CaO at T = (1384 ± 3) K.

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