Synthesis and Thermodynamic Study of CaYb₂Ge₄O₁₂ and CaLu₂Ge₄O₁₂ Germanates in the Range 320–1050 K

Germanates CaYb₂Ge₄O₁₂ and CaLu₂Ge₄O₁₂ have been prepared via firing the solid precursor oxides CaO, Yb₂O₃ (Lu₂O₃), and GeO₂ in air at 1223–1423 K. The X-ray diffraction crystal structures of the prepared germanates have been determined. The high-temperature heat capacity in the range 320–1050 K has been measured by differential scanning calorimetry (DSC). The measured heat capacities are well fitted by the Maier–Kelley equation: C_p(CaYb₂Ge₄O₁₂) = \(\left( {{\text{416}}{\text{.4}{\; \pm \;0}}{\text{.40}}} \right) + \left( {{\text{72}}{\text{.67}{\; \pm \;2}}{\text{.30}}} \right){{ \times 1}}{{{\text{0}}}^{{{\text{-3}}}}}T - \left( {{\text{50}}{\text{.13}{\; \pm \;0}}{\text{.19}}} \right){{ \times 1}}{{{\text{0}}}^{{\text{5}}}}T{{~}^{{{\text{-2}}}}}\) , and C_p(CaLu₂Ge₄O₁₂) = \(\left( {{\text{450}}{\text{.0}{\; \pm \;1}}{\text{.75}}} \right) + \left( {{\text{15}}{\text{.46}{\; \pm \;1}}{\text{.90}}} \right){{ \times 1}}{{{\text{0}}}^{{{\text{-3}}}}}T - \left( {{\text{78}}{\text{.67}{\; \pm \;1}}{\text{.60}}} \right){{ \times 1}}{{{\text{0}}}^{{\text{5}}}}{{T}^{{~{\text{-2}}}}}\) . These results have been used to calculate selected thermodynamic properties of the prepared germanates.