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

Abstract

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) = \(\left( {{{416}}{{.4\; \pm \;0}}{{.40}}} \right) + \left( {{{72}}{{.67\; \pm \;2}}{{.30}}} \right){{ \times 1}}{{{{0}}}^{{{{-3}}}}}T - \left( {{{50}}{{.13\; \pm \;0}}{{.19}}} \right){{ \times 1}}{{{{0}}}^{{{5}}}}{{T}^{{{{-2}}}}}\), and C_p(CaLu₂Ge₄O₁₂) = \(\left( {{{450}}{{.0\; \pm \;1}}{{.75}}} \right) + \left( {{{15}}{{.46\; \pm \;1}}{{.90}}} \right){{ \times 1}}{{{{0}}}^{{{{-3}}}}}T - \left( {{{78}}{{.67\; \pm \;1}}{{.60}}} \right){{ \times 1}}{{{{0}}}^{{{5}}}}{{T}^{{{{-2}}}}}\). These results have been used to calculate selected thermodynamic properties of the prepared germanates.