Thermal Conductivity Investigation of $$\mathbf{Ca}_\mathbf{9} \mathbf{RE}(\mathbf{VO}_\mathbf{4})_\mathbf{7}$$ Ca 9 RE ( VO 4 ) 7 (RE $$=$$ = La, Nd, Gd) and $$\mathbf{Ca}_\mathbf{10}$$ Ca 10 M(VO $$_\mathbf{4})_\mathbf{7}$$ 4 ) 7 (M $$=$$ = Li, Na, K) Single Crystals

The $$\hbox {Ca}_{9}\hbox {RE}(\hbox {VO}_{4})_{7}$$ (RE $$=$$ La, Nd, Gd) and $$\hbox {Ca}_{10}\hbox {M}(\hbox {VO}_{4})_{7}$$ (M $$=$$ Li, Na, K) single crystals have been grown by the Czochralski method. The binary vanadates are isostructural to “whitlockite” mineral (rhombohedral symmetry, R3c space group). Their thermal conductivity has been investigated in the range 50 K–300 K parallel to the c axis. For $$\hbox {Ca}_{9}\hbox {Gd}(\hbox {VO}_{4})_{7}$$ crystals, the thermal conductivity has been investigated in the range 300 K–550 K also. Additionally, for the $$\hbox {Ca}_{10}\hbox {M}(\hbox {VO}_{4})_{7}$$ (M $$=$$ Li, Na, K) crystals the heat capacity has been studied in the temperature range 80 K–300 K. The character of the temperature dependence of thermal conductivity is close to that of glasses. The possible reasons of the observed features of the thermal conductivity have been analyzed. Raman spectra of $$\hbox {Ca}_{10}\hbox {M}(\hbox {VO}_{4})_{7}$$ (M $$=$$ Li, Na, K) crystals have been measured and discussed. The spectral lines were broad and similar to polycrystalline or amorphous solids. These crystals are expected to be suitable for application as efficient nonlinear optic and laser materials.