Influence of lithium and magnesium on the real structure and dielectric properties of Ca9Y(VO4)7 single crystals
Grown by the Czochralski method, pure magnesium and lithium doped Ca9Y(VO4)7 single crystals (Ca9Y(VO4)7 (C1), Ca9Y(VO4)7:Li (C2) and Ca9Y(VO4)7:Mg (C3), respectively) are characterized by means of chemical analysis, X-ray diffraction analysis and high-temperature dielectric spectroscopy. All crystals demonstrate good structural performance with almost isotropic micromosaics, free of extended defects and mechanical stress. Evidences for mobile domain structure are observed below the ferroelectric Curie temperature at 1213 ± 5 K. The temperatures of ferroelectric-paraelectric phase transitions in Ca9Y(VO4)7, Ca9Y(VO4)7:Li and Ca9Y(VO4)7:Mg almost coincide. In the paraelectric phase in these crystals, there is another phase transition at 1293 ± 5 K. The electrical nature and temperatures of both transitions are in agreement with the sequence of phases R3c → Rc → Rm earlier suggested to occur at the same temperatures in Ca9Y(VO4)7 on the basis of other experimental evidences. In all temperature modifications of R3c, Rc, and Rm, the Ca9Y(VO4)7:Li crystal has the highest activation energy and two-orders lower Ca2+ ionic conductivity in comparison with nominally pure Ca9Y(VO4)7 with the conductivity of Ca9Y(VO4)7:Mg lying in between. The observed influence of guest atoms is connected with different structural roles of lithium and magnesium in whitlockite crystal lattices where atoms of lithium all reside in unique M4 atomic sites crossing ion-conduction pathways. Magnesium atoms iso-structurally substitute for calcium in position M5 and have a smaller effect on fast Ca2+ transport.