Influence of lithium and magnesium on the real structure and dielectric properties of Ca 9 Y(VO 4 ) 7 single crystals

Grown by the Czochralski method, pure magnesium and lithium doped Ca 9 Y(VO 4 ) 7 single crystals (Ca 9 Y(VO 4 ) 7 ( C1 ), Ca 9 Y(VO 4 ) 7 :Li ( C2 ) and Ca 9 Y(VO 4 ) 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 Ca 9 Y(VO 4 ) 7 , Ca 9 Y(VO 4 ) 7 :Li and Ca 9 Y(VO 4 ) 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 R 3 c → R 3̄ c → R 3̄ m earlier suggested to occur at the same temperatures in Ca 9 Y(VO 4 ) 7 on the basis of other experimental evidences. In all temperature modifications of R 3 c , R 3̄ c , and R 3̄ m , the Ca 9 Y(VO 4 ) 7 :Li crystal has the highest activation energy and two-orders lower Ca 2+ ionic conductivity in comparison with nominally pure Ca 9 Y(VO 4 ) 7 with the conductivity of Ca 9 Y(VO 4 ) 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 Ca 2+ transport.