Effect of Cationic Substitutions on Ferroelectric Properties of Pb5Ta10O30: Generation of the Relaxor Behavior

Phases of the type ACa9(VO4)7 have been prepared where A can be a trivalent rare earth cation or Bi3+. The structure of BiCa9 (VO4)7 was determined from single crystal X-ray diffraction data. The space group is R3 with a=10.8511(2) and c=38.0505(9) at 120 K and Z=6. The structure is related to that of Ca3(VO4)2, but a site partially occupied by Ca in Ca3(VO4)2 is completely vacant in BiCa9(VO4)7. All the ACa9(VO4)7 phases prepared showed a significant second harmonic signal. For the rare-earth compounds, the magnitude of the signal was comparable to that of KH2PO4; a value three times higher was found for BiCa9 (VO4)7.