Syntheses and Crystal Structures of the Lanthanum Titanium Oxyselenides La4Ti2O4Se5 and La6Ti3O5Se9

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.