Phase relations and crystal structures in the systems (Bi,Ln)2WO6 and (Bi,Ln)2MoO6 (Ln=lanthanide)

Several outstanding aspects of phase behaviour in the systems (Bi,Ln)2WO6 and (Bi,Ln)2MoO6 (Ln=lanthanide) have been clarified. Detailed crystal structures, from Rietveld refinement of powder neutron diffraction data, are provided for Bi1.8La0.2WO6 (L-Bi2WO6 type) and BiLaWO6, BiNdWO6, Bi0.7Yb1.3WO6 and Bi0.7Yb1.3WO6 (all H-Bi2WO6 type). Phase evolution within the solid solution Bi2−xLaxMoO6 has been re-examined, and a crossover from γ(H)-Bi2MoO6 type to γ-R2MoO6 type is observed at x∼1.2. A preliminary X-ray Rietveld refinement of the line phase BiNdMoO6 has confirmed the α-R2MoO6 type structure, with a possible partial ordering of Bi/Nd over the three crystallographically distinct R sites.