The structure of a PWO:La3+ crystal

A lead tungstate crystal, with 1% mol La3+ doping, was grown by the Czochralski method and its structure determined by single-crystal X-ray diffraction techniques. It retains the scheelite-type structure with cell constants a=b=5.4587(6) ?, c=12.038 (1) ?, V=358.7(1) ?3, Z=4. The trivalent La3+ ions are located at the 4(b) positions with occupancy 0.01 and share this site, statistically, with Pb2+ ions. The structural discrepancy between the La3+ doped and the undoped PWO comes from the positions of the oxygen atoms, which results that the PLO8 polyhedron and WO4 tetrahedron rotate anticlockwise by about 57.8° along their S4 axis compared with those of the undoped PWO crystal. The reasons for this are discussed.