Influence of growth conditions on structural parameters of scheelite PbTO4 (T = Mo, W) crystals

• PbTO4 (T = Mo, W) crystal structures were refined by neutron and X-ray diffraction.
• Symmetry reduction from space group I41/a   to I4_ was first found for PbWO4 crystals.
• The type and concentration of point defects were determined.
• Coloration is caused by the Pb and O vacancy formation and Mo6 + → Mo5 + transition.

Lead molybdates and tungstates with general chemical formula PbTO4 (PTO), where T is Mo (PMO) or W (PWO), were grown by the Czochralski method in an oxygen (PTO-A) or inert (PTO-N2) atmosphere from a stoichiometric mixture of corresponding oxides or mixture containing an excess of PbO (PMO-N2(Pb)). The crystals were investigated by X-ray and neutron single crystal and X-ray powder (Rietveld method) diffraction methods. For PWO-A and PWO-N2 crystals, the presence of additional reflections (50%) was revealed, indicating the symmetry reduction from space group I41/a   to I4_ caused by kinetic (growth) order–disorder phase transition associated with an ordering/disordering of oxygen vacancies. In the structures of PMO samples, the vacancies are absent in the Mo and O sites (PMO-A and PMO-N2) and they are present in the Pb sites (except for PMO-N2(Pb)); the defect content being higher in the PMO-N2 crystal grown in an inert atmosphere. It is shown that the light yellow color of PTO crystals is due to the O and Pb vacancies, that may be accompanied by the transition Pb2 + → Pb3 +, and the gray-purple color of PMO-N2(Pb) is due to the transition Mo6 + → Mo5 +.

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