Tungstate Scintillators Growth, Structures and Properties

Sammanfattning: Tungstate crystals are well-known scintillators, which can be used as detector material for high-energy radiation and are often chosen as references in measurements on new luminescent materials. For measurements of excitation and relaxation properties, single crystals are desirable. Crystals, up to 1 cm in length, of MgWO₄, CaWO₄ (pure and doped with Cr³⁺ and Bi³⁺), BaWO₄, Zn_1-xCd_xWO₄ (0 < x <1), and NaBi(WO₄)₂ were grown from Na₂W₂O₇ fluxes. Powder of HgWO₄ was prepared by boiling a mixture of HgO and H₂WO₄ in water. Crystals of HgWO₄ and Na₂W₂O₇.½H₂O up to 1 mm in length, were flux-grown under high pressure and temperature.

The solid solution system Zn_1-xCd_xWO₄ was studied using single crystal X-ray diffraction. The structure of HgWO₄ was refined from neutron powder diffraction and single crystal X-ray diffraction data. Only small differences were seen in the refinements. When the structures were compared, it was seen that the WO₆ octahedron is almost constant throughout the series Zn, Cd, and HgWO₄, while the MO₆ octahedron (M = Zn, Cd, Hg) changes substantially.

The luminescence measurements using synchrotron radiation were made to study relaxation of electronic excitations. Tungstate crystals are excited in the 5-7 eV region by direct optical creation of oxyanion molecular excitons. Higher energies increase the efficiency of the emissions associated with lattice defects or impurities, due to multiplication of electronic excitations. Polarised emissions and phosphorescence showed anisotropic effects for CdWO₄ and PbWO₄, with the main emissions polarised in a plane normal to the c axis, and along the c axis, respectively.