Tungstate Scintillators Growth, Structures and Properties
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, Zn1-xCdxWO4 (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 Zn1-xCdxWO4 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.
neutron powder diffraction