Investigation of ZnWO4 Crystals Doped with Tm3+ and Li+ Ions at Different Concentrations

A series of Tm,Li:ZnWO₄ single crystals with different combinations of thulium and lithium concentrations is grown by the Czochralski method. Actual concentrations of both dopants are measured using inductively coupled plasma mass spectrometry. Segregation coefficients of thulium and lithium in the crystals are calculated by the initial point method. It is shown that introduction of lithium into the crystal composition increases the segregation coefficient of thulium between the crystal and the melt by at least a factor of 2, from 0.2 to 0.4. At the same time, the segregation coefficient of lithium monotonically decreases from 0.18 to 0.08 with its increasing concentration. Thus, to produce a sample with considerable and equimolar Tm and Li concentrations in the crystal, the nominal concentration of lithium is required to be very high, severalfold higher than that of thulium. Polarized optical absorption spectra of the crystals are measured at room temperature for all possible combinations of orientations of the strength vectors of the test beam’s electric and magnetic fields with respect to the optical indicatrix axes of the crystals. It is shown that at the electric field orientation E || N_m the shape of the absorption spectra appreciably depends on the magnetic field orientation as well. The influence of the lithium presence/absence in the crystal on the shape of the absorption spectra is determined. Absorption cross sections of thulium in the given crystal are refined and found to be considerably larger than previously published. Mechanical strength characteristics, microhardness and fracture toughness, are measured for the series of Tm,Li:ZnWO₄ crystals using the indentation method. It is shown that these characteristics are noticeably deteriorated when Tm³⁺ ions are introduced into the crystal without simultaneously introducing the charge compensator—Li⁺ ions—because there arise zinc vacancies that loosen the crystal structure. Additional introduction of lithium in equimolar amounts results in that the strength characteristics of the crystals are not only restored but even exceed those of undoped ZnWO₄, since this simultaneous doping does not lead to formation of zinc vacancies. Thus, conditions are revealed that allow fabrication of heavily thulium-doped crystals with maximum possible mechanical strength characteristics of ZnWO₄.