ИСТИНА

Yb:ZnWO4 single crystal with wolframite structure is promising laser material. Meanwhile, heterovalent doping of ZnWO4 by Yb3+ without a proper charge compensation was earlier observed to result in sharp decrease in mechanical strength characteristics of the crystal, because of formation of Zn2+ vacancies that weakens the lattice. Yb segregation coefficient in the crystals is also rather low (about 0.3) in these conditions. In frame of the research we optimized the content of Li+ charge compensator in Yb,Li:ZnWO4 crystal from point of view of Yb segregation coefficient and of mechanical strength characteristics. We have grown 11 Yb,Li:ZnWO4 single crystals with the nominal Yb and Li contents varying from 0 to 7 at.%. Depending on the particular Yb and Li contents combination, the intensity of crystals cracking varies from complete absence of cracks to very strong cracking. The actual concentrations of the dopants were measured by atomic emission spectrometry with inductively coupled plasma and derived from the optical absorption spectra (only for Yb3+). Yb and Li segregation coefficients have been evaluated from the ratios between nominal and actual dopants concentrations. It was found that Yb3+ segregation coefficient in ZnWO4 crystal is ~ 0,25 without special charge compensators, while, it increases by factor of 2 in heavily Li+ codoped crystals. Li+ segregation coefficient in ZnWO4 crystal equals to ~ 0,2 and almost independent on Li+ concentration. In order to reach the same Yb3+ and Li+ contents in the crystal, the melt for Yb,Li:ZnWO4 Czochralski growth should contain much higher Li+ concentration than Yb3+ one. Hardness and fracture toughness of the crystals were measured at (010) crystallographic plane by indentation method. Yb,Li:ZnWO4 crystals, grown from the melts containing 5-5,5 at.% of Yb3+ and 8-12 at.% of Li+, have the best mechanical strength. Less Li+ content results in too big difference between the actual Yb3+ and Li+ contents, and, hence, too large amount of Zn vacancies in the crystal. It substantially weakens the crystal lattice and leads to reduction of the crystal mechanical strength. The same weakening is in the case of too high concentrations of Li+ and Yb3+ ions, which are have quite pronounced difference with Zn2+ ion in ionic radii and in chemical nature.