Impurity paramagnetic centers in a single crystal of zinc tungstate doped with thulium

A thulium-doped ZnWO$_4$ crystal grown by the Czochralski method was studied by EPR at room temperature. Accidental impurities of paramagnetic ions Fe$^{3+}$, Mn$^{2+}$, Cr$^{3+}$ and Gd$^{3+}$ were found in the crystal. Trivalent ions are represented by both single monoclinic centers and triclinic dimeric associates. Spin Hamiltonians of all observed centers were constructed, which describe the experimentally obtained results quite well, and the concentration ratio of these centers was estimated.