Effect of impurities on the adsorption spectrum of NaGd(WO$_{4}$)$_{2}$ crystal

Three NaGd(WO$_{4}$)$_{2}$ single crystals have been grown by Czochralski technique using the initial chemicals with different purity. Impurity concentrations were determined by the inductively coupled plasma mass-spectrometry. The optical absorption spectra of the crystals were measured and the influence of residual impurities' contents on optical properties and coloration of the crystals was determined. It was demonstrated that the green coloration which is sometimes observed for NaGd(WO$_{4}$)$_{2}$ crystals results from the presence of residual impurities of $d$-elements, for which the cromium impurity is the most important