The Study On Luminescent Properties Of Cr³⁺,Mn²⁺ Doped CdWO₄Single Crystals

It is well known that CdWO₄single crystals are largely studied as competitive host materialsin the optical field due to their promising applications for scintillators or laser. The opticalproperties of CdWO₄can be largely improved when it was doped with different ions. Previousstudies showed that Cd²⁺site in the CdWO₄is readily replaced by cation with low valence stateand W⁶⁺site is readily replaced by cation with high valence state. This paper used Bridgmanmethod to get Cr³⁺and Mn²⁺-doped CdWO₄single crystals, and its spectroscopic properties havebeen recorded and discussed.The high quality Cr³⁺-doped CdWO₄single crystals in the size of～Φ25mm×100mm has been grown by Bridgman method under the technical condition ofchoosing CdO:WO₃: Cr₂O₃raw chemical composition in molar ratio of100:100:0.5, taking～50°Ctemperature gradient of solid-liquid interface and0.05mm/h growth rate. The X-ray diffraction（XRD）, absorption, excitation and emission spectra of different part of the unannealed andannealed Cr³⁺-doped CdWO₄were investigated. It is obvious that the absorption spectrum ischaracterized by two broad and strong optical absorption bands at wavelengths of about472nmand708nm, which are corresponding to the transitions⁴A_2gâ†'⁴T_1gand⁴A_2gâ†'⁴T_2g. The weak⁴T_2gâ†'²Egtransition（the R-line） at632nm can also be observed. The crystal-fieldparameter Dq, the Racah parameter B and C were estimated to be1412.4cm^-1,776.8cm^-1and3427.6cm^-1according to the absorption spectra and crystal splitting theory. The emission spectrawere recorded by pumping the samples at675nm, the broadband fluorescence at about1000nm due to⁴T_2gâ†'⁴A_2gtransition is present. It can be deduced from the absorption andemission spectra that the Cr³⁺ion is probable to replace the Cd²⁺ion that lies in the octahedralneighborhood of oxygen O2-ions. Annealing processes are advantageous to change thequality of the crystal and increasing of the emission intensity. Because the annealingprocess can reduce the defects of oxygen vacuum in CdWO₄crystal, the effective lightabsorption of Cr³⁺ions enhances obviously. It results into strengthening emissionintensity.A high-quality Mn²⁺-doped CdWO₄single crystal at a size of25×100mm isgrown by Bridgman method with CdO, WO₃and MnO as raw materials and their molar ratio of 100:100:0.5. The temperature gradient of solid-liquid interface is～60°C and the growth rate is0.05mm/h. The XRD absorption, excitation and emission spectra of different parts of the as-grownand annealed Mn²⁺-doped CdWO₄crystals were investigated. An optical absorption bandsof about490nm is observed. An narrow emission band at about591nm was found,which was attributed to the electron transition of⁴T1gâ†'6A1glevel. After being annealed inair and O2atmosphere, the effective light absorption of Mn²⁺is evidently enhanced and emissionintensity is also strengthened. Because the annealing process can reduce the defects ofoxygen vacuum in CdWO₄crystal, the effective light absorption of Mn²⁺ions enhancesobviously. It results into strengthening emission intensity.