Growth and optical properties of Cr3+-doped CdWO4 single crystals

Abstract

A high-quality Cr 3+ :CdWO 4 single crystal at a size of approximately Φ 25×80 mm is grown using the Bridgman method with CdO, WO 3 , and Cr 2 O 3 as raw materials and their molar ratio of 100:100:0.5. The temperature gradient of solid-liquid interface at growth is approximately 50• C/cm and the growth rate is 0.05 mm/h. The X-ray diffraction (XRD), absorption, excitation, and emission spectra of different parts of the as-grown and O2-annealed crystals are investigated. Two strong broad optical absorption bands of about 472 and 708 nm are observed, and they are associated with the transitions 4 A2 → 4 T1 and 4 A2 → 4 T2. The weak 4 T2 → 2 E transition (the R-line) at 632 nm is also observed. The crystal-field parameter Dq and the Racah parameters B and C are estimated to be 1 412.4, 776.8, and 3 427.6 cm −1 , respectively, according to the absorption spectra and crystal-splitting theory. A broadband fluorescence at about 1 000 nm due to 4 T 2 → 4 A 2 transition is produced by exciting the samples at 675 nm. After being annealed in an O 2 atmosphere, the crystals become more transparent, while the effective light absorption of Cr 3+ ions is evidently enhanced and the emission intensity is also strengthened due to the reduction of oxygen vacancies in the CdWO 4 crystal after annealing.OCIS Metal tungstates are important for their applications in scintillators and other optical devices [1] . In general, the properties of metal tungstates can be effectively modified by introducing cationic ions during the crystal growth. In recent years, metal tungstates doped with functional ions have been widely studied because they are competitive host materials in the optical field due to their promising applications for laser, phosphors, or scintillators [2] . CdWO 4 crystal is commercially viable and important among the metal tungstates. Previous studies have demonstrated that the Cd 2+ site in the CdWO 4 tends to be occupied by cations with a low valence state, while the W 6+ site tends to be replaced by cations with a higher valence state [3] . Active lanthanide ions (RE=Nd, Dy, Ho, Er, Tm or Yb) have been used as dopants to form active materials for laser purposes [2] . The Cr 3+ ion is one of the transition metal ions used as activators in materials for tunable solid-state lasers [4] . Cr 3+ -doped crystals and optical properties have been reported [5−7] . Tunable solid-state lasers have a wide range of applications, such as medicine and life science, telecommunication, and environmental monitoring [8−10] . Almost all CdWO 4 single crystals were grown using the Czochralski (CZ) technique in previous works [11,12] . Some difficulties occur in this growing method for CdWO 4 single crystals. For instance, there is a continuous composition change of melts due to the volatilization of CdO and WO 3 . In addition, cracking readily occurs in the as-grown crystals due to cleavage. We recently prepared CdWO 4 crystals using the Bridgman method [13] . In our process, harmful volatilization was minimized by sealing the crucibles, an...

Spin-Forbidden Transitions in the Spectra of Transition Metal Ions and Nephelauxetic Effect

“…A survey of the literature data on the emission transitions of these ions reveals that for the Cr 3+ ions the energy of the 2 E g → 4 A 2g emission transition varies between 11975 cm −1 (839 nm) in KZnClSO 4 • 3H 2 O 27 to 15823 cm −1 (632 nm) in CdWO 4 28 and 15734 cm −1 (634 nm) in KMgF 3 , 29 with the difference in the wavelengths of more than 200 nm. As far as the Mn 4+ ions in solids are concerned, the energy of the same 2 E g → 4 A 2g transition can be tuned from 16207 cm −1 (617 nm) in Na 2 SiF 6 30 to 13827 cm −1 (723 nm) in SrTiO 3 , 31 thus covering a spectral range with a width of more than 100 nm.…”

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