Growth and characterization of Scheelite crystals

Abstract

A large number of tungstate (AWO4) and molybdate (AMoO4), where ?A? stands for large size divalent ions like Ca, Pb or Ba are naturally occurring crystals. Their scheelite structure is characterized by tetrahedrally coordinated (WO4)2 /(MoO4)2- group positioned at four corners of a unit cell and the divalent A2- located at bcc is octahedrally coordinated with oxygen ions [1]. The research interest in scheelite crystals dates back to early sixties, with investigation on CaWO4 used as a laser host, where the laser action was achieved by doping the crystal with Nd3+ ions. Subsequently, the second harmonic generation in this crystal was also demonstrated [2]. Crystals of PbWO4 (PWO) are extensively used as scintillation detectors. The scintillation emission in this case lies in blue region and it arises due to electronic transitions within the WO4 groups. PbMoO4 (PMO) crystals are used as acousto-optic modulator and this usefulness lies in independence of the deflection angle on polarization and also being a soft material. BaWO4 (BWO) is the most promising universal Raman active crystal among all scheelite crystals [3]. Its important features are high gain in steady as well as in transient states and much lower decrease in Raman level even in pico-second pump laser pulses. Another iso-structural material viz. NaBi (WO4)2 (NBW), where mono and trivalent ions may occupy the same bcc position with equal probability, is reported as a promising Cherenkov detector for LHC experiments [4]. Rare-earth doped double tungstates [Re:NaT(WO4)2 with T being a Trivalent ion are also laser crystals having high conversion efficiency [5]. The applicability of nano-particles as photo-catalysts and ionic conductors of scheelites happens to be a subject of current interest. Though scheelites have been investigated for over 50 years, issues related to their luminescent emission still remained unresolved. Some of them are listed as follows: (i) PMO crystal exhibits blue emission band only at low temperatures.