Dipole centres in CaMoO4 crystals

Abstract

In CaMoO4, crystals with oxygen deficiency thermally stimulated depolarization (TSD) currents are investigated in dependence on polarization time as well as on polarization temperature. The results can be well understood on the base of dipole orientation. Optical absorption measurements yield a band near 460 nm and another very broad one at around 850 nm, either band showing anisotropic absorption of plane polarized light. From molecular orbital theory the infrared band is assumed to be due to a transition within (MoO4)3− centre. The other absorption band possibly arises from oxygen vacancies which have trapped one or two electrons. The gradual disappearance of either band due to annealing procedure can be correlated to that of the TSD signal. Thus the proposed models of dipole centres involve absorbing defects, too.

Impedance Spectroscopy Analysis of Doped PbWO4 Single Crystals

“…However, only dc current response was measured and the temperature is low (80-340 K) in the TSPC/TSDC test, while the ac impedance spectroscopy is a powerful tool in studying the defect cluster and dielectric relaxation [10,11]. Defect associates also have been investigated intensively in alkali halides, metal oxide crystals and CaMoO 4 crystals [12][13][14]. Considering the similarity of ionic radius of Y 3+ (0.93 A) and Pb 2+ (1.20 A), it is expected that Y 3+ ions occupy the Pb 2+ sublattice sites, just like La 3+ ions do [6].…”

Structural and optical characterization of Er-doped CaMoO4down-converting phosphors

“…In the unit cell of CaMoO 4 , the Mo ions are coordinated to four oxygens and calcium ions have eight oxygen neighbors, forming tetrahedral MoO 4 and distorted snub disphenoid deltahedral CaO 8 units in such a way that each CaO 8 unit shares its corners with eight adjacent MoO 4 units. Table 3 gives From the short-range structural data in Table 3, it is concluded that sample CM-80 has longer Ca-O bond lengths (2.45 and 2.47 Å ) than the 800 C annealed samples which show contraction of Ca-O bond lengths with the heat treatment (Bö hm et al, 1981). The values of Mo-O and Ca-O coordination numbers, Mo-O and Ca-O bond lengths and O-Mo-O and O-Ca-O bond angles do not show significant variation with the addition of Er 2 O 3 in CaMoO 4 because Er 3+ does not replace Ca 2+ but co-exists as a separate phase (Er 2 O 3 ) with CaMoO 4 .…”

“…2). This is due to the presence of defects such as oxygen vacancies in the CaMoO 4 sample dried at 80 C (Bö hm et al, 1981). Upon annealing at 800 C, these defects get removed and the unit cell contracts (Malevu & Ocaya, 2015) and hence the unit-cell volume decreases (Table 2, discussed below).…”

“…Both the annealing treatment and Er 2 O 3 addition produce only small changes in the optical absorption edge (band gap) of the material. A small increase in band gap from 4.76 eV in CM-80 to 4.83 eV in CM-800 is due to the presence of defects such as oxygen vacancies in the CaMoO 4 sample dried at 80 C (CM-80) (Bharat et al, 2017;Bö hm et al, 1981), which are removed upon annealing at 800 C, resulting in the widening of the band gap in sample CM-800 (Vidya et al, 2012). However, on doping with Er 2 O 3 , defects are induced which produce new energy levels within the band gap (Longo et al, 2008;Cavalcante et al, 2012).…”

Thermally Activated Processes in CaMoO₄ Crystals

“…As-grown crystals exhibit a dark-blue colour disappearing after annealing in the oxygen flow atmosphere. The absorption bands in a visible-infrared spectral range are explained by presence of anomalously valence molybdenum ions (Mo 5+ ) in the bulk crystal [6] and formation of (MoO 4 ) 3or (MoO 3 )absorption centres [7,8]. Annealing is followed by disappearance of these centres.…”