Growth defect W3+-W2+ of CdWO4 crystals

Abstract

CdWO4 crystals grown by the Czochralski method at the low-temperature gradient were investigated with electron spin resonance (ESR) spectroscopy. ESR spectra did not contain the spectra of impurity ions typical for the CdWO4 structure, i.e., Fe3+, Mn2+, and Cr3+. At the same time, in the studied crystals a complex ESR spectrum having the hyperfine structure due to two nonequivalent tungsten atoms was observed (W183;I=1/2; natural abundance, 14.28%). Angular dependence analysis and simulation of ESR spectra have shown that this novel spectrum is described by a spin-Hamiltonian with the following parameters:D=839 G,E=80 G,gxx=2.01,gyy=1.97,gzz=1.987 and electron spinS=7/2. There is one magnetically nonequivalent position of the center in the crystal structure and the direction ofDzz andgzz corresponds to the direction of Wn-Wn+2 (or Cdn-Cdn+2) in the crystal structure. Because of the fact that it is in principle impossible to achieve the electron stateS=7/2 for the d-shell of one transition metal ion and taking into account the fact that such electron state is realized for two nonequivalent tungsten atoms, we suppose the defect structure to be the chain W2+-M+-W3+. In the structure of this defect the ion M+ is diamagnetic, the ions W2+ and W3+ have electron spinS=2 andS=3/2, respectively. The necessary condition for such defect to exist is to place this chain of ions in cadmium positions for the charge compensation. the reason for such defects to form is supposed to be the incorporation of M+ ions into the CdWO4 lattice. The presence of W2+ and W3+ in Cd positions in the defect structure provides the charge compensation and the lowering of the lattice stress.