EPR STUDY OF NI+ CENTERS IN CSCAF3

Ni+ centers produced by room-temperature x-ray irradiation of CsCaF3 s ingle crystals have been studied by EPR. Three types of centers have b een detected, all of them with tetragonal symmetry and the \x(2)-y(2)] ground state. The models for these centers, which are similar to thos e observed in other fluoride crystals, consist of a Ni+ ion surrounded by a Jahn-Teller elongated fluorine octahedron [Ni+(I)] and the same model, but with one and two nearest-neighbor fluorine vacancies, for N i+(II) and Ni+(III). The spin-Hamiltonian parameters (g and superhyper fine tensors) have been measured and compared with the values predicte d from molecular-orbitals calculations of the NiF65- and NiF43- comple xes. The results allow us to determine the values of the Ni+-F- distan ces. An anomalous behavior is found for Ni+(I), The orthorhombic chara cter of the superhyperfine interaction with the equatorial fluorines a lso has been theoretically estimated and is in good agreement with the experimental results. Besides, an explanation based on the spin polar ization of filled orbitals has been proposed to account for the superh yperfine interaction with the axial fluorines. Dynamical effects obser ved between 20 and 30 K in the Ni+(I) EPR signal have been discussed o n the basis of a dynamical Jahn-Teller effect.