LOW-TEMPERATURE ESR-SPECTRA OF NICKEL-DOPED NACL-CRYSTALS

Paramagnetic resonance of Bridgman-grown NaCl crystals nominally doped with divalent nickel was studied over the temperature range from 4 K to 300 K. The broad isotropic EPR line with g = 2.12 divided by 2.23 a nd Delta H-pp = 280 divided by 780 G was detected at temperatures high er than 130 K for samples of various thermal history. The origin of th ese spectra was attributed to aggregated and precipitated Ni2+ ions. A t temperatures lower than 40 K another ESR signal was registered. It w as an anisotropic one, the intensity of which strongly depends upon th e temperature. This spectrum results from substitutional Ni+ ions with tetragonally distorted surrounding along a [100] direction. The angul ar dependence of the ESR line positions was fitted to an appropriate s pin-Hamiltonian with the g-factor components g(parallel to) and g(perp endicular to) equal to 2.86 and 2.10, respectively. The temperature-de pendence of the signal intensity at temperatures between 4 and 110 K h as been interpreted in terms of a transition from the static to dynami c Jahn-Teller effect.