Inversion of spin levels in Ni2+: Zn(BF4)(2) center dot 6H(2)O under uniform compression and the effect of transition coincidence

The electron paramagnetic resonance (EPR) spectra of Ni2+ ions substituting for Zn2+ ions in Zn(BF4)(2) . 6H(2)O crystals are studied over a wide rang e of temperatures under uniform compression. Measurements are performed in the X- and Q-bands. The parameter D, which characterizes the initial splitt ing, undergoes considerable variations with changes in temperature and pres sure, whereas the g factor remains virtually unchanged. An increase in the temperature is accompanied by a nonlinear increase in D. Under uniform comp ression, the initial splitting varies linearly and the parameter D changes its sign at 3.5 kbar, which indicates inversion of the spin levels. The coi ncidence of the EPR lines associated with different transitions leads to th e appearance of line-profile dips in the spectra due to cross-relaxation in side the spin system. (C) 2001 MAIK "Nauka / Interperiodica".