Inversion of spin levels in Ni2+: Zn(BF4)2 · 6H2O 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 · 6H2O crystals are studied over a wide range of temperatures under uniform compression. Measurements are performed in the X-and Q-bands. The parameter D, which characterizes the initial splitting, undergoes considerable variations with changes in temperature and pressure, whereas the g factor remains virtually unchanged. An increase in the temperature is accompanied by a nonlinear increase in D. Under uniform compression, the initial splitting varies linearly and the parameter D changes its sign at 3.5 kbar, which indicates inversion of the spin levels. The coincidence of the EPR lines associated with different transitions leads to the appearance of line-profile dips in the spectra due to cross-relaxation inside the spin system.