ELECTRON-PARAMAGNETIC-RESONANCE SPECTRA OF NI2+-DOPED ZN(BF4)2-CENTER-DOT-6H2O - TEMPERATURE AND CONCENTRATION-DEPENDENCE

The single-ion spin Hamiltonian parameters have been determined for zinc fluoroborate hexahydrate doped with Ni2+ (3d8, S = 1) and compared with those obtained for nickel fluoroborate hexahydrate. To account for the differences observed, EPR for several concentrations (x = 0.005 to 1) was studied from room to liquid-helium temperature. Between 4.2 and 1.5 K, for low doping, they are constant with g(parallel-to) = 2.229 +/- 0.006, g(perpendicular-to) = 2.261 +/- 0.025, and D = - (0.1291 +/- 0.0008) cm-1. Appropriate values for nickel fluoroborate are nearly the same but they are weakly temperature dependent, attributed to a ferromagnetic interaction. At higher temperatures these parameters are remarkably concentration dependent. Two main lines arise in parallel orientation. For concentrations up to x almost-equal-to 0.04 and at 77 K, they show asymmetrically distributed satellite lines outside the more intense central portion. The satellite lines seem to increase with concentration but only those assigned to a ferromagnetic-coupled first-neighbor pair survive the simultaneous broadening effect. As the broadening is asymmetric, the lines' centers shift smoothly outwards. At x almost-equal-to 0.7, we observe maximum broadening; for larger x no structural detail is seen and the spectra are progressively exchange narrowed. At the same time, the two lines are drawn together. The linewidth dependence on concentration indicates that the Ni ions have randomly occupied the Zn sites as expected, and that the contribution from spin-lattice relaxation practically does not change with concentration. We conclude that the differences in the spin Hamiltonian parameters are due essentially to magnetic interactions throughout the whole temperature range.