Pressure dependence of the crystal-field spectrum of KNiF3: Single and double excitations

This work investigates the Ni-F distance dependence of the crystal-field (CF) transitions of Ni2+ in KNiF3 by high-pressure spectroscopy. All peaks shift to higher energy with pressure according to trends foreseen by the Tanabe-Sugano diagram. At ambient conditions, we obtain Racah and CF splitting parameters of B = 0.118 eV, 10Dq = 0.908 eV; C/B = 4.4 (10Dq/B = 7.7). B and 10Dq vary with pressure as partial derivative B/partial derivative P = -0.11 meV GPa(-1) and a partial derivative 10Dq/partial derivative P = 24 meV GPa(-1). Similar to KCoF3, the slight decrease of B with pressure reflects the strong ionic character of the Ni-F bond and its high stability against compression. We have correlated the measured pressure dependence of 10Dq with the Ni-F bond distance, showing that it follows a potential law as 10Dq = CR-n with an exponent n = 6.6 +/- 0.5, thus providing experimental data for checking the suitability of theoretical models aiming to explain the slight deviations of observed R dependencies of 10Dq from the CF theory (n = 5). We have applied the experimental 10Dq(R) relationship to determine the real Ni-F bond distances in fluoroperovskites ABF(3) : Ni2+ from the spectroscopically measured 10Dq as an alternative method for determining bond distances, RNi-F, in impurity systems. We show that the so-obtained RNi-F deviates from the bond distance of the host site, RB-F, proportionally to the difference RB-F - R-0, with R-0 being the sum of ionic radii RF- + R-Ni(2+). The behavior is compared to that found for Mn2+ along the fluoroperovskite series ABF(3): Mn2+. Finally, weak UV peaks observed below the charge-transfer band gap (E-g <= 10 eV) in the absorption spectrum, the assignment of which still remains controversial, have been assigned to single and double excitation transitions. The assignment was unveiled on the basis of their energy and pressure shift.

Automated summary

This study investigates the Ni-F distance dependence of Ni2+ crystal-field transitions in KNiF3 through high-pressure spectroscopy, revealing a correlation between Ni-F bond distance and 10Dq. The experimental data provides insight into the suitability of theoretical models and alternative methods for determining bond distances in impurity systems. Furthermore, the analysis of weak UV peaks below the charge-transfer band gap in the absorption spectrum sheds light on single and double excitation transitions.