Covalency Effects in KNiF3. II. Optical Studies

Abstract

The optical spectra of KNi${\mathrm{F}}_3$ and KMg${\mathrm{F}}_3$: ${\mathrm{Ni}}^{2+}$ crystals have been measured between 5000 and 40 000 ${\mathrm{cm}}^{-1\mathrm{}}$ and from 300 to 4.2°K. Five bands were observed and used to determine three parameters in the ligand field analysis. The cubic crystal field splitting $10Dq$ was determined to be 7250 ${\mathrm{cm}}^{-1\mathrm{}}$ and the Racah parameters $B=955\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and $C=4230\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. In KNi${\mathrm{F}}_3$, below the Néel temperature of 275°K, there was an average additional shift of 320 ${\mathrm{cm}}^{-1\mathrm{}}$ to higher energies as compared with the magnetically disordered ${\mathrm{Ni}}^{2+}$ in KMg${\mathrm{F}}_3$. This is explained on a molecular field model. An unexpected splitting below $T_N$ of the ${}_{}{}^3{}_{}{}^{}A_2^{}{}_{}{}^{}\to {}_{}{}^1{}_{}{}^{}E$ transition in KNi${\mathrm{F}}_3$, which was not observed in the magnetically dilute crystals has been observed.