Study of the Dependence of the Racah Parameter B on the Coordination-Bond Length in Cs₃CoCl₅ and Cs₃CoBr₅ Crystals by the Effect of Uniaxial Stress on Optical Absorption Spectra, and by Others

Abstract

Shifts of the average transition-energy E_av of spectra ⁴A₂→²E(²D) and ²T₂(²D) induced by applied uniaxial stress P, ΔE_av⁄|ΔP|, are investigated by using P up to 2.5 kbar at 4.2 K. Spectra ⁴A₂→²E(²D) of Cs₃CoCl₅ exhibit a small blue-shift, while those of Cs₃CoBr₅ exhibit a large red-shift. This is explained by the difference mainly in (ΔB⁄B₀)⁄(ΔR⁄R₀) between two compounds, where B₀ is B for a free Co²⁺ ion and ΔB is the change of B induced by that of the coordination-bond length R₀, ΔR. ΔE_av⁄|ΔP| of two spectra ⁴A₂→²E(²D) and ²T₂(²D) gives nearly the same value of (ΔB⁄B₀)⁄ΔR⁄R₀), i.e., 1.38±0.15 for Cs₃CoCl₅ and 2.20±0.15 for Cs₃CoBr₅. On the other hand, values of B and R₀ in excited cubic-field terms are determined from absorption-energies of zero-phonon lines and from absorption-intensities of vibrational site-lines, respectively. (ΔB⁄B₀)⁄ΔR⁄R₀) obtained from this procedure agrees with (ΔB⁄B₀)⁄ΔR⁄R₀) obtained from ΔE_av⁄|ΔP|.