D. Hernandez, F. Rodriguez, M. Moreno and H. U. Güdel
Pressure dependence of the crystal field spectrum of the NH₄MnCl₃ perovskite: Correlation between 10Dq, Ne and Nt, and the Mn-Cl distance in MnCl₆^4- complexes
Physica B 265, 186-190 (1999)                 

Abstract: This work investigates the pressure-dependence of the crystal field (CF) spectrum of MnCl₆^4- complexes in NH₄MnCl₃. The aim is to determine the variation of 10Dq as well as the bonding parameters Ne and Nt deduced from the absorption spectra, with the Mn_¯Cl distance. The selected crystal provides a perfect O_h symmetry for Mn²⁺ allowing a direct correlation between the pressure and the Mn_¯Cl distance through the bulk modulus. Precise values of the variations of the spectroscopic parameters with P were obtained from the linear behaviour exhibited by the six single CF excitations in the 0_¯60 kbar range. It is worth pointing out that an R-dependence of 10Dq on the basis of a KR^–n variation with n close to 7 was attained from the pressure-induced shifts. A remarkable feature is the reduction of Ne and Nt upon pressure. Although Ne < Nt due to the greater bonding character of the e_g-orbitals, its variation with pressure however is smaller than that of the p-bonding t_2g-related Nt parameter.