Pressure dependence of the crystal field spectrum of the NH4MnCl3 perovskite:: correlation between 10Dq, Ne and Nt, and the Mn-Cl distance in MnCl64- complexes

This work investigates the pressure-dependence of the crystal field (CF) spectrum of MnCl64- 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-b 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 pi-bonding t(2g)-related Nt parameter. (C) 1999 Elsevier Science B.V. All rights reserved.