VIBRATIONAL EFFECTS ON THE MAGNETIC PROPERTIES OF HIGH-SPIN Fe2+ IN OCTAHEDRAL D4h SITES

We used a dynamic crystal-field formalism to describe the magnetic properties of ~ e ~ + in K2ZnF4 in which the lowest orbital states of ~ e ~ + ( 5 ~ ) are Eg and B2, (ground state). True Jahn-Teller effect inside Eg (involving \ , vibrational modes big and b2g) and pseudo Jahn-Teller effect between Bag and Eg (involving vibrational mode eg) were treated with linear coupling models, together with spin-orbit coupling (exact diagonalization in a "spin-orbit-vibrations" basis). The magnetocrystalline properties of isolated transition ions are usually referred to a competition between spin-orbit coupling and crystal-field. Here we introduce the idea that orbit-lattice coupling is also concerned. Following Ham's paper [I] we recently introduced a linear vibronic coupling in the cluster approach for octahedral high-spin l?e2+ in Dlh symmetry whose two lowest orbital states are Eg and Bz, (ground state) separated by 6. This way, we refined the analysis of the thermal dependence of the Mijssbauer quadrupole splitting in '?F'e doped K2ZnF4 and Ba2ZnFs : the Jahn-Teller active modes Eg (bl,, b,,) -Eg, together with spin-orbit coupling, enabled a spectacular fitting of the data [2]. However, the problem remained open [3] of the pseudo Jahn-Teller mode B2, e,-E,, whose high phonon energy (w 350 cm-') , recently deK2 Z n FL : ~ e ' * 0 r ( a 1 s t a t i c f i e l d