Separation of Cubic and Axial Parts of the Crystal Field from Hydrostatic and Uniaxial Pressure Dependence of the Parameter a_o

Abstract

The spin Hamiltonian of S-state ions of the d-group (Mn", Fe3+, Cr') in trigonal crystal field is given /1,2/ by H, = g p S.S+D+ m m which could also be written in a unique system using the O n spin operators where a. = -(a-F) is the combination of cubic and axial parts of the crystal field. When EPR measurements are made with the magnetic field either along o r perpendicular to the trigonal axis the term a. can be obtained from the line position but it is not possible to determine a and F separately. It is possible to distinguish the effects of the terms in a and F by making measurements in other directions, but the interpretation of the measurements depends on the knowledge of the position of the? , q , obtain /l/e Therefore most experimental results of S-state ions in trigonal field a r e about ao, not a and F, separately /1,3/. Since a and F are both fourth order crystal field terms, it can be expected that F is considerably smaller than a if it arises from a small perturbation of the cubic field. So in theoretical calculations it is often assumed ao* -a to evaluate the value of a . from the formula of a /4/. However, this approximation sometimes may lead to great e r r o r , especially, in explaining the pressure dependence of a variation of F is not much smaller than that of a although F is often muchsmalle r than a. axis, which is sometimes difficult to because the pressure 0' 1) Chengdu, Sichuan, People's Republic of China. 9physica (b)