STRESS DEPENDENT FREQUENCY SHIFT OF THE RESONANT BANDMODE IN NaCl:Cu+ G. BUSSE, W. PRETTL and L. GENZEL

of the vibrations of the magnetic ion alone, AO, at 2’ = 0 is 1.2% of the observed zero-field splitting. Similarly, we find AO/D = 0.027 at room temperatures. For 54Fe3+, 66Fe3+, and 67Fe3+ in calcite, we use TD = 4’70oK [a], and obtain AO/D = 0.007 at T = O°K and 0.03 at 300°K. We fail to account for the temperature independence of 6AO from 4.2’K as found experimentally [l]. Instead, our calculations show that 6U decreases by a f ctor of 2 at T = 112OK*. For 155Gd3+ and “3 I67Gd + in ThO2, as before [4], we assume that the lattice vibrations pertinent to our problem may be represented by the vibrations of the Th ions alone with [9] TD = 270’K. In this case, we get AO/D = 0.006 at T = O°K and 0.033 at T = = 300’K. We also find that the isotope shift approximately decreases by a factor of 2 at 2000K. This result agrees with Marshall’s experimental data [3]. Up to now, we have only considered the motion of the magnetic ions alone. If we take into account the motion of the neighboring ions, we might expect the phonon-induced splitting at 300°K to be more than 10% of the observed zerofield splitting. However, in many cases [l-3. lo],