The calculated power law exponents for the system Pr3" -02-zre shown to be in agreement wIth the nrevfouslv determined exnorimentaI values. thus confirming that these expnents are greater than those ?or the q&tern Pr3' -F-z ft is customary to describe the variation of crystal field parameters with interionic distance R tn terms of the power law Rmtn_ Such distance variations can be realised experimentally either by pressure Il.1 or by using different isomorphic host crystals [Z]. Distance variations can also be determined by careful analysis of static crystal field parameters in complex hosts using the superposition model [3,4]. A knowledge of the inveree power iaw exponents is useful in predictlng the interaction between the electronic states of the ion with lattice vibrations 153 ss well as giving an extra criterion to test ab inftio crystal field theories [6].
It is this last problem which concerns its here, Power Iaw exponents have previously been determin.ed by ab initio calculations for the systems Pr3+ -Cl' 172 snd Pr3+ -F' [a]. In the latter case the ab initio calculation has been shown to be in good agreement with experiment bs!j+Indirect evidence also shows that the -Cl' calcu:atton is at least approximately corrlect [Bj.
The previous ab initio results obtained by us, t&en together with the l;t values obtained from the analysis of garnet crystal fields, pose a problem. Ab initio calculations [7,8] give $4 lytg elO.5 for Pr3+ -Cl', whereas tq= #6= 5.7 for Pr2+ -F'. .This difference can be readiIy explained as a consequence of the difference in