DETERMINATION OF 2-ELECTRON CRYSTAL-FIELD PARAMETERS IN SOME NEODYMIUM COMPOUNDS

Two-electron, fourth rank (k = 4), crystal field parameters are fitted for four neodymium compounds : LiYF4: Nd3+, Nd2O2S, Y2O3: Nd3+ and Nd 2O3, displaying a strong experimental/calculated discrepancy for the H -2(11/2) level. The final mean deviation for all the levels is 2 cm- 1 (20 %) lower than that attained with the standard (one-particle) theo ry. The compound parameters b(q){k1k2)k involve odd and even combinati ons of k1 and k2. The geometrical dependence (q) seems to indicate a d rift towards a C(infinity v) site symmetry caused by axial ligands. Th e diagonal matrix elements of the correlated crystal field are smaller than the off-diagonal matrix elements except for twin terms.