DETERMINATION OF THE LEADING CRYSTAL-FIELD PARAMETER A20 IN NDFE11TIN1-DELTA COMPOUNDS

The value of the leading second-order crystal-field coefficient A20 of the Nd ion in NdFe11TiN1-delta, (A20 = + 170 K a0(-2) for delta = 0.5), has been determined from our bonding charge model (BCM). The enhancement of neodymium anisotropy and the change of sign of A20 after nitrogenation (A20 = -30 K a0(-2) for NdFe11Ti) can be explained by the Nd-N bonding, and is well accounted for within the framework of the BCM. The nitrogen-induced shift in A20 is DELTA-A20 = +200 K a0(-2) for Nd with one nitrogen atom as its neighbour. This shift is opposite in sign and more than three times larger in magnitude than that observed in Sm2Fe17N3-delta, -63 K a0(-2). A detailed discussion of the structural relationship between the tetragonal ThMn12-type compounds (for NdFe11TiN1-delta) and the rhombohedral Th2Zn17-type compounds (for Sm2Fe17N3-delta) leads to a physical understanding of the differences in the effect of nitrogenation on A20 in these phases. The value of A20, +170 K a0(-2), obtained in NdFe11TiN1-delta with delta = 0.5 yields a room-temperature anisotropy field of 7.2 T compared to that estimated from experimental data, 8 T. We predict a value of +370 K a0(-2) for A20 in fully nitrogenated NdFe11TiN (delta = 0.5), with a corresponding predicted uniaxial anisotropy of 13 T at room temperature.