The absorption spectrum of single crystal NdF3 of tysonite structure was recorded at liquid helium temperature up to 33 800 cm−1. 127 Stark levels were obtained and fitted to a mean square deviation of 24 cm−1 with a Hamiltonian involving 12 free atom parameters and four D3h crystal field parameters. The fit was improved to a mean square deviation of 14 cm−1 by adding five supplementary crystal field parameters to approach the real C2 symmetry of the site. The paramagnetic susceptibilities parallel and perpendicular to the trigonal axis in the structure were measured from 4.2 to 1100 K and compared with the values derived from the wave vectors of the 18 lowest Kramers doublets through the application of the Van Vleck formula. An overall good agreement was found. The material is anisotropic, χ∥ being larger than χ⊥.

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A-Nd2O3 is a layer structure built on the (NdO)n+n cation. Data for the 4.2 K optical spectrum and the strongly anisotropic parallel and perpendicular paramagnetic susceptibilities between 4 and 1000 K measured on single crystals, are reproduced using a Hamiltonian, that involves 12 free atom parameters (three Racah’s parameters, the spin–orbit coupling constant, Trees’ parameters α and β, and Judd’s six Tk operators) and six C3v crystal field parameters. The Hamiltonian operates on the 364 kets in 4f3, that is including J mixing. 88 Stark levels (from 0 to 34 000 cm−1) were fitted to a mean square deviation of 17 cm−1. The crystal field parameters (Wybourne’s formalism) are B20=−836 cm−1, B40=634 cm−1, B43=−1606 cm−1, B60=752 cm−1, B63=237 cm−1, B66=672 cm−1. The spectrum is strongly displaced to the red with respect to Nd3+ in LaCl3 and this is due to a lowering of Slater’s integrals F2 and F4 by 2% to 3%. The F6 integral is unchanged. The spin–orbit coupling constant is also lowered by 2%. The interaction parameters are slightly affected.

The absorption spectrum of pure rhombohedral NdAlO3 was determined at 4.2 K from 10 000 to 34 000 cm−1. Adding the levels determined for the 4I multiplet by other authors, a total of 117 Stark levels was obtained. They were fitted to a mean square deviation of 13 cm−1 with a Hamiltonian involving 12 free atom parameters and six D3 crystal field parameters applied on the 364 kets basis in the 4f3 configuration. The values are B02=−481 cm−1, B04 =481 cm−1, B34=−390 cm−1, B06=−1700 cm−1, B36=−950 cm−1, B66=−1080 cm−1. Free atom parameters are very close to those of Nd3+ in LaCl3. The powder paramagnetic susceptibility was measured from 4 to 1000 K. The 1/χ versus T curve shows a pronounced curvature, even below 300 K, which is fairly well reproduced by a crystal-field-only calculation involving the complete wave vectors of the 18 lowest Kramers doublets.

The energy level schemes of the neodymium oxyhalides (NdOX, X = F, Cl, and Br) were studied and simulated with a phenomenological model accounting simultaneously for both the free ion interactions and the crystal fleld effect. The former included the electrostatic and interconfigurational interactions as well as the spin-orbit coupling. The simulations were carried out by using the data from the optical absorption and luminescence as well as the inelastic neutron scattering measured at low temperatures between 2.5 and 77 K. The values of the Slater integral F 2 describing the eJectrostatic interactions decrease while F 4 and F6 increase as a function of the ionic radius of the halide anion. The strength of the spin-orbit coupling is quite the same in all three matrices. The crystal field effect -measured as the crystal fleld strength parameter S -is almost twice as strong in the hexagonal NdOF matrix (650 cm -1 ) than in the tetragonal NdOCI or NdOBr (367 and 378 cm -1 , respectively). Similar evolution was obtained for the shortand mid-range crystal field strengths related to the spatial extension of the interaction.