Application of Bond Valence Model to Stability of RE Oxycompounds

The bond valence model was used to study the stability of the non-stoichiometric $LaO_{1-x}F_{1+2x}$ phases, the solid solubility in the ($La_{1-x}Gd_{x}$) OCl system and the phase transformation in the REOCl ($RE^{3+}=La^{3+} -Nd^{3+}, Sm^{3+}-Gd^{3+}, Ho^{3+}, and Y^{3+}$) series. The stability of the non-stoichiometric $LaO_{1-x}F_{1+2x}$ (0 ≤ x ≤ 0.3) phase decreases with increasing excess fluoride. The global instability index values close to 0.2 indicate the instability of the non-stoichiometric phase. The relative stability of the ($La_{1-x}Gd_{x}$)OCl (0 ≤ x ≤ 1.0) solid solutions achieved its minimum in the middle of the series. However, the X-ray powder diffraction results indicated complete solid solubility in the whole ($La_{1-x}Gd_{x}$)OCl series and no phase separation was observed. The bond valence model was used to explain the structural transformation from the tetragonal oxychlorides, REOCl (RE = La-Er, and Y), to hexagonal beyond ErOCl. The calculated global instability index values did not show any clear trend across the REOCl series probably due to the inaccuracies and incoherencies in the original structural data

Energy Level Scheme of Nd3+\text{}^{3+} Ion in Rare Earth Oxyhalides, REOX (X = F, Cl, and Br)

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 field 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$\text{}^{2}$ describing the electrostatic interactions decrease while F$\text{}^{4}$ and F$\text{}^{6}$ 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 field strength parameter S - is almost twice as strong in the hexagonal NdOF matrix (650 cm$\text{}^{-1}$) than in the tetragonal NdOCI or NdOBr (367 and 378 cm$\text{}^{-1}$, respectively). Similar evolution was obtained for the short-and mid-range crystal field strengths related to the spatial extension of the interaction