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.