Application of Bond Valence Model to Stability of RE Oxycompounds

J. Hölsäa, K. Koskia, R.-J. Lamminmäkia, H. Rahialab and E. Säilynojaa aUniversity of Turku, Department of Chemistry, FIN-20014 Turku, Finland bÅbo Akademi University, Department of Physical Chemistry, FIN-20500 Turku, Finland

Received: June 24, 1996

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The bond valence model was used to study the stability of the non-stoichiometric LaO1-xF1+2x phases, the solid solubility in the (La1-xGdx) OCl system and the phase transformation in the REOCl (RE3+=La3+ -Nd3+, Sm3+-Gd3+, Ho3+, and Y3+) series. The stability of the non-stoichiometric LaO1-xF1+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 (La1-xGdx)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 (La1-xGdx)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.

DOI: 10.12693/APhysPolA.91.563 PACS numbers: 61.10.-i, 61.66.-f, 81.40.-z