Abstract

Single phase zircon type rare earth (RE^III) chromates(V), containing Gd^III, Yb^III and Y^III ions, were synthesized by pyrolysis of the precursors prepared from the mixed solutions of RE^III and Cr^VI in the ratio of RE/Cr = 0.90/1.00. With other RE/Cr ratios, RE/Cr ≥ 0.95 or RE/Cr ≤ 0.85, the pyrolysis products contained a secondary phase of RE₂O₃ or Cr₂O₃. The composition of the compounds was determined to be RE_0.9CrO_3.85 by chemical analyses, with small standard deviations. The ESR spectrum of Y_0.9CrO_3.85 was typical of Cr^V ions in solids with g = 1.968 and ΔH = 140 mT, and did not show the signals of Cr^II or Cr^III. These compounds have a defect structure which differs from other zircon type compounds containing RE^III of larger ionic radius than Gd^III. The detailed structure was determined by the powder X-ray Rietveld method. The measured densities suggested that the defect structure is RE_0.9(CrO₄)_0.85(CrO₃)_0.15 (RE = Gd, Yb and Y). Apparently this structure contains CrO₃˙ with a dangling bond. Based on the structural analyses and ab initio MO calculations, these species might be stabilized by forming Cr₂O₇ (CrO₃–O–CrO₃) units in RE_0.9CrO_3.85 due to the oxygen sharing between CrO₃˙ and the neighboring CrO₄ tetrahedron. The present results conclude that the phase diagram of rare earth chromate(V) series reported so far should be corrected so that the compounds containing Gd^III and smaller ions have the defect zircon type structure.