Crystal chemistry of cerium titanates,tantalates and niobates

Cerium dioxide has been found to react with other oxides at high temperatures in an open air environment with the formation of Ce⁺³, Ce⁺⁴ or mixed valence phases. The compound Ce⁺⁴Ti₂O₆ forms with the brannerite structure $\text{a}\text{=9.804,}\text{b}\text{=3.758,}\text{c}\text{=6.914, β=119° 8.7′}$, however, with the addition of sodium, the black pseudocubic perovskite compound NaCe⁺³Ti₂O₆ is formed, $\text{a}\text{=3.864}\text{A}$. Single crystals of the Ce⁺³Ta₇O₁₉ reveal that this compound is hexagonal, P6₃/mcm, $\text{a}\text{=6.232,}\text{c}\text{=19.985}\text{A}$. Single crystals of the compound Ce⁺³TaO₄ are light green of the LaTaO₄-type, P2₁/c with $\text{a}\text{=7.618,}\text{b}\text{=5.531,}\text{c}\text{=7.767}\text{A}\text{, β=100° 56.3}$. On oxidizing at low temperature, ≈600°C, the crystals turn black and change to monoclinic, P2/m, $\text{a}\text{=7.617,}\text{b}\text{=5.491,}\text{c}\text{=3.851}\text{A}\text{, β=102° 30.5'}$, with a corresponding change to ≈CeTaO_4.174. Another phase which is also light yellow is formed by oxidizing at 350°C for long periods of time and corresponds to CeTaO_4.50. The compound CeNb₅O₁₄ is orthorhombic Pmnb, $\text{a}\text{=20.12,}\text{b}\text{=12.474,}\text{c}\text{=7.744}\text{A}$. Ce⁺³NbO₄ has a fergusonite unit cell when quenched to room temperature $\text{a}\text{=5.544,}\text{b}\text{=11.434,}\text{c}\text{=5.177, β=94° 41.8'}$. On oxidation in air the cell size is $\text{a}\text{=5.364,}\text{b}\text{=11.424,}\text{c}\text{=5.129}\text{A}\text{, β93° 22.7'}$ at room temperature and corresponds to ≈CeNbO_4.25.