Abstract
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 +4Ti 2O 6 forms with the brannerite structure a =9.804, b =3.758, c =6.914, β=119° 8.7′ , however, with the addition of sodium, the black pseudocubic perovskite compound NaCe +3Ti 2O 6 is formed, a =3.864 A . Single crystals of the Ce +3Ta 7O 19 reveal that this compound is hexagonal, P6 3/mcm, a =6.232, c =19.985 A . Single crystals of the compound Ce +3TaO 4 are light green of the LaTaO 4-type, P2 1/c with a =7.618, b =5.531, c =7.767 A, β=100° 56.3 . On oxidizing at low temperature, ≈600°C, the crystals turn black and change to monoclinic, P2/m, a =7.617, b =5.491, c =3.851 A, 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 CeNb 5O 14 is orthorhombic Pmnb, a =20.12, b =12.474, c =7.744 A . Ce +3NbO 4 has a fergusonite unit cell when quenched to room temperature a =5.544, b =11.434, c =5.177, β=94° 41.8' . On oxidation in air the cell size is a =5.364, b =11.424, c =5.129 A, β93° 22.7' at room temperature and corresponds to ≈CeNbO 4.25.
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