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 +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.