Nioboaeschynite-(Ce), Ce(NbTi)O(6)

Abstract

Nioboaeschynite-(Ce), ideally Ce(NbTi)O(6) [cerium(III) niobium(V) titanium(IV) hexa-oxide; refined formula of the natural sample is Ca(0.25)Ce(0.79)(Nb(1.14)Ti(0.86))O(6)], belongs to the aeschynite mineral group which is characterized by the general formula AB(2)(O,OH)(6), where eight-coordinated A is a rare earth element, Ca, Th or Fe, and six-coordinated B is Ti, Nb, Ta or W. The general structural feature of nioboaeschynite-(Ce) resembles that of the other members of the aeschynite group. It is characterized by edge-sharing dimers of [(Nb,Ti)O(6)] octa-hedra which share corners to form a three-dimensional framework, with the A sites located in channels parallel to the b axis. The average A-O and B-O bond lengths in nioboaeschynite-(Ce) are 2.471 and 1.993 Å, respectively. Moreover, another eight-coordinated site, designated as the C site, is also located in the channels and is partially occupied by A-type cations. Additionally, the refinement revealed a splitting of the A site, with Ca displaced slightly from Ce (0.266 Å apart), presumably resulting from the crystal-chemical differences between the Ce(3+) and Ca(2+) cations.

Fig. 1.

The crystal structure of nioboaeschynite-(Ce). Purple octahedra and small blue spheres (with arbitrary radius) represent the [(Nb,Ti)O6] groups and C-site cations, respectively. Large green displacement ellipsoids at the 99% probability level represent the A-site cations.

Fig. 2.

The crystal structure of nioboaeschynite-(Ce) represented with displacement ellipsoids at the 99% probability level. Blue, purple and green ellipsoids represent (Nb,Ti), A-site Ce, and O atoms, respectively. Purple spheres, with arbitrary radius, represent C-site Ce atoms. For clarity, the A-site splitting is not shown.

Fig. 3.

Variations of the two shortest C—O bond lengths with the Ti content in the C-site of aeschynite-(Y) and nioboaeschynite-(Ce). Nioboaeschynite-(Ce) data points are from this study and all other data points for aeschynite-(Y) are taken from Bonazzi & Menchetti (1999).