R. Caracas

Université Catholique de Louvain, Laboratoire de Géologie et Minéralogie, 3, place Louis Pasteur, 1348 Louvain-la-Neuve, Belgium

Keywords: coordination chemistry, oxides, Nb-bearing phases, NbO₆ octahedron, Nb-O bond.

Oxides are the main host phases for Nb, where it is usually 6-fold coordinated by oxygen. In order to find out the crystallochemical trends of the behavior of Nb in octahedral coordination 920 octahedra have been selected from the 731 NB-bearing oxides included in the ICSD database [1].

Several geometric parameters have been measured for every octahedron and the statistics of all the values is presented in Table I.

The bond length is defined as the distance between the mass centers of the atoms. The O-Nb-O angles are considered only between neighbor O atoms. The Nb-(O,O,O) solid angle is measured considering a unit sphere centered on the Nb atom. The directions of the three Nb-O bonds define the 3 apices of a spherical triangle on the surface of the sphere. The ratio between the area of this spherical triangle and the area of the sphere provides the solid angle. The O-O lengths are considered only between two O neighbors, 12 for each octahedron. The longest O-O distance between not-neighbor O atoms defines the apical dimension.

The ideal values for bond length and surface are obtained from an undistorted octahedron with the same volume as the measured octahedron.

The main relationships between the different measured quantities are summarized below :

Bond length deviation = 0.3356 x Total surface deviation - 0.0429, with R²=0.611;

Half of the measured Nb-O bond lengths are in the range 1.9333 - 2.0584 with an average value 2.0027. This suggests a slightly covalent character, the ideal ionic Nb-O distance being 2.01 [2,3].

The electronic clouds of the coordinating O atoms do not overlap in general, half of the O-O distances being in the range 2.7583 - 3.0592 with an average of 2.8163.

Most of the octahedra are deformed. All the octahedra from the structures which contain also transitional cations are deformed. The deformation is measured both in bond length and in O-Nb-O angle. The maximum of the deformation in bond length is found for Ti-, W-, Mn- and K- bearing phases. The maximum of the deformation in O-Nb-O angle is found for Bi-, Ag-, W- and Sr- bearing phases.

Table I. Statistics of the principal geometric parameters of the NbO₆ octahedron

Parameter	Unit	Average	Median	St.dev.	Min.	Max.
Nb-O bond length	A	2.0027	1.9841	0.1423	1.072	2.8059
Dd/av.d [*]	-	0.1439	0.1381	0.0888	0	0.7037
Ideal-Experimental bond length	A	0.1152	0.1238	0.0253	0.0019	0.3733
O-Nb-O angle	Rad	90.5545	90.0104	10.2518	49.3207	149.8607
Da/av.a [**]	-	0.2946	0.2438	0.1847	0	0.9860
Nb-(O,O,O) solid angle (rad)	Rad	3.1184	3.1283	0.0939	1.7740	4.2627
O-O-O surface	A²	3.4142	3.3998	0.2813	1.3337	5.9148
Ds/av.s [***]	-	0.1408	0.1229	0.0871	0	1.0216
Apical dimension	A	4.0539	4.0222	0.1419	3.7647	5.1158
O-O length	A	2.8163	2.8076	0.1800	1.1185	4.3207
Do/av.o. [****]	-	0.1508	0.1381	0.0904	0	0.8205
Ideal-Experimental O-O distance	A	0.1845	0.1867	0.0544	0.0040	2.9766
Volume	A³	12.6429	12.5575	1.0063	8.0449	20.7894

[*] The deformation in bond length is defined as the difference between the minimum and maximum Nb-O bond lengths for each octahedron, Dd divided by the average value of Nb-O bond length av.d of the octahedron .

[**] The deformation in O-Nb-O angle is defined as the difference between the minimum and maximum O-Nb-O angle for each octahedron, Da divided by the average value of O-Nb-O angle av.a of the octahedron .

[***] The deformation in O-O-O surface is defined as the difference between the minimum and maximum O-O-O surface for each octahedron, Ds divided by the average value O-O-O surface av.s of the octahedron .

[****] The deformation in O-O distance is defined as the difference between the minimum and maximum O-O distance for each octahedron, Do divided by the average value O-O distance av.o of the octahedron .