Figure 1. Projection on (010) of the structure of CeTaO The Ta cation 4° are located at the centers of the distorted octahedra outlined The y-coordinate (x100) is given for each atom.

in figure.

Figure 2. Projection on

Projection on (010) of the structure of CeNb04. The solid lines identify the Nb polyhedron, the broken lines, the Ce polyhedron. The y-coordinate (x100) is indicated for each atom. The relevant Ce-0 and Nb-0 distances are also given.

In the structure of CeNbo the oxygen atom 0(1) is surrounded by two niobium and two cerium atoms and the 0(2) atom is surrounded by one niobium and two cerium atoms. Consequently, if one does not take into account the individual distances, the former anion is overbonded

([S1 = 2.42), whereas the latter one is underbonded ([S1 = 1.58).

i

i

Therefore, in this structure the differences in these distances are mainly due to balancing the electrostatic charge.

5+

As we have mentioned earlier Nd Tao and CeNb04 are isostructural. A difference which is worthwhile to point out lies in the coordination of the pentavalent cations. As can be seen from table 9 and 10 the first two sets of M-0 distances increase (from 1.851 and 1.920 to 1.861 and 1.941 A), whereas the third set decreases (from 2.482 to 2.353 A) on going from the niobate to the tantalate. These values indicate that this third set of oxygen atoms behave, at least in the niobate, as second nearest neighbors. Therefore the coordination number +5

of these cations should be taken as 4+2, although the Ta to be approaching six-fold coordination.

ion appears

The change in structure which takes place in the tantalum series is probably due to the size of the rare-earth cation. When this size becomes too large for the A-site of the fergusonite-like structure, the tantalates crystallize with the LaTa04-type structure. The same structural change does not take place in the niobates because of the preference 5+ of the Nb cations for coordination smaller than six.

Table 1.

Experimental conditons used to collect the powder intensity data for CeTa04, CeNbO4, and NdTao4·

Monochromatic beam: reflection 220 of a Cu monochromator in
transmission geometry.

Mean neutron wavelength: 1.5416(3) A
Å

Horizontal divergences

(i)

In-pile collimator: 10 min. arc;

(ii) Monochromatic beam collimator: 20 min. arc;
(iii) Diffracted beam collimator: 10 min. arc.

Monochromator mosaic spread: 15 min. arc.

Sample container: V can of 1 cm. diameter.

(**) Yelon, W.B. (1977). Neutron Diffraction Newsletter.

In the above formulas N is the number of statistically independent observations, P the number of parameters refined, C the number of constraints, I the integrated intensities and w the weights.

The parameters U, V and W appear in the equation

where H is the full width at half maximum of a reflection occurring at the Bragg angle 8. The theoretical values of U, V and W have been calculated with the expressions derived by Caglioti, Paoletti and Ricci, Nucl. Instr., 3, 223-228 (1958).

The errors on the cell dimensions determined by neutron diffraction do not include the error in the neutron wavelength.