Iron-containing niobates of rare earths and cadmium with pyrochlore structure

The phases of the composition CdLn(Fesub(0.5)Nbsub(1.5))Osub(7) (where Ln=La-Lu), crystallizing in the structural type of pyrochlore, have been synthesized and investigated. Solid phase synthesis has been performed by stepwise roasting at 800, 1000 and 1200 deg C. The chemical composition of the roasting products and their melting points have been determined. Experimental data have been processed with the aid of the computer using the special program: the crystal lattice parameters have been found by the least square method with a simultaneous use of the extrapolation function and accounting for the measurement weights. The changes in the crystal lattice parameters, the refraction indices, melting points, X-ray and pycnometric densities have been compared according to the order number of the rare-earth elements. Comparison of physico-chemical constants of ferroniobates with those of cadmium pyroniobates points to a noticeable increase in the crystal lattice parameters of iron-containing pyrochlores. In the series of rare-earth elements, when passing from the La-based compositions to those on the basis of Lu, a regular decrease in the crystal lattice parameters takes place; density increases and refraction indices decrease with a decreasing value of ionic radii and increasing atomic mass