Vibrational spectra and structure of niobates and tantalates of Ln/sub 3/MO/sub 7/ type

Infrared and vibrational spectra of polycrystalline niobates and tantalates of rare earth elements with the general formula of Ln/sub 3/MO/sub 7/ (Ln = La, Pr, Sm, Gd; M=Nb, Ta) have been investigated. Experimental and calculated data allow an assumption to be made that the structure of the Ln/sub 3/MO/sub 7/ compounds, with the exception of lanthanum and praseodymium tantalates, changes from the structure close to webnerite to the structure based on pyrochlore. Deformation of the crystalline lattice along the main axis to cubic syngony is due to an increase of the covalent bonds of Ln-O in the series from La to Gd.