RAMAN-SCATTERING STUDY OF BA2NANB5(1-X)TA5XO15

The concentration dependences of ferroelectricity and ferroelasticity of Ba2NaNb5(1-x)Ta5xO15 were studied by Raman spectroscopy. The A1(z) mode spectra clearly show the difference between ferro- and paraelectr ic phases. With the increase of x starting from 0.1, the A1(z) modes a t 278, 640 cm-1 and at x=0.1 show softening below x=0.7, and then beco me higher above x=0.7. It is concluded that the ferroelectric phase be comes a paraelectric one at a concentration between x=0.6 and 0.7. Wit h regard to the ferroelasticity the B2(y) mode at 32 cm-1, which is co upled to the ferroelasticity, was investigated. When the concentration x increases the mode at 32 cm-1 fades and then disappears at a concen tration around x=0.9. This strongly suggests that the ferroelastic pha se disappears around the concentration of 0.9.