Concentration-Phase Transitions in Solid Solutions Li_xNa_(1-x)Ta_(0.1)Nb_(0.9)O_3, Li_(0.12)Na_(0.88)Ta_yNb_(1-y)O_3, and NaTa_yNb_(1-y)O_3 and Their Manifestations in Raman Spectra

摘要

Using the Raman-spectroscopy method, we have studied concentration-phase transitions in the solid solutions Li_xNa_(1-x)Ta_(0.1)Nb_(0.9)O_3, Li_(0.12)Na_(0.88)Ta_yNb_(1-y)O_3, and NaTa_yNb_(1-y)O_3 (x = 0-0.16, y = 0-1). It has been revealed that, for the solid solutions Li_xNa_(1-x)Ta_(0.1)Nb_(0.9)O_3 and Li_(0.12)Na_(0.88)Ta_yNb_(1-y)O_3, the concentration-phase transition is a transition between the ferroelectric and antiferroelectric states. It is accompanied by the disappearance from the spectrum of a line that corresponds to stretching bridge vibrations of oxygen atoms along the polar axis, which is forbidden by selection rules in the centrosymmetric phase, and by splitting into two components of a line that corresponds to librational vibrations of oxygen octahedra as a whole, which can be caused by doubling of the unit cell in the antiferroelectric phase. Manifestation and variation of intensities of lines with frequencies 860-873 and 900-905 cm~(-1) upon variation of the concentration of tantalum for solid solutions Li_(0.12)Na_(0.88)Ta_yNb_(1-y)O_3, and NaTa_yNb_(1-y)O_3 is caused by the formation of polar clusters in the medium, which is centrosymmetric in general due to disordering in the sublattice of niobium and tantalum.