Far-infrared soft mode behavior in PST thin films

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Temperature dependences of the optic phonons in PbSc₁/₂Ta₁/₂O₃ sol-gel films deposited on sapphire substrates were studied by means of Fourier transform far-infrared transmission spectroscopy in the temperature range 20-900. Four films displaying different B-site order with both ferroelectric and relaxor behavior were studied. In all cases the TO mode near 80 at 10 softens on heating to ≈45 following the Cochran law with extrapolated critical temperature near 700 (400 above the temperature of dielectric maximum, T m), but above 600 its frequency remains stabilized. It can be assigned to the A₁ component of the ferroelectric soft mode inside polar clusters which form below the Burns temperature near 700. In the ordered PST film another mode activates below Tₘ in infrared spectra near 60 exhibiting also anomalous temperature dependence due to its coupling with the former mode. It is assigned to the A₁ component of the F₂g Raman active mode. Central mode, which appears below the Burns temperature in the THz range, is assigned to the dynamics of polar clusters. It slows down on cooling and vanishes from our spectral range below T m. Another overdamped excitation assigned to the E component of the soft mode appears near 30 at low temperatures.