Hiroki Hasebe

Raman Scattering Study of SrTi18O3

Antiferrodistortive and ferroelectric soft modes of isotoped ferroelectric SrTi 18 O 3 x 16 O 3(1- x ) were observed for a single crystal of x =0.87 with Raman scattering. The frequency of one of t...

Brillouin and Raman scattering study of the isotopically induced ferroelectric phase transition of SrTi18O3

The ferroelectric phase transition of the isotope 18 O exchanged strontium titanate SrTi 18 O 3 has been studied by both Brillouin- and Raman scattering near T m where the dielectric constant takes a maximum value. The Brillouin scattering study elucidates anomalous temperature dependence of the acoustic modes; the c 44 mode shows a drastic softening though the c 33 mode is almost independent of temperature. The Raman scattering study below T m shows a weak softening behavior of the ferroelectric mode. Existence of the ferroelectric micro-region is suggested in the isotopically induced ferroelectricity of SrTi 18 O 3 .

Ferroelectric phase transition of SrTi18O3 studied by brillouin scattering

Abstract Ferroelectric phase transition of SrTi18O3 (87% 18O-exchanged SrTiO3) has been investigated by the 90° Brillouin scattering experiment in the VV and VH scattering geometries. The c 44 pure transverse acoustic mode splits into two modes below T c≈25 K, indicating that the phase transition takes place (or begins) at T c and that the phase realized below T c has lower crystal symmetry than tetragonal one.

Brillouin Scattering Study of Ferroelectric SrTi( 18 O X 16 O 1− X ) 3

The 90° Brillouin scattering experiment has been performed in two SrTi( 18 O x 16 O 1 m x ) 3 single crystal plates with x = 0.86 and x = 0.53. The c 44 transverse acoustic mode takes maximum intensity at T c (∼ 26 K for x = 0.86 and ∼ 19 K for x = 0.53) and splits into two modes below T c . Moreover, the central peak clearly appears showing maximum intensity around T c . These facts indicate that the crystal symmetry becomes lower than tetragonal one below T c , and that the ferroelectric phase transition really occurs at T c .

Light Scattering Study in Ferroelectric SrTi 18 O 3

The light scattering experiment has been performed in SrTi( 18 O x 16 O 1 m x ) 3 single crystal with the exchange rate of x = 0.86. The ferroelectric E u optical soft mode can be observed above the ferroelectric phase transition temperature T c , 27 K. It splits into two modes below T c . One of these two modes, which is supposed to be the A 1 (or B 2 ) mode, hardens with decrease in temperature. This experimental result implies that the phase transition of SrTi 18 O 3 is of the displacive type.

Phase Transition of SrTi 18 O 3 Studied with Raman Scattering

The spectra characteristic of ferroelectric micro regions (FMRs) were studied in the paraelectric phase of isotoped ferroelectric SrTi 18 O 3 x 16 O 3(1 m x) with x = 0.47 and non ferroelectric one with x = 0.21. The Frequency of ferroelectric mode does not reach vanishing value in specimen of x = 0.21, whereas that in x = 0.47 decreases to zero around 21 K with decreasing temperature. The discussion on the phase transition mechanism is given.

Isotopically Induced Ferroelectric Phase Transition in SrTi 18 O 3 Studied by Brillouin and Raman Scattering

The isotopically induced ferroelectric phase transition of SrTi 18 O 3 , where 87% of oxygen atoms 16 O are exchanged for its isotope 18 O, has been studied by both Brillouin- and Raman scattering. On cooling run, at several degrees above the temperature where the dielectric constant takes a maximum value, the Brillouin spectrum of the c 44 mode in the paraelectric-tetragonal phase begins to split into two transverse acoustic modes. One of them shows a drastic softening with a dip on the spectrum characteristic for coupling with a central peak component. Though the ferroelectric soft A 2 u mode is Raman inactive in the paraelectric phase, it has been observed in the x ( yz )- x geometry indicating a local symmetry breaking. These anomalies suggest existence of a local distribution of the ferroelectric transition point caused by the ferroelectric micro-region.

Central peak in SrTi18O3 studied by light scattering

A central peak (CP) has been found mainly in the VH light scattering spectra of SrTi(18 O 0.87 16 O 0.13)3 (87% 18O-exchanged SrTiO3). Both of the intensity and relaxation time of the CP show broad peaks around T m ≈ 22 K (below the transition temperature T c ≈ 25 K). The physical origin of the CP is determined not to be the polarization fluctuation, representing that this CP is not a sign of a phase transition of order-disorder type. The stochastic fluctuation of the spatial derivatives of the polarization would be the physical origin of the CP.