Acoustic phonon anomalies in Ca doped SrTiO₃quantum ferroelectrics as studied by Brillouin light scattering

Abstract

The cubic-tetragonal structural phase transition has been investigated in 3% Ca doped SrTiO3 crystals with perovskite structure by Brillouin light scattering in the temperature range between 90 and 500 K. The frequency and the width of longitudinal acoustic (LA) and transverse acoustic phonon modes show anomalous behaviour at Ttr = 275 K. The observed anomalies are ascribed to the structural phase transition from cubic to tetragonal phases in Ca doped SrTiO3. The behaviour of LA phonon in the critical region close to Ttr is discussed within the frame work of Landau–Khalatnikov relaxation theory. The relaxation time of the order parameter, which is calculated on the basis of the Landau–Khalatnikov type of relaxation mechanism, exhibits critical slowing down towards Ttr.

Anomalies of Brillouin Light Scattering in Selected Perovskite Relaxor Ferroelectric Crystals

“…In the temperature range 500–323 K, a linear increase in the frequency of the LA phonon with the near constancy of its width is ascribed to the normal anharmonic process. Below 323 K, the LA phonon shows anomalous behavior with a sharp minimum in the frequency followed by a sharp maximum in width at the ferroelastic phase transition temperature T ct = 280 K [ 83 ]. Similar elastic anomalies are observed in nondoped SrTiO 3 crystal by picosecond ultrasound spectroscopy [ 84 ].…”

“… Brillouin spectra of SCT-0.03 measured with FSR of 100 GHz at ( a ) 473, 373 and 303 K and ( b ) 253 and 223 K. ( Figure 16 a. Reprinted with kind permission from Ref. [ 83 ]. Copyright (2020) The Japan Society of Applied Physics).…”

“…(Reprinted with kind permission from Ref. [ 83 ]. Copyright (2020) The Japan Society of Applied Physics).…”

Lattice instability of B-site substituted SrTiO3 crystals studied by terahertz time-domain spectroscopic ellipsometry

“…4,5) Doping of Ca into SrTiO 3 (STO) is also very sensitive to the lattice distortion because the Ca content raises the staggered structural phase transition temperature at 105 K of STO. 6,7) Owing to the Ca 2+ homovalent substitution of the Asite, the free electrons are introduced only by the oxygen vacancy. However, heterovalent doping introduces free electrons without any oxygen vacancy.…”

Study of A-site substituted quantum paraelectric strontium titanate crystals by terahertz time-domain spectroscopic ellipsometry

“…7,8) Recently various kinds of instability such as ferroelectricity, ferroelasticity, and super conductivity have been reported by the doping small amount of various kinds of cations into STO. [9][10][11] Such various instability may be caused by the effects of dopants on the tolerance factor and the unit cell volume (chemical pressure).…”