Lattice-Dynamical Study of the 110°K Phase Transition in SrTiO 3
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The lattice-dynamical characteristic of the 110\ifmmode^\circ\else\textdegree\fi{}K transition in SrTi${\mathrm{O}}_{3}$ has been elucidated by inelastic neutron scattering measurements. The transition is caused by a ${\mathrm{T}}_{25}$ soft-phonon mode at the [111] zone boundary, confirming the model recently proposed by Fleury, Scott, and Worlock. The square of the frequency of this soft mode is proportional to $T\ensuremath{-}{T}_{0}$ above the transition temperature ${T}_{0}$. At the transition, this zone boundary becomes a superlattice point, enlarging the unit cell. The results of elastic intensity measurements at the superlattice points at 78\ifmmode^\circ\else\textdegree\fi{}K are consistent with the structure given by Unoki and Sakudo in its space group $\frac{I4}{mcm({{D}_{4h}}^{18})}$ as well as in the oxygen parameter. This structure is shown to be a logical consequence of the condensed soft mode.Keywords:
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Inelastic neutron scattering measures the dynamical processes in materials, i.e. the spontaneous fluctuations that experiences samples when interacting with neutrons. An inelastic neutronspectrum is obtained from measurements of both energy and the momentum transfer between the neutron and the sample. This short dissertation explains fundamental concepts of the inelastic neutron scattering process, describes instrumentation, and gives a couple of materials applications were inelastic neutron scattering has been proven to be useful like ionic conductors and thermoelectric materials.
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Li2Ge4O9 is known to be a ferroelectric phase with Curie temperature at Tc ∼ −93 °C. In this study, we found both the lowest-frequency phonon and heat-capacity anomalies around 100 °C in the Li2Ge4O9 phase, demonstrating their presence in the structural phase-transition. The phase transition is probably triggered by regularization of distorted GeO6 octahedra in the paraelectric phase, resulting in another phase consisting of the regular octahedral GeO6 unit, which was supported by in situ measurement of emission of 2E → 4A2 transition in the Mn4+-doped phase. It is deduced that the phase transition is more dynamic than ferroelectric phase-transition originating in ordering of the Li ions.
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Dynamic structure factor
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The modified SbSI (ceramic) has been grown by Bridgman method. The dielectric permitivity of modified SbSI along c(z)-axis has been measured using LCR meter as a function of temperature at fixed frequency 1 KHz. The nature of ferroelectric phase transition in the modified SbSI has been discussed using ab initio calculations. Theoretical investigations reveal how ferroelectricity and phase transition in the modified SbSI are closely related to deformation of the unit cell and the phonon interaction. Dielectric measurements reveal that substitution of the individual (I−) with (Cl−) ions creates the deformation of the unit cell and increase the temperature of the ferroelectric phase transition TC. Deformations of the unit cell change the interaction between phonons and the anharmonicity of soft B1u normal mode.
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Structural phase transitions drive several unconventional phenomena including some illustrious ferroic attributes which are relevant for technological advancements. With this note, we have investigated the structural transition of perovskite-type trigonal Ba2ZnTeO6 using Raman spectroscopy, across Tc ~ 150 K, which is also accompanied by a para- to ferroelastic transition. We have observed the presence of central peak (quasi-elastic Rayleigh profile), strong anharmonicity in the soft mode, hysteretic phonon behavior, and signatures of coexistent phases. The existence of central peak in Ba2ZnTeO6 is manifested by a sharp rise in the intensity of the Rayleigh profile in concomitant with the damping of the soft mode near Tc, shedding light on the lattice dynamics during the phase transition. While most of the phonons show splitting below Tc confirming the phase transition, we have observed thermal hysteretic behavior of phonon modes that signifies the first-order nature of the transition and presence of coexisting phases, which are corroborated by our temperature-dependent x-ray diffraction and specific heat measurements. Further, an evidence of the concomitant ferroelastic transition appears in the form of a very strong anharmonicity in the thermal response of the soft phonon mode at ~ 31 cm-1 which is remarkable compared to the hitherto known behavior of soft modes in well-known ferroelectrics.
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