The structure of SrTiO3 has been the subject of X-ray, [1] infrared, [2] ESR, [3–5] neutron, [6] and Raman [7–10] spectroscopic investigations since 1962. While the high-temperature phase is acknowledged to be simple cubic perovskite 0 h 1 , with one formula group per unit cell, there have been, until recently, apparent inconsistencies in the diverse data and a lack of agreement concerning the symmetry of the crystal at low temperatures. Following discovery of anomalies in the sound velocity of SrTiO3 near 110°K [11], ESR studies [3] showed the presence of a cubic to tetragonal phase transition. This was verified by means of X-ray analysis [1]; in the latter study the c/a ratio was determined to be 1.000 56, and two other phase transitions were inferred to be at 65 and 35°K. Infrared and neutron studies [2, 6] supported the X-ray symmetry assignment of structure at 77 °K: the crystal was viewed as having domain structure with one of the equivalent (100) axes in each domain slightly elongated in the tetragonal phase; below 110°K the crystal was thought to retain a single formula group per unit cell and all atoms at inversion centers. The distortion from cubic symmetry was apparently so slight, in fact, that even the predicted F1u → Eu + A2u doubling below 110°K of each of the Oh -phase IR-active vibrations was unobservable in infrared studies. [2]

Soft Phonon Modes and the 110°K Phase Transition in SrTiO3

Structural phase transitions can be studied by a variety of techniques; macroscopic fields, X-ray, light and neutron diffraction, and resonance methods. The information which can be obtained by these different techniques about the order parameter and susceptibility at structural phase transitions is described and compared. Landau theory coupled with the ‘soft mode’ concept provides a simple picture of many structural phase transitions in terms of relatively few phenomenological constants. A review is given of the application of this type of theory to the wide variety of different phenomena which can occur at structural phase transitions. In particular, emphasis is placed on the nature of the fluctuations within Landau-Ginzburg theory and on the role of secondary order parameters. The nature of incommensurate phases and of lock-in phase transitions are discussed using both the conventional lattice dynamical approach and the soft soliton theories. The parameters of the Landau theory can, in princip...

Structural phase transitions I. Landau theory

We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature (Tc) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO3 layers in BaTiO3/SrTiO3 superlattices are not only ferroelectric (with Tc as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO3 layers adjacent to them. Tc was tuned by approximately 500 kelvin by varying the thicknesses of the BaTiO3 and SrTiO3 layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity.

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Probing Nanoscale Ferroelectricity by Ultraviolet Raman Spectroscopy

A quadrupole coupling induced $^{\mathrm{47}}\mathrm{T}\mathrm{i}$ and $^{\mathrm{49}}\mathrm{T}\mathrm{i}$ satellite background which transforms into well-defined satellite lines below ${T}_{c}$ in the ferroelectric phase has been observed in the cubic phase of an ultrapure ${\mathrm{B}\mathrm{a}\mathrm{T}\mathrm{i}\mathrm{O}}_{3}$ single crystal. The results demonstrate the coexistence of a displacive and order-disorder component in the phase transition mechanism and tetragonal breaking of the cubic symmetry due to biased Ti motion between off-center sites in the paraelectric phase above ${T}_{c}$.

NMR evidence for the coexistence of order-disorder and displacive components in barium titanate.

The structure of our material world is characterized by a large hierarchy of length scales that determines material properties and functions. Increasing spatial resolution in optical imaging and spectroscopy has been a long standing desire, to provide access, in particular, to mesoscopic phenomena associated with phase separation, order, and intrinsic and extrinsic structural inhomogeneities. A general concept for the combination of optical spectroscopy with scanning probe microscopy emerged recently, extending the spatial resolution of optical imaging far beyond the diffraction limit. The optical antenna properties of a scanning probe tip and the local near-field coupling between its apex and a sample provide few-nanometer optical spatial resolution. With imaging mechanisms largely independent of wavelength, this concept is compatible with essentially any form of optical spectroscopy, including nonlinear and ultrafast techniques, over a wide frequency range from the terahertz to the extreme ultraviolet. ...

J. M. Worlock

Papers

Electric-Field-Induced Raman Scattering in SrTi O 3 and KTa O 3