F. H. Dacol

Glassy polarization behavior in ferroelectric compounds Pb(Mg13Nb23)O3 and Pb(Zn13Nb23)O3

Abstract We report measurements of the temperature dependence of the optic index of refraction, n(T), at several wave lengths in two ferroelectric compounds that have the simple perovskite ABO3 structure. The compounds are Pb(Mg 1 3 Nb 2 3 ) O 3 and Pb(Zn 1 3 Nb 2 3 ) O 3 where the B-site ions are disordered. Deviations from the high temperature linear n(T) can be observed starting at a temperature Td (far above the ferroelectric phase transition temperature Tc). We assume a model in which these deviations arise from very local randomly oriented polarization (i.e. glassy polarization), Pd. Then, using the quadratic electrooptic effect and the known coefficients, we determine Pd and compare it to the normally observed ferroelectric reversible spontaneous polarization Pr. Also, from infrared reflectivity data, we show that these materials are microscopically homogeneous. We believe that our model of the phase transition contains the basic physics describing ferroelectrics with diffuse phase transitions.

Measurements of alloy composition and strain in thin GexSi1−x layers

The utility of Raman spectroscopy for the simultaneous determination of composition and strain in thin GexSi1−x layers has been investigated. Using data from the literature and new data for the strain shift of the Si‐Si phonon mode presented here, we show how Raman spectra provide several different means of measuring composition and strain in samples as thin as 200 A. We demonstrate that for largely relaxed layers with compositions near x=0.30, Raman scattering can measure the composition, x, with an accuracy of ±0.015 and the strain, e, with an accuracy ±0.0025. The accuracy of the alloy composition obtained from Raman spectra is comparable or, in the case of very thin layers, superior to that measured by other techniques such as x‐ray diffraction, electron microprobe, and Auger electron spectroscopy.

Ferroelectrics with a glassy polarization phase

Abstract In this paper we discuss some of the measurements and properties of materials that are crystalline ferroelectrics with a glassy polarization phase. We review measurements of the temperature dependence of the optical index of refraction, n(T), which were first used to observe these properties. Then we discuss a model of these polarization effects in terms of a local, randomly oriented polarization (Pr) that may occur hundreds of degrees above Tc. This local polarization is allowed by the strong breakdown of the translational symmetry that occurs in these systems. We also show how the temperature dependence of the strain, x(T), complements the n(T) results. Lastly, we discuss some recent x-ray and EXAFS measurements and how they are in agreement with the model, and the connection between these high temperature effects and the low-temperature glass-like excitations that are found in these systems.

Raman scattering analysis of relaxed GexSi1−x alloy layers

We have used Raman scattering to evaluate thick epitaxial GexSi1−x layers with 0.20≤x≤0.43 grown on Si (100) substrates. We show that a detailed consideration of the composition dependencies of the relative intensities of the various phonon modes can enhance the sensitivity of Raman scattering to variations in composition and strain. We find that samples are uniform on a scale of ≂1 μm laterally and <1000 A in the growth direction.

Polarization in the cubic phase of BaTIO3

Abstract We have measured, at three different wave lengths, the temperature dependence of the optic index of refraction, n(T), in ferroelectric BaTiO 3 for T > T c (T c ≡ the cubic to tetragonal transition temperature). n(T) deviates from the high temperature extrapolated value between T c and ≈ T c + 180°C. Assuming that these deviations are caused by the quadratic electro-optic effect, and taking its dispersion into account, we obtain, with no adjustable parameters, a root mean square polarization, | P |, vs. temperature in the cubic phase. At T + c , |P| is about 30% to 50% of the value of the reversible spontaneous polarization at T − c . These results are consistent with the recently published infrared studies of BaTiO 3 for T>T c and two proposals for their origin are discussed.

Mechanism of the ferroelastic transition of BiVO4

Abstract BiVO4 has a pure ferroelastic transition at Tc = 528°K and atmospheric pressure. We elucidated the mechanism of this transition by studying the q≈0 soft optical phonon, with the symmetry of the ferroelastic strain, under large hydrostatic pressures at room temperature. A free-energy analysis, including the optical-acoustical phonon couplings, shows that the transition is driven by the q≈0 soft optical phonon.

Phonon shifts in ion bombarded GaAs: Raman measurements

Abstract When 15 MeV ions bombard single crystal GaAs (111), (100), and (110) faces, they leave a strained, crystalline, surface layer with many defects (as well as a buried amorphous layer). Using Raman spectroscopy, we measure the shits and line widths of the optic phonons of these strained crystalline layers. Using simple models, the possible sources of the phonon shifts are quantitatively considered. We conclude that the strains, and a change in the ionic plasma frequency (LO-TO splitting) due to a ratio of interstitials, or antisites, to atoms in the crystals of ≈ 2% − 3% account for the major portion of the phonon shifts. These effects have been ascribed previously to phonon confinement.

Raman and infrared results on YBa2Cu3O7−x type materials

Abstract We report both Raman and infrared results on the same semiconducting YBa2Cu3O7-x material. With the appropriate heat treatment, this material can be reversibly changed from an orthorhombic superconductor (x≈0.0) to a semiconductor (x>0.5). In the semiconducting material it is easier to measure the phonon features using either technique, and we observe almost the number of modes allowed. By simultaneously considering the modes observed by both techniques, using group theory to sort out the modes in both phases, and using projection operators to determine the symmetry adapted vectors, we arrive at an understanding of some of the modes. We find few differences when comparing our results to other measurements in the superconducting phase. The two general conclusions that we arrive at are: 1) splittings of the modes due to lowering the symmetry from tetragonal to the superconducting orthorhombic phase are not observed; 2) no frequency renormalizations, of the type that have been discussed theoretically, are observed.

Studies on annealing and quenching of strontium barium niobate (SBN) single crystals: A-site cation ordering-disordering effect

Abstract Quenching and annealing studies have been carried out to investigate the effects of the A-site occupancy in the tungsten bronze structure on the polarization, pyroelectric coefficient, dielectric permittivity, and phase transition (Curie temperature range) behavior of ferroelectric relaxor Sr1−xBa x Nb2O6 (1−x = 0.75, 0.60, 0.50) single crystals. Quenching possibly generates a random distribution of vacancies between A1 and A2 sites and enhances the ordering of the cations in A-site. As a consequence of that, a Tc increase of as much as about 30°C above the Tc of non-quenched state has been observed. Quenching at a higher temperature, i.e., approximately about 1000°C results in reducing the ordering of the A-site because of random distribution of Sr2+, Ba2+, as well as vacancies. A continuous Tc increase accompanied by a decrease in dielectric permittivity and a broadened phase transition is reported and is believed to be due to strain built up because of energetically unfavored distribution of B...

Soft optical phonon in ferroelastic BiVO4

Abstract We report the observation of an underdamped q∼O soft optical phonon in the Raman spectra of the paraelastic and ferroelastic phases of BiVO4. This mode has the same symmetry as the ferroelastic strain. The temperature dependence of the soft optical phonon energy indicates that the ferroelastic transition is continuous and that the order parameter has a Landau-type behavior over a wide temperature range.