Gerald Burns

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.

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.

Index of refraction in ‘dirty’ displacive ferroelectrics☆

Abstract We report measurements of the optic index of reflection as a function of temperature, n(T), for several highly disordered ferroelectrics: PLZT ceramic [Pb1−xLax(Zr0.65Ti0.35)1−x/4O3 where x = 0.08] and single crystals of Pb3(MNb2)O9 where M = Zn and Mg. The results are qualitatively different from the unusual displacive ferroelectric behavior and the difference is clearly due to local disorder.

The tungsten bronze field in the system K2O|Li2O|Nb2O5

A limited portion of the liquidus field and the entire subsolidus stability region has been determined for tungsten bronze-type solid solutions in the K2O|Li2O|Nb2O5 system. The tungsten bronze phase does not occur with completely filled alkali cation sites at the composition K3Li2Nb5O15, but exists exclusively in the high niobia region above the KNbO3|LiNbO3 pseudojoin. Equilibrium relationships defining conditions for the growth of electrooptic crystals, and dielectric, optical, and x-ray measurements establishing the influence of composition on structure and properties are presented and discussed.

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.

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.

FERROELECTRIC AND OPTICAL PROPERTIES OF KSr2Nb5O15

A new transparent tetragonal crystal, KSr2Nb5O15, of the tungsten bronze type has been found to be ferroelectric with a Curie temperature of ≈160°C. At room temperature the electro‐optic coefficient, r∥, is 1.3 × 10−8 cm/V and e∥ is approximately 1,000.

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.

‘Dirty’ displacive ferroelectrics

Abstract Using the powder Raman technique we have measured most of the lattice vibrational modes in the Pb 1− x La x Ti 1− x /4 O 3 perovskite solid solution series. From the data for these disordered systems the clamped dielectric constant at zero frequency, ϵ m , was determined and compared with the clamped dielectric constant, ϵ cap , measured by standard capitance techniques. We find that ϵ cap /ϵ m exceeds one, and increases as the transition temperature is approached from below. Strikingly, we also find similar behavior for the ‘classical’ ferroelectrics BaTiO 3 , LiNbO 3 and LiTaO 3 , as well as for ferroelectrics with the tungsten bronze crystal structure.

Crystal Fields at Rare‐Earth Ions

The object of this paper is to find out why the calculated rare‐earth crystal‐field parameters do not agree with those determined experimentally. As is well known, in explaining the 4f spectra in the rare‐earth compounds, one needs relatively few crystal‐field constants, Vnms. These Vnms have been measured for many rare‐earth ions in LaCl3, ethylsulfates and other lattices. One can study the Vnms in a given lattice as the nuclear charge is varied (starting from Ce, Z=58 to Yb, Z=70). For a given n and m one finds that the Vnms vs Z vary fairly smoothly. However, when the crystal fields are calculated using an ionic model one cannot get good agreement with the Vnms. In general, neither the magnitude nor the Z dependence of the Vnms can be reproduced. The calculated crystal‐field parameters are written as: (1−αn)〈rn〉Anm, where αn is the shielding factor (the outer 5s2p6 electrons to some extent shield the inner 4f electrons from the crystal field), 〈rn〉 is the expectation value of rn over the 4f elect...