Steven Carl Louis Meschia

Conductivity across random barrier distribution as origin of large low-frequency dielectric peak in perovskite crystals and ceramics

Abstract Several perovskite crystals and ceramics show very large dielectric (ϵ′) peaks at high temperature T and low frequency ƒ . In some cases these peaks are in the cubic phase far above any ferroelectric transition. Even at the peaks, the lossy part ϵ″ is larger than the real part ϵ′. The ϵ′ vs T curves for different ƒ follow the same d.c. (low-ƒ) envelope down to some T(ƒ) below which the curve for that ƒ falls below the envelope. Similarly, the conductivity (or ϵ″) data show d.c. and a.c. (high-frequency) envelopes for which data at different ƒ overlap. As a first approximation to a crystal with random barriers impeding conductivity, a model with barriers B (in T units) every lattice constant a = 4 A and barriers B + Δ every distance d is assumed. The model is fit to permittivity and conductivity data for a strontium titanate single crystal, and a good qualitative fit is obtained.

Random barrier height model for phase shifted conductivity in perovskites

Abstract A large dielectric permittivity peak which occurs at low frequency and high temperature in many perovskite crystals and ceramics, and an associated difference in dc and ac conductivity, is attributed to phase shifted conductivity resulting from mobility barriers of different heights. A model is developed which has intrinsic barriers, and higher and more widely spaced extrinsic barriers, and special cases are examined. Model predictions show good agreement with experimental results of Stumpe, Wagner, and Bauerle for an SrTiO3 single crystal 1.02 mm thick. The additional peaks seen by them at higher temperature for a crystal 0.24 mm thick are qualitatively accounted for by adding a third set of still higher barriers with spacing equal to the crystal thickness.

Anomalous and normal protonic conductivity in Cs1-x(NH4)xH2PO4, Cs1-x(ND4)xD2PO4, and K1-x(NH4)xH2PO4

Abstract Dielectric permittivity has been measured for mixed crystals K 1− x (NH 4 ) x H 2 PO 4 (KADP) and Cs 1− x (NH 4 ) x H 2 PO 4 (CADP) and its deuterated analog (DCADP), as well as for two parent crystals, ferroelectric CsH 2 PO 4 (CDP) and antiferroelectric NH 4 H 2 PO 4 (ADP). The measured response contains two components, the dielectric response associated with the ferroelectric or antiferroelectric behavior, and the protonic conductivity response which is the subject of this report. Both normal and anomalous features are seen in the protonic conductivity. The normal c -axis conductivity for CADP is characterized by activation energies ranging from 0.56 eV for CDP to 0.32 eV for CADP.4 (40 mol.% NH 4 in the growth solution, about 0.06 mol.% in crystal), so ammonium admixture increases the conductivity but decreases the activation energy. Deuteration has little effect, at least on the c -axis conductivity. The conductivity is anisotropic, and considerably larger along the ferroelectric b axis than along a or c . The most important anomaly is a large hump in conductivity in the 210 K to 270 K range for both CADP and KADP, about 20 K wide. Heating runs in CADP produced much larger and more irregular anomalies than cooling runs. These anomalies could be nearly eliminated by applying a dc bias field. The KADP runs did not extend far enough above the humps to determine whether, or where, normal protonic conductivity sets in.

An electret accelerometer for use in active vibration control systems

An electret accelerometer was designed which can easily be integrated with piezoelectric actuators in order to actively control sub-audio vibrations.

DIELECTRIC, NMR AND X-RAY DIFFRACTION STUDY OF PSEUDO-ONE-DIMENSIONAL CS1-X(NH4)XH2PO4

Abstract Mixed crystals Cs1−x (NH4) x H2PO4 of the ferroelectric CsH2PO4 (CDP) and the antiferroelectric NH4H2PO4 were grown with x = 0.2 (CADP0.2) in solution. The structural properties of the crystal were analyzed by means of X-ray diffraction. Dielectric measurements at several temperatures and frequencies have been performed along the three crystallographic axes in this sample and also in the fully deuterated CADP0.2 sample (DCADP0.2). Dielectric and NMR experiments in a powdered sample were also performed. The shift of the transition temperature as a function of x and deuteration, and the changes in the properties of the different phases together with, the thermally-activated conductivity found in the paraelectric phase will be discussed and related with the several relaxation mechanisms measured in the NMR experiments.

Ferroelectricity and protonic conductivity in Cs1−x(NH4)xH2PO4

Mixed crystals Cs1−x(NH4)xH2PO4 of the ferroelectric CsH2PO4 (CDP) and the antiferroelectric (NH4)H2PO4 were grown with x=0.2 (CADP0.2) in solution. The structural properties of the crystal were analyzed by means of x-ray diffraction. Dielectric measurements at several temperatures and frequencies have been performed along the three crystallographic axes in this sample and also in the fully deuterated CADP0.2 sample (DCADP0.2). Dielectric and NMR experiments in a powdered sample were also performed. The shift of the transition temperature as a function of x and deuteration, and the changes in the properties of the different phases together with the conductivity found in the paraelectric phase will be discussed.

NMR spin-lattice relaxation study of Cs1‑x(NH4)xH2PO4

Abstract Spin-lattice relaxation time T1 for 31P was measured as a function of temperature at 28.2 MHz in a powder sample of Cs1-x (NH4) x H2PO4 (CADP) crystallized from a solution with a molar ratio x = 0.2. This crystal has the same structure as CsH2PO4 (CDP), which exhibits a low-temperature pseudo-one-dimensional ferroelectric phase transition at 159 K and a superionic transition at 504 K on heating. The measured temperature dependence is explained in terms of hydrogen bond dynamics.