High-Temperature Phase Transitions in PbZrO3

A high-temperature X-ray diffraction study of a high-purity PbZrO₃ specimen at 25° to 236°C is reported. The symmetry of the perovskite sub-cell of PbZrO₃ was determined unambiguously by observing the splitting of the line groups N = 12 and N = 16 when the diffraction pattern was indexed on the basis of the perovskite cell being a unit cell for the structure. Diffractometer studies were conducted of the N = 12 line group using Fe K α radiation and of the N = 16 line group using Cu K α radiation. The ferroelectric phase of PbZrO₃ possessed a rhombohedral sub-cell with the angle α being acute. Stable two-phase mixtures and considerable thermal hysteresis were observed at the antiferroelectric α ferroelectric transition temperature. A very weak "extra" diffraction line was observed in the diffraction patterns of the ferroelectric and paraelectric phases, indicating that the unit cells of these structures may be multiples of the perovskite subcell.     

Investigation of Rare-Earth Doped Barium Titanate

In this study the effect of additions of 0.0015 to 0.0030 mole fraction of rare-earth oxides on the d.-c. resistivity of sintered barium titanate was investigated. The substitution may be represented by ( X ₂O₃)_M (BaTiO₃)_1-M where X is the rare earth. The rare earths samarium, gadolinium, and holmium were introduced singly into the titanate, and the resistivity was measured as a function of temperature from −170° to +330°C. An anomalous increase near the tetragonal - cubic transition temperature at 120°C. occurred which in some cases amounted to an increase in the resistivity of 4000 times the value in the tetragonal phase. The thermoelectric power of the material changed sign at the Curie temperature. The tetragonal phase exhibited n -type behavior whereas the cubic phase was p -type. The rhombohedra1 and orthorhombic phases exhibited conduction activation energies of the order of 0.2 e.v. whereas that in the tetragonal phase was approximately 0.1 e.v.     

Dielectric Properties and Phase Transitions of Sodium-Strontium-Cadmium Niobates

Solid solution was observed in the system Na₁-xSr_N Cd_PNbO₃ where N + P = x/2 between ≤ x 0.40 when the ratio of N:P was within the limits 1:2 → 2:1. Solid solution was observed at N:P ratios outside these limits but these materials were not studied extensively. A ferroelectric pseudomonoclinic perovskite phase was observed at 25° C when ≤ x & le; 0.10 in the compositional series studied. At higher values of x a ferroelectric pseudotetragonal phase with c/a 1 appeared between the pseudomonoclinic ferroelectric phase and the para-electric pseudocubic phase which was stable at high temperatures. The pseudotetragonal phase, which exhibited much larger values of €/€₀ than the pseudomonoclinic phase, had a large peak in the electrical permittivity when it transformed to the pseudocubic state. The high-temperature phase observed in these materials became a small-cell perovskite structure when the subcell edge length attained 3.942 ± 0.002 A as a result of thermal expansion.     

The Organizational Ecology of Strategic Groups in the American Brewing Industry: A Comment

This paper comments on the work of Glenn R. Carroll and AnandSwaminathan (1992), who integrate ideas from strategic grouptheory and organizational ecology in order to better analyzeperformance heterogeneity in the American brewing industry.Although they provide a novel approach, this comment outlinesseveral weaknesses with their analysis. In addition, an alternativeview of the evolution of the brewing industry is presented,one that is more consistent with previous research, is not refutedby Carroll and Swaminathan's empirical findings, and is substantiallydifferent from their view of the industry.     

Structural and Dielectric Investigation of the PbTiO3-BaZrO3 System

Thermal expansion measurements, dielectric measurements, and X-ray diffraction studies were made of the PbTiO₃-BaZrO₃ system. A dielectric constant maximum of 11,300 was found at the Curie temperature for the Pb_0.80-Ba_0.20Ti_0.80Zr_0.20O₃ composition. The transition from a ferroelectric tetragonal phase to a para-electric cubic phase for the 10, 20, and 30 mole % BaZrO₃ in PbTiO₃ compositions was of first order. X-ray diffraction determination of the lattice parameters (25°C) showed that this tetragonal-to-cubic phase transition occurred at a composition of 44 mole % BaZrO₃.