TRANSITIONS IN FERROELECTRIC KNBO3
PENNSYLVANIA STATE UNIV UNIVERSITY PARK X-RAY AND CRYSTAL ANALYSIS LAB
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A preliminary dielectric study of KNbO3 single crystals, prepared without flux, showed a sharp peak in the dielectric constant at -50 C on cooling and -35 C on heating, indicating the existence of a phase transition at this point. Further study was carried out on the dielectric, structural, and thermal properties of this lowest phase. KNbO3 single crystals were prepared as described by E. A. Wood Acta Cryst. 4353, 1951, using KCO3 as a flux and cooling down from 1000 C. Dielectric tests were made on crystals 2 to 3 mm on edge and about 0.3 mm thick. Powder photographs of KNbO3 were taken with CuK a radiation in a Norelco powder camera of 11.4-cm diam. Orthorhombic cell dimensions a5.721 A, b3.973 A, c5.695 A were obtained at room temperature. An examination was made of the lowest temperature phase using CuK a radiation. Diffraction patterns at -140 C showed pseudocubic lines of perovksite type. Definite line spittings were observed in a few high angle lines such as 422, 332, and 420. A study was made of the specific heat anomaly at the 3 transitions in KNbO3. The measurements were carried out by heating the specimens continuously at a rate of 0.5 to 1 Cmin. Sharp peaks in the specific heats appeared at the 3 transition temperatures.
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