Kwang-Sei Lee

High-temperature transformation in KH2PO4 and RbH2PO4 crystals

The high-temperature phenomena exhibited by KH2PO4 and RbH2PO4 have been investigated by differential thermal analysis, thermogravimetric methods, and thermo-polarizing microscopy. The thermal transformations which appear at Tp = 196 °C in KH2PO4 and Tp = 96 °C in RbH2PO4 are endothermic in addition to showing weight loss. On heating further to beyond Tp, the thermal transformation shows several endothermic peaks and there is weight loss in KH2PO4 and RbH2PO4. It has been observed by thermo-polarizing microscopy that until Tp is exceeded, uniaxial interference figures are exhibited by crystals of KH2PO4 and RbH2PO4, with cracks and chemical change at the surface of KH2PO4 near Tp~192 °C and RbH2PO4 near Tp~92 °C. The high-temperature phenomena exhibited by KH2PO4 and RbH2PO4 near Tp could indicate not changes from tetragonal to monoclinic structure but chemical decomposition at the surface of the crystals such as that described by nMH2PO4→MnH2PnO3n + 1 + (n-1)H2O (M = K, Rb)

Surface instability and possible polymerization in at high temperatures

We have investigated in detail the high-temperature phenomena of by means of optical microscopy, differential scanning calorimetry (DSC), and impedance spectroscopy. The so-called high-temperature transformation temperature is scattered widely over the temperature range of 82-117 , depending strongly upon the experimental conditions such as sample quality and experimental probes. It has been observed by thermomicroscopy that, upon heating above 110 , the cracks appeared at the surface and the crack pattern grew up with increasing temperature above 110 , and on suitable prolonged heat treatment at constant temperature near 110 . The activation energy is estimated from DSC, which revealed the wide scatter of over the range 82-117 . The complex ac impedance spectra near 113 were fitted by superposition of two Cole-Cole types of relaxations. The fast component is interpreted as proton migration in the bulk, while the slow component is accounted for as a cluster formation due to breaking and reforming of the hydrogen bond at the surface. Our results show evidence that the high-temperature phenomenon of near is not a polymorphic structural phase transition of the tetragonal phase to a presumed monoclinic modification, but a slow partial polymerization of into (probably n = 2) at reaction sites at the surface of crystal. The high-temperature phenomenon is believed to be controlled by the topochemical factors.

Impedance relaxation of at high temperatures

Measurements of complex ac impedance of have been carried out. The dielectric constant showed a high-temperature anomaly around for the c axis and for the a axis). An equivalent circuit model based on two parallel G - C circuits was adopted to describe the impedance relaxational behaviour observed below and above . From this method we could estimate the relaxation times consisting of two components: the fast component due to proton migration and the slow component resulting from electrode - crystal interfacial polarization. The slow component relaxation time tends to increase markedly on crossing , the possible origin of which is considered to be the partial decomposition of crystal on the surface of crystal.

Strontium metaborate, SrB2O4

Single crystals of strontium metaborate, SrB2O4, were grown for the first time from the melt by the Czochralski method and a detailed structure analysis was carried out. The Sr atom is dodecahedrally coordinated by eight O atoms from the neighbouring infinite planes consisting of borate molecules so that all O atoms in the borate molecules participate in the coordination with Sr⋯O distances ranging from 2.522 (7) to 2.728 (5) A.

Nature of the high-temperature dielectric anomaly in TlH2PO4

The dielectric constant of TlH 2 PO 4 along the b axis, e b ′ , was measured as a function of temperature between 290 and 443 K. The e b ′ displayed a new high-temperature anomaly at about 425 K ( T P ) on heating above the ferroelastic-paraelastic transition at 356 K ( T U ), while it showed a quasi-irreversibility on subsequent cooling to room temperature. After prolonged heat treatment at and above T P , microscopic spots, which grew into multigrains, were observed, indicating an additional tendency towards the incipient partial decomposition at temperatures above T P . The appearance of spots and the persistence of the high-temperature state on cooling back to room temperature suggest that the high-temperature anomaly near T P is topochemical in nature and that polymerization due to dehydration can take place on tempering near T P . The quasi-irreversibility of e b ′ seems to result from the nonstoichiometry at the surface.

Dielectric Properties of the Phase Transitionin TlH2AsO4

The dielectric properties of TlH 2 AsO 4 powder pellet are investigated over the temperature range from 80 to 433 K. With respect to the form of the temperature dependence of complex dielectric constant, TlH 2 AsO 4 crystal shows much the same characteristics as with the TlH 2 PO 4 crystal. The results are indicative of a first-order antiferroelectric phase transition at 250.5 K (the lower phase transition point, T L ) with thermal hysteresis of 1.4 K, and of a second-order ferroelastic phase transition at about 390 K (the upper phase transition point, T U ).

Ferroelectric Properties of La Doped Na1/2Bi1/2TiO3 (NBT) Lead-Free Ferroelectric Ceramics

La doped Na 1/2 Bi 1/2 TiO3 (NBT), Na 1/2 Bi1/2 − xLaxTiO3 (NBLT; x = 0, 0.01, 0.03, 0.05, 0.07, 0.10) ceramics, were prepared by solid-state reaction method. The effect of La doping on dielectric and ferroelectric properties of lead free NBT ferroelectric ceramics were investigated. NBT and NBLT single-phase were confirmed by X-ray diffraction. The La doping was influenced the dielectric behavior and ferroelectric property. With increasing La doping, the dielectric peak of phase transition becomes broaden and the peak disappeared up to x = 0.07. The ferroelectric P-E hysteresis loop changed from a leaky shape to slim shape.

Growth and dielectric properties of SrB2O4 single crystal

Abstract Single crystals of strontium metaborate SrB2O4 have been obtained and their electrical and optical properties studied. The wide transparency range of SrB2O4 extends from 100 nm to 3250 nra. The result of dielectric and thermal measurements confirms that no structural phase transition exhibits in the temperature range from 50 K to 1200 K. The dielectric dispersion above 600 K and a pronounced dielectric anomaly at 1136 K analyzed using an equivalent circuit model are a result of the low frequency dielectric relaxation given rise by the super ionic conduction at high temperature and by the space charge occurred by the softening, respectively.

The relaxant phase transition of

Alternating-current complex impedance measurements have been carried out to characterize a relaxant phase transition of solid solution crystal as a function of frequency in the temperature range from to . The results were described using an equivalent-circuit model based on simple combinations of ideal elements of a pure resistor R and an ideal capacitor C. The ferroelectric transitions exhibit unusual properties compared with those of a ferroelectric relaxor. The diffusive character in the phase transition is a result of polarization fluctuation due to two distinguishable cations. No -shift results from the absence of confirmable slowing down coupled with the soft mode. The dielectric relaxation occurs on the high-temperature side of the dielectric maximum with the result that the point defect thermally activated at high temperature gives rise to the low-frequency relaxation. The dielectric dispersion that occurs on heating is due to the high degree of ionic hopping transport accompanied with a large thermal expansion.

Sr2+-doping effect on the phase transition in Pb5Ge3O11 single crystals

Abstract The dielectric and the pyroelectric measurements of Sr 2+ -doped Pb 5 Ge 3 O 11 single crystals were carried out in the frequency range 100 Hz to 15 MHz from room temperature up to 600°C. With increasing Sr 2+ -concentration, ferroelectric phase transition temperatures shifted to lower temperature and the phase transitions were diffused. The deviation from Curie-Weiss behavior of the dielectric constant was analyzed on the basis of Benguiguis theory [L. Benguigui, Solid State Commun. 14, 669 (1974)]. Estimation of the local order parameter suggests that the diffuse phase transition of Sr 2+ -doped Pb 5 Ge 3 O 11 single crystal is caused by the local polarization due to a positional disorder.