Isomorphous Phase Transition Research Articles

Hysteresis Loops in Ferroelectrics Undergoing an Isomorphous Phase Transition

Ferroelectric hysteresis loops are discussed in terms of free energy expanded to the eighth order in polarization. For suitable choice of the coefficients, deformed triple hysteresis loops will be observed. There is a possibility that such deformed loops are utilized for nondestructive readout memory.

Temperature dependence of spontaneous strain in ferroelastic thallous nitrate

TlNO3 is a ferroelastic crystal belonging to the orthorhombic Aizu species m3mFmmm(ss) at room temperature. Lattice-parameter measurements by X-ray powder diffraction at various temperatures are reported for this crystal. Spontaneous-strain components are calculated from the lattice parameters. The spontaneous strain has a large magnitude of 7.80 x 10−2 at room temperature. Its two components are found to be rather insensitive to change of temperature. This may be due in part to the improper nature of this ferroelastic. Anomalies indicating either new isomorphous phase transitions or crossover behaviour are observed at 309, 329 and 399 K. Between 352 and 416 K the crystal belongs to the rhombohedral Aizu species m3mF3. The critical exponent of the spontaneous strain for the rhombohedral phase is found to change from 0.72 (3) to 0.60 (5) at 399 K.

Brillouin Scattering Study in SnCl2·2H2O near the Isomorphous Phase Transition Temperature

Angular dependence in the a * c plane of Brillouin scattering spectra in SnCl 2 ·2H 2 O has been studied over a temperature range through the isomorphous phase transition temperature T c =-53.8^° C . Temperature dependence of elastic stiffness constants c 11 , c 13 , c 15 , c 33 , c 35 and c 55 has been obtained. Anomalies of c 13 and c 33 have been found at T c . A phenomenological discussion is presented in order to explain these anomalies in connection with the layered structure.

Energy-dispersive x-ray diffraction with synchrotron radiation at cryogenic temperatures

A facility is described which has been developed for the rapid acquisition of structural information through the use of heterochromatic synchrotron radiation from a sample pressurized in a diamond-anvil cell and simultaneously cooled to cryogenic temperature. The system employs a closed-cycle He refrigerator, which can be continuously operated, independent of any liquid cryogens, from a remote station. The compressive contact force between the diamonds, and hence the sample pressure, is also externally controlled, thereby providing remote control capabilities for both the pressure and temperature. NaCl has been used as an internal pressure calibrant and existing empirical equation-of-state calculations for NaCl have been extended to reduced temperatures for this purpose. Preliminary data on the lower pressure critical point associated with the isomorphic phase transition in Ce0.8Th0.1La0.1 are presented.

Finite temperature lattice softening in CeSn 3

Results of temperature dependent Mossbauer effect measurements on Sn in CeSn 3 and LaSn 3 are presented. An anomalous increase in the Sn mean squared displacement (MSD) in CeSn 3 in the 50K region surrounding 135K has been observed. A phenomological model is developed which relates the results to previous measurements on CeSn 3. This model discusses lattice softening effects near continuous, isomorphic phase transitions such as occur in a number of rare earth compounds.

Is there an isomorphous phase transition in cubic nickel iodine boracite at 120K?

It is suggested that there is a phase transition in nickel iodine boracite at approximately 120K, which is associated with no change of symmetry-an isomorphous transition. The cause of this transition is a condensation of OI atoms from a gas-like state to a four-state disorder.

Shift ofKx-ray lines associated with valency change and with isomorphous phase transitions in rare earths

Studies are reviewed that employ the effect of an anomalously strong shift of the fundamental K-series x-ray lines associated with a change in the number of 4f electrons in a given atom for investigating the phenomenon of variable valency in compounds of the rare-earth elements in general, and the so-called isomorphous phase transitions in these compounds in particular. The mechanism of the shift effect and its theory are described. The available data on the shifts of the K-lines of the different compounds of all the rare-earth elements (except promethium) are discussed. A summary table gives the valencies (including the anomalous ones), the occurrence of which has been proved by this method. Application of the K-shift method for studying isomorphous (or isostructural) transitions allows one to establish unambiguously that the mechanism of these transitions involves a change in the number of 4f electrons, and to measure the magnitude of these changes. The review concludes with a brief description of the principles and design of the required experimental apparatus-crystal diffraction spectrometers.

Смещение рентгеновских <i>K</i>-линий при изменениях валентности и изоморфных фазовых переходах в редких землях

Смещение рентгеновских <i>K</i>-линий при изменениях валентности и изоморфных фазовых переходах в редких землях

Two-Sublattice Model of Ferroelectric Phase Transitions

Dielectric properties of two nonequivalent sublattice polarizations are studied. Assuming a strong coupling favoring an antiparallel alignment of polarizations, the model reveals some new features of a ferroelectric transition with no multiple change of the unit cell volume. The Curie-Weiss constant is unusually small and this fact directly accounts for a possible sign change of the spontaneous polarization by only a slight variation of model parameters with temperature. The model also can describe a para-ferroelectric phase transition followed by an isomorphous phase transition between two ferroelectric phase with no change of symmetry. Temperature dependence of susceptibility is derived with the emphasis on the critical region in which the applied pressure or electric field smears out the difference between two isomorphous phases. The model can explain some properties of ammonium sulfate, dicalcium strontium and lead propionate.

Phenomenological Theory of an Isomorphous Transition in Polar Crystals – Application to the II-III Transition in Ferroelectric Ca2Pb(C2H5COO)6

A phenomenological theory is presented for a possible isomorphous phase transition in polar crystals. The elastic Gibbs function G is expanded in a power series of a deviation of the polarization P from a fixed value P 0 as; G = G 0 + a 0 ( T - T 0 - K p ) Δ P + b 0 ( p - p 0 )( Δ P ) 2 + c ( Δ P ) 3 + d ( Δ P ) 4 , where T and p are temperature and hydrostatic pressure, respectively, and Δ P ≡ P - P 0 . A first order isomorphous phase transition takes place at pressures less than a critical value of p crit = p 0 +3 c 2 /(8 b 0 d ), and above the critical pressure no phase transition exists. The critical behavior of the order parameter, dielectric constant, and specific heat is discussed. By choosing proper values of parameters, a qualitative agreement is obtained betweent the calculated and the observed pressure and temperature dependence of low frequency dielectric constant of Ca 2 Pb(C 2 H 5 COO) 6 in the vicinity of the critical point.

Critical Behavior at a Valence Instability

The isomorphic phase transition in the system ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Th}}_{x}$, which is characterized by a delocalization of the $4f$ electron and a concomitant volume change, has been studied through measurements of the density and resitivity. For samples near the critical composition ($x\ensuremath{\simeq}0.264$) derivatives of these quantities diverge with mean-field exponents. Such mean-field behavior implies a long-force range, and suggests that electroelastic coupling dominates the critical behavior.

Investigation of the Electronic Mechanism of Isomorphic Phase Transitions Using the "Chemical" Shifts of X-Ray Lines.

Investigation of the Electronic Mechanism of Isomorphic Phase Transitions Using the "Chemical" Shifts of X-Ray Lines.

Исследование электронного механизма изоморфных фазовых переходов методом "химических" смещений рентгеновских линий

Исследование электронного механизма изоморфных фазовых переходов методом "химических" смещений рентгеновских линий