Phase Transition Of Order-disorder Type Research Articles

Vanishing of Transition Temperature in Hydrogen-Bond Crystals Under High Pressure

Some of hydrogen-bond crystals are well known to undergo order-disorder type phase transitions at ambient pressure. After recent experimental studies, however, transition temperatures T c of these crystals vanish at respective critical pressures p c contrary to the third law of thermodynamics. The Curie constants in KDP (KH 2 PO 4 ) and DKDP(KD 2 PO 4 ) were shown to hold finite values in contrast to the vanishing of T c for p > p c . This fact has been explained as the crossover of phase transition mechanism from the order-disorder type to the displacive one under high pressure. The saturation of the divergence in the dielectric constant k c near T = 0 in both crystals has been fitted with the Barrett formula given in terms of three parameters used to discuss the quantum paraelectricity. The pressure dependences of the three parameters have been evaluated for p > p c . The mechanism of phase transition in hydrogen-bond crystals under high pressure has been discussed on the basis of these experimental results.

Vanishing of Transition Temperature in Hydrogen-Bond Crystals Under High Pressure

Some of hydrogen-bond crystals are well known to undergo order-disorder type phase transitions at ambient pressure. After recent experimental studies, however, transition temperatures T c of these crystals vanish at respective critical pressures p c contrary to the third law of thermodynamics. The Curie constants in KDP (KH 2 PO 4 ) and DKDP(KD 2 PO 4 ) were shown to hold finite values in contrast to the vanishing of T c for p > p c . This fact has been explained as the crossover of phase transition mechanism from the order-disorder type to the displacive one under high pressure. The saturation of the divergence in the dielectric constant k c near T = 0 in both crystals has been fitted with the Barrett formula given in terms of three parameters used to discuss the quantum paraelectricity. The pressure dependences of the three parameters have been evaluated for p > p c . The mechanism of phase transition in hydrogen-bond crystals under high pressure has been discussed on the basis of these experimental res...

Structural Study of Semiconductive CdTe-ZnTe Alloy by High-Energy X-ray Diffraction

Semiconductive cadmium telluride and zinc telluride mixture (Cd 1 m x Zn x Te) is observed to have ferroelectric behaviors such as the D-E hysteresis measurements, dielectric measurements and thermal measurements that suggest the existent of ferroelectric phase transition. The paraelectric structure in high-temperature phase is zinc-blende structure. An order-disorder type phase transition occurs and the structure of Cd 1 m x Zn x Te crystal changes to ferroelectric pseudo cubic structure. However, the detail of the mechanism of the phase transition and the structure of ferroelectric phase have not been revealed. We performed X-ray diffraction measurements with synchrotron radiation and obtained a clear evidence of short range order or intermediate range order structure in ferroelectric phase. The role of glassy structure in the mechanism of ferroelectric behavior is discussed.

Calorimetric investigations of overcritical behaviour in (TEA)2CuCl4 crystals

The results of experimental investigation of molar heat capacity of [N(C2H5)4]2CuCl4 - ferroelasto-ferroelectric crystals in a wide temperature range are presented. The typical anomaly for the first order phase transition of order-disorder type at 258 K and additional anomaly with maximum at 195 K has been observed. We suggest that the low temperature anomalies of physical properties of [N(C2H5)4]2CuCl4 crystals can be attributed to over critical traces of isomorphous phase transitions (similar to that observed in liquid-gas system). Results of numerical analysis of phenomenological model of phase transition in [N(C2H5)4]2CuCl4 crystals are presented and compared with the experimentally determined temperature dependence of the specific heat and entropy changes.

Raman spectra and phase transition in betaine potassium iodide dihydrate

Polarized Raman spectra of betaine potassium iodide dihydrate ([(CH3)3N+CH2 COO-]2KI · 2H2O) have been studied in the temperature range 40 - 290 K. The behaviour of betaine external modes and of water internal vibrations provides evidence for a first order phase transition of order-disorder type at 100 K. This phase transition is related to the freezing of the successive reorientations of water molecules. The temperature dependence of the halfwidth of the water stretching band centred at 3489 cm-1 (40 K) obeys an Arrhenius-type law corresponding to different activation energies below and above the critical temperature. The comparison of Raman and dielectric data allows us to identify two types of reorientation of water molecules.

Vanishing of order-disorder type phase transition in DKDP at high pressure

Temperature dependences of the dielectric constant along the ferroelectric c-axis have been measured at various pressures up to 7.9 GPa in DKDP. The vanishing of the ferroelectric state has been observed at 6.3 GPa. The Curie constant C has been saturated to about three fourths of that at atmospheric pressure above 4 GPa even if the transition temperature T becomes 0. The vanishing of Tc with finite C is theoretically derived from the temperature dependence of electric polarizability in displacive type ferroelectrics. The crossover of the order-disorder type to the displacive one under high pressure is discussed on the mechanism of phase transition to explain these experimental results in DKDP.

Dipole ordering and critical behavior of the static and dynamic properties in three-dimensional and layered MM′P2 X 6 crystals (M,M′ – Sn, Cu, In; X – S, Se)

The thermodynamic properties and lattice dynamics of crystals MM′P2X6 with cations in different valence states have been investigated. For Sn2P2S(Se)6 crystals with a three-dimensional lattice, the interaction between polarization and deformation plays an important role in the mechanisms of the displacive transitions from a paraelectric to ferroelectric or incommensurate phase in the vicinity of a Lifshitz point on the state diagram. Layered CuInP2S6 crystals undergo a first order and order-disorder type phase transition to a ferrielectric phase. For these crystals, different polarization mechanisms within a wide frequency range were clearly observed and found to be interrelated via dielectric and Raman spectroscopy measurements. These mechanisms include the internal vibrations of (P2S6)4- anions; the external lattice modes in which In3+ and Cu+ ions participate; the relaxational critical slowingdown due to Cu+ ions which yield the order – disorder phase transition; and the low frequency relaxational dynamics which causes the “throw” of Cu+ ions into the interlayer space thus providing ionic conductivity.

Isotopic effects in hydrogen-bonded crystals with order–disorder type phase transitions

The influence of geometric isotopic effects on the Curie temperature of hydrogen-containing crystals was considered. The experimental pressure and concentration dependencies of the Curie temperature have been explained in a framework of the pseudo-spin Ising model, by taking into account the changes in geometry of hydrogen bonds under pressure and due to isotopic exchange.

Needle twins and right-angled twins in minerals; comparison between experiment and theory

Transformation twinning in minerals forms isolated twin walls, intersecting twin walls with corner junctions, and wedge-shaped twins as elements of hierarchical patterns. When cut perpendicular to the twin walls, the twins have characteristic shapes, right-angled and needle-shaped wall traces, which can be observed by transmission electron microscopy or by optical microscopy. Theoretical geometries of wall shapes recently derived for strain-related systems should hold for most displacive and order-disorder type phase transitions: (1) right-angled twins show curved junctions; (2) needle-shaped twins contain flat wall segments near the needle tip if the elastic behavior of the mineral is dominated by its anisotropy; (3) additional bending forces and pinning effects lead to curved walls near the junction that make the needle tip appear more blunt. Experimental studies confirmed that these features occur in a wide range of materials. Bent right-angled twins were analyzed in Gd 2 (MoO 4 ) 3 . Linear needle tips were found in WO 3 , [N(CH 3 ) 4 ] 2 .ZnBr 4 CrAl, BiVO 4 , GdBa 2 Cu 3 O 7 , and PbZrO 3 . Parabolic tips occur in K 2 Ba(NO 2 ) 4 , and GeTe whereas exponential curvatures appear in BaTiO 3 , KSCN, Pb 3 (PO 4 ) 2 , CaTiO 3 , alkali feldspars, YBa 2 Cu 3 O 7 ?, and MnAl. The size and shape of the twin microstructure relates to its formation during the phase transition and the subsequent annealing history. The mobility of the twin walls after formation depends not only on the thermal activation but also on the structure of the wall, which may be pinned to impurities on a favorable structural site. Depinning energies are often large compared with thermal energies for diffusion. This leads to kinetic time scales for twin coarsening that are comparable to geological time scales. Therefore, transformation twins that exhibit needle domains not only indicate that the mineral underwent a structural phase transition but also contain information about its subsequent geological history.

A model for the reflectivity spectra of TmTe

A simple one dimensional diatomic chain model is proposed to explain the reflectivity spectra of TmTe as observed by Wardet al. It is suggested that the system undergoes a structural phase transition of order-disorder type at 4.2 K, where the new phase assumes an anti-ferroelectric type of arrangement of the atoms. The results, we obtained, agree well qualitatively with the experimental results.

Brillouin scattering of the order-disorder type phase transition in NaNO 3

NaNO 3 undergoes the order-disorder type phase transition with the anti-parallel order of NO 3 molecules along the three-fold z axis at T c = 276.5°C. A Brillouin scattering experiment was performed to observe the c 33 pure longitudinal acoustic mode coupled with the fluctuation of the anti-parallel order in the temperature range between 23.22°C and 299.55°C. The relaxation time of the fluctuation was determined as a function of temperature below T c to be τ f/( T c − T), where τ fis(6.0 ± 0.7) × 10 −11s deg. τ f is one order lower than the previous result reported by the ultrasonic study.

Phase transitions in some clathrate compounds1

Abstract Low temperature properties of clathrate compounds of hydro-quinone and thiourea and of clathrate hydrates are discussed using experimental results from calorimetry, neutron scattering and nuclear resonance experiments. All of them undergo phase transitions of order-disorder type, involving orientational ordering of the guest molecules in the first two and ordering of both the guest and host molecules in the third. Even though the basic situation as to the motional freedom of the guest molecule in the cavity is the same in the three clathrate systems, the time scales of the reorientational motion range from 1 ps to more than hours even at a fixed temperature of 20 K. In the slowest case, the substance behaves as a glass. For clathrate hydrates, not only the host lattice determines the motion of the guest molecule, but the guest molecule controls ordering of the orientation of the water molecules in the host lattice.

Temperature dependence of Raman spectra in 1,2-dinitrotetrachlorobenzene

The existence of order-disorder type phase transitions around 353 K in 1,2-dinitrotetrachlorobenzene (1,2-DNTCB), found earlier during DSC and dielectric studies, was confirmed on the basis of temperature dependence of selected Raman spectral parameters. It was established that the positions and widths of the studied bands do not vary noticeably near the transition temperature, but the change in their intensity is pronounced and a variation in their shape takes place. As was expected, Raman bands gain gaussian character in the vicinity of the phase transition. The observed properties were consistent with the concept of superstructure formation during phase transition in the 1,2-DNTCB crystal proposed already on the basis of X-ray and dielectric measurements. It was assumed that in the high temperature rotational phase a coupling of molecular movements leads to the collective rearrangement.

Heat transport properties of ferroelectrics and related materials

Abstract A review of experimental data on heat transport properties of crystals with ferroelectric and ferrodistortive phase transitions is presented. Special attention is paid to the temperature dependence of the thermal conductivity coefficient in order-disorder and displacive type phase transitions. An interpretation and a qualitative approach to the description of the heat transport coefficients anomalies are discussed.

Dynamics of the ferroelectric transition and acentricity in PbHPO4

Abstract A Raman spectroscopy study has shown no evidence for a large structural acentricity in the paraelectric phase of PbHPO4 (LHP) and that the crystal growth conditions are not a major factor in determining the phase transition properties of LHP. It is also found that the spectral response in the region of the central-mode frequency is well fitted by a single Debye relaxation function, indicating that a relaxational soft mode triggers the order-disorder type phase transition, which is probably due to polarization fluctuations of distorted PO4 tetrahedra.

Microwave dielectric dispersion in PbHPO4

Abstract The paper presents the results of the microwave dielectric investigations of the ferroelectric PbHPO4. The full dielectric spectrum of the ferroelectric Debye-type relaxational dispersion over the range 106 to 1012 Hz confirms that the relaxational soft mode triggers the order-disorder type phase transition and accounts for the static permittivity.

Dielectric Studies on HgCl2: 2 KCl Crystals

Abstract The dielectric properties of HgCl2 : 2 K Cl crystals have been measured as a function of frequency (102-105 Hz) and temperature (304-415 K), along with the d.c. conductivity, in two mutually perpendicular (X and Y) directions. Along the X direction a simple Debye type relaxation mecha­nism is observed at 304 K. A large dielectric anisotropy has been noticed at all frequencies and temperatures. Phase transitions of ferroelectric and order-disorder type are observed along the X and Y direction, respectively.

Dielectric dispersion and IR spectra of tris (dimethylammonium) nonachlorodiant imonate (III)

Abstract The complex electric permittivity of tris (dimethy lammonium) nonachlorodiant imonate (III) has been studied over the frequency range 105-109 Hz close to the ferroelectric phase transition at 242 K. The behaviour of the relaxation time and the IR absorption bands ascribed to the NH2 (CH3)2 + vibrations confirm that the order-disorder type phase transition is related to the motion of NH2 (CH3)2 + cations.

Central mode in hyper-Raman spectra of hydrogen-bonded ferroelectric PbHPO 4

A central mode of the hydrogen-bonded ferroelectric PbHPO 4 is found in the paraelectric phase by hyper-Raman scattering. From the polarization dependence, the observed central mode is assigned to B u mode. From the line shape analysis, the central mode is related with the order-disorder type phase transition. The temperature dependences of the intensity and the half width of this central mode follow the Curie-Weiss law. Because of the weak SHG in paraelectric phase, it is concluded that the space group of the paraelectric phase belongs to C 4 2 h .

Internal rotation and phase transitions in lanthanum tetrahydroborates

The investigation carried out shows that in the low-symmetry compouds La(BH4)3 and La(BH4)3·3THF, as in the high-symmetry MBH4, phase transitions occur. An unexpected result is the fact that the symmetry of the environment of the boron ions for these phase transitions practically does not change and remains close to cubic. The activation energy for reorientation of the BH − 4 ion (1.2 kcal/mole) is somewhat overstimated, if we start from the difference in the electronegativity of the boron and hydrogen ions (2.0 and 2.15). This may mean that the nature of the barrier to reorientation of the BH − 4 anion in lanthanum tetrahydroborates is connected with the van der Waals interaction of the tetrahedral BH − 4 groups with the environment. From the139La NMR data it follows that the coordination sphere of the triply-charged lanthanum is characterized by low-symmetry of the arrangement of the BH4− and THF (OC4H8) ligands, and the La3+-(BH4)− bond has predominantly ionic character. Consequently, the potential energy of the BH − 4 anions in the lattice is the sum of the strong isotropic ionic component and the substantially weaker anisotropic van der Waals component. In the low-temperature phases, for both compounds we observe structurally nonequivalent positions for the BH − 4 anions, retained in the high-temperature phase. The phase transitions in the studied compounds are connected with a change in the nature of the hydrogen atom distribution. Due to the low height of the barriers, the reorientational mobility sets in even at low temperatures, and in the phase transition region the value of υc reaches 1010–1011 Hz. For such a high reorientational frequency, there may be substantial effects from correlation rotation of the anions, leading to a change in the hydrogen atom distribution in the structure and an order-disorder type phase transition.