Order-disorder Processes Research Articles

Vibrational study of HgGa2S4 under high pressure

In this work, we report on high-pressure Raman scattering measurements in mercury digallium sulfide (HgGa2S4) with defect chalcopyrite structure that have been complemented with lattice dynamics ab initio calculations. Our measurements evidence that this semiconductor exhibits a pressure-induced phase transition from the completely ordered defect chalcopyrite structure to a partially disordered defect stannite structure above 18 GPa which is prior to the transition to the completely disordered rocksalt phase above 23 GPa. Furthermore, a completely disordered zincblende phase is observed below 5 GPa after decreasing pressure from 25 GPa. The disordered zincblende phase undergoes a reversible pressure-induced phase transition to the disordered rocksalt phase above 18 GPa. The sequence of phase transitions here reported for HgGa2S4 evidence the existence of an intermediate phase with partial cation-vacancy disorder between the ordered defect chalcopyrite and the disordered rocksalt phases and the irreversibility of the pressure-induced order-disorder processes occurring in ordered-vacancy compounds. The pressure dependence of the Raman modes of all phases, except the Raman-inactive disordered rocksalt phase, have been measured and discussed.

Elastic relaxation behavior, magnetoelastic coupling, and order-disorder processes in multiferroic metal-organic frameworks

Resonant ultrasound spectroscopy has been used to analyze magnetic and ferroelectric phase transitions in two multiferroic metal-organic frameworks (MOFs) with perovskite-like structures [(CH${}_{3}$)${}_{2}$NH${}_{2}$]M(HCOO)${}_{3}$ (DMA[M]F, M $=$ Co, Mn). Elastic and anelastic anomalies are evident at both the magnetic ordering temperature and above the higher temperature ferroelectric transition. Broadening of peaks above the ferroelectric transition implies the diminishing presence of a dynamic process and is caused by an ordering of the central DMA ([(CH${}_{3}$)${}_{2}$NH${}_{2}$]${}^{+}$) cation which ultimately causes a change in the hydrogen bond conformation and provides the driving mechanism for ferroelectricity. This is unlike traditional mechanisms for ferroelectricity in perovskites which typically involve ionic displacements. A comparison of these mechanisms is made by drawing on examples from the literature. Small elastic stiffening at low temperatures suggests weak magnetoelastic coupling in these materials. This behavior is consistent with other magnetic systems studied, although there is no change in ${Q}^{\ensuremath{-}1}$ associated with magnetic order-disorder, and is the first evidence of magnetoelastic coupling in MOFs. This could help lead to the tailoring of MOFs with a larger coupling leading to magnetoelectric coupling via a common strain mechanism.

Temperature dependence of the molecular conformations of dilauroyl phosphatidylcholine: A density functional study

AbstractBorn–Oppenheimer molecular dynamics (BOMD) in combination with density functional theory, augmented with a damped empirical dispersion term, has been used to study the behavior of dilauroyl phosphatidylcholine (DLPC) isomers with temperature. In contrast to dimyristoyl phosphatidylcholine (DMPC), the BOMD results show the presence of various conformations at different temperatures. The molecular order–disorder process, quantified by the distance‐fluctuation criterion as a function of temperature, is characterized by two transitions. This is in line with the known two‐phase transitions of DLPC bilayers, involving trans to gauche conformational changes in the alkane chains. The different temperature dependence of the DLPC and DMPC molecules suggests that the experimentally observed unusual dynamics of DLPC bilayers compared to that of longer chain lipids is governed to a large extent by the intramolecular dynamics. A first‐principles methodology applied at the molecular level can thus be an appropriate tool for microscopic analysis of the order–disorder transitions, which are related to the molecular structural transformations within large assemblies. © 2012 Wiley Periodicals, Inc.

High-pressure optical and vibrational properties of CdGa2Se4: Order-disorder processes in adamantine compounds

High-pressure optical absorption and Raman scattering measurements have been performed in defect chalcopyrite (DC) CdGa2Se4 up to 22 GPa during two pressure cycles to investigate the pressure-induced order-disorder phase transitions taking place in this ordered-vacancy compound. Our measurements reveal that on decreasing pressure from 22 GPa, the sample does not revert to the initial phase but likely to a disordered zinc blende (DZ) structure the direct bandgap and Raman-active modes of which have been measured during a second upstroke. Our measurements have been complemented with electronic structure and lattice dynamical ab initio calculations. Lattice dynamical calculations have helped us to discuss and assign the symmetries of the Raman modes of the DC phase. Additionally, our electronic band structure calculations have helped us in discussing the order-disorder effects taking place above 6–8 GPa during the first upstroke.

Disorder and phase transitions in oxyfluoride (NH 4) 3Ta(O 2) 2F 4

Calorimetric, X-ray, dielectric and DTA under pressure measurements have been performed on oxyfluoride (NH 4) 3Ta(O 2) 2F 4. The succession of nonferroelectric phase transitions was found associated with the order-disorder processes. The comparative analysis tantalate with related niobate has revealed the important role of the central atom in the physical properties behavior, mechanism of structural distortions and barocaloric effect in oxyfluorides with the eight-coordinated anionic polyhedra.

Human LDL core cholesterol ester packing: three-dimensional image reconstruction and SAXS simulation studies

Human LDL undergoes a reversible thermal order-disorder phase transition associated with the cholesterol ester packing in the lipid core. Structural changes associated with this phase transition have been shown to affect the resistance of LDL to oxidation in vitro studies. Previous electron cryo-microscopy studies have provided image evidence that the cholesterol ester is packed in three flat layers in the core at temperatures below the phase transition. To study changes in lipid packing, overall structure and particle morphology in three dimensions (3D) subsequent to the phase transition, we cryo-preserved human LDL at a temperature above phase transition (53°C) and examined the sample by electron microscopy and image reconstruction. The LDL frozen from 53°C adopted a different morphology. The central density layer was disrupted and the outer two layers formed a "disrupted shell"-shaped density, located concentrically underneath the surface density of the LDL particle. Simulation of the small angle X-ray scattering curves and comparison with published data suggested that this disrupted shell organization represents an intermediate state in the transition from isotropic to layered packing of the lipid. Thus, the results revealed, with 3D images, the lipid packing in the dynamic process of the LDL lipid-core phase transition.

Order–disorder phase transition in crystalline di-tert-butylphosphinic acid dimer: The role of the ice rules and quantum tunneling

Heat capacities of di-tert-butylphosphinic acid (abbreviated as DTPA) crystal were measured by adiabatic calorimetry. Phase transition of a second-order type was discovered at 37.8K. The entropy of the transition was estimated to be 2.6JK−1mol−1, and the magnitude was interpreted as suggesting that the transition is attributed to the order–disorder process of the hydrogen-bonded protons of a DTPA dimer and that the protons rearrange with always keeping the ice rules. It was discussed that the real state of the protons should be described on the consideration of the tunneling between the two configurations which are represented classically as each proton is located close to either of the two oxygen atoms forming a hydrogen bond.

Cation ordering and phase transitions in feldspars along the join CaAl2Si2O8-SrAl2Si2O8: a TEM, IR and XRD investigation

Two feldspars of composition Ca0.3Sr0.7Al2Si2O8 (An30SrF70) and Ca0.6Sr0.4Al2Si2O8 (An60SrF40) were synthesized from gel and annealed at temperatures between 1150 and 1480ºC for durations between 0.025 (1.5 min) and 1073 h. The An60SrF40 feldspar is triclinic for all annealing temperatures, whereas the An30SrF70 possesses, at room temperature, a monoclinic symmetry, C2/m in the shortest annealing runs and I2/c after slightly longer heating, and becomes triclinic after prolonged annealing. A TTT plot for the triclinic–monclinic transition shows an activation energy of 101±12 kcal, less than, but still close to, that observed for Al-Si order in anorthite. TEM investigation shows antiphase domains and Carlsbad twins the size of which increases with annealing. IR spectroscopy was performed on a series of samples of An30SrF70, annealed at T = 1350ºC. The linewidth in hard mode is related to the spontaneous strain of the monoclinic–triclinic transition via . The evolution of the order parameter with time can be modelled as lnt, as predicted in order-disorder processes at intermediate degrees of order. It is shown that even after the longest annealing runs in An30SrF70, equilibrium was not obtained whereas, under the same conditions, equilibrium Al-Si order can be obtained in anorthite and Sr-feldspar.

Acoustic dissipation associated with phase transitions in lawsonite, CaAl2Si2O7(OH)2{middle dot}H2O

Resonant ultrasound spectra of a single crystal and a polycrystalline sample of lawsonite [CaAl2Si2O2(OH)2·H2O] have been measured at room temperature and at low temperatures in the region 20-300 K. The influence of known phase transitions at 125 and 270 K is seen in the frequency variations of the resonance peaks, which are indicative of elastic stiffening, and in values for the quality factor QQF, which are indicative of dissipation. Two dissipation peaks, at ~250 and ~210 K, are interpreted as being due to the proton order-disorder processes associated with the two species of hydrogen atoms in the structure: one in hydroxyl OH groups and one in the H2O molecules. These occur below the Cmcm ↔ Pmcn transition point but coincide with changes in the shear elastic constants and in features of IR spectra reported elsewhere. A third, much smaller, dissipation peak occurs immediately below the Pmcn ↔ P21cn transition point. The combination of these anomalies in acoustic dissipation and in elastic constants is consistent with the view that the Cmcm ↔ Pmcn transition is driven both by displacive and proton ordering effects. For the Pmcn ↔ P21cn transition, dissipation and the transition are more closely related, consistent with the view that the transition is driven essentially by proton ordering.

Calorimetric Finding of Phase and Glass Transitions Concerning the Ordering of the Water Located between the Silicate Layers of the Clay Na-RUB-18

Abstract The behavior of the water accommodated between the silicate layers of a clay Na8[Si32O64(OH)8]·32H2O was investigated by using adiabatic calorimetry. A phase transition was observed showing a heat-capacity peak at 194 K and its entropy was roughly estimated to be 5.2 J K−1 (H2O-mol)−1. The transition was interpreted to be essentially due to an order–disorder process involving two orientations of the water molecule which can form hydrogen bonds with siloxane-oxygen atoms. It is thought that the water molecules move with a large amplitude of vibration as well as disorder between the two permissible orientations at room temperature and become gradually ordered to take a more stable orientation of the two below 194 K. A glass transition was found at a temperature Tg=106 K, due to freezing of the water molecule in the orientations, which causes a little disorder to remain. The potential barrier (Δεa) of the reorientation was assessed from the Tg to be about 34 kJ mol−1, which corresponds to the energy to break two hydrogen-bonds. There remains a possibility that the silanol-hydrogen atom takes a tunneling state involving two accessible sites between two silanol-oxygen atoms; however, it was not confirmed in this work.

Neutron Powder Diffraction Studies of Order-Disorder Phase Transitions and Kinetics

One of the major applications of neutron diffraction in mineral sciences has been in the study of order-disorder processes and phase transitions. Neutron scattering methods provide unique insights into the origins and mechanisms of these processes, and have enabled mineralogists to develop new models of phase transformation behavior. The time-temperature dependence of processes such as phase transformations, exsolution, cation ordering and disordering in minerals has considerable potential geophysical, geochemical and petrological importance. Let’s take atomic ordering as an example. Order-disorder transformations and similar structural phase transitions are typically some of the most efficient ways a mineral can adapt to changing temperature or chemical composition. Disorder of distinct species across different crystallographic sites at high temperature provides significant entropic stabilization of mineral phases relative to low-temperature ordered structures. Positional or orientational disordering can have similar drastic effects. For example, the calcite-aragonite phase boundary shows a significant curvature at high temperature due to disorder of CO3 groups within the calcite structure, associated with an orientation order-disorder phase transition (Redfern et al. 1989). This leads to an increased stability of calcite with respect to aragonite over that predicted by a simple Clausius-Clapeyron extrapolation of the low pressure-temperature thermochemical data. Understanding of the structural characteristics of the phase transition in calcite developed in tandem with studies of the analogous transition in nitratine, NaNO3, but it was not until Dove and Powell (1989) carried out high-temperature neutron diffraction experiments on powdered calcite that there was direct experimental evidence linking the thermodynamic and structural nature of the transition in CaCO3. The key to the success of their study was the fact that neutrons penetrate the entire volume of samples held at extreme conditions (in this case very close to the melting temperature and under a confining CO2 pressure). Cation ordering …

Phase transition, molecular motions, structural changes and low-frequency vibrations in [Cu(NH 3) 5](ClO 4) 2

The phase transition in [Cu(NH 3) 5](ClO 4) 2 at T c h = 150 . 6 K while heating and at T c c = 141 . 6 K while cooling was determined by means of differential scanning calorimetry (DSC), by extrapolating the T peak h and T peak c vs. rate of sample heating and cooling to the scanning rate value of 0 K min −1, respectively. The presence of ca. 9 K hysteresis in the phase transition temperature suggests that the detected phase transition is of the first order type. Large transition entropy value indicates considerable disordering of high-temperature phase. Changes in spectra were observed and two solid phases were identified by Fourier transform far- and middle-infrared spectroscopy (FT-FIR and FT-MIR), which indicate that the detected phase transition is related to a crystal structure change. Structural character of this phase transition also confirms the existence of some kind of soft modes in the low-temperature phase. The results of X-ray and neutron powder diffraction measurements (XRPD and NPD) proved that the crystal structure change took place. The cubic phase ( Fm 3 ¯ m ) transforms into the monoclinic phase ( P2 1/ m). The inelastic, incoherent neutron scattering (IINS) results, compared with infrared and Raman spectra at low-frequency vibrations region, additionally authenticate that statement. The temperature dependence of the full-width at half-maximum (FWHM) of the band connected with δ as(HNH) mode indicates that the observed phase transition is not related to dynamical order–disorder process of the NH 3 ligands, which make fast reorientational motions in both detected phases. Reorientational correlation time becomes longer than 10 −10 s only at 25 K, which was detected by quasi-elastic neutron scattering (QENS) measurements.

Order parameter variation through theC2/m-P21/mphase transition in cummingtonite

The C2/m-P2 1 /m phase transition in natural cummingtonites, with different compositions, has been investigated by single-crystal X-ray diffraction and powder absorption IR spectroscopy. Variations in the intensity of type b (superlattice) reflections above room temperature are consistent with the transition being thermodynamically continuous and conform to the solution to a 2-4-6 Landau potential. Parameters extracted from IR spectra collected both above and below room temperature suggest that the local structural evolution differs slightly from the average macroscopic behavior observed by X-ray diffraction. Changes in wavenumber of an isolated absorption band at ∼1130 cm -1 and changes in the integrated intensity of a band at ∼760 cm -1 can be described by the solution to a 2-4 Landau potential with saturation below room temperature. Absorption bands due to O-H vibration and libration, however, appear to show discontinuities in the variation of their wavenumbers at the transition point. A local mechanism for the transition might involve a framework distortion providing the main driving force, with coupling to an order-disorder process for the protons

Phase transitions and molecular motions in [Ni(ND 3) 6](ClO 4) 2

[Ni(ND 3) 6](ClO 4) 2 has three solid phases between 100 and 300 K. The phase transitions temperatures at heating ( T C1 h=164.1 K and T C2 h=145.1 K) are shifted, as compared to the non-deuterated compound, towards the lower temperature of ca. 8 and 5 K, respectively. The ClO 4 − anions perform fast, picosecond, isotropic reorientation with the activation energy of 6.6 kJ mol −1, which abruptly slow down at T C1 c phase transition, during sample cooling. The ND 3 ligands perform fast uniaxial reorientation around the Ni–N bond in all three detected phases, with the effective activation energy of 2.9 kJ mol −1. The reorientational motion of ND 3 is only slightly distorted at the T C1 phase transition due to the dynamical orientational order–disorder process of anions. The low value of the activation energy for the ND 3 reorientation suggests that this reorientation undergoes the translation–rotation coupling, which makes the barrier to the rotation of the ammonia ligands not constant but fluctuating. The phase polymorphism and the dynamics of the molecular reorientations of the title compound are similar but not quite identical with these of the [Ni(NH 3) 6](ClO 4) 2.

Fourier transform infrared spectroscopy study on order–disorder transition in Langmuir–Blodgett films of 7-(2-octadecyloxycarbonylethyl)guanine before and after recognition to cytidine

Order–disorder transitions of 9-monolayer Langmuir–Blodgett (LB) films of 7-(2-octadecyloxycarbonylethyl)guanine (ODCG) before and after recognition to cytidine were investigated by Fourier transform infrared (FTIR) spectroscopy. The different order–disorder transitions suggest that molecular recognition between ODCG and cytidine influence these two LB films on the order–disorder process of alkyl tailchain. Cleavage of the multi-hydrogen bonds was also observed by the infrared spectroscopy at elevated temperature.

Phase transitions, structural changes and molecular motions in [Zn(NH 3) 4](BF 4) 2 studied by neutron scattering, X-ray powder diffraction and nuclear magnetic resonance

Nuclear magnetic resonance ( 1H NMR and 19F NMR) measurements performed at 90–295 K, inelastic incoherent neutron scattering (IINS) spectra and neutron powder diffraction (NPD) patterns registered at 22–190 K, and X-ray powder diffraction (XRPD) measurements performed at 86–293 K, provided evidence that the crystal of [Zn(NH 3) 4](BF 4) 2 has four solid phases. The phase transitions occurring at: T C3=101 K, T C2=117 K and T C1=178 K, as were detected earlier by differential scanning calorimetry (DSC), were connected on one hand only with an insignificant change in the crystal structure and on the other hand with a drastic change in the speed of the anisotropic, uniaxial reorientational motions of the NH 3 ligands and BF 4 − anions (at T C3 and at T C2) and with the dynamical orientational order–disorder process (“tumbling”) of tetrahedral [Zn(NH 3) 4] 2+ and BF 4 − ions (at T C1). The crystal structure of [Zn(NH 3) 4](BF 4) 2 at room temperature was determined by XRPD as orthorhombic, space group Pnma (No. 62), a=10.523 Å, b=7.892 Å, c=13.354 Å and Z=4. Unfortunately, it was not possible to determine the structure of the intermediate and the low-temperature phase. However, we registered the change of the lattice parameters and unit cell volume as a function of temperature and we can observe only a small deviation from near linear dependence of these parameters upon temperature in the vicinity of the T C1 phase transition.

How Cr3+ and Fe3+ affect Mg?Al order?disorder transformation at high temperature in natural spinels

Three natural Mg(Al2-yCry)O4 spinels (y ∼0.07–0.16), highly ordered in terms of Mg–Al, and one Mg(Al2−yFe3+y)O4 spinel (y∼0.08), highly ordered also in terms of Fe3+, were studied by means of X-ray single-crystal diffraction. All samples were heated in situ from 25 to 1000 °C in order to follow both thermal expansion and evolution of the structural state of spinel with temperature. Thermal expansion was monitored by means of the variation of cell edge a with temperature, and found to be well represented throughout the temperature range by a regression line a = a0 (1+αΔT), slightly different at lower and higher temperatures. Thermal expansion coefficient α1, referring to the lower temperature range (i.e. during pure thermal expansion), was slightly lower than α2, calculated only over the highest temperatures. The trend showed different slopes for individual crystals. Structural evolution with temperature was studied by means of the variation of oxygen positional parameter u, which is strongly influenced by intersite cation exchange and thus closely correlated with inversion parameter x. In particular, in the three Cr samples, in which Cr resides only in the octahedral site, u parameter variations and hence the order–disorder process, started at about 700 °C. Instead, in the Fe3+ sample, this process was triggered at lower temperatures, starting at 550 °C with Fe3+–Mg exchange followed at higher temperatures by that of Mg–Al. Cr contents in the Cr samples affected the occupancy of Al in the tetrahedral site at the highest temperatures. In both Mg–Al–Cr and Mg–Al–Fe3+ compositions, if Cr∼Fe3+, parameter u reached the same value only when the Mg–Al exchange was dominant, i.e. at the highest temperatures, but not before. Cation distribution at each temperature was obtained by the bond-length model, applying thermal expansion to pure bond lengths. This method is applied here to complex compositions for the first time.

High‐pressure metastable phase transitions in β‐Ge3N4 studied by Raman spectroscopy

AbstractWe studied polymorphic transitions occurring in β‐Ge3N4 metastably compressed to high pressures (P = 40 GPa) at room temperature. Previous studies under high‐pressure–high‐temperature conditions have shown that this phase transforms into a newly discovered spinel‐structured material (γ‐Ge3N4) above 10 GPa and 1200°C. However, ab initio theoretical studies have indicated that the phenacite‐structured low‐pressure β‐Ge3N4 polymorph should also undergo a series of metastable transformations upon compression at low temperatures. Here we studied these transformations by in situ Raman spectroscopy in a diamond anvil cell. The phase transitions were first predicted to involve a sequence of second‐order phase changes driven by soft phonon modes at the Brillouin zone centre (the Γ point: q = 0) resulting in displacive motions of the N atoms, causing the symmetry to descend from P63/m through P6 to P3. The two transitions were predicted to occur at P = 20 and 28 GPa. However, it was also noted that a direct first‐order transition between P63/m and P3 phases might occur at P = 23 GPa if order–disorder processes or phonon condensations away from the Brillouin zone centre were considered. Here we present experimental evidence for a phase transition occurring at P = 20 GPa, within β‐Ge3N4 that has been metastably compressed at ambient temperature. From the number of Raman modes observed, this transition most likely corresponds to the direct P63/m–P3 transformation, and it is therefore first order in character, but with a small activation energy barrier as ascertained by little or no hysteresis observed upon decompression. However, when all the minor features appearing in the Raman spectrum are accounted for, the number of observed resonances is greater than the number of zone centre modes expected even for the P3 structure. This indicates that the true unit cell is larger than expected from zone‐centre mode softening, and that order–disorder processes or phonon instabilities at q≠0 must have occurred during the transition. Copyright © 2003 John Wiley & Sons, Ltd.

Study of the low-frequency Raman scattering in NaNbO3crystal

The Raman scattering spectrum of the sodium niobate crystal, in both P and Rphases, has been investigated from room temperature up to 440°C.The dependence of the low-frequency (LF) spectrum clearly reveals, for the firsttime, over a wide temperature range, the presence of a strong quasi-elasticscattering below a LF zone centre phonon. The phase transition mechanism isdiscussed, considering an order–disorder process induced by the relaxation of theNb ions.

Rate equations for non-convergent order-disorder processes - a review and application to orthopyroxene

Rate equations for non-convergent order-disorder processes - a review and application to orthopyroxene