Displacive Phase Transition Research Articles

T-induced displacive phase transition of end-member Pb-lawsonite

AbstractPb-lawsonite, PbAl2[(OH)2|Si2O7]·H2O, space group Pbnm, was synthesized as crystals up to 15 μm × 5 μm × 5 μm in size by a piston cylinder technique at a pressure of ∼4 GPa and a temperature of 873 ± 10 K. Temperature-dependent powder and single-crystal X-ray diffraction (XRD) analyses partly using synchrotron radiation as well as Raman spectroscopic investigations reveal a phase transition around 445 K resulting in the Cmcm high-temperature structure. The transformation temperature is considerably higher than that of lawsonite around 273 K, which is characterized predominantly by proton order/disorder. The transition is confirmed using principal component analysis and subsequent hierarchical cluster analysis on both the powder XRD patterns and the Raman spectra. Furthermore, a non-uniform change is observed around 355 K, which is not as pronounced as the 445 K transition and apparently comes from enhanced hydrogen bonding, which stops the atom shifts in Pb-lawsonite. These are the same bonds that mainly characterize the phase transition in lawsonite around 273 K. In contrast, the structural transition of Pblawsonite at 445 K seems to originate from the interaction of the SiO4tetrahedra and AlO6octahedra framework with the Pb2+cation. The structural environment of Pb2+can be described by a 12-fold coordination above 445 K, which changes towards irregular ten-fold coordination below this temperature. An assignment of the O–H stretching Raman bands confirms moderately strong H bonds in Pb-lawsonite, whereas both strong and weak H bonds exist in lawsonite. Therefore, a further phase transition of Pblawsonite, similar to that of lawsonite around 273 K, is not expected.

Magnesium hydrides and their phase transitions

The mechanism of phase transitions in magnesium dihydrides is explored with the aim how to find the ways for lowering crystal structure stability and facilitate hydrogen desorption. After recapitulation of transition metal hydrides properties, a possible mechanism of fluorite-rutile type displacive phase transition is proposed.

Low temperature structural modification in Rb2ZrF6: Investigations by perturbed angular correlation spectroscopy

Temperature dependent perturbed angular correlation (PAC) measurements in crystalline compounds Rb2ZrF6 and Cs2HfF6 have been performed in the temperature range 298–753K. In Rb2ZrF6, four discrete quadrupole interaction frequencies have been observed at room temperature which correspond to four minor structural modifications. From previous measurements, on the other hand, two structural modifications of this compound were known. A displacive phase transition, probably, occurs at low temperature due to rotation of the ZrF62− octahedron and produces different structural modifications. From present measurements in Rb2ZrF6, two quadrupole interaction frequencies [ωQ=26.1(3)Mrad/s, η=0.55(2), δ=5(1)% and ωQ=148.7(3)Mrad/s, η=0.538(5), δ=1.2%] have been found at room temperature which were not found from previous studies. In Cs2HfF6, these new structural modifications have not been observed.

Molecular Ferroelectric Pyridin-4-ylmethanaminium Perchlorate Undergoes Paraelectric–Ferroelectric and Ferroelectric–Ferroelectric Phase Transitions

Molecule-based ferroelectrics are a class of highly desirable intelligent materials for their rich switchable physical properties, easy and environmentally friendly processing, especial lightweight and mechanical flexibility. In the current work, a new molecule-based ferroelectrics pyridin-4-ylmethanaminium perchlorate was discovered undergoing paraelectric–ferroelectric phase transition at Tc = 258.40(8) K and ferroelectric–ferroelectric phase transition at T1 = 255.93(2) K. It crystallizes in monoclinic crystal system with a centrosymmetric space group of C2/c at 293 K and also crystallizes in monoclinic crystal system but with a polar space group of Cc at 223 K. The spontaneous polarization can reach 1.25 μC/cm2 and the coercive field is about 2.6 kV/cm below 254 K. The paraelectric–ferroelectric phase transition belongs to displacive phase transition and the nearby ferroelectric–ferroelectric phase transition contains order–disorder features, which consistent with the dynamic process including deforma...

Vibrational contributions to the phase stability of PbS-PbTe alloys

The thermoelectric figure of merit $(ZT)$ of semiconductors such as PbTe can be improved by forming nanostructures within the bulk of these materials. Alloying PbTe with PbS causes PbS-rich nanostructures to precipitate from the solid solution, scattering phonons and increasing $ZT$. Understanding the thermodynamics of this process is crucial to optimizing the efficiency gains of this technique. Previous calculations of the thermodynamics of PbS-PbTe alloys [(J. W. Doak and C. Wolverton, Phys. Rev. B 86, 144202 (2012)] found that mixing energetics alone were not sufficient to quantitatively explain the thermodynamic driving force for phase separation in these materials: first-principles calculations of the thermodynamics of phase separation overestimate the thermodynamic driving force for precipitation of PbS-rich nanostructures from PbS-PbTe alloys. In this work, we re-examine the thermodynamics of PbS-PbTe, including the effects of vibrational entropy in the free energy through frozen-phonon calculations of special quasirandom structures (SQS) to explain this discrepancy between first-principles and experimental phase stability. We find that vibrational entropy of mixing reduces the calculated maximum miscibility gap temperature ${T}_{G}$ of PbS-PbTe by 470 K, bringing the error between calculated and experimental ${T}_{G}$ down from 700 to 230 K. Our calculated vibrational spectra of PbS-PbTe SQS exhibit dynamic instabilities of S ions that corroborate reports of low-$T$ ferroelectriclike phase transitions in solid solutions of PbS and PbTe, which are not present in either of the constituent compounds. We use our calculated vibrational spectra to obtain phase transition temperatures, which are in qualitative agreement with experimental results for PbTe-rich alloys, as well as to predict the existence of a low-$T$ displacive phase transition in PbS-rich PbS-PbTe, which has not yet been experimentally investigated.

Locating Gases in Porous Materials: Cryogenic Loading of Fuel-Related Gases Into a Sc-based Metal-Organic Framework under Extreme Pressures.

An alternative approach to loading metal organic frameworks with gas molecules at high (kbar) pressures is reported. The technique, which uses liquefied gases as pressure transmitting media within a diamond anvil cell along with a single-crystal of a porous metal-organic framework, is demonstrated to have considerable advantages over other gas-loading methods when investigating host-guest interactions. Specifically, loading the metal-organic framework Sc2BDC3 with liquefied CO2 at 2 kbar reveals the presence of three adsorption sites, one previously unreported, and resolves previous inconsistencies between structural data and adsorption isotherms. A further study with supercritical CH4 at 3-25 kbar demonstrates hyperfilling of the Sc2 BDC3 and two high-pressure displacive and reversible phase transitions are induced as the filled MOF adapts to reduce the volume of the system.

Locating Gases in Porous Materials: Cryogenic Loading of Fuel‐Related Gases Into a Sc‐based Metal–Organic Framework under Extreme Pressures

AbstractAn alternative approach to loading metal organic frameworks with gas molecules at high (kbar) pressures is reported. The technique, which uses liquefied gases as pressure transmitting media within a diamond anvil cell along with a single‐crystal of a porous metal–organic framework, is demonstrated to have considerable advantages over other gas‐loading methods when investigating host–guest interactions. Specifically, loading the metal–organic framework Sc2BDC3 with liquefied CO2 at 2 kbar reveals the presence of three adsorption sites, one previously unreported, and resolves previous inconsistencies between structural data and adsorption isotherms. A further study with supercritical CH4 at 3–25 kbar demonstrates hyperfilling of the Sc2BDC3 and two high‐pressure displacive and reversible phase transitions are induced as the filled MOF adapts to reduce the volume of the system.

High-temperature phase transition and local structure of a hydrous anorthoclase

The in situ Raman spectra of a hydrous anorthoclase at temperatures of 20–800 °C have been measured using a LABRAM-HR spectrometer and Linkam TS 1500 heating stage. The frequencies of modes at 54, 99, 130 and 162 cm−1 related to M–O vibrations decrease sharply and then increase drastically or keep steady at temperatures above 200 °C. A knee point can be clearly seen at about 200 °C for those modes. The frequency of the mode at 282 cm−1 shows little temperature dependence. However, for the two strongest modes at 471 and 512 cm−1, the frequencies decrease linearly with increasing temperature. From evolution of the frequencies of modes at 54, 99, 130 and 162 cm−1 with temperature, the following conclusions can be drawn: (1) The distance of the local M–O bond shortens rather than lengthens at temperatures above 200 °C; (2) The abrupt changes of the local structure of M site induce a collapse of the framework structure and displacive phase transition at 200 °C; and (3) The H atoms incorporated in anorthoclase are located at the M site. These results are indicative for the structure and properties of anorthoclase at deep earth conditions.

Pressure-induced transformation processes in ferroelastic Pb3(P1–x As x O4)2, x = 0 and 0.80

Abstract The high-pressure behaviour of two representative palmierite-type Pb3(P x As1–x O4)2 ferroelastic compounds with x = 0 and 0.80 was analyzed by combined single-crystal X-ray diffraction and Raman spectroscopy. Single-crystal diffraction measurements on the As-rich compound Pb3(P0.20As0.80O4)2 show that it undergoes the same macroscopic monoclinic to trigonal phase transition as previously observed in pure Pb3(PO4)2 but with a significantly lower transition pressure, p c = 0.90(4) GPa for x = 0.80 as opposed to p c = 1.81(1) for x = 0.0. Synchrotron X-ray diffraction experiments reveal that both compounds exhibit significant diffuse scattering in a pressure interval of ~1.5 GPa above the corresponding pressure-induced transitions, indicating the persistence of monoclinic nanodomains within the macroscopically trigonal phase above the phase-transition point, similar to the high-temperature structural state. Raman spectroscopy reveal quite distinct lattice dynamics for x = 0 and x = 0.80, indicating different pathways of pressure-driven structural alteration. The pure phosphate compound shows a displacive phase transition of thermodynamically second-order type, whereas the As-rich compound exhibits an order-disorder phase transition with multistep structural changes on the mesoscopic scale. The pressure evolution of the Pb phonon modes as well as the broadening of the X-ray diffraction peaks suggests a further pressure-induced phase transition occurring in the range 5–7 GPa for Pb3(P0.20As0.80O4)2, whilst no indication for a second phase transition up to 10 GPa was observed for Pb3(PO4)2.

Compressibility and crystal–fluid interactions in all-silica ferrierite at high pressure

The high-pressure behavior of a synthetic siliceous ferrierite has been studied by in situ single-crystal and powder synchrotron X-ray diffraction with a diamond anvil cell, using four different P-transmitting fluids: the non-penetrating silicone oil and the potentially pore-penetrating methanol:ethanol:H2O = 16:3:1 mixture, ethylene glycol and 2methyl-2propen-1ol. The high-pressure experiment in silicone oil shows a remarkable flexibility of the FER framework. Two displacive phase transitions, following the path Pmnn-to-P121/n1-to-P21/n11 with pressure, were observed. The three polymorphs were found to share a virtually identical bulk compressibility, though showing a different anisotropic pattern. The experiments with potentially penetrating media enhanced the occurrence of a complex scenario, from which the P-induced intrusion of fluid molecules into the FER structural voids can be assumed by the different phase-transition paths and compressibility patterns, by the calculated residual electron density and by the different deformation mechanisms at the atomic scale, observed as a function of the used medium. The starting orthorhombic polymorph was always restored upon decompression in all the experiments. The roles of the different surface area in single crystal and polycrystalline samples, and of the process kinetics on the compressibility and crystal–fluid interactions, are discussed.

Twin displacive phase transitions in amino acid quasiracemates.

Three quasiracemates, L-norleucine:D-methionine, L-norvaline:D-norleucine, and L-norvaline:D-methionine, were crystallized to see how they differ from regular racemates in terms of crystal structure (studied by single-crystal X-ray diffraction) and of thermally induced phase transitions (studied by differential scanning calorimetry). Two types of transitions are detected between 100 and 450 K and structurally characterized: (1) displacive transitions of the molecular bilayers that form the crystal and (2) continuous or discontinuous disordering transitions in the amino acid side chains. Uniquely for the quasiracemates, the displacive transition proceeds in two close steps as only one surface of each molecular bilayer slides at first, upon forming an intermediate phase, while the other surface follows at a slightly higher temperature. Altogether, 18 new single-crystal structure-refinement data sets are reported for these three quasiracemates.

Synthesis and properties of lightweight fibrous ceramics with a 3D skeleton structure prepared by infiltration

In our report, a fibrous ceramics with the mullite fibers as the matrix and SiO2–AlPO4 sol as the high-temperature binder were fabricated by the infiltration method. The effects of sintering temperature on the properties, such as porosity, phase transition, microstructure, compressive strength and fracture mechanism, were investigated. Compared with the fibrous ceramics with the SiO2–B2O3 binder, the fibrous ceramics with the SiO2–AlPO4 binder exhibited lower densities (0.560–0.595g/cm3), lower thermal conductivities (0.157–0.165W/mK), high strain (7–8%), high rebound-resilience (77–90%) and the same level of compressive strengths (1.1–2.1MPa). During the cooling stage of the heat treatment process, α→β-cristobalite displacive phase transition in the SiO2–AlPO4 binder was successfully suppressed. The fracture behavior of the ceramics tends to be a layer-by-layer failure, which is quite different from that of the traditional ceramics. The non-brittle fracture mode and the excellent elasticity made this fibrous ceramics promising high-temperature elastic insulation materials.

Crystal and magnetic structures, phase transitions in quasi-one-dimensional pyroxenes Na0.5Li0.5FeGe2O6

The possibility of cation substitution in pyroxenes allows the investigation the gradual change of structural and magnetic properties under doping in these compounds. Here, Na0.5Li0.5FeGe2O6 was prepared by the standard solid-phase reaction method and has been investigated using X-ray and neutron powder diffraction, and further characterized by magnetic and calorimetric measurements. The crystal structure of Na0.5Li0.5FeGe2O6 at 300K is monoclinic C2/c (a=10.0333(1), b=8.8136(1), c=5.5295(9)Å, β=108.921(1)°). Calorimetric investigations indicate a displacive first order phase transition at T=271±1K which is accompanied by the appearance of superstructure reflections in the X-ray patterns. At this transition Na0.5Li0.5FeGe2O6 undergoes a space group change from C2/c to P21/c (a=9.9692(3), b=8.8545(3), c=5.4752(2)Å, β=108.494(1)°). Magnetic order has been found below the Néel temperature TN≈18K and has been refined from neutron diffraction. The quasi-low-dimensional magnetic spin system Na0.5Li0.5FeGe2O6 exhibits a collinear antiferromagnetic structure with the space group Pa21/c and the doubling of the unit cell along the crystallographic a-axis of the pyroxene crystal (propagation vector k=(1/2, 0, 0)). The critical modes which are responsible for the phase transition from C2/c to P21/c symmetry and for the magnetic transition from paramagnetic to antiferromagnetic state have been determined and their role in the transition is described. They are also the relevant mechanism driving the magnetic order in LiFeGe2O6.

Using Raman spectroscopy to understand the origin of the phase transitions observed in [(C3H7)4N]2Zn2Cl6 compound

Phase transitions of the centrosymmetric compound, [(C3H7)4N]2Zn2Cl6, were studied by differential scanning calorimetry (DSC), X-ray diffraction, Raman spectroscopy and dielectric measurements. Two reversible order–disorder and displacive phase transitions are observed at T1=327K and T2=347K with 3K and 4K hysteresis respectively, indicating a first order character. The evolution of Raman line shifts, “ν”, and the half-width, “Δν”, versus temperature show some singularities associated with the transitions, suggesting that they are governed by the reorientational and the displacement of the organic part. Besides the results of the dielectric permittivity study confirms the conclusion drawn from the calorimetric and Raman measurements that the phase transition located in the vicinity of the temperature of the dielectric proprieties is characterized by change of dynamical state of cation.

Contradistinct Thermoresponsive Behavior of Isostructural MIL-53 Type Metal–Organic Frameworks by Modifying the Framework Inorganic Anion

The influence of simple framework inorganic anions on the thermoresponsive behavior of the isostructural MIL-53 type metal–organic frameworks [AlF(bdc)] and [Al(OH)(bdc)] has been determined using a combination of diffraction and computational techniques. [AlF(bdc)] has an orthorhombic large pore structure from 500 to ∼175 K at which point it undergoes a subtle distortion to form a monoclinic large pore structure that remains stable to 11 K. The orthorhombic large pore form of [AlF(bdc)] exhibits negative thermal expansion from 175–500 K. [Al(OH)(bdc)] has an orthorhombic large pore structure from 500 to 125 K at which point it undergoes a displacive phase transition, a breathing effect, to form a nonporous monoclinic structure. The orthorhombic large pore form of [Al(OH)(bdc)] exhibits positive thermal expansion from 150 to 500 K. The presence of a breathing effect in [Al(OH)(bdc)], and not [AlF(bdc)], is related to the additional contributions to attractive interactions across the shortest dimension of ...

High-pressure crystal chemistry of coesite-I and its transition to coesite-II

Abstract The high-pressure crystal chemistry of coesite was studied by means of single crystal X-ray diffraction in the pressure interval ∼2–34 GPa and at ambient temperature. We compressed the samples using diamond-anvil cells loaded with neon as pressure-transmitting medium and collected X-ray diffraction data using synchrotron radiation. The thermodynamically stable coesite – coesite-I – was observed up to ∼20 GPa, with the following unit-cell parameters: a = 6.6533(12) Å, b = 11.9018(10) Å, c = 6.9336(10) Å, β = 121.250(20)° and V = 469.38(15) Å3. The volume-pressure data of coesite-I are described by means of a third-order Birch-Murnhagan EoS with parameters V 0 = 547.26(66) Å3, K T0 = 96(4) GPa, K ′ T o ${K'_{To}}$ = 4.1(4). Above such pressure we witness the formation of a well crystallized coesite-II, previously observed only by spectroscopic studies. The structure of the novel high-P polymorph was determined and refined at ∼28 and ∼31 GPa with final R indices of 8% and 12%, respectively. Coesite-II has P21/n symmetry and a unit cell that is “doubled” along the b-axis with respect to that of the initial coesite-I: a = 6.5591(10) Å, b = 23.2276(14) Å, c = 6.7953(9) Å, β = 121.062(19)° and V = 886.84(19) Å3 at ∼28 GPa. All Si atoms are in tetrahedral coordination. The displacive phase transition I->II is likely driven by the extreme shortening (0.05 Å or 3.2%) of the shortest and the most compressible Si1-O1 bond, related to the stiff 180° Si1-O1-Si1 angle. Under compression the linear angle bends, resulting in two independent angles, one of which, however, retains almost linear geometry (∼178°). The requirement of this angle to be close to linear likely causes further Si-O compression down to an extremely short distance of ∼1.52 Å which prompts subsequent structural changes, with the formation of a triclinic phase at ∼31 GPa, coesite-III.

An unusual structural phase transition in Rb2HfF6

An unusual structural phase transition in the crystalline compound Rb2HfF6 near room temperature has been observed from perturbed angular correlation (PAC) spectroscopy. Our measurements in this compound produce two different crystalline configurations characterized by ωQ=74.1(1)Mrad/s, η~0, δ~0 and ωQ=24.7(2)Mrad/s, η=0.53(1), and δ=4(2)%. From PAC measurements in different samples, it is found that crystal structure corresponding to ωQ=74Mrad/s, η~0 transforms to the other quite arbitrarily with temperature and no definite temperature corresponding to this transition has been observed. This can possibly be attributed to displacive phase transition.

Phase transitions and the piezoelectricity around morphotropic phase boundary in Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 lead-free solid solution

In this paper, two displacive phase transitions around the morphotropic phase boundary (MPB) in Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) ceramics were detected by inspecting two anomalies of the Raman Ti4+-O2− longitudinal optical mode (∼725 cm−1). Further, permittivity and X-ray diffraction results demonstrated these two phase transitions originate from tetragonal (T) to rhombohedral (R) through an intermediate orthorhombic (O) phase. Importantly, we found that the maximum piezoelectric response (d33 = 545pC/N) was achieved at the boundary between the T and O phase, indicating that the giant piezoelectricity of BZT-xBCT may mainly stem from the T-O phase boundary due to easier polarization rotation and larger lattice softening.

Effect of elevated temperature on the properties of geopolymer synthesized from calcined ore-dressing tailing of bauxite and ground-granulated blast furnace slag

A geopolymer was prepared using calcined ore-dressing tailing of bauxite and ground-granulated blast furnace slag. To explore the effect on its properties of heating the geopolymer to 1200°C, the physical evolutions, including the appearance, length, compressive strength and porosity, and the chemical processes involving the decomposition and crystallization of specimens after heating were investigated. After exposure to elevated temperatures up to 1000°C, the studied geopolymer mortars exhibited the following changes. The linear length initially showed shrinkage, responding to free water loss and gel dehydration, and then, it exhibited expansion, resulting from a volume increase due to the displacive and reconstructive phase transition of quartz in the mortars. The compressive strength of the mortars decreased due to the increasing cracks and porosity arising from the dehydration, dehydroxylation and thermal mismatch between the contracting gels and expanding quartz. Additional conversion of the X-ray amorphous gels, which exhibit excellent thermal stability below 800°C, into such crystalline phases as calcium aluminum oxide and anorthite (CaAl2(SiO4)2) was found at 1000°C, resulting in a severe deterioration of strength and a reduction of approximately 40.0MPa. After exposure to 1200°C, a strength gain was observed as a result of the densification and self-healing caused by the viscous sintering and the formation of ceramic phases, such as anorthite (sodian, ordered, (Ca,Na)(Al,Si)2Si2O8).

A combined high-pressure diffraction and computational study on scandium MOFs

Previous high-pressure experiments have shown that pressure-transmitting fluids composed of small molecules can be forced inside the pores of metal organic framework materials, where they can cause phase transitions and amorphization and can even induce porosity in conventionally non-porous materials.1 Here we present a combined high-pressure diffraction and computational study of the structural response to methanol uptake at high pressure on a scandium terephthalate MOF (Sc2BDC3, BDC=1,4-benzenedicarboxylate)2 and its nitro-functionalized derivative (Sc2(NO2-BDC)3)3 and compare it to direct compression behaviour in a non-penetrative hydrostatic fluid, Fluorinert-77. In Fluorinert-77, Sc2BDC3 displays amorphization above 0.1 GPa, reversible upon pressure release, whereas Sc2(NO2-BDC)3 undergoes a phase transition (C2/c to Fdd2) to a denser but topologically-identical polymorph. In the presence of methanol, the reversible amorphization of Sc2BDC3 and the displacive phase transition of the nitro-form are completely inhibited (at least up to 3 GPa). Upon uptake of methanol on Sc2BDC3, the methanol molecules are found by diffraction to occupy two sites, with preferential relative filling of one site compared to the other: grand canonical Monte Carlo simulations support these experimental observations and molecular dynamics simulations reveal the likely orientations of the methanol molecules, which are controlled at least in part by H-bonding interactions between guests. As well as revealing the atomistic origin of the stabilization of these MOFs against non-penetrative hydrostatic fluids at high pressure this study demonstrates a novel high pressure approach to study adsorption within a porous framework as a function of increasing guest content, and so to determine the most energetically favourable adsorption sites.