First-order Structural Phase Transition Research Articles

Concentration- and temperature-induced phase transitions in NdAlO3−TbAlO3 and NdAlO3−DyAlO3 systems

Phase and structural behaviour in the NdAlO3–RAlO3 (R = Tb, Dy) systems has been studied in a wide concentration and temperature ranges. Two kinds of solid solutions Nd1−xTbxAlO3 and Nd1−xDyxAlO3 with rhombohedral (space group No 167 R3¯c) and orthorhombic (space group No 62 Pbnm) perovskite structure are formed in these systems. Morphotropic phase transition between two perovskite-like structures occurs at x = 0.1 for Nd1−xTbxAlO3 and at x = 0.05–0.1 for Nd1−xDyxAlO3. First-order structural phase transition Pbnm↔R3¯c has been detected in Nd0.8Tb0.2AlO3, N0.6Tb0.4AlO3, Nd0.8Dy0.2AlO3 and Nd0.7Dy0.3AlO3 samples at 540 K, 1174 K, 687 K and 900 K, respectively. Phase diagrams of the pseudo-binary systems NdAlO3−TbAlO3 and NdAlO3−DyAlO3 have been constructed.

Features of colloidal disperse structure formation in petroleum bitumen

Temperature-modulated differential scanning calorimetry has been employed to analyze the structure-related thermal properties of petroleum bitumen. This method enables one to distinguish between “order–disorder” and glass transitions, thereby making it possible to monitor and identify structure-related phase transformations, the signals from which are invisible or overlapped in the thermograms of conventional differential scanning calorimetry. Bitumen has been shown to be a colloidal disperse system only under certain temperature–time conditions. Its dispersed phase may be represented by aggregates of two types with colloidal sizes. Saturated hydrocarbons form a solid crystalline phase in accordance with the regularities of first-order structural phase transitions and nucleation mechanism of phase separation. Asphaltenes and resins form a solid amorphous phase for a relatively long time as a result of a structural relaxation glass transition by the spinodal mechanism of phase separation.

Synthesis and study of structural, thermodynamic, and magnetic properties of Na x Li1–x FeGe2O6 (x = 0.1–0.9) compounds

The properties of NaxLi1–xFeGe2O6 (x = 0.1–0.9) solid solutions obtained via a solid-phase synthesis have been measured by X-ray diffraction, calorimetry, and magnetic method. The order–disorder transformations in low-dimensional NaxLi1–xFeGe2O6 (x = 0.1–0.9) spin systems with predominately antiferromagnetic exchange interaction have been revealed in the low-temperature susceptibility dependences. The study of thermal and physical properties has confirmed that substituting the sodium ions with the lithium ones induces the first-order structural phase transitions of the displacement type which are characterized by a symmetry change in monoclinic crystals from high-temperature C2/c space group to low-temperature P21/c space group.

Pressure dependence of the structure and electronic properties ofSr3Ir2O7

We study the structural evolution of Sr$_3$Ir$_2$O$_7$ as a function of pressure using x-ray diffraction. At a pressure of 54 GPa at room temperature, we observe a first-order structural phase transition, associated with a change from tetragonal to monoclinic symmetry, and accompanied by a 4% volume collapse. Rietveld refinement of the high-pressure phase reveals a novel modification of the Ruddlesden-Popper structure, which adopts an altered stacking sequence of the perovskite bilayers. As the positions of the oxygen atoms could not be reliably refined from the data, we use density functional theory (local-density approximation+$U$+spin orbit) to optimize the crystal structure, and to elucidate the electronic and magnetic properties of Sr$_3$Ir$_2$O$_7$ at high pressure. In the low-pressure tetragonal phase, we find that the in-plane rotation of the IrO$_6$ octahedra increases with pressure. The calculations further indicate that a bandwidth-driven insulator-metal transition occurs at $\sim$20 GPa, along with a quenching of the magnetic moment. In the high-pressure monoclinic phase, structural optimization resulted in complex tilting and rotation of the oxygen octahedra, and strongly overlapping $t_{2g}$ and $e_g$ bands. The $t_{2g}$ bandwidth renders both the spin-orbit coupling and electronic correlations ineffectual in opening an electronic gap, resulting in a robust metallic state for the high-pressure phase of Sr$_3$Ir$_2$O$_7$.

High-resolution spectroscopy of HoFe3(BO3)4 crystal: a study of phase transitions

The transmission spectra of HoFe3(BO3) multiferroic single crystals are studied by optical Fourier-transform spectroscopy at temperatures of 1.7–423 K in polarized light in the spectral range 500–10 000 cm–1 with a resolution up to 0.1 cm–1. A new first-order structural phase transition close to the second-order transition is recorded at Tc = 360 K by the appearance of a new phonon mode at 976 cm–1. The reasons for considerable differences in Tc for different samples of holmium ferroborate are discussed. By temperature variations in the spectra of the f–f transitions in the Ho3+ ion, we studied two magnetic phase transitions, namely, magnetic ordering into an easy-plane structure as a second-order phase transition at TN = 39 K and spin reorientation from the ab plane to the c axis as a first-order phase transition at TSR = 4.7 ± 0.2 K. It is shown that erbium impurity in a concentration of 1 at % decreases the spin-reorientation transition temperature to TSR = 4.0 K.

Infrared study of lattice dynamics and spin-phonon and electron-phonon interactions in multiferroicTbFe3(BO3)4andGdFe3(BO3)4

We present a comparative far-infrared reflection spectroscopy study of phonons, phase transitions, spin-phonon and electron-phonon interactions in isostructural multiferroic iron borates of gadolinium and terbium. The behavior of phonon modes registered in a wide temperature range is consistent with a weak first-order structural phase transition (Ts = 143 for GdFe3(BO3)4 and 200 K for TbFe3(BO3)4) from high-symmetry high-temperature R32 structure into low-symmetry low-temperature P3121 one. The temperature dependences of frequencies, oscillator strengths, and damping constants of some low-frequency modes reveal an appreciable lattice anharmonicity. Peculiarities in the phonon mode behavior in both compounds at the temperature of an antiferromagnetic ordering (TN = 32 K for GdFe3(BO3)4 and 40 K for TbFe3(BO3)4) evidence the spin-phonon interaction. In the energy range of phonons, GdFe3(BO3)4 has no electronic levels but TbFe3(BO3)4 possesses several ones. We observe an onset of new bands in the excitation spectrum of TbFe3(BO3)4, due to a resonance interaction between a lattice phonon and 4f electronic crystal-field excitations of Tb3+. This interaction causes delocalization of the CF excitations, their Davydov splitting, and formation of coupled electron-phonon modes.

Reversible Phase Transition Triggered by Order–Disorder Transformation of Carboxyl Oxygen Atoms Coupled with Distinct Reorientations in [HN(C4H9)3](fumrate)0.5·(fumaric acid)0.5

A new reversible molecular phase transition material [HN(C4H9)3](fumrate)0.5·(fumaric acid)0.5 (1) has been successfully synthesized. Differential scanning calorimetry measurement shows a pair of reversible peaks at 181.9 and 178 K on heating and cooling modes, respectively. The large thermal hysteresis of ∼3.9 K discloses its reversible first-order structural phase transition. Specific heat capacity and dielectric constant measurements around Tc further confirm its phase transition behaviors. The detailed structural analyses of 1 at variable temperatures reveal that its structural phase transition is mainly accomplished by the order–disorder transformations of carboxyl oxygen atoms and distinct reorientations among cations and anion-acid infinite sheets together with proton-dynamic motion. All these results open a new way to construct potential phase transition materials through the selection of flexible aliphatic cations with simple carboxyl-based anions.

Temperature-triggered order–disorder phase transition in molecular-ionic material N-butyldiethanolammonium picrate monohydrate

We report an organic–ionic material that undergoes a first-order structural phase transition, induced by order–disorder of oxygen atoms in picrate anion. This strategy offers a potential pathway to explore new switchable dielectric materials.

Role of Temperature and Pressure on the Multisensitive Multiferroic Dicyanamide Framework [TPrA][Mn(dca)3] with Perovskite-like Structure.

A multistimuli response to temperature and pressure is found in the hybrid inorganic-organic perovskite-like [TPrA][Mn(dca)3] compound, which is related to a first-order structural phase transition near room temperature, Tt ≈ 330 K. This phase transition involves a transformation from room temperature polymorph I, with the noncentrosymmetric space group P4̅21c, to the high temperature polymorph II, with the centrosymmetric space group I4/mcm, and it implies ionic displacements, order-disorder phenomena, and a large and anisotropic thermal expansion (specially along the c-axis). As a consequence, [TPrA][Mn(dca)3] exhibits a dielectric anomaly, associated with the change from a cooperative to a noncooperative electric behavior (antiferroelectric (AFE)-paraelectric (PE) transition). The former implies an AFE distribution of electric dipoles in polymorph I, related to the described off-shift of the apolar TPrA cations and the order-disorder of the polar dca ligands mechanisms, that are different from those reported, up to now, for others perovskite-type hybrid compounds. Such cooperative electric order, below Tt ≈ 330 K, coexisting with long-range antiferromagnetic ordering below T = 2.1 K render the [TPrA][Mn(dca)3] a new type-I multiferroic material. In addition, the obtained experimental results reveal that this compound is also a multistimuli-responsive material, with a very large sensitivity toward temperature and applied external pressure, δTt/δP ≈ 24 K kbar(-1), even for small values of pressure (P < 2 kbar). Therefore, this material opens up a potential interest for future technological applications, such as temperature/pressure sensing.

Rapid and robust spatiotemporal dynamics of the first-order phase transition in crystals of the organic-inorganic perovskite (C12H25NH3)2PbI4.

The dynamics of the thermally induced first-order structural phase transition in a high-quality single crystal of the organic-inorganic perovskite (C12H25NH3)2PbI4 was investigated by optical microscopy. The propagation of the straight phase front (habit plane) during the phase transition along the cooling and heating pathways of the thermal hysteresis was observed. The thermochromic character of the transition allowed monitoring of the thermal dependence of average optical density and aided the visualization of the interface propagation. The thermal hysteresis loop is 10 K wide, and the interface velocity is constant at V ≈ 1.6 mm s–1. The transition is accompanied with sizeable change in crystal size, with elongation of ~6% along the b axis and compression of ~ –2% along the a axis, in excellent agreement with previously reported X-ray diffraction data. The progression of the habit plane is at least 160 times faster than in spin-crossover materials, and opens new prospects for organic-inorganic perovskites as solid switching materials. Moreover, the crystals of (C12H25NH3)2PbI4 are unusually mechanically robust and present excellent resilience to thermal cycling. These hitherto unrecognized properties turn this and possibly similar hybrid perovskites into perspective candidates as active medium for microscopic actuation.

Orthorhombic YBaCo4O8.4 crystals as a result of saturation of hexagonal YBaCo4O7 crystals with oxygen

Hexagonal YBaCo4O7 crystals (sp. gr. P63mc, ah = 6.3058(4) A, ch = 10.2442(7) A, Z = 2) are saturated with oxygen to the YBaCo4O8.4 composition and studied by X-ray diffraction (XRD) analysis. The saturation is completed by a structural first-order phase transition to orthorhombic crystals (sp. gr. Pbc21, ao = 31.8419(2) A, bo = 10.9239(5) A, co = 10.0960(5) A, Z = 20). The connection of two lattices is expressed in terms of the action of matrix (500/120/001) on the hexagonal basis. Five structural fragments of the same type but with different degrees of order alternate along the long axis of the oxygen-saturated orthorhombic structure. The XRD data on single crystals differ from the results obtained by other researchers on ceramic samples; possible causes of these differences are discussed.

Homogeneous and heterogeneous nucleations in the surface phase transition: Si(111)4 × 1-In

Homogeneous and heterogeneous nucleations in a reduced-dimensional system undergoing a first-order structural phase transition were examined by using low electron energy diffraction and scanning tunneling microscopy. The high-temperature 4 × 1 phase of a Si(111)-In surface was supercooled at temperatures below the transition temperature () and evolved slowly into a low-temperature 8 × 2 phase with time. The transition rate decreased significantly as the temperature approached . The kinetics of the observed homogeneous nucleation was analyzed by classical nucleation theory. The introduction of oxygen atoms reduced the hysteresis and accelerated nucleation significantly, showing that the -raising oxygen impurity plays the role of a nucleation seed for heterogeneous nucleation.

Phase transitions, band structures, elastic and lattice dynamic properties of CdSnV2 (V=P, As, Sb) under pressure from first principles

Ab initio self-consistent calculations have been performed to study the pressure-induced effects on the structural, electronic, elastic and lattice dynamic properties of CdSnV2 (V=P, As, Sb) semiconductors using norm-conserving pseudo-potentials and generalized-gradient approximation in the frame of density functional theory. The optimized structural parameters are well consistent with available experimental and theoretical data. Enthalpy calculations show that CdSnV2 semiconductors undergo a pressure-induced first-order structural phase transition from the chalcopyrite (CH) to the rocksalt (RS) phase. The evolutions of fundamental band-gap energies, single-crystalline elastic constants, polycrystalline elastic moduli, phonon dispersion spectra, IR absorption and Raman scattering spectra, as well as zone-center phonon frequencies with pressure have been investigated in detail. Our results show that the underlying physical origin of CH→RS phase transition might be in close relation with the unstable low-frequency phonon modes and the softening shear elastic modulus.

High-temperature superconductivity stabilized by electron-hole interband coupling in collapsed tetragonal phase ofKFe2As2under high pressure

We report a high-pressure study of simultaneous low-temperature electrical resistivity and Hall effect measurements on high quality single-crystalline KFe2As2 using designer diamond anvil cell techniques with applied pressures up to 33 GPa. In the low pressure regime, we show that the superconducting transition temperature T_c finds a maximum onset value of 7 K near 2 GPa, in contrast to previous reports that find a minimum T_c and reversal of pressure dependence at this pressure. Upon applying higher pressures, this T_c is diminished until a sudden drastic enhancement occurs coincident with a first-order structural phase transition into a collapsed tetragonal phase. The appearance of a distinct superconducting phase above 13 GPa is also accompanied by a sudden reversal of dominant charge carrier sign, from hole- to electron-like, which agrees with our band calculations predicting the emergence of an electron pocket and diminishment of hole pockets upon Fermi surface reconstruction. Our results suggest the high-temperature superconducting phase in KFe2As2 is substantially enhanced by the presence of nested electron and hole pockets, providing the key ingredient of high-T_c superconductivity in iron pnictide superconductors.

Pressure-induced conductivity and yellow-to-black piezochromism in a layered Cu-Cl hybrid perovskite.

Pressure-induced changes in the electronic structure of two-dimensional Cu-based materials have been a subject of intense study. In particular, the possibility of suppressing the Jahn-Teller distortion of d(9) Cu centers with applied pressure has been debated over a number of decades. We studied the structural and electronic changes resulting from the application of pressures up to ca. 60 GPa on a two-dimensional copper(II)-chloride perovskite using diamond anvil cells (DACs), through a combination of in situ powder X-ray diffraction, electronic absorption and vibrational spectroscopy, dc resistivity measurements, and optical observations. Our measurements show that compression of this charge-transfer insulator initially yields a first-order structural phase transition at ca. 4 GPa similar to previous reports on other Cu(II)-Cl perovskites, during which the originally translucent yellow solid turns red. Further compression induces a previously unreported phase transition at ca. 8 GPa and dramatic piezochromism from translucent red-orange to opaque black. Two-probe dc resistivity measurements conducted within the DAC show the first instance of appreciable conductivity in Cu(II)-Cl perovskites. The conductivity increases by 5 orders of magnitude between 7 and 50 GPa, with a maximum measured conductivity of 2.9 × 10(-4) S·cm(-1) at 51.4 GPa. Electronic absorption spectroscopy and variable-temperature conductivity measurements indicate that the perovskite behaves as a 1.0 eV band-gap semiconductor at 39.7 GPa and has an activation energy for electronic conduction of 0.232(1) eV at 40.2 GPa. Remarkably, all these changes are reversible: the material reverts to a translucent yellow solid upon decompression, and ambient pressure powder X-ray diffraction data taken before and after compression up to 60 GPa show that the original structure is maintained with minimal hysteresis.

Brownmillerite Ca2Co2O5: Synthesis, Stability, and Re-entrant Single Crystal to Single Crystal Structural Transitions

Ca2Co2O5 in the brownmillerite form was synthesized using a high-pressure optical-image floating zone furnace, and single crystals with dimensions up to 1.4x0.8x0.5 mm3 were obtained. At room temperature, Ca2Co2O5 crystallizes as a fully ordered brownmillerite variant in the orthorhombic space group Pcmb (No. 57) with unit cell parameters a=5.28960(10) {\AA}, b=14.9240(2) {\AA}, and c=10.9547(2) {\AA}. With decreasing temperature, it undergoes a re-entrant sequence of first-order structural phase transitions (Pcmb to P2/c11 to P121/m1 to Pcmb) that is unprecedented among brownmillerites, broadening the family of space groups available to these materials and challenging current approaches for sorting the myriad variants of brownmillerite structures. Magnetic susceptibility data indicate antiferromagnetic ordering in Ca2Co2O5 occurs near 240 K, corroborated by neutron powder diffraction. Below 140 K, the specimen exhibits a weak ferromagnetic component directed primarily along the b axis that shows a pronounced thermal and magnetic history dependence.

Magnetic field driven quantum critical phase transition in CeAl

The resistivity and heat capacity of CeAl, a well-known Kondo system, is probed under high magnetic fields of up to 14 T. The system is known to undergo a first-order structural phase transition from hexagonal to monoclinic phase at 110 K. The current study has revealed that this structural transition is a robust one under high magnetic fields. However, clear dispersion in resistivity and heat capacity are seen in the presence of high magnetic fields below 30 K, whose analysis suggest its nearness to quantum critical point (QCP). Systematic vanishing of Kondo ordering ( behaviour) and the appearance of non-Fermi liquid behaviour (+ behaviour) around 6–9 T region at low temperatures suggest that the system settles for a QCP. In line with the resistivity, the heat capacity also revealed signatures reminiscent of a non-Fermi liquid. The first-time confirmation about the existence of a QCP in stoichiometric CeAl makes this already-known compound more interesting because it could lead to the discovery of possible new ground states.

Temperature dependent nanoscale atomic correlations in Ir1−xPtxTe2 (x = 0.0, 0.03 and 0.04) system

X-ray absorption near-edge structure (XANES) spectroscopy has been used to investigate the unoccupied electronic states and local geometry of Ir1−xPtxTe2(x = 0.0, 0.03 and 0.04) as a function of temperature. The Ir L3-edge absorption white line, as well as high energy XANES features due to the photoelectron multiple scatterings with near neighbours, reveal clear changes in the unoccupied 5d-electronic states and the local geometry with Pt substitution. We find an anomalous spectral weight transfer across the known first-order structural phase transition from the trigonal to monoclinic phase in IrTe2, which characterizes the reduced atomic structure symmetry below the transition temperature. No such changes with temperature are seen in the Pt substituted superconducting samples. In addition, a gradual increase of the spectral weight transfer is observed in IrTe2 with a further decrease in temperature below the transition, indicating that the low temperature phase is likely to have a symmetry lower than the monoclinic one. The results suggest that the interplay between inter-layer and intra-layer atomic correlations should have a significant role in the properties of an Ir1−xPtxTe2 system.

Metal-insulator transition upon heating and negative-differential-resistive-switching induced by self-heating in BaCo0.9Ni0.1S1.8

The layered compound BaCo1−xNixS2−y (0.05 < x < 0.2 and 0.05 < y < 0.2) exhibits an unusual first-order structural and electronic phase transition from a low-T monoclinic paramagnetic metal to a high-T tetragonal antiferromagnetic insulator around 200 K with huge hysteresis (∼40 K) and large volume change (∼0.01). Here, we report on unusual voltage-controlled resistive switching followed by current-controlled resistive switching induced by self-heating in polycrystalline BaCo1−xNixS2−y (nominal x = 0.1 and y = 0.2). These were due to the steep metal to insulator transition upon heating followed by the activated behavior of the resistivity above the transition. The major role of Joule heating in switching is supported by the absence of nonlinearity in the current as function of voltage, I(V), obtained in pulsed measurements, in the range of electric fields relevant to d.c. measurements. The voltage-controlled negative differential resistance around the threshold for switching was explained by a simple model of self-heating. The main difficulty in modeling I(V) from the samples resistance as function of temperature R(T) was the progressive increase of R(T), and to a lesser extend the decrease of the resistance jumps at the transitions, caused by the damage induced by cycling through the transitions by heating or self-heating. This was dealt with by following systematically R(T) over many cycles and by using the data of R(T) in the heating cycle closest to that of the self-heating one.

Estimation of entropy change at the first order martensitic transition in Ni–Mn–X based ferromagnetic shape memory alloys

Change in entropy (ΔS) has been estimated at the first order martensitic transition in selected Ni–Mn–X (Ga, Sn, and In) based ferromagnetic shape memory alloys (FSMAs). During the first-order martensitic structural phase transition, discontinuity in entropy leads to a large ΔS. ΔS can be estimated from differential scanning calorimeter curves. Magnetic characterization reveals that the magnetic contribution ∆SM, is dominant contributor to ΔS. ∆SM is also responsible for the giant magnetocaloric effect observed in these FSMAs. By controlling the valence electron to atom ratio (e/a), it is possible to realize a large entropy change near ambient temperature in these alloys. Estimation of ∆S and ∆SM in a variety of promising FSMAs provides a means to identify alloys suitable for applications such as magnetic sensors, actuators and magnetic refrigeration.