Order-disorder Structural Phase Transition Research Articles

Investigation of the temperature behavior of the vibrational spectrum of the (CuSO4)(C2H8N2) · 2H2O organometallic crystal by Raman spectroscopy

In the paper, the temperature behavior of the vibrational spectrum of the ( CuS O 4 ) ( C 2 H 8 N 2 ) ⋅ 2 H 2 O crystal has been analyzed by Raman scattering. At a temperature of about 70 K the appearance of new lines was detected in the Raman spectra. The appearance of additional lines is associated with the fixation of the torsional vibration of the ethylenediamine chelate ring. Such a fixation testifies to the implementation of an order–disorder structural phase transition in these crystals. A model of the mechanism of the phase transition in the ( CuS O 4 ) ( C 2 H 8 N 2 ) ⋅ 2 H 2 O crystal involving the chelate ring of ethylenediamine is proposed.

Structural phase transition in a charge-transfer compound: tropylium hexafluoridoantimonate(V)-1,4-dimethylnaphthalene (1/1).

Molecular motion in crystals has attracted much attention for the development of stimuli-responsive materials. The most studied are molecules with few atoms or highly symmetrical molecules. To develop molecules with new motion characteristics, we synthesized a charge-transfer compound, namely, tropylium hexafluoridoantimonate(V)-1,4-dimethylnaphthalene (1/1), (C7H7)[SbF6]·C12H12, and studied its structural phase transition. In this compound, the tropylium cation and the 1,4-dimethylnaphthalene molecule have planar geometry, but the latter has low symmetry. They are stacked as a one-dimensional chain structure through π-π charge-transfer interactions. Weak intermolecular interactions and planar molecular geometry result in a large degree of freedom of in-plane motion. Upon heating, due to the in-plane rotation of the molecules, the compound undergoes an order-disorder structural phase transition (phase-transition temperature = 334 K). The space group of the room-temperature phase is P21/m and the space group of the high-temperature phase is P4/mmm. This phase transition is accompanied by significant dielectric anomalies. The current investigation shows that the structural features of the title compound can be used to construct functional materials with phase transitions, such as molecular ferroelectrics.

Hexamethylenetetramine-tridecanedioic acid (1/1): temperature-induced order-disorder structural phase transitions with phase co-existence.

The cocrystal hexamethylenetetramine-tridecanedioic acid (1/1) (HMT-C13), C6H12N4·C13H24O4, was investigated by single-crystal X-ray diffraction techniques at several temperatures during cooling and heating processes. Our results show the formation of two crystalline phases, separated by a large temperature phase co-existence between 290 and 340 K. Phase I, stable above 341 K, presents an orthorhombic structure described in the space group Bmmb, with one N4(CH2)6·C13H22O4 adduct in its asymmetric unit. Phase II, stable below 290 K, presents a monoclinic symmetry described by the space group P21/c, with two N4(CH2)6·C13H22O4 adducts in its asymmetric unit. The phase co-existence is observed both upon cooling and heating, and seems to be related to a complex domain-growth dynamic within the crystal.

Large dielectric switch effects induced by an order-disorder transformation in cyclopropylamine perchlorate crystals.

Solid-state crystals with two distinct dielectric states can be a physical practice in binary-based technologies. A large dielectric switch effect up to 103 caused by an order-disorder structural phase transition is found in cyclopropylamine perchlorate (CPA-ClO4) crystals at temperatures around 230 K (Tc) and 220 K (T'c). Large dielectric switch effects here can be compared to that of the famous ceramic oxide dielectrics. As far as we know, this is the highest dielectric switch effect in simple organic salt crystals and organic-metal compounds so far. If the phase transition temperature can be adjusted by molecular manipulation, one of the most promising candidates for technological applications may emerge in the future.

Stepwise Dielectric Switching Occurs in Two Photo-Responsive Complexes Possessing Two-Dimensional Structures.

Two organic-inorganic hybrid complexes, (CH3NH3)Na[Fe(CN)5NO]·H2O (1) and (CH3NH3)2[Fe(CN)5NO] (2), which exhibit stepwise dielectric switching as well as photo-induced structural transformation, are synthesized and examined. In these two compounds, the photo-responsive complex anions, [Fe(CN)5NO]2-, connected by Na+ through N-Na coordination bonds or CH3NH3+ through N···H-N hydrogen bonds, form two-dimensional structures. One organic cation, CH3NH3+, that resides in the intralaminar cavity and plays a role as a template, undergoes a temperature-controlled order-disorder structural phase transition. As the frozen-thawed state change of the polar organic cations modifies the polarizability of materials, stepwise dielectric switching is observed at the phase transition temperature. Furthermore, the photo-induced linkage isomerism of [Fe(CN)5NO]2- building block survives in the new compounds at the low-temperature range, which is verified by variable-temperature IR spectra after photo-irradiation. The coexistence of switchable dielectric properties and photo-induced structural variation suggests multiple optical-electric roles of the present materials.

Impact of the Copper-Induced Local Framework Deformation on the Mechanism of Structural Phase Transition in [(CH3)2NH2][Zn(HCOO)3] Hybrid Metal–Formate Perovskite

Dimethylammonium zinc formate [(CH3)2NH2][Zn(HCOO)3] (DMAZn) treated as a model dense metal–organic framework is a compound that has been intensively studied in the last decade. It undergoes a first-order order–disorder structural phase transition at T0 ∼ 167K, considered as the improper ferroelectric one. The complex mechanism of this phase transition is attributed to the ordering of the hydrogen bonds between (CH3)2NH2+ (DMA+) and the formate groups as well as distortion of the zinc–formate framework. Here, the analysis of the impact of zinc substitution by Cu doping on the order–disorder phase transition is presented. The Jahn–Teller effect of Cu2+ ions results in a local framework distortion, which consequently perturbs the structural dynamics. Based on the dielectric, thermal, structural, electron paramagnetic resonance, and IR spectroscopy measurements, it is shown that doping with Cu2+ ions leads to a decrease of the phase transition entropy and temperature as well as a change in the nature of the ...

Structural, phonon, magnetic and optical properties of novel perovskite-like frameworks of TriBuMe[M(dca)3] (TriBuMe = tributylmethylammonium; dca = dicyanamide; M = Mn2+, Fe2+, Co2+, Ni2+).

We report the synthesis, crystal structures, thermal, optical and phonon properties of four new metal dicyanamide frameworks templated using tributylmethylammonium cations (TriBuMe+). Our results show that these compounds crystallize in a three-dimensional perovskite-like monoclinic structure, space group P21/n, with two inequivalent TriBuMe+, one well-ordered and the second one showing distinct disorder. DSC and X-ray diffraction show that TriBuMeMn undergoes an order-disorder structural phase transition near 380 K associated with an increase in the symmetry to the orthorhombic Pnma. Phase transitions are also observed for Ni and Fe analogues. The associated change in the phase transition entropy is very large (up to 66.7 J kg-1 K-1 for TriBuMeNi), suggesting that these compounds might be promising barocaloric materials. Raman and IR data confirm the same monoclinic structures for all compounds and show that all changes on heating to 430-450 K are reversible after cooling the samples to room temperature. Magnetic studies indicate that all studied compounds are paramagnets at least down to 2 K. Optical studies indicate that TriBuMeMn has an energy band gap of 4.95 eV and exhibits orange emission under 420 nm excitation with the activation energy for thermal quenching of 0.0673 eV.

An optoelectronic duple bistable phosphate with ultrahigh thermal stability

A new inorganic CsCdPO4 shows quadratic nonlinear optical and dielectric duple bistability, resulting from a high-temperature order–disorder structural phase transition.

Order-disorder structural phase transition and magnetocaloric effect in organic-inorganic halide hybrid (C2H5NH3)2CoCl4

We report a detailed experimental and theoretical investigation of structural, optical, magnetic and magnetothermal properties of single crystals of a new organic-inorganic hybrid (C2H5NH3)2CoCl4. Grown by slow evaporation method at room temperature, the compound crystallizes in centrosymmetric orthorhombic structure (Pnma) which undergoes a reversible phase transition at 235/241K (cooling/heating) to noncentrosymmetric P212121 space group symmetry associated with order-disorder transformation of carbon atoms of the ammonium cations as well as molecular rearrangement. Electronic absorption spectra of the compound are typical of geometrically distorted [CoCl4]2- tetrahedra having spin-orbit coupling effect. The isolated nature of [CoCl4]2- tetrahedra in the crystal reflect in paramagnetic behaviour of the compound. Interestingly, field induced spin flipping behaviour is observed at low temperature. First principles density functional calculations reveal weak magnetic interaction among cobalt spins with ferromagnetic state being the ground state. The entropy change associated with the spin flipping has been experimentally estimated by magnetic and heat capacity measurements which has a maximum value of 16JKg−1K−1 at 2.5K under 7T magnetic field. To the best of our knowledge, this is the first report on magnetocaloric effect observed in an organic-inorganic halide compound. The estimated value is sizable and is comparable to that of well-known transition metal molecular cluster magnets Mn12 or Fe14. The overall findings promise to enlighten new routes to design and constitute multifunctional organic-inorganic halide materials.

Temperature-induced order-disorder structural phase transitions of two-dimensional isostructural hexamethylenetetramine co-crystals.

Hexamethylenetetramine-benzoic acid (1/2) (HBA) and hexamethylenetetramine-4-methylbenzoic acid (1/2) (HMBA) co-crystals undergo order-disorder structural phase transition from a low-temperature monoclinic crystal structure to a high-temperature orthorhombic crystal structure at the transition temperatures of 257.5 (5) K (Pn ↔ Fmm2) and 265.5 (5) K (P21/n ↔ Cmcm), respectively, using variable-temperature single-crystal X-ray diffraction analysis. The observed phase transitions were confirmed to be reversible first-order transitions as indicated by the sharp endothermic and exothermic peaks in the differential scanning calorimetry measurement. The three-molecule aggregate of HBA and HMBA consists of a hexamethylenetetramine molecule and two benzoic acid or two 4-methylbenzoic acid molecules, respectively. The acid molecules are ordered at the low-temperature phase and are equally disordered over two positions, which are related by a mirror symmetry, at the high-temperature phase. The two-dimensional supramolecular constructs common to both co-crystals are formed by three-molecule aggregates via weak intermolecular C-H...O and C-H...π interactions into molecular trilayers parallel to the ac plane with small XPac dissimilarity indices and parameters. The PIXEL interaction energies of all corresponding molecular contacts were calculated and the results are comparable between HBA and HMBA co-crystals, resulting in similar lattice energies and transition temperatures despite their two-dimensional isostructural relationship. The observed phase transitions of these two energetically similar co-crystals are triggered by similar mechanisms, i.e. the molecular rotator ordering and structural order-disorder transformation, which induced non-merohedral twinning with similar twin matrices in the low-temperature crystal form of both co-crystals.

Single Crystal Electron Paramagnetic Resonance of Dimethylammonium and Ammonium Hybrid Formate Frameworks: Influence of External Electric Field

We present a continuous wave electron paramagnetic resonance (EPR) study of a Mn2+ doped [(CH3)2NH2][Zn(HCOO)3] hybrid dense metal–organic framework (MOF) that exhibits an order–disorder structural phase transition at Tc = 163 K. The W-band EPR measurements of a powder sample are performed to verify the previously reported spin Hamiltonian parameters of the Mn2+ centers in the low-temperature phase. The temperature dependent single crystal X-band EPR experiments reveal that Mn2+ probe ions are susceptible to the phase transition, as the spectrum changes drastically at Tc. The angular dependent EPR spectra of Mn2+ centers are obtained by rotating the single crystal sample about three distinct directions. The simulation of the determined angular dependences reveals six MnO6 octahedra in the ordered phase that originate from a severe crystal twinning of the [(CH3)2NH2][Zn(HCOO)3] MOF. The possible ferroelectric origin of the crystalline twins is investigated by single crystal EPR measurements with an applied...

Suppressing FePt order-disorder structural phase transition temperature by Ag doping: First principles investigation for enhancing magnetic recording density

ABSTRACTIn this work, we used density functional theory to study the structures, formation energy and order–disorder structural transition temperature (To) of L10 FePt with Ag additive. The results show that an Ag atom substituting for a Fe site is more energetically preferable than that of substituting for a Pt, thus Ag atom should replace Fe site. We found that To is significantly suppressed by Ag impurity because of the increasing in configurational entropy, whereas the magnetism tends to decrease due to the absence of some Fe atom in doping processes. The decrease in To could open the opportunity to prepare FePt with lower temperature and small grain size for the high density magnetic recording media application.

Orientational ordering of guest induced structural phase transition coupled with switchable dielectric properties in a host–guest crystal: bis(thiourea) thiazolium chloride

A host–guest crystal: (thiazolium)[(thiourea)2Cl] undergoes order-disorder structural phase transition at 213.7 K, accompanied by distinct switchable dielectric responses.

Structural phase transition in perovskite metal-formate frameworks: a Potts-type model with dipolar interactions.

We propose a combined experimental and numerical study to describe an order-disorder structural phase transition in perovskite-based [(CH3)2NH2][M(HCOO)3] (M = Zn(2+), Mn(2+), Fe(2+), Co(2+) and Ni(2+)) dense metal-organic frameworks (MOFs). The three-fold degenerate orientation of the molecular (CH3)2NH2(+) (DMA(+)) cation implies a selection of the statistical three-state model of the Potts type. It is constructed on a simple cubic lattice where each lattice point can be occupied by a DMA(+) cation in one of the available states. In our model the main interaction is the nearest-neighbor Potts-type interaction, which effectively accounts for the H-bonding between DMA(+) cations and M(HCOO)3(-) cages. The model is modified by accounting for the dipolar interactions which are evaluated for the real monoclinic lattice using density functional theory. We employ the Monte Carlo method to numerically study the model. The calculations are supplemented with the experimental measurements of electric polarization. The obtained results indicate that the three-state Potts model correctly describes the phase transition order in these MOFs, while dipolar interactions are necessary to obtain better agreement with the experimental polarization. We show that in our model with substantial dipolar interactions the ground state changes from uniform to the layers with alternating polarization directions.

EPR Study of Structural Phase Transition in Manganese-Doped [(CH3)2NH2][Zn(HCOO)3] Metal–Organic Framework

We present an electron paramagnetic resonance (EPR) study of [(CH3)2NH2][Zn(HCOO)3] metal–organic framework (MOF) powder doped with a small amount of paramagnetic Mn2+ ions. Our EPR measurements indicate a successful incorporation of local Mn2+ probes into the structure allowing us to detect and investigate an order–disorder structural phase transition in the studied MOF. The temperature-dependent continuous wave (CW) X- and Q-band EPR measurements reveal a sudden change in the spectra at a phase transition temperature T0 = 163 K. Simulations were performed to determine the spin Hamiltonian parameters of the spectra which reflect the local symmetry of the Mn2+ probes in the disordered and ordered phases. The temperature dependence of the axial zero-field splitting parameter D demonstrates that the observed phase transition at T0 is discontinuous. Additionally, this dependence follows the prediction of the Landau theory. We also performed preliminary pulse EPR measurements which reveal a rather long phase ...

Evidence of local structural order and spin-lattice coupling in the frustrated pyrochloreY2Ru2O7

We present an extended x-ray absorption fine structure study of the pyrochlore ${\mathrm{Y}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ (8--298 K). We find evidence, on a local scale, of a significant magnetoelastic coupling at the N\'eel temperature ${T}_{\mathrm{N}}\ensuremath{\sim}77\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ pointed out by a huge Debye-Waller ${\ensuremath{\sigma}}^{2}$ factor deviation from a correlated temperature dependent Debye-like local order behavior plus a temperature independent static contribution. Moreover, we notice the occurrence of a potential local order-disorder structural phase transition at ${T}^{*}=150$ K. This anomalous behavior is consistent with the pyrochlore's predisposition towards structural disorder and with a strong spin-phonon correlation. Remarkably the low-temperature order competes with the tendency of magnetic frustration to induce a less symmetric local structure.

Order-disorder structural phase transition in the system of intercalated Ni atoms in the Ni0.5TiSe2 compound

Changes in the crystal structure of the Ni0.5TiSe2 intercalated compound have been thoroughly investigated using powder X-ray diffractometry in the temperature range from 93 to 700 K. It has been shown that upon heating to the temperature of about 400 K, this compound undergoes a phase transition from the monoclinic structure described by space group I12/m1 to the trigonal structure \((P\bar 3 m1)\), which is associated with the disorder in the subsystem of intercalated Ni atoms. The supercooling effect with the formation of the metastable state, which is characteristic of first-order phase transitions such as melting and crystallization, has been revealed in the region of the order-disorder phase transition, and the anomalous behavior of the resistivity has been observed.

Conformational polymorphism of a zinc complex with enalapril antihypertensive drug

The crystal structure of an enalapril-Zn complex was determined by X-ray diffraction at various temperatures. The complex consisted of the drug enalapril coordinated to a Zn2+ ion by the oxygen atom of the carboxylate group in the monodentate mode. The metal ion produced a tetrahedral geometry, indicating that the enalapril complexation with a Zn2+ ion in vitro is similar to that suggested in vivo. The crystal structure of the enalapril-Zn complex showed an irreversible order–disorder structural phase transition in the temperature range of 150–200K during the cooling process. A symmetry change from C222 at room temperature (Phase I) to P212121 at 150K (Phase II) was observed. Phase I presented pronounced occupational disorder, whereas Phase II was slightly disordered.

EPR study of methyl radical in van-der-Waals solids

High-resolution EPR spectra of CH3 radicals trapped in solid N2 matrix were obtained in a temperature range 7–28K. The analysis of the spectrum suggests that, in the whole temperature range, the radical performs both fast rotational motion around the C3-symmetry axis and libration motion about the C2-symmetry axes. A comparison study of the methyl libration motion in gas matrices of linear molecules, N2, CO, N2O and CO2, was carried out using a model of the infinite potential well with flat bottom. As a result, averaged angular deviations of the radical orientation and dimensions of the potential wells were estimated. We assessed contributions of both the classical motion and the quantum-mechanical correction to the radical reorientation. A possibility of utilizing the trapped CH3 radical as a spin probe to study order–disorder structural phase transition at the microscopic level was discussed. An empirical formula obtained earlier for the matrix shift of the isotropic hyperfine coupling (hfc) constant of CH3 in matrices of spherical particles was shown to hold true for matrices of linear molecules as well. The experimental results and their treatment made it possible to report an estimate of the hfc tensor anisotropy for the free methyl radical. This information is of special interest to the theory of atoms and molecules.

Quantum-chemical modeling of ferroelectric solids with hydrogen-bond networks of different dimensionalities and their deuterated analogs

The application of quantum chemistry methods in the microscopic theory of H/D-bonded ferroelectrics and antiferroelectrics is illustrated using a comparative analysis of low-temperature order-disorder structural phase transitions in zero-dimensional materials of the K3H(SO4)2 family, zero-dimensional crystals of 5-halo derivatives of 9-hydroxyphenalenone (5Hal-9HPO), and three-dimensional crystals of potassium dihydrogen phosphate (KDP) and potassium dideuterium phosphate (DKDP) as examples. In the framework of the Ising model with tunneling, it is demonstrated that (in agreement with experimental data) the transition to the low-temperature ordered phase in zero-dimensional materials is possible only in the case of their deuteration, whereas the quantum paraelectric behavior is characteristic of undeuterated samples. This behavior for KDP crystals is impossible due to the sharp increase in the Ising parameters as compared to zero-dimensional materials. The factors responsible for the increase in these parameters are considered.