Mechanism Of Structural Phase Transition Research Articles

Decoupled ultrafast electronic and structural phase transitions in photoexcited monoclinic VO2.

Photoexcitation has emerged as an efficient way to trigger phase transitions in strongly correlated materials. There are great controversies about the atomistic mechanisms of structural phase transitions (SPTs) from monoclinic (M1-) to rutile (R-) VO2 and its association with electronic insulator-metal transitions (IMTs). Here, we illustrate the underlying atomistic processes and decoupling nature of photoinduced SPT and IMT in nonequilibrium states. The photoinduced SPT proceeds in the order of dilation of V-V pairs and increase of twisting angles after a small delay of ~40 fs. Dynamic simulations with hybrid functionals confirm the existence of isostructural IMT. The photoinduced SPT and IMT exhibit the same thresholds of electronic excitations, indicating similar fluence thresholds in experiments. The IMT is quasi-instantaneously (<10 fs) generated, while the SPT takes place with time a constant of 100 to 300 fs. These findings clarify some key controversies in the literature and provide insights into nonequilibrium phase transitions in correlated materials.

The construction of a two-dimensional organic-inorganic hybrid double perovskite ferroelastic with a high T c and narrow band gap.

Two-dimensional (2D) hybrid double perovskites have attracted extensive research interest for their fascinating physical properties, such as ferroelectricity, X-ray detection, light response and so on. In addition, ferroelastics, as an important branch of ferroic materials, exhibits wide prospects in mechanical switches, shape memory and templating electronic nanostructures. Here, we designed a 2D phase-transition double perovskite ferroelastic through a structurally progressive strategy. This evolution is core to our construction process from 0D to 1D and AgBi-based 2D. In this way, we successfully synthesized 2D lead-free ferroelastic (DPA)4AgBiBr8 (DPA = 2,2-dimethylpropan-1-aminium) with a high Curie temperature (Tc), which shows a narrower band gap than 0D (DPA)4Bi2Br10 and 1D (DPA)5Pb2Br9. Moreover, the mechanism of structural phase transition and molecular motion are fully characterized by temperature dependent solid-state NMR and single crystal XRD. (DPA)4AgBiBr8 injects power into the discovery of new ferroelastics or the construction and dimensional adjustment in new hybrid double perovskites.

Theoretical insights into the crystal and fundamental properties of MgAl9Zn compound: First-Principles calculations

Mg-Al-Zn series Alloys are a very significant class of magnesium alloys and play a vital role in future industrial and economic development due to its outstanding physical properties, such as excellent machinability, fine corrosion resistance, low cost-effectiveness, and so on. However, the structural phase transition mechanism and the evolution of electronic, mechanical and thermal properties under high pressure remain poorly known. Here, we have searched a series of crystal structures of MgAl9Zn alloy compound at pressure range from 0 GPa to 200 GPa by CALYPSO method. Our energetic and dynamic calculations suggest that the ground state structure of MgAl9Zn under ambient pressure is crystallized in the hexagonal phase with space group of P63 symmetry. As the pressure increased up to 54 GPa, a phase transition occurred, i.e., the P63 phase transforms to R3c phase. The electronic band structure results indicated that both P63 and R3c phases are metallic. The mechanical and thermal results show that, compared with P63 phase, the R3c phase possesses better strength and stronger abilities to absorb or release heats. The resent findings pave a way for future design of advanced alloyed materials.

Energy-driven multi-step structural phase transition mechanism to achieve high-quality p-type nitrogen-doped β-Ga2O3 films

It is a big challenge to grow β-Ga2O3 films of good p-type conductivity and stability due to the large acceptor ionization energy, strong hole-trapping effect, low hole mobility, and self-compensation effects. In this study, we demonstrate for the first time the success in the growth of high-quality p-type N-doped β-Ga2O3 films with an acceptor ionization energy of 0.165 eV, Hall resistivity of 17.0 Ωcm, Hall hole mobility of 23.6 cm2V−1s−1, hole concentration of 1.56 × 1016 cm−3 and good stability, and in the fabrication of high-performance p-type β-Ga2O3 films-based field effect transistors. The success paves the way to fabricate full β-Ga2O3 films based p-n diodes and will advance the applications of next-generation high-performance β-Ga2O3-based optoelectronic and electronic devices. In addition, we discover the growth mechanism of multi-step structural phase transitions from hexagonal P63mc GaN to rhombohedral R3¯c α-GaNxO3(1-x)/2 and finally to monolithic C2/m N-doped β-Ga2O3, which improves the crystalline quality, facilitates the acceptor doping, increases the acceptor activation efficiency, and thus enhances the p-type conductivity and stability of the N-doped β-Ga2O3 films. The novel p-type oxide film growth technique and mechanism open a new horizon to the other wide bandgap oxide semiconductors.

State of charge dependent ordered and disordered phases in a Li[Ni1/3Co1/3Mn1/3]O2cathode material

Suppression of Ni pop-up-initiated phase transition in an NCM 111 cathode by Zr-doping based on the understanding of the structural phase transition mechanism in the NCM structure.

Phonon properties and mechanism of order-disorder phase transition in formamidinium manganese hypophosphite single crystal.

The detailed temperature-dependent IR and Raman spectra were used to study and understand the mechanism of structural phase transition occurring at 175K in manganese hypophosphite templated with formamidinium (FA+) ions, [FA]Mn(H2POO)3, which adopts a perovskite-like architecture. The structural transformation between the C2/c and the P21/c monoclinic phases has a complicated nature and is mainly driven by re-orientational motions of the FA+ cations but it is also accompanied by a significant distortion of the MnO6 octahedral units as well as steric-forced changes of the PH2 groups determining the off-center shifts of FA+ cations in the cages. The re-orientational motions of formamidinium cations at 175K are followed by slight changes of their geometry and re-arrangement of hydrogen bonds (HBs). The strong temperature-dependences of bands corresponding to vibrations involving hydrogen bonding reveal the highly-dynamic character of this phase transition and strong nature of created HBs. The most pronounced changes are observed for the modes corresponding to the formamidinium cation, proving that the phase transition has an order-disorder character.

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 ...

Kinetics of the γ-δ phase transition in energetic nitramine-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine.

The solid, secondary explosive nitramine-octahydro-1,3,5,7-tetranitro-1,3,5,7 or HMX has four different stable polymorphs which have different molecular conformations, crystalline structures, and densities, making structural phase transitions between these nontrivial. Previous studies of the kinetics of the β-δ HMX structural transition found this to happen by a nucleation and growth mechanism, where growth was governed by the heat of fusion, or melting, even though the phase transition temperature is more than 100 K below the melting point. A theory known as virtual melting could easily justify this since the large volume difference in the two phases creates a strain at their interface that can lower the melting point to the phase transition temperature through a relaxation of the elastic energy. To learn more about structural phase transitions in organic crystalline solids and virtual melting, here we use time-resolved X-ray diffraction to study another structural phase transition in HMX, γ-δ. Again, second order kinetics are observed which fit to the same nucleation and growth model associated with growth by melting even though the volume change in this transition is too small to lower the melting point by interfacial strain. To account for this, we present a more general model illustrating that melting over a very thin layer at the interface between the two phases reduces the total interfacial energy and is therefore thermodynamically favorable and can drive the structural phase transition in the absence of large volume changes. Our work supports the idea that virtual melting may be a more generally applicable mechanism for structural phase transitions in organic crystalline solids.

Probing the pathway of an ultrafast structural phase transition to illuminate the transition mechanism in Cu2S

Disentangling the primary order parameter from secondary order parameters in phase transitions is critical to the interpretation of transition mechanisms in strongly correlated systems and quantum materials. Here, we present a study of structural phase transition pathways in superionic Cu2S nanocrystals that exhibit intriguing properties. Utilizing ultrafast electron diffraction techniques sensitive to both the momentum-space and the time-domain, we distinguish the dynamics of crystal symmetry breaking and lattice expansion in this system. We are able to follow the transient states along the transition pathway, and so observe the dynamics of both the primary and secondary order parameters. Based on these observations, we argue that the mechanism of structural phase transition in Cu2S is dominated by the electron-phonon coupling. This mechanism advances the understanding from previous results, where the focus was solely on dynamic observations of the lattice expansion.

Phase Transition Mechanism of Poly(l-lactic acid) among the α, δ, and β Forms on the Basis of the Reinvestigated Crystal Structure of the β Form

The crystal structure of poly(l-lactic acid) β form has been reinvestigated on the basis of the 2-dimensional X-ray diffraction diagram measured for the sample obtained by stretching the highly crystalline α form at a high temperature. The six helical chains of (3/1) conformation are packed in a rectangular unit cell of the space group P1 with the complicated but systematic packing mode of the upward and downward chains. This structural model is different from the previously reported trigonal model [Puiggali Polymer 2000, 41, 8921]. The structural phase transition mechanism from the α form to the β form via the δ form has been proposed by assuming the cooperative displacements of the upward and downward helical chains as well as the conformational change. To support this mechanism, the two types of experiments were performed: (i) The highly oriented regular α form was stretched at about 165 °C to the various drawing ratios and cooled to the room temperature with the sample length fixed constantly. The X-ray diffraction data of these samples revealed the transition from the α to the mechanically deformd α (αd), to the δ form, and then to the β form depending on the drawing ratio. (ii) The α sample was suspended vertically with a constant load in the heating process, and the X-ray diffraction pattern was measured as a function of time (temperature). The original α form was found to melt at about 200 °C via the transition to the αd form and then to the δ form, followed by the recrystallization into the highly oriented β form. From these two experiments, the tension-induced structural transition from the α to the β form was found to occur via the disordering of the α form to the αd and to δ form under tension. The structural change process was derived on the basis of all of the knowledge collected from the X-ray structure analysis and the ex-situ and in-situ X-ray diffraction measurements: the molecular chains experience the cooperative translational slippage along the diagonal directions in the ab-plane to give the complicatedly mixed packing of the upward and downward chains to give the characteristic structure of the β form. At the same time, the chain conformational changes from the regular (10/3) to the disordered (10/3) and then to the (3/1) form during the disordering in the chain packing mode.

Roger Arthur Cowley. 24 February 1939 — 27 January 2015

Roger Cowley was one of the leading solid-state physicists of his generation. He was a highly versatile scientist who made important contributions to the understanding of the motion of atoms in solids and liquids, of the mechanisms of structural phase transitions, and of a range of magnetic phenomena, especially in systems with quenched disorder. Adept at both experiment and theory, he had the rare gift of being able to see through layers of complexity that often cloud real-world materials and capture the essence of their behaviour in simple models. His style was instinctively collaborative so he often worked together with some of the other leading people in his field, both nationally and internationally. His collaborative style also meant that he was frequently called on to serve in administrative roles. He was head of department at the University of Edinburgh for seven years and Chairman of Physics at Oxford for eight years. He played a large role in the administration of Oxford as a whole. Many of his graduate students went on to distinguished careers of their own, both within Britain and abroad.

Crystal structure and phase transitions of a layered perovskite-like CsScF4crystal

This work is devoted to the complex research on temperature phase transitions in a CsScF4 crystal. The crystal structure was solved and refined at different temperatures by using the Rietveld method. Structural phase transitions were investigated by using the following spectroscopic methods, some of them for the first time: Brillouin spectroscopy, Raman spectroscopy, IR absorption spectroscopy and NMR. The symmetry analysis of the Brillouin zone center of all phases is presented. The vibrational spectra of the crystal in three phases have been calculated. The structural phase transition mechanism was determined. The transitions at T1 = 475 K and T2 = 317.5 K are of displacement type. The Raman soft modes have been associated with rotations of the ScF6 octahedral group.

Experimental and theoretical methods to study structural phase transition mechanisms in K3WO3F3 oxyfluoride

The results of structural phase transitions mechanisms study in K3WO3F3oxyfluoride are represented by different experimental and theoretical methods. The structural phase transition anomalies at T1=452K and T2=414K of Raman and IR spectra have been analyzed. Using vibrational spectroscopy methods, the NMR-experiment has been done to clarify the nature of found phase transitions: displacive types or order-disorder types. The model of “disordered” crystal was proposed, and the results of lattice dynamics calculation in frameworks of the generalized Gordon–Kim method of ordered (R3) and “disordered” crystals were compared. The high pressure phases were studied by the Raman technique too.

Phonon dynamics and inelastic neutron scattering of sodium niobate

Sodium niobate (NaNbO3) exhibits most complex sequence of structural phase transitions in perovskite family and therefore provides as excellent model system for understanding the mechanism of structural phase transitions. We report temperature dependence of inelastic neutron scattering measurements of phonon densities of states in sodium niobate. The measurements are carried out in various crystallographic phases of this material at various temperatures from 300 K to 1048 K. The phonon spectra exhibit peaks centered around 19, 37, 51, 70 and 105 meV. Interestingly, the peak around 70 meV shifts significantly towards lower energy with increasing temperature, while the other peaks do not exhibit an appreciable change. The phonon spectra at 783 K show prominent change and become more diffusive as compared to those at 303 K. In order to better analyze these features, we have performed first principles lattice dynamics calculations based on the density functional theory. The computed phonon density of states is found to be in good agreement with the experimental data. Based on our calculation we are able to assign the characteristic Raman modes in the antiferroelectric phase to the A1g symmetry, which are due to the folding of the T (w=95 cm-1) and delta(w=129 cm-1) points of the cubic Brillouin zone.

The 7th European Conference on Mineralogy and Spectroscopy

The rapid advancement in technology offers new methods for performing investigations in the broad field of mineralogy. New spectroscopic techniques open frontiers in both basic science and in applied research. These range from a basic understanding of the mechanisms of structural phase transitions to the compositional effect on thermodynamic and transport properties of crystalline and amorphous natural solids, to the development of protocols for the remote-sensing detection of the presence (even in small amounts) of dangerous contaminants or of precious ores, to the explanation of unusual materials behaviour thus solving century-long debates, or to unravelling the complex history of famous gemstones. All of this incredible technical development is accompanied by more and more effective computational techniques. These are crucial for interpreting complicated experimental results at the microscopic level, and are often the only means to explore minerals and materials behaviour at conditions of the deep Earth, barely accessible or even inaccessible to …

Electronic structures and structural phase transition mechanism of from first-principles calculations

Abstract We investigate the electronic structures of the cubic and tetragonal phases of Sr 2 MgWO 6 . Calculated electronic structures show that the two phases are both wide indirect band gap semiconductors. Our phonon calculations indicate that the structural phase transition from cubic to tetragonal phase is because the phonon frequencies of Γ 4 + modes become imaginary. Our variable cell-shape relaxation results imply that a symmetry-allowed intermediate trigonal phase could be realized.

Organic-inorganic compounds with strong nonlinear optical properties based on 2,4,6-trimethylpyridinium and tetrahedral BF4−networks

A different organic-inorganic crystal---[2,4,6-trimethylpyridinium][BF${}_{4}$]---of nonlinear optical properties at room temperature was synthesized and characterized. The compound is built up of the organic [2,4,6-trimethylpyridinium] cations incorporated into inorganic, tetrahedral BF${}_{4}$ anions. It crystallizes at room temperature in the polar space group $\mathit{Pmn}{2}_{1}$, and undergoes three first-order phase transitions at [cooling (heating)] 241 (245) K, 297 (328) K, and 389 (406) K. The lowest temperature ferroic phase transition (ferroelastic; $\mathit{mm}2\ensuremath{\rightarrow}m$ type) is related to the significant pyroelectric effect. The compound was studied by single-crystal x-ray diffraction at several temperatures, using thermal (differential scanning calorimetry and thermogravimetric analysis) methods and dielectric spectroscopy. The piezoelectric, pyroelectric, and second-harmonic generation (SHG) properties were determined. Density-functional theory calculations in two stable phases are given. The [2,4,6-trimethylpyridinium][BF${}_{4}$] crystal exhibits a SHG efficiency of 1.7 times that of KDP. The mechanism of structural phase transitions in the title compound is discussed.

Crystallography and mechanisms of structural phase transitions: the use of symmetry-adapted modes

Crystallography and mechanisms of structural phase transitions: the use of symmetry-adapted modes

Pressure- and temperature-induced structural phase transition mechanisms of nitrogen

Pressure- and temperature-induced structural phase transition mechanisms of nitrogen

Theoretical description of pressure- and temperature-induced structural phase transition mechanisms of nitrogen

A theoretical description is proposed for the pressure- and temperature-induced structural transitions of nitrogen. Three regions of the phase diagram are distinguished corresponding to different types of transition mechanisms and parent structures. Combined ordering and displacive reconstructive transitions are found in the lowest-pressure region, from the parent disordered $\ensuremath{\beta}$ structure to the ordered $\ensuremath{\alpha}$ and $\ensuremath{\gamma}$ structures. In a second region, which extends from about 2 to 140 GPa at room temperature, group-subgroup related structures occur, which realize ferroelastic or ferroelectric distortions of the $\ensuremath{\delta}$ parent structure. Space-group symmetries, consistent with the assumed structural mechanisms, are proposed for the ${\ensuremath{\delta}}_{\text{loc}}$, $\ensuremath{\zeta}$, $\ensuremath{\kappa}$, $\ensuremath{\iota}$, and $\ensuremath{\theta}$ molecular phases. Above 140 GPa, in the molecular dissociation region, the local structure of the $\ensuremath{\eta}$ phase is assumed to realize a link between the molecular $\ensuremath{\zeta}$ and $\ensuremath{\kappa}$ phases and the high-temperature cubic-gauche (cg)-N phase. An analogy is pointed out between the amorphous $\ensuremath{\eta}$ to superhard cg-N transition and the mixed vortex state to superconducting transition of type II superconductors.