First-order Structural Phase Transition Research Articles

Structural phase transition and optical properties of zero-dimensional cobalt bromide hybrid material

Zero-Dimensional (0D) metal halide materials have recently emerged as a class of potential optoelectronic materials because of the structural diversity and superior photophysical properties. Here we report a 0D cobalt bromide hybrid material, [Br(CH2)3NH3]2CoBr4 (BPA-CoBr4, BPA=Br(CH2)3NH3, 3-Bromopropylamine), in which the isolated metal halide tetrahedron (CoBr42-) is completely separated each other and surrounded by the organic ligand Br(CH2)3NH3+. Differential scanning calorimetry (DSC) measurements exhibit BPA-CoBr4 undergoes a first-order structural phase transition, as clearly visible to a pair of weak peaks at 340 K (heating) and 317 K (cooling). Single crystal X-ray diffractions were carried out at 293 K and 360 K to explain the phase transformation mechanism, indicating it is an isostructural order–disorder type. The crystal structures which both crystalize in a monoclinic P21/c crystal symmetry and the most distinct difference between room-temperature and high-temperature structures is the order–disorder transition of the 3-Bromopropylamine, which is the driving force of the phase transition. The UV–vis absorption spectra reveal that BPA-CoBr4 has a steep absorption edge at 280 nm corresponding to photo energy of 4.4 eV. The density functional theory (DFT) calculations were performed to study the electronic structure with narrower indirect bandgap 3.9 eV which is lower than the experimental values. PDOS plots show that the electronic states of Co-d and Br-p orbitals are mainly contributed for the VBM and CBM of BPA-CoBr4.

Emergence of the isotropic Kitaev honeycomb lattice α− RuCl3 and its magnetic properties

We present a comprehensive investigation of the crystal and magnetic structures of the van der Waals antiferromagnet α− RuCl3 using single crystal x-ray and neutron diffraction. The crystal structure at room temperature is a monoclinic ( C2/m ). However, with decreasing temperature, a remarkable first-order structural phase transition is observed, leading to the emergence of a rhombohedral ( R3ˉ ) structure characterized by three-fold rotational symmetry forming an isotropic honeycomb lattice. On further cooling, a zigzag-type antiferromagnetic order develops below TN=6∼6.6 K. The critical exponent of the magnetic order parameter was determined to be β=0.11(1) , which is close to the two-dimensional Ising model. Additionally, the angular dependence of the magnetic critical field of the zigzag antiferromagnetic order for the polarized ferromagnetic phase reveals a six-fold rotational symmetry within the ab− plane. These findingsreflect the symmetry associated with the Ising-like bond-dependent Kitaev spin interactions and underscore the universality of the Kitaev interaction-dominated antiferromagnetic system.

π Phase Interlayer Shift and Stacking Fault in the Kagome Superconductor CsV_{3}Sb_{5}.

The stacking degree of freedom is a crucial factor in tuning material properties and has been extensively investigated in layered materials. The kagome superconductor CsV_{3}Sb_{5} was recently discovered to exhibit a three-dimensional CDW phase below T_{CDW}∼94 K. Despite the thorough investigation of in-plane modulation, the out-of-plane modulation has remained ambiguous. Here, our polarization- and temperature-dependent Raman measurements reveal the breaking of C_{6} rotational symmetry and the presence of three distinct domains oriented at approximately 120° to each other. The observations demonstrate that the CDW phase can be naturally explained as a 2c staggered order phase with adjacent layers exhibiting a relative π phase shift. Further, we discover a first-order structural phase transition at approximately 65K and suggest that it is a stacking order-disorder phase transition due to stacking fault, supported by the thermal hysteresis behavior of a Cs-related phonon mode. Our findings highlight the significance of the stacking degree of freedom in CsV_{3}Sb_{5} and offer structural insights to comprehend the entanglement between superconductivity and CDW.

Study of the structure, structural transition, interface model, and magnetic moments of CrN grown on MgO(001) by molecular beam epitaxy

Structural phase transition is studied in high quality CrN thin films grown by molecular beam epitaxy on MgO(001) substrates. Cross-sectional transmission electron microscopy and x-ray diffraction reveal that the epitaxial relationship between CrN film and MgO substrate is [100]CrN/[100]MgO, [110]CrN/[110]MgO, and [001]CrN/[001]MgO. The films show tensile strain/compression at the CrN/MgO(001) interface, which relaxes gradually with the film growth. Temperature dependent x-ray diffraction measurements show a first-order structural phase transition. In addition to the experimental measurements, first-principles theoretical calculations have been carried out for finding a stable model for the CrN/MgO interface. These calculations determine two possible models for the interface, where a monolayer of chromium oxide is formed between the CrN and MgO layers.

Metallization and superconductivity with Tc > 12 K in transition metal dichalcogenide HfS2 under pressure

The application of high pressure not only drives the comprehension of the structure and triggers exotic electronic states in transition metal dichalcogenides, but also promotes the discovery of intriguing phenomena. Here, we show that HfS2 exhibits highly tunable electronic property under pressure with sequence of gradual narrowing of band-gap at <40 GPa, followed by pressure-induced metallization at above 40 GPa, and ultimately superconductivity starting at ∼115 GPa. Raman and x-ray diffraction experiments provide strong evidence for first-order structural phase transitions from a trigonal (P3‾m1) to an orthorhombic (Immm) at around 18 GPa and then to a tetragonal structure (I4/mmm) above 40 GPa. The disappearance of all the Raman modes supported the observed metallization at above 40 GPa. At the similar pressure, the carrier-type of the sample is found to transform from electron to hole according to the Hall coefficient. At a critical pressure of 115 GPa a superconducting state sets with a transition temperature (Tc) of 3 K. Further compression dramatically increases Tc up to a maximum value of 12.2 K at 173 GPa, setting a record of Tc for superconducting transition metal dichalcogenides with zero-resistance. The pressure-induced high Tc superconductivity is closely linked to structural reconstructions which trigger changes in electronic states near the Fermi surface. The efficient and remarkable manipulation on the transport properties of 1T-HfS2 not only shed lights on the broad perspective of layered materials but also provide crucial information towards their practical applications.

Optical spectroscopy and band structure calculations of the structural phase transition in the vanadium-based kagome metal ScV6Sn6

In condensed matter physics, materials with kagome lattice display a range of exotic quantum states, including charge density wave (CDW), superconductivity, and magnetism. Recently, the intermetallic kagome metal ${\mathrm{ScV}}_{6}{\mathrm{Sn}}_{6}$ was discovered to undergo a first-order structural phase transition with the formation of a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\ifmmode\times\else\texttimes\fi{}3$ CDW at around 92 K. The bulk electronic band properties are crucial to understanding the origin of the structural phase transition. Here, we conducted an optical spectroscopy study in combination with band structure calculations across the structural transition. Our findings showed abrupt changes in the optical reflectivity/conductivity spectra as a result of the structural transition, without any observable gap formation behavior. The optical measurements and band calculations actually reveal a sudden change of the band structure after transition. It is important to note that this phase transition is of the first-order type, which distinguishes it from conventional density-wave type condensations. Our results provide an insight into the origin of the structural phase transition in this new and unique kagome lattice intermetallic material.

In situ high‐temperature Raman scattering study of monoclinic Ag2Mo2O7 microrods

In this work, we present a temperature-dependent behavior of monoclinic silver dimolybdate (m-Ag2Mo2O7) microrods using in situ Raman scattering. The m-Ag2Mo2O7 microrods were obtained by the conventional hydrothermal method at 423 K for 24 h. The structural and morphological characterization of the sample has been done by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Temperature-dependent Raman scattering measurements were performed on m-Ag2Mo2O7 microrods, and the results show an irreversible first-order structural phase transition at 698 K-723 K and the melting process at 773 K. Changes in the Raman spectra confirm the phase transition from the P21/c monoclinic structure to the P-1 triclinic structure. No morphological changes were observed during the structural phase transition of the sample at 723 K. Time-dependent optical microscopy at 773 K showed the growth of nanowires on the Ag2Mo2O7 microrods in the triclinic structure.

First Principles Study of Structural Phase Transition in Ag2S under High Pressure

We have studied the structural behavior of Ag2S under pressure (P) using density functional theory (DFT). By comparing the enthalpy differences of the different structures under pressure it can be seen that at zero pressure, the lowest energy structure calculated for Ag2S was the same as that found experimentally at low temperatures, namely, P21/c monoclinic. While up to pressure above P = 30 GPa, Ag2S undergoes two different first-order structural phase transitions. The first one that observed at P = 9.3 GPa is from monoclinic P21/c (phase I) structure to orthorhombic (phase II) P212121 structure and the second transition at P = 13.8 GPa is computable to transition from orthorhombic P212121 to monoclinic (phase III) P21/c structure.

Microscopic mechanism of nanoscale shear bands in an energetic molecular crystal (α-RDX): A first-order structural phase transition

Nanoscale shear bands formed in many energetic molecular crystals upon shock compression [including 1,3,5-trinitro-$s$-triazine (RDX)] are considered as a defect-free mechanism for formation and growth of hot spots which control detonation initiation. Using classical molecular dynamics, we predict the formation of similar nanoscale shear bands in the \ensuremath{\alpha}-RDX crystal subjected to quasistatic isothermal uniaxial compression indicating a common mechanism of shear strain localization under both shock and quasistatic conditions. In the framework of the Ginzburg-Landau phenomenology coupled with the coarse-grained (CG) Helmholtz free energy of the crystal from first principles, we explore the thermodynamics of stress-induced lattice transformations under quasistatic uniaxial load. We show that the shear banding exhibits a critical behavior associated with a first-order structural phase transition with bands of localized twinning strain as transient microstructure. Analysis of the CG Helmholtz free energy suggests that the stress-induced core softening of the effective intermolecular interaction is a fundamental mechanism for a structural phase transition leading to the nanoscale shear bands.

Optical spectroscopy and ultrafast pump-probe study of the structural phase transition in 1T′−TaTe2

1T'-TaTe2 exhibits an intriguing first-order structural phase transition at around 170 K. Understanding the electronic structural properties is a crucial way to comprehend the origin of the structural phase transition. We performed a combined optical and ultrafast pump-probe study on the compound across the transition temperature. The phase transition leads to abrupt changes of both optical spectra and ultrafast electronic relaxation dynamics. The measurements revealed a sudden reconstruction of band structure. We elaborate that the phase transition is of the first order and can not be attributed to the conventional density-wave type instability. Our work is illuminating for understanding the origin of the structural phase transition.

Specificity of the Transformation of End Methyl Groups in the Interlamellar Regions of Tetracosane during the Solid-Solid Phase Transition as revealed by FTIR Spectroscopy

The kinetics of the development of the first-order diffuse structural phase transition in monodisperse samples of tetracosane C24H50 was studied by FTIR spectroscopy. The temperature dependences of the frequencies and intensities of the C-H-stretching vibrations in the spectral region ν=2800-3000 cm-1 were studied for 4 different modes in the methylene CH2 trans sequences of the main chain and for 4 different modes corresponding to the end methyl CH3-groups. Specific changes in the vibrational frequencies of all investigated modes are found, which are caused by changes in the symmetry of the molecular packing during the first-order phase transition. Keywords: n-alkanes, phase transitions, theory of diffuse phase transitions, IR spectroscopy.

Commensurate Stacking Phase Transitions in an Intercalated Transition Metal Dichalcogenide.

Intercalation and stacking-order modulation are two active ways in manipulating the interlayer interaction of transition metal dichalcogenides (TMDCs), which lead to a variety of emergent phases and allow for engineering material properties. Herein, the growth of Pb-intercalated TMDCs-Pb(Ta1+x Se2 )2 , the first 124-phase, is reported. Pb(Ta1+x Se2 )2 exhibits a unique two-step first-order structural phase transition at around 230 K. The transitions are solely associated with the stacking degree of freedom, evolving from a high-temperature (high-T) phase with ABC stacking and R3m symmetry to an intermediate phase with AB stacking and P3m1, and finally to a low-temperature (low-T) phase again with R3msymmetry, but with ACB stacking. Each step involves a rigid slide of building blocks by a vector [1/3, 2/3, 0]. Intriguingly, gigantic lattice contractions occur at the transitions on warming. At low-T, bulk superconductivity with Tc ≈ 1.8 K is observed. The underlying physics of the structural phase transitions are discussed from first-principle calculations. The symmetry analysis reveals topological nodal lines in the band structure. The results demonstrate the possibility of realizing higher-order metal-intercalated phases of TMDCs and advance the knowledge of polymorphic transitions, and may inspire stacking-order engineering in TMDCs andbeyond.

Solid-state phase transition in n-alkanes of different parity

The kinetics of the first-order solid-state structural transition in monodisperse n-alkanes samples of tricosane C23H48 and tetracosane C24H50 was studied by DSC and FTIR spectroscopy. The initial nuclei location of the new phase was revealed. The process of crystal structure rearrangement is initiated in the interlayers between neighboring lamellar for odd tricosane, while the nanonuclei in even tetracosane arise in the crystalline lamella cores. Thus, the influence of the number evenness of carbon atoms in the n-alkanes chains on the first-order structural phase transition has been proved.

Magnetic order, disorder, and excitations under pressure in the Mott insulator Sr2IrO4

Protected by the interplay of on-site Coulomb interactions and spin-orbit coupling, ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ at high pressure is a rare example of a Mott insulator with a paramagnetic ground state. Here, using optical Raman scattering, we measure both the phonon and magnon evolution in ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ under pressure and identify three different magnetically-ordered phases, culminating in a spin-disordered state beyond 18 GPa. A strong first-order structural phase transition drives the magnetic evolution at $\ensuremath{\sim}10$ GPa with reduced structural anisotropy in the ${\mathrm{IrO}}_{6}$ cages, leading to increasingly isotropic exchange interactions between the Heisenberg spins and a spin-flip transition to $c$-axis-aligned antiferromagnetic order. In the disordered phase of Heisenberg ${J}_{\mathrm{eff}}=1/2$ pseudospins, the spin excitations are quasielastic and continuous to 10 meV, potentially hosting a gapless quantum spin liquid in ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$.

Pressure-induced structural phase transition and suppression of Jahn-Teller distortion in the quadruple perovskite structure

By means of in situ synchrotron x-ray diffraction and Raman spectroscopy under hydrostatic pressure, we investigate the structural stability of the quadruple perovskite $\mathrm{La}{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$. At 34 GPa, the data unveil a first-order structural phase transition from monoclinic $I2/m$ to cubic $Im\overline{3}$ symmetry characterized by a pronounced contraction of the unit cell and by a significant modification in the Raman phonon modes. The phase transition is also marked by the suppression of a Jahn-Teller distortion which is present in the ambient monoclinic phase. In addition, above 20 GPa pressure, a sudden and simultaneous broadening is observed in several Raman modes which suggests the onset of a sizable electron-phonon interaction and incipient charge mobility. Considering that $\mathrm{La}{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$ is a paramagnetic insulator at ambient conditions, and that the Jahn-Teller distortion is frozen in the high-pressure $Im\overline{3}$ phase, we argue that this phase could be a potential candidate to host a purely electronic insulator-metal transition with no participation of the lattice.

Evolution of Structural and Electronic Properties of TiSe2 under High Pressure.

A pressure-induced structural phase transition and its intimate link with the superconducting transition was studied for the first time in TiSe2 up to 40 GPa at room temperature using X-ray diffraction, transport measurement, and first-principles calculations. We demonstrate the occurrence of a first-order structural phase transition at 4 GPa from the standard trigonal structure (S.G.P3̅m1) to another trigonal structure (S-G-P3̅c1). Additionally, at 16 GPa, the P3̅c1 phase spontaneously transforms into a monoclinic C2/m phase, and above 24 GPa, the C2/m phase returns to the initial P3̅m1 phase. Electrical transport results show that metallization occurs above 6 GPa. The charge density wave observed at ambient pressure is suppressed upon compression up to 2 GPa with the emergence of superconductivity at 2.5 GPa, with a critical temperature (Tc) of 2 K. A structural transition accompanies the emergence of superconductivity that persists up to 4 GPa. The results demonstrate that the pressure-induced phase transitions explored by the experiments along with the theoretical predictions may open the door to a new path for searching and controlling the phase diagrams of transition metal dichalcogenides.

Evidence of a structural phase transition in the triangular-lattice compound CuIr2Te4

Synthesized $\mathrm{Cu}{\mathrm{Ir}}_{2}{({\mathrm{Te}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x})}_{4}$ samples were comprehensively characterized using various techniques. At ambient pressure, $\mathrm{Cu}{\mathrm{Ir}}_{2}{\mathrm{Te}}_{4}$ undergoes an anomalous phase transition at ${T}_{s}\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with a large thermal hysteresis of $\mathrm{\ensuremath{\Delta}}{T}_{s} \ensuremath{\sim}40\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and a superconducting transition at ${T}_{c}\ensuremath{\sim}3\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ during magnetization and electrical-resistivity measurements. We determined this transition to be a first-order structural phase transition from high-temperature hexagonal ($P\overline{3}m1$) to low-temperature triclinic ($P\overline{1}$) symmetry. Both external physical pressure and chemical doping largely enhance ${T}_{s}$ and $\mathrm{\ensuremath{\Delta}}{T}_{s}$ and suppress ${T}_{c}$, thereby indicating a strong correlation between ${T}_{s}$ and ${T}_{c}$. Critically, no superlattice is observed below ${T}_{s}$ as per the electron-diffraction examinations. Thus, the anomalous phase transition at ${T}_{s}$, which exhibits a large thermal hysteresis in resistivity and magnetization measurements, is structural rather than a charge-density-wave formation.

Decoupling the metal insulator transition and crystal field effects of VO2

VO2 is a highly correlated electron system which has a metal-to-insulator transition (MIT) with a dramatic change of conductivity accompanied by a first-order structural phase transition (SPT) near room temperature. The origin of the MIT is still controversial and there is ongoing debate over whether an SPT induces the MIT and whether the Tc can be engineered using artificial parameters. We examined the electrical and local structural properties of Cr- and Co-ion implanted VO2 (Cr-VO2 and Co-VO2) films using temperature-dependent resistance and X-ray absorption fine structure (XAFS) measurements at the V K edge. The temperature-dependent electrical resistance measurements of both Cr-VO2 and Co-VO2 films showed sharp MIT features. The Tc values of the Cr-VO2 and Co-VO2 films first decreased and then increased relative to that of pristine VO2 as the ion flux was increased. The pre-edge peak of the V K edge from the Cr-VO2 films with a Cr ion flux ≥ 1013 ions/cm2 showed no temperature-dependent behavior, implying no changes in the local density of states of V 3d t2g and eg orbitals during MIT. Extended XAFS (EXAFS) revealed that implanted Cr and Co ions and their tracks caused a substantial amount of structural disorder and distortion at both vanadium and oxygen sites. The resistance and XAFS measurements revealed that VO2 experiences a sharp MIT when the distance of V–V pairs undergoes an SPT without any transitions in either the VO6 octahedrons or the V 3d t2g and eg states. This indicates that the MIT of VO2 occurs with no changes of the crystal fields.

Pressure-Driven Structural Phase Transitions and Superconductivity of Ternary Hydride MgVH6

A systematic investigation of ternary hydride MgVH6 within the pressure range of 0–200 GPa has been performed by means of the particle swarm optimization algorithm and density functional theory. Results of an extensive structure search and full relaxation further indicate that three new phases, P21/m, C2/m, and Pmn21, appear successively with the increasing of pressure, which contain two first-order structural phase transitions from P21/m to C2/m about 32 GPa and from C2/m to Pmn21 about 93 GPa. Three phases are all metallic phases by analyzing their band structures and density of electronic states and are also dynamically and mechanically stable based on their phonon spectra and elastic properties. More importantly, the electron–phonon coupling calculation indicates that MgVH6 with Pmn21 symmetry is a promising superconductor with an estimated superconducting transition temperature of 27.6 K at 150 GPa, which is similar to the superconducting transition temperature of 35 K at 300 GPa of ternary hydride LaSH6 (Phys. Rev. B 2019, 100, 184502). Further analysis of the electron–phonon coupling mechanism shows that the superconductivity is mainly derived from the strong electron–phonon interaction of heavier transition metal V atoms.

Structural Transition with a Sharp Change in the Electrical Resistivity and Spin–Orbit Mott Insulating State in a Rhenium Oxide, Sr3Re2O9

We report the successful synthesis, crystal structure, and electrical properties of Sr3Re2O9, which contains Re6+ with the 5d1 configuration. This compound is isostructural with Ba3Re2O9 and shows a first-order structural phase transition at ∼370 K. The low-temperature (LT) phase crystallizes in a hettotype structure of Ba3Re2O9, which is different from that of the LT phase of Sr3W2O9, suggesting that the electronic state of Re6+ plays an important role in determining the crystal structure of the LT phase. The structural transition is accompanied by a sharp change in the electrical resistivity. This is likely a metal-insulator transition, as suggested by the electronic band calculation and magnetic susceptibility. In the LT phase, the ReO6 octahedra are rotated in a pseudo-a0a0a+ manner in Glazer notation, which corresponds to C-type orbital ordering. Paramagnetic dipole moments were confirmed to exist in the LT phase by muon spin rotation and relaxation measurements. However, the dipole moments shrink greatly because of the strong spin-orbit coupling in the Re ions. Thus, the electronic state of the LT phase corresponds to a Mott insulating state with strong spin-orbit interactions at the Re sites.