Order Structural Phase Transition Research Articles

Second Order Structural Phase Transitions in [(Ch3)4p]3m2cl9 (M=Bi and Sb) Hybrid Compounds Revealed by Temperature-Controlled Raman Spectroscopy and X-Ray Diffraction

Second Order Structural Phase Transitions in [(Ch3)4p]3m2cl9 (M=Bi and Sb) Hybrid Compounds Revealed by Temperature-Controlled Raman Spectroscopy and X-Ray Diffraction

Structural and magnetic transitions in the planar antiferromagnet Ba4Ir3O10

We report the structural and magnetic ground state properties of the monoclinic compound barium iridium oxide Ba$_4$Ir$_3$O$_{10}$ using a combination of resonant x-ray scattering, magnetometry, and thermodynamic techniques. Magnetic susceptibility exhibits a pronounced antiferromagnetic transition at $T_{\text{N}}$ $\approx$ 25K, a weaker anomaly at $T_{\text{S}}$ $\approx$ 142K, and strong magnetic anisotropy at all temperatures. Resonant elastic x-ray scattering experiments reveal a second order structural phase transition at $T_{\text{S}}$ and a magnetic transition at $T_{\text{N}}$. Both structural and magnetic superlattice peaks are observed at $L$ = half integer values. The magnetization anomaly at $T_{\text{S}}$ implies the presence of magneto-elastic coupling, which conceivably facilitates the symmetry lowering. Mean field critical scattering is observed above $T_{\text{S}}$. The magnetic structure of the antiferromagnetic ground state is discussed based on the measured magnetic superlattice peak intensity. Our study not only presents essential information for understanding the intertwined structural and magnetic properties in Ba$_4$Ir$_3$O$_{10}$, but also highlights the necessary ingredients for exploring novel ground states with octahedra trimers.

Магнитные и магнитокалорические эффекты в системах с реверсивными переходами первого рода

Within the framework of the model of interacting parameters of magnetic and structural orders, a theoretical analysis of magnetostructural reversible first-order phase transitions is carried out. Reversible phase transitions are characterized by a jump-like appearance of magnetic order with decreasing temperature (as in a first-order phase transition), and with a reverse increase in temperature, the magnetic order gradually disappears (as in a second-order phase transition). Such transitions are observed in some alloys of the Mn_{1-x}Cr_{x}NiGe magnetocaloric system under pressure (x = 0.11) and without (x = 0.18) and are accompanied by specific magnetic and magnetocaloric features. A phenomenological description of these features is carried out within the concept of a soft mode for the structural subsystem undergoing first-order structural phase transition (P6_{3}/mmc-P_{nma}) and the Heisenberg model for the spin subsystem. For systems with magnetostructural instability within the molecular field approximation for the spin subsystem and the shifted harmonic oscillator approximation for the lattice subsystem, it is shown that the reversible phase transitions arise when the temperature of magnetic disordering is in the temperature hysteresis region of the 1st order structural phase transition P6_{3}/mmc-P_{nma}. It is also shown that the two-peak form of the isothermal entropy, which is characteristic of reversible transitions, is due to the separation of the structural and magnetic entropy contributions.

Pressure induced semimetallic B2 phase of alkaline earth tellurides

We report pressure induced structural, topological quantum phase transitions and Fermi surface topology in MTe (M = Ca, Sr, Ba) compounds under high pressure using first principles calculations based on density functional theory. MTe compounds undergo a first order structural phase transition from NaCl-type (B1) to CsCl-type (B2) phase at 24.2, 14.7, 4.9 GPa respectively and are consistent with the experimental observations. Electronic structure of MTe compounds were calculated using new parameterization of Tran Blaha-modified Becke Johnson (TB-mBJ) potential and the obtained band gaps are improved over LDA/GGA functionals. The three compounds under investigation are indirect band gap semiconductors at ambient conditions. Under high pressure, B2 phase of (Ca/Sr)Te and BaTe are found to be node-line and Dirac semimetals, respectively without inclusion of spin-orbit coupling.

Phase transition induced spectral tuning of dual luminescent crystalline zinc-phthalocyanine thin films and OLEDs

We analyze the impact of the first order structural phase transition between the α- and β-phases in crystalline zinc-phthalocyanine thin films on their luminescent behavior. Upon optical excitation, two intensity maxima governing the dual fluorescence can be attributed to thin film excimer emission of the α-phase at 930 nm and bulk Frenkel exciton emission of the corresponding β-phase at 780 nm. Comprehensive temperature and time dependent studies on the structural phase transition reveal a controllable shift of spectral weight between these two maxima. Applying this approach to thin film devices, we demonstrate the preparation of dual luminescent organic light emitting diodes with tunable emission characteristics in the near infrared, which are of interest as electro-optical units in future hybrid photonic devices.

PRETRANSITIONAL PHENOMENA IN REGION OF STRUCTURAL PHASE TRANSITION IN SODIUM PERCHLORATE

Structural and dynamic properties and molecular relaxation processes in crystalline sodium NaClO4 perchlorate in the temperature range from 300 K to 650 K were studied by Raman spectroscopy. The temperature dependences of the position of the maximum v (frequency), the width w and the intensity I of the spectral band, corresponding to the fully symmetric vibration v1(A) of the ClO4–1 perchlorate ion, in the spectral range from 933 cm–1 to 944 cm–1 were analyzed. The frequency v and intensity I decrease, and the width w increases with the increasing temperature. It is shown that these temperature dependences have certain features at a temperature of 460 K. With a further increase in temperature, the frequency v decreases more rapidly, the width w increases, and the intensity I decreases more intensively. In the temperature range from 460 K to the temperature Tstr = 581 K of the first order structural phase transition, we observe a deviation of the temperature dependence of the frequency and width from the linear dependences characteristic of lower temperatures. These deviations appear at a temperature of 460 K and increase with the increasing temperature and approaching the phase transition temperature. It has been established that in the crystalline sodium perchlorate NaClO4 a structural first-order phase transition is stretched. At the phase transition temperature (Tstr = 581 K), the width increases sharply, and the frequency decreases sharply, decreasing with a further increase in temperature. The existence of a pretransitional region in the studied crystalline sodium perchlorate NaClO4 was found. This pre-transition region occurs in the temperature range from 460 K to Tstr = 581 K.

Temperature dependence of the IR mode frequency calculated from the volume data for DMNaFE

We analyze the infrared (IR) frequencies of the internal mode of asymmetric close to phase transition in DMNaFe. By means of the mode Grüneisen parameter , the IR frequencies are calculated from the observed volume data given in the literature. Calculated frequencies are fitted to the observed data for both below and above as the varies with the temperature in the low and high temperature phases in this compound. Our results show that the critical behavior of the observed IR frequencies can be explained from the volume data to describe a first order structural phase transition in DMNaFe.

Negative linear compressibility and structural phase transition in energetic silver azide under pressure: A first principles study

In this work, we have investigated high pressure behavior of silver azide within the framework of dispersion corrected density functional theory. We have predicted a pressure induced first order structural phase transition from orthorhombic (Ibam) → tetragonal (I4∕mcm) phase at 3.1 GPa. Ambient phase exhibits negative linear compressibility along a-axis in agreement with the experiments. Calculated phonon dispersion relations confirm the dynamical stability of ambient and high pressure phases. We have also analyzed bonding nature of alkali and transition metal azides, this provides an insight on possible correlation between structure-bonding-stability. The bonding nature between metal and terminal nitrogen of azide plays a predominant role in determining the order of stability in inorganic metal azides.

Barkhausen effect in the first order structural phase transition in type-II Weyl semimetal MoTe2

We report the first observation of the non-magnetic Barkhausen effect in van der Waals layered crystals, specifically, during transitions between the Td and 1T′ phases in type-II Weyl semimetal MoTe2. Thinning down the MoTe2 crystal from bulk material to about 25 nm results in a drastic strengthening of the hysteresis in the phase transition, with the difference in critical temperature increasing from ~40 K to more than 300 K. The Barkhausen effect appears for thin samples and the temperature range of the Barkhausen zone grows approximately linearly with reducing sample thickness, pointing to a surface origin of the phase pinning effects. The distribution of the Barkhausen jumps shows a power law behavior, with its critical exponent α = 1.27, in good agreement with existing scaling theory. Temperature-dependent Raman spectroscopy on MoTe2 crystals of various thicknesses shows results consistent with our transport measurements.

Structural phase transition and ferroelasticity in (H2NNH3)3CdBr5 crystal

Single crystals of (H2NNH3)3CdBr5 were grown and their properties were investigated. DSC, DTG, structural studies, optical polarized microscopy observations and dielectric studies are presented. TGA showed the stability of the substance up to about 455 K and decomposition above this temperature. DSC studies revealed the first order structural phase transitions at 294/292 on heating/cooling runs. X-ray structural studies showed a change of symmetry from orthorhombic to monoclinic and the transition seems to belong to the mmmF2/m Aizu's species. In optical studies performed with a polarizing microscope, the ferroelastic type of domain structure and the phase front appearance were observed. Dielectric measurements clearly evidenced the first-order phase transition as a jump-like decrease of permittivity on cooling and increase on heating. Structural and dielectric studies indicate the order-disorder character of the phase transition.

Structural, vibrational, and electrical properties of 1T−TiTe2 under hydrostatic pressure: Experiments and theory

We report the structural, vibrational and electrical transport properties up to 16 GPa of the 1T-TiTe2, a prominent layered 2D system, which is predicted to show a series of topologically trivial - nontrivial transitions under hydrostatic compression. We clearly show signatures of two iso-structural transition at 2 GPa and 4 GPa obtained from the minima in c/a ratio concomitant with the phonon linewidth anomalies of Eg and A1g modes at around the same pressures, providing strong indication of unusual electron-phonon coupling associated to these transitions. Resistivity presents nonlinear behavior over similar pressure ranges providing a strong indication of the electronic origin of these pressure driven isostructural transitions. Our data thus provide clear evidences of topological changes at A and L point of the Brillouin zone predicted to be present in the compressed 1T-TiTe2. Between 4 GPa and 8 GPa, the c/a ratio shows a plateau suggesting a transformation from an anisotropic 2D layer to a quasi 3D crystal network. First principles calculations suggest that the 2D to quasi 3D evolution without any structural phase transitions is mainly due to the increased interlayer Te-Te interactions (bridging) via the charge density overlap. In addition to the pressure dependent isostructural phase transitions, our data also evidences the occurrence of a first order structural phase transition from the trigonal (P-3m1) phase at higher pressures. We estimate the start of this structural phase transition to be 8 GPa and the symmetric of the new high-pressure phase to be monoclinic (C2/m).

Structure and Lattice Dynamics of Calcium Chalcogenides under High Pressure

We have investigated the structural and lattice dynamics of alkaline-earth chalcogenides namely CaX (X = S, Se Te) compounds under high pressure up to 50 GPa using plane wave pseudo potential approach based on density functional theory. It is found that CaS, CaSe and CaTe compounds undergo first order structural phase transitions from NaCl-type (B1) to CsCl-type (B2) phase at 34.9, 31.8 and 24.2 GPa respectively and the obtained transitions pressures are consistent with the experimental results. We have also calculated the phonon dispersions of these three compounds in B1 phase and are found to be dynamically stable even after the phase transition pressure to maximum pressure of the present study (50 GPa) for CaS, CaSe whereas CaTe is stable up to 30 GPa

Lattice and magnetic instabilities in Cu3Bi(SeO3)2O2X ( X=Br,Cl )

We performed Raman studies and a dielectric characterization of the pseudo-kagome Cu3Bi(SeO3)2O2X (X = Cl, Br). These compounds share competing nearest-neighbour ferromagnetic exchange and frustrating next-nearest-neighbour antiferromagnetic exchange as well as highly noncollinear magnetic ground state. However, at low temperature they differ with respect to the existence of inversion symmetry. For both compounds there exists a pronounced interplay of polar phonon modes with quantum magnetic fluctuations. A novel Raman mode appears for temperatures below the Neel temperature with a Fano lineshape and an enormous intensity that exceeds most of the phonon lines. We discuss a possible contribution of longitudinal magnons to this signal. In contrast, one magnon scattering based on linear transvers magnons is excluded based on a symmetry analysis of spin wave representations and Raman tensor calculations. There exists evidence that in these pseudo-kagome compounds magnetic quantum fluctuations carry an electric dipole moment. Our data as well as a comparison with previous far-infrared spectra allow us to conclude that Cu3Bi(SeO3)2O2Cl changes its symmetry most likely from Pmmn to P21mn with a second order structural phase transition at T*=120 K and becomes multiferroic. Cu3Bi(SeO3)2O2Br represents an interesting counter part as it does not show this instability and stays inversion symmetric down to lowest temperatures, investigated.

Dramatic change of photoexcited quasiparticle relaxation dynamics across Yb valence state transition in YbInCu4

YbInCu$_4$ undergoes a first order structural phase transition near $T_v$=40 K associated with an abrupt change of Yb valence state. We perform ultrafast pump-probe measurement on YbInCu$_4$ and find that the expected heavy fermion properties arising from the \emph{c-f} hybridization exist only in a limited temperature range above $T_v$. Below $T_v$, the compound behaves like a normal metal though a prominent hybridization energy gap is still present in infrared measurement. We elaborate that those seemingly controversial phenomena could be well explained by assuming that the Fermi level suddenly shifts up and becomes far away from the flat \emph{f}-electron band as well as the indirect hybridization energy gap in the mixed valence state below $T_v$.

Infield X-ray diffraction studies of field and temperature driven structural phase transition in Nd0.49Sr0.51MnO3+δ

Comprehensive X-ray diffraction (XRD) studies have been performed at different temperature (T) (4.2–300K) and magnetic field (H) (0–8T) to understand the evolution of crystal structure of Nd0.49Sr0.51MnO3+δ (NSMO) under non ambient conditions. The T dependent XRD results show the abrupt change in the lattice parameters without any change in lattice symmetry at ∼200K, which is associated with the first order structural phase transition from ferromagnetic to antiferromagnetic phase. This phase transition is strongly H dependent and shifted to lower temperature (∼150K) on the application of 8T field with phase coexistence (high temperature phase ∼18%), even down to 4.2K. Isothermal XRD results at 150K under different H clearly illustrate the H induced first order structural phase transition. The critical H at which this phase transformation starts is ∼1T, with rapid growth above 4T with hysteretic nature during increasing and decreasing H. These results are supported with the resistivity and magnetoresistance results and affirm the strong spin-lattice coupling in NSMO. Our detail studies reveal the structural correlations to the observed colossal magnetoresistance and magnetocaloric effect in this material.

Polymorphic Layered MoTe2 from Semiconductor, Topological Insulator, to Weyl Semimetal

Large size (∼2 cm) single crystals of layered MoTe2 in both 2H- and 1T′-types were synthsized using TeBr4 as the source of Br2 transport agent in chemical vapor transport growth. The crystal structures of the as-grown single crystals were fully characterized by X-ray diffraction, Raman spectroscopy, scanning transmission electron microscopy, scanning tunneling microscopy (STM), and electrical resistivity (ρ) measurements. The resistivity ρ(T), magnetic susceptibility χ(T), and heat capacity Cp(T) measurement results reveal a first order structural phase transition near ∼240 K for 1T′-MoTe2, which has been identified to be the orthorhombic Td-phase of MoTe2 as a candidate of Weyl semimetal. The STM study revealed different local defect geometries found on the surface of 2H- and Td-types of MoTe6 units in trigonal prismatic and distorted octahedral coordination, respectively.

Band Structure, Density of States, Structural Phase Transition and Metallization in CdO Under High Pressure

The structural phase transition and metallization of the group II-VI compound semiconductor cadmium oxide (CdO) is investigated through its band structure obtained using the full potential linear muffin-tin orbital (FP-LMTO) method. The ground state properties and band gap values are compared with the experimental results. Under high pressure CdO undergoes a first order structural phase transition from the NaCl structure (B1) to the CsCl structure (B2). The band structure, density of states (DOS) and total energy are computed as a function of reduced volume for both NaCl and CsCl phases. The values of pressure under reduced volume are calculated using Birch-Murnaghan’s equation of state. The pressure corresponding to structural phase transition from NaCl structure (B1) to the CsCl structure (B2) is 0.9 Mbar in CdO. The metallization pressure PM is 1.2 Mbar. DOI: http://dx.doi.org/10.17807/orbital.v0i0.884

Studies on magnetic field and temperature driven magneto-structural phase transition in La0.5Sr0.5MnO3+δ

Abstract The influence of magnetic field on the crystal structure of colossal magnetoresistance (CMR) material La 0.5 Sr 0.5 MnO 3+δ has been investigated. Low temperature high-magnetic field powder X-ray diffraction (XRD) measurements were carried out on La 0.5 Sr 0.5 MnO 3+δ in temperature range from 4.2 to 300 K and magnetic field range from 0 to 8 T. A first order structural phase transition from tetragonal I 4/ mcm to orthorhombic Fmmm phase, coupled with reported electronic and magnetic phase transition from ferromagnetic metal to antiferromagnetic insulator state has been started at ∼200 K and completed at ∼100 K. On the application of 8 T magnetic field this magneto-structural phase transition shifted to lower temperature (∼162 K) with phase coexistence regime down to 4.2 K. Isothermal X-ray diffraction measurements at 150 K infer the evidence of magnetic field driven first order structural phase transition from Fmmm to I 4/ mcm . These results consistent with the resistivity and magnetoresistance results; present the microscopic evidence of strong spin-lattice coupling and reveal the magnetic field driven structural phase transition as the origin of observed colossal magnetoresistance in this material.

In situ synchrotron study of ordered and disordered LiMn1.5Ni0.5O4 as lithium ion battery positive electrode

In situ powder synchrotron diffraction and X-ray absorption spectroscopy have been used to investigate cation ordered as well as disordered modifications of the LiMn1.5Ni0.5O4 spinel-type compound as a Li-ion battery (LIB) cathode during electrochemical cycling. The structural state depends on the adopted heat treatment conditions and is ascertained by Raman and neutron diffraction data. The degree of Mn-Ni ordering (none, partial, complete) is one parameter that influences the electrochemical behaviour. The in situ data reveal significant differences in behaviour with respect to structural phase transitions during electrochemical cycling. Ordered Li2Mn3NiO8 (P4332) undergoes two consecutive first order structural phase transitions between spinel type phases during charging - discharging. These are most probably connected with steps in oxidation state for the Ni-atoms as supported by XANES data. The disordered phases (Fd-3m) show a mixed “solid solution - two phase” behaviour accompanied by a smooth decrease in the unit cell volume during charging over the two redox steps involved, Ni2+/Ni3+/Ni4+, however, also with volume jumps at first order transitions. This combined behaviour is explained for the first time for spinel type LiMn1.5Ni0.5O4 materials. It is not clear why ordered and disordered phases show different phase behaviors depending on the state of (dis)charge. This could be due to higher valent Mn taking part in the redox activity at higher voltages, however, future verification is required.

Switchable dielectric behavior induced by a disorder–order structural phase transition in (C3H12N2)2(HSO4)2[Mn(H2O)4(SO4)2

The inorganic–organic hybrid compound (C3H12N2)2(HSO4)2[Mn(H2O)4(SO4)2] (1) was synthesized under slow evaporation conditions. Differential scanning calorimetry (DSC) measurement indicated that compound 1 undergoes a reversible phase transition at 278K with a hysteresis width of 3K. Two-temperature single crystal X-ray diffraction data showed that the unit cell contains the (C3H12N2)2+ cation, with discrete [Mn(H2O)4(SO4)2]2− and HSO4− anions. The unit cell volume of 1 become double on decreasing the temperature. The disorder–order transformation of the C atoms of the (C3H12N2)2+ cation may drive the structural phase transition. The dielectric permittivity of compound 1 can be switched between two states, which could be used in switchable dielectric devices.