Low-temperature Structural Transition Research Articles

Competing electronic instabilities in the quadruple perovskite manganite PbMn7O12

Structural behaviour of PbMn$_{7}$O$_{12}$ has been studied by high resolution synchrotron X-ray powder diffraction. This material belongs to a family of quadruple perovskite manganites that exhibit an incommensurate structural modulation associated with an orbital density wave. It has been found that the structural modulation in PbMn$_{7}$O$_{12}$ onsets at 294 K with the incommensurate propagation vector $\mathbf{k}_s=(0,0,\sim2.08)$. At 110 K another structural transition takes place where the propagation vector suddenly drops down to a \emph{quasi}-commensurate value $\mathbf{k}_s=(0,0,2.0060(6))$. The \emph{quasi}-commensurate phase is stable in the temperature range of 40K - 110 K, and below 40 K the propagation vector jumps back to the incommensurate value $\mathbf{k}_s=(0,0,\sim2.06)$. Both low temperature structural transitions are strongly first order with large thermal hysteresis. The orbital density wave in the \emph{quasi}-commensurate phase has been found to be substantially suppressed in comparison with the incommensurate phases, which naturally explains unusual magnetic behaviour recently reported for this perovskite. Analysis of the refined structural parameters revealed that that the presence of the \emph{quasi}-commensurate phase is likely to be associated with a competition between the Pb$^{2+}$ lone electron pair and Mn$^{3+}$ Jahn-Teller instabilities.

Displacive structural phase transitions and the magnetic ground state of quadruple perovskite YMn7O12

We present an experimental study of the structural and magnetic phase transitions of the quadruple perovskite ${\mathrm{YMn}}_{7}{\mathrm{O}}_{12}$. On cooling below 640 K a displacive structural phase transition occurs, lowering the crystal symmetry from cubic $(Im\overline{3})$ to monoclinic $(I2/m)$, driven by long range B site ${\mathrm{Mn}}^{3+}$ orbital order. A second, unconventional structural phase transition takes place on further cooling below 200 K, which is characterized by static atomic displacements $(P{2}_{1}/n)$ as well as a dipolar-glass-like state of yttrium. This low temperature structural transition is driven by the underbonding of yttrium, which is present due to the rigid framework of octahedra tilts unique to the quadruple perovskites that primarily accommodates A site manganese ions. A ground state ferrimagnetic structure was also determined, similar to that found in other $R{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$ compounds that do not present a low temperature structural phase transition.

Thermodynamic Properties of a First-Generation Carbosilane Dendrimer with Terminal Phenylethyl Groups

The heat capacity of a first-generation carbosilane dendrimer with terminal phenylethyl groups as a function of temperature in the range from 6 to 520 K is studied for the first time via precision adiabatic vacuum calorimetry and differential scanning calorimetry. Physical transformations, such as low-temperature structural anomaly and glass transition are detected in the above-mentioned range of temperatures, and their standard thermodynamic characteristics are determined and analyzed. The standard thermodynamic functions of the studied dendrimer in the range of T → 0 to 520 K are calculated from the experimental data, as is the standard entropy in the devitrified state at T = 298.15 K. The standard thermodynamic characteristics of the carbosilane dendrimers studied in this work and earlier are compared.

Sub-State Conformations of the Mesophilic and Psychrophilic Lactate Dehydrogenases Preceding Irreversible Thermal Inactivation

The thermodynamics of oxamate binding and the temperature stability of the glycolytic enzyme lactate dehydrogenase from porcine heart, phLDH (mesophilic Sus scrofa) and from mackerel icefish, cgLDH (psychrophilic Chamapsocephalus gunnari) have been investigated. A novel fluorescence assay was utilized that simultaneous monitors changes to the global protein structure, structural changes near the active site, and aggregation of the enzyme in response to increasing temperature and increasing concentration of the natural osmolyte, trimethyl amine N-oxide (TMAO), a stabilizer of protein structure. Using this assay, the reverse changes of stability and affinity for oxamate were established for both, phLDH and cgLDH. Importantly, a low-temperature (pre-denaturation) structural transition was found that precedes the high-temperature (denaturation) transition for both LDHs and coincides with increasing enzymatic activity. The structural transitions of the global protein structure and the active site are concerted for the rigid (phLDH) and not concerted for the flexible (cgLDH) LDHs. The profound contribution of entropy to ∆G along with the higher structural flexibility increases functional plasticity of the psychrophilic cgLDH. TMAO increases stability and shifts all structural transitions to the higher temperatures for both orthologs and simultaneously reduces their catalytic activity. The multiple active and inactive along with intermediate sub-state conformations of the enzyme exist in equilibrium at the stage preceding irreversible thermal inactivation. This equilibrium is an essential selective factor for the adaptation of an enzyme to the environmental temperature. It seems also possible that thermal adaptation of proteins may be complemented by evolution of the cellular milieu.

Structure and property correlations in FeS

For iron-sulfide (FeS), we investigate the correlation between the structural details, including its dimensionality and composition, with its magnetic and superconducting properties. We compare, theoretically and experimentally, the two-dimensional (2D) layered tetragonal (“t-FeS”) phase with the 3D hexagonal ("h-FeS") phase. X-ray diffraction reveals iron-deficient chemical compositions of t-Fe0.93(1)S and h-Fe0.84(1)S that show no low-temperature structural transitions. First-principles calculations reveal a high sensitivity of the 2D structure to the electronic and magnetic properties, predicting marginal antiferromagnetic instability for our compound (sulfur height of zS= 0.252) with an ordering energy of about 11meV/Fe, while the 3D phase is magnetically stable. Experimentally, h-Fe0.84S orders magnetically well above room temperature, while t-Fe0.93S shows coexistence of antiferromagnetism at TN= 116 and filamentary superconductivity below Tc= 4K. Low temperature neutron diffraction data reveals antiferromagnetic commensurate ordering with wave vector km=(0.25,0.25,0) and 0.46(2)µB/Fe. Additionally, neutron scattering measurements were used to find the particle size and iron vacancy arrangement of t-FeS and h-FeS. The structure of iron sulfide has a delicate relationship with the superconducting transition; while our sample with a = 3.6772(7)Å is a filamentary superconductor coexisting with an antiferromagnetic phase, previously reported samples with a > 3.68Å are bulk superconductors with no magnetism, and those with a≈3.674Å show magnetic properties.

Effect of Semiconductor Element Substitution on the Electric Properties of Barium Titanate Ceramics

Abstract The investigated ceramics were prepared by a solid-state reaction from simple oxides and carbonates with the use of a mixed oxide method (MOM). The morphology of BaTi0.96Si0.04O3 (BTSi04) ceramics was characterised by means of a scanning electron microscopy (SEM). It was found that Si+4 ion substitution supported the grain growth process in BT-based ceramics. The EDS results confirmed the high purity and expected quantitative composition of the synthesized material. The dielectric properties of the ceramics were also determined within the temperature range (ΔT=130-500K). It was found that the substitution of Si+4 ions had a significant influence on temperature behavior of the real (ε’) and imaginary (ε”) parts of electric permittivity as well as the temperature dependence of a.c. conductivity. Temperature regions of PTCR effect (positive temperature coefficient of resistivity) were determined for BTSi04 ceramics in the vicinity of structural phase transitions typical for barium titanate. No distinct maximum indicating a low-temperature structural transition to a rhombohedral phase in BTSi04 was found. The activation energy of conductivity was determined from the Arrhenius plots. It was found that substitution of Si ions in amount of 4wt.% caused almost 50% decrease in an activation energy value.

Superconducting Properties of FeSe $$_{1-x}$$ 1 - x S $$_{x}$$ x Crystals for x up to 0.19

We report on the transport properties of FeSe\(_{1-x}\)S\(_{x}\) single crystals for x = 0.04, 0.09, and 0.19. These compounds show a consequent increase in the superconducting transition temperature to about 10.5 K for x = 0.04 and 0.09 followed by a decrease to 8.5 K for x = 0.19, completing the superconducting dome of the FeSe\(_{1-x}\)S\(_{x}\) series phase diagram. The sample with the highest substitution level x = 0.19 shows no transition to an orthorhombic phase at low temperatures. Therefore, the suppression of the low temperature structural transition in FeSe\(_{1-x}\)S\(_{x}\) coincides with the start of the steep decrease in the superconducting transition temperature.

Electronic transport properties of MFe2As2 (M = Ca, Eu, Sr) at ambient and high pressures up to 20 GPa

We experimentally investigated the electronic transport properties of four iron pnictide crystals, namely, EuFe2As2, SrFe2As2, and CaFe2As2 parent compounds, and superconducting CaFe1.94Co0.06As2 at ambient and high pressures up to 20 GPa. At ambient pressure we examined the electrical resistivity, Hall and magnetoresistance effects of the samples in a temperature range from 1.5 to 380 K in high magnetic fields up to 13.6 T. In this work we carried out the first simultaneous investigations of the in-plane and out-of-plane Hall coefficients, and found new peculiarities of the low-temperature magnetic and structural transitions that occur in these materials. In addition, the Hall coefficient data suggested that the parent compounds are semimetals with a multi-band conductivity that includes hole-type and electron-type bands. We measured the pressure dependence of the thermoelectric power (the Seebeck effect) of these samples up to 20 GPa, i.e. across the known phase transition from the tetragonal to the collapsed tetragonal lattice. The high-pressure behavior of the thermopower of EuFe2As2 and CaFe2As2 showing the p-n sign inversions was consistent with the semimetal model described above. By means of thermopower, we found in single-crystalline CaFe2As2 direct evidence of the band structure crossover related to the formation of As–As bonds along the c-axis on the tetragonal → collapsed tetragonal phase transition near 2 GPa. We showed that this feature is distinctly observable only in high-quality samples, and already for re-pressurization cycles this crossover was strongly smeared because of the moderate deterioration of the sample. We also demonstrated by means of thermopower that the band structure crossover that should accompany the tetragonal → collapsed tetragonal phase transition in EuFe2As2 near 8 GPa is hardly visible even in high-quality single crystals. This behavior may be related to a gradual valence change of the Eu ions under pressure that leads to an injection of free electrons and the steady shift of the conduction to n-type.

Covalency driven low-temperature structural distortion and its effect on electronic structure ofHg2Ru2O7

We investigate theoretically the possible low temperature structural distortion in the pyrochlore based ruthenate compound ${\mathrm{Hg}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$. Our study reveals a signature of structural distortion leading to charge disproportionation between the Ru ions in the unit cell. The charge disproportion is found to be driven by the energy gain due to large covalency originating from the overlap of extended Ru $5d$ and O 2p orbitals combined with Hg $d$-Ru $d$ hybridization. The exchange interaction operative between the charge disproportionated Ru ions turns out to be of antiferromagnetic nature, which drives the insulating state and stabilizes the noncollinear magnetic structure. Our study sheds light on the microscopic mechanism of the low temperature structural transition in this compound.

Low temperature structural transitions in dipolar hard spheres: The influence on magnetic properties

We investigate the structural chain-to-ring transition at low temperature in a gas of dipolar hard spheres (DHS). Due to the weakening of entropic contribution, ring formation becomes noticeable when the effective dipole–dipole magnetic interaction increases. It results in the redistribution of particles from usually observed flexible chains into flexible rings. The concentration (ρ) of DHS plays a crucial part in this transition: at a very low ρ only chains and rings are observed, whereas even a slight increase of the volume fraction leads to the formation of branched or defect structures. As a result, the fraction of DHS aggregated in defect-free rings turns out to be a non-monotonic function of ρ. The average ring size is found to be a slower increasing function of ρ when compared to that of chains. Both theory and computer simulations confirm the dramatic influence of the ring formation on the ρ-dependence of the initial magnetic susceptibility (χ) when the temperature decreases. The rings due to their zero total dipole moment are irresponsive to a weak magnetic field and drive to the strong decrease of the initial magnetic susceptibility.

Photo-induced low temperature structural transition in the “114” YBaFe4O7 oxide

Synchrotron irradiation of the oxide YBaFe4O7.0 below 190K converts the low temperature monoclinic structure to a higher symmetry tetragonal form analogous to the room temperature structure. This photo-induced metastable tetragonal form is stable even in the absence of irradiation over the range 4–60K, however, above 60K the photo-transition is reversible. These structural phenomena are correlated to the magnetic behaviour of this system, suggesting possible spin-lattice coupling. A scenario explaining the low temperature photo-induced transition is proposed, based on the different distributions of the valence electrons in the iron sub-lattice of the monoclinic and tetragonal phases.

Depth profile of the ferromagnetic order in a YBa2Cu3O7/La2/3Ca1/3MnO3superlattice on a LSAT substrate: A polarized neutron reflectometry study

Using polarized neutron reflectometry (PNR) we have investigated a YBa2Cu3O7(10nm)/La2/3Ca1/3MnO3(9nm)]10 (YBCO/LCMO) superlattice grown by pulsed laser deposition on a La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) substrate. Due to the high structural quality of the superlattice and the substrate, the specular reflectivity signal extends with a high signal-to-background ratio beyond the fourth order superlattice Bragg peak. This allows us to obtain more detailed and reliable information about the magnetic depth profile than in previous PNR studies on similar superlattices that were partially impeded by problems related to the low temperature structural transitions of the SrTiO3 substrates. In agreement with the previous reports, our PNR data reveal a strong magnetic proximity effect showing that the depth profile of the magnetic potential differs significantly from the one of the nuclear potential that is given by the YBCO and LCMO layer thickness. We present fits of the PNR data using different simple block-like models for which either a ferromagnetic moment is induced on the YBCO side of the interfaces or the ferromagnetic order is suppressed on the LCMO side. We show that a good agreement with the PNR data and with the average magnetization as obtained from dc magnetization data can only be obtained with the latter model where a so-called depleted layer with a strongly suppressed ferromagnetic moment develops on the LCMO side of the interfaces. The models with an induced ferromagnetic moment on the YBCO side fail to reproduce the details of the higher order superlattice Bragg peaks and yield a wrong magnitude of the average magnetization. We also show that the PNR data are still consistent with the small, ferromagnetic Cu moment of 0.25muB that was previously identified with x-ray magnetic circular dichroism and x-ray resonant magnetic reflectometry measurements on the same superlattice.

First-principles studies for structural transitions in ordered phase of cubic approximantCd6Ca

Recently a low-temperature structural transition has been reported for complex cubic compoundsCd6M (M = Ca, Yb, Y, rare earth) and it is believed that the transition is due toorientational ordering of an atomic shell in the icosahedral cluster inCd6M. The first-principles electronic structure calculations and structuralrelaxations are carried out to investigate structures and orientationalordering of the innermost tetrahedral shell of the icosahedral cluster inCd6Ca. The very short interatomic distances in the experimental average structures are relaxedand the innermost tetrahedral shell of an almost regular shape is obtained. Three types oforientation for the tetrahedral shell and eight different combinations of them for theclusters at a vertex and body-centre of a cubic cell are obtained. A possible modeldescribing the orientational ordering at low temperatures or high pressures is discussed.

Transitions between secondary structures in isolated polyalanines

Monte Carlo simulations of gas-phase polyalanine peptides have been carried out with the Amber ff96 force field. A low-temperature structural transition takes place between the α-helix stable conformation and β-sheet structures, followed by the unfolding phase change. The transition state ensembles connecting the helix and sheet conformations are investigated by sampling the energy landscape along specific geometric order parameters as putative reaction coordinates, namely the electric dipole μ, the end-to-end distance d, and the gyration radius Rg. By performing series of shooting trajectories, the committor probabilities and their distributions are obtained, revealing that only the electric dipole provides a satisfactory transition coordinate for the α↔β interconversion. The nucleus at the transition is found to have a high helical content.

Ab initio studies on orientational ordering in cubic Zn–Sc

Abstract Complex cubic compounds Zn6Sc and Cd6M (M = Yb, Ca, Y, rare earth) are approximant crystals of Zn- and Cd-based quasicrystals. A low-temperature structural transition has been observed for these compounds universally and is considered to be due to orientational ordering of an atomic shell in the icosahedral cluster. To get microscopic information on structure and orientational ordering of the atomic shell in the icosahedral cluster, the first-principles structural relaxation for Zn6Sc is carried out. We obtain variety of favorable structures for different lattice constants. The results are compared with those for isostructural Cd6Ca obtained in the previous work. We stress differences between Zn6Sc and Cd6Ca and discuss their possible consequences on phase diagrams.

Low temperature neutron diffraction study ofMgCr2O4 spinel

The low temperature structural and magnetic transition in the normal spinelMgCr2O4 has been studied using high resolution neutron powder diffraction. The transition from acubic, paramagnetic phase (space group , a = 8.3329(1) Å at 19 K) to a tetragonal, antiferromagnetically ordered structure(I41/amd,a = 5.8961(1) and c = 8.3211(1) Å at 10 K) occurs via a sharp first order transition at 12.4 K. A small hysteresis of 0.15 Kbetween cooling and warming experiments is observed. The structural and magnetictransitions are coincident and their order parameters (the tetragonal strain and Crmoment) scale with each other. The structural transition partially relieves magneticfrustration in the Cr network, although remaining frustration is evident from the low value(1.63 µB) of thesaturated Cr3+ moment.

Growth of CuS platelet single crystals by the high-temperature solution growth technique

Millimeter size CuS single crystals with a dark indigo blue color and a plate hexagonal shape have been successfully grown by the high-temperature solution growth technique using the KCl–LiCl eutectic as solvent. Surface microtopographic studies of the crystals indicated that the growth is made by the lateral spreading of the layers. Electrical resistivity measurements clearly show an anomaly at T∼55 K, related with the low-temperature structural transition, a high residual resistivity ratio of ∼400 and a sharp superconducting transition at T≈1.7 K confirming the very good quality of the crystals.

Direct measurement of nanofluxes and structural relaxations of perfluorinated ionomer membranes by scanning probe microscopy

This nanorheological study addresses the water transport hindrance in perfluorinated ionomer membranes (Nafion ® H + form) in conjunction with structural relaxation properties of the polymer matrix. A low-temperature relaxation of the “dry” Nafion ® matrix at 79 °C, about 35 °C below the glass transition temperature, was found to be responsible for a steady improvement of the water transport above 70 °C. The low-temperature structural transition at 79 °C was observed for the dehydrated membrane, and thus, is specific to the polymer matrix and not a sole consequence of the kinetics of the solvent-containing aggregates within the apolar perfluorocarbon matrix. Water was found to be trapped within a hydrated perfluorinated ionomer membrane below 70 °C. The direct observation of water transport is possible with a novel scanning force microscopy (SFM) method, flux-lateral force microscopy (F-LFM). With this technique, local fluid permeation fluxes are directly obtained at the membrane downstream surface. The structural transition properties were acquired by another highly sensitive nanoscopic technique, shear modulation force microscopy (SM-FM).

Assessment of the La–Sr–Mn–O system

In the La–Sr–Mn–O oxide system, the perovskite phase (La,Sr)MnO 3 and the two Ruddlesden–Popper phases, (La,Sr) 2MnO 4 and (La,Sr) 3Mn 2O 7, are known to show substitution of Sr 2+ for La 3+. All three phases show very interesting electronic and magnetic properties, such as giant magnetoresistivity (GMR) that strongly depend on the oxidation state of Mn. Using the CALPHAD approach and applying the compound energy formalism we model the La-solubility in the Ruddlesden–Popper phases in the simplest possible way, with La 3+ substituting for Sr 2+ and Mn 4+ simultaneously being reduced to Mn 3+. Due to the lack of experimental data we model no oxygen nonstoichiometry, even though they probably show both oxygen excess and deficiency. In the case of the perovskite phase more experimental data exists and we model all possible nonstoichiometries, oxygen deficiency, oxygen excess, Sr-solubility and also cation nonstoichiometry, which means that the ratio (La+Sr)/Mn can deviate from one. The model required might seem quite complex; however it is simply a combination of the models previously used to describe the LaMnO 3 and SrMnO 3 perovskites. Two interaction parameters are sufficient to model all experimental data on the (La,Sr)MnO 3 perovskites to within experimental uncertainty. In this paper we concentrate on the high temperature properties of the perovskite and do not model the low temperature structural and magnetic transitions.

Paramagnetic behavior and correlation between high- and low-temperature structural and magnetic transitions inLa1−xSrxMnO3(x∼1/8)single-crystal perovskites

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