Low-temperature Structural Phase Transition Research Articles

Cause, Consequence, and Control of Ag Vacancies in BaAg2-xAs2.

The impact of transition metal (Ag) deficiencies on the structural and transport properties of ThCr2Si2-type arsenides are investigated. We experimentally confirm a partial occupancy of Ag in BaAg2-xAs2, which can be predictably controlled within 0.053(5) ≤ x ≤ 0.19(1) by varying the quenching temperature during the crystal growth. Density functional theory calculations reveal that substoichiometric concentrations of Ag lower the density of states at the Fermi level, providing an electronic cause for the tendency of Ag to form vacancies. This vacancy concentration is linked to a characteristic kink in electrical resistivity that can be substantially shifted from 22 to 125 K and is established as the metric of a low-temperature structural phase transition (SPT) in BaAg2-xAs2. Along with electrical resistivity, the Seebeck coefficient and heat capacity for selected BaAg2-xAs2 samples are presented, which also exhibit anomalies due to the SPT.

Structural transition, orientational order, and anomalous specific heat in a two-dimensional dimer crystal of core-softened particles.

Systems featuring hard-core-soft-shell repulsive pair potentials can form ordered phases, where particles organize themselves in aggregates with nontrivial geometries. The dimer crystal formed by one such potential, namely, the hard-core plus generalized exponential model of order 4, has been recently investigated, revealing a low-temperature structural phase transition, with the onset of nematic ordering of the dimers. In the present work, we aim to characterize this phase transition via a mean-field theory, by which a detailed analysis of the low-temperature properties of the system is carried out under quadrupole approximation. We determine the transition temperature and identify its order parameter, highlighting the link between the structural transition and the nematic ordering of the system. The first-order character of the transition is established and supported by the Landau expansion of the free energy in powers of the order parameter. The theory is subsequently generalized to take into account lattice vibrations and dimer length fluctuations. Finally, we provide an explanation for the anomalous behavior displayed by the specific heat in the vanishing-temperature limit, which is also supported by Monte Carlo simulations.

Catalogue of phonon modes in several cuprate high-temperature superconductors from density functional theory

Cuprates are promising candidates for study in developing higher temperature superconductors. A thorough understanding of a material’s phonon modes enables further investigation of its emergent properties, however, no complete reference of the phonon modes exists. Here, using density functional theory, we evaluate the phonon frequencies and atomic displacements for La2CuO4, Bi2Sr2CuO6, and Bi2Sr2CaCu2O8, in their tetragonal structures. The phonon modes for all materials agree with those expected from space group symmetry and display instabilities corresponding to known low-temperature structural phase transitions.

Effect of low temperature structural phase transitions in BaTiO3 on electrical transport through a metal–ferroelectric–metal multilayer of AuCr/BaTiO3/Nb:SrTiO3

In this paper we report an investigation of electronic transport through the metal–ferroelectric–metal (MFM) multilayer consisting of AuCr/BaTiO3/Nb:SrTiO3 over a temperature range of 100 K–300 K where BaTiO3 (BTO) shows a series of structural phase transitions leading to change of magnitude as well as the orientation of the polarization . We observed that the bias dependent barrier heights associated with the interfaces carry strong signature of the phase transitions in the BTO layer which lead to a strong temperature dependent asymmetric transport, when cooled down below room temperature. Specifically, it is observed that the temperature dependence is closely correlated to low temperature transitions in the BTO layer as revealed through the temperature dependent x-ray diffraction (XRD), capacitance as well as resistivity behavior of the BTO layer. There is substantial enhancement of the asymmetry in the device current that occurs at or close to temperatures T 2 ∼ 190 K where BTO shows a crystallographic phase change to the low temperature rhombohedral phase. The temperature dependent changes occur due to barrier modulation at the interfaces of AuCr/BaTiO3 as well as BaTiO3/Nb:SrTiO3 that softens on cooling due to inhomogenities present there. The change in barrier on change of the bias direction has been observed below T 2 which arises from alignment of the polarization in-plane or out-of-plane as determined by tensile or compressive character of the in-plane strain in the BTO film. We also discuss the effect of space charge determined by the oxygen vacancies in the interface region, regulated by the applied bias.

Cryostat for cooling samples in the study of low-temperature structural and magnetic phase transitions by neutron diffraction

A vertical cryostat was developed built around a Sumitomo RDK-101D cold head. The cryostat is intended for investigating the structure and phase transformations of different materials via neutron diffraction experiments on samples under high vacuum of up to 10−5 mbar. A working temperature of the sample close to 2 K was reached. The drift diameter of the head is 54 mm, making it possible to cool samples with a height of 55 mm and 15 mm in diameter. Test measurements were conducted and illustrative spectra were recorded on the example of a sample of HoFeO3. The cryostat was designed and manufactured at the Frank Laboratory of Neutron Physics of the Joint Institute for Nuclear Research.

On the Origin of the Negative Thermal Expansion Behavior of YCu

Among the intermetallics and alloys, YCu is an unusual material because it displays negative thermal expansion without spin ordering. The mechanism behind this behavior that is caused by the structural phase transition of YCu has yet to be fully understood. To gain insight into this mechanism, we experimentally examined the crystal structure of the low-temperature phase of YCu and discuss the origin of the phase transition with the aid of thermodynamics calculations. The result shows that the high-temperature (cubic CsCl-type) to low-temperature (orthorhombic FeB-type) structural phase transition is driven by the rearrangement of three covalent bonds, namely, Y-Cu, Y-Y, and Cu-Cu, which compete for the bonding energy and phonon entropy. At low temperatures, the mixing of Y and Cu does not take place easily because of the weak attractive force between these atoms expected from the small negative mixing enthalpy. This causes all three interactions to take part in the bonding, and Y and Cu are segregated to form an FeB-type structure, which is stabilized by internal energy. At higher temperatures, Cu ions are bound loosely with Y ions due to the large Y-Cu distance (3.01 Å), which results in large vibration entropy and stabilizes a CsCl-type crystal structure. In addition, the CsCl-type structure is reinforced by the Y-Y interaction between next-nearest neighbors, resulting in a smaller unit cell volume. The crystal structure has the simple cubic framework of Y containing Cu ions bound loosely at the cavity sites. The calculated frequency of the Y-like phonon modes is much higher than that of the Cu-like modes, indicating the presence of Y-Y covalent interactions in the CsCl-type phase.

Contrasting the magnetism in La2−xSrxFeCoO6(x=0,1,2) double perovskites: The role of electronic and cationic disorder

The magnetism of the double perovskite compounds \SLFCOx\ ($x$ = 0, 1, 2) are contrasted using magnetization, neutron diffraction and electron paramagnetic resonance with the support from density functional theory calculations. \LFCO\ is identified as a long-range ordered antiferromagnet displaying a near-room temperature transition at $T_N$ = 270~K, accompanied by a low temperature structural phase transition at $T_S$ = 200~K. The structural phase transformation at $T_S$ occurs from $R\overline{3}c$ at 300~K to $Pnma$ at 200~K. The density functional theory calculations support an insulating non-compensated AFM structure. The long-range ordered magnetism of \LFCO\ transforms to short-range glassy magnetism as La is replaced with Sr in the other two compounds. The magnetism of \LFCO\ is differentiated from the non-equilibrium glassy features of \SFCO\ and \SLFCO\ using the {\em cooling-and-heating-in-unequal-fields} (CHUF) magnetization protocols. This contransting magnetism in the \SLFCOx\ series is evidenced in electron paramegnetic resonance studies. The electronic density-of-states estimated using the density functional theory calculations contrast the insulating feature of \LFCO\ from the metallic nature of \SFCO. From the present suite of experimental and computational results on \SLFCOx, it emerges that the electronic degrees of freedom, along with antisite disorder, play an important role in controlling the magnetism observed in double perovskites.

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.

A high-resolution neutron powder diffraction study of the low-temperature structural phase transitions in RbCaF3 perovskite

The temperature dependence of the crystal structure and thermoelastic properties of RbCaF3 perovskite are reported from a high-resolution neutron powder diffraction investigation in the temperature interval 2–300 K. Two phase transitions have been identified from these data, the first from space group Pm3¯m to I4/mcm, at 200 K, has been well characterized in earlier studies, the second, from space group to I4/mcm to Pbnm has previously been less well understood. New results from the former transition show the order parameter evolves with a critical exponent of 0.284(5), and the phase is characterized by an Einstein temperature of 192(8) K, and a Debye temperature of 270(10) K. The temperature variation of the tetragonal spontaneous strain in this phase is consistent with an additional fourth order invariant in the Landau potential. The second phase transition is found to be strongly first order and a temperature dependent, and heating rate/cooling rate dependent phase coexistence is found at all temperatures below ~ 80 K on heating, and ~ 65 K on cooling. The evolution of the orthorhombic unit cell is atypical in the region of the phase transition, with a and b exhibiting strong negative linear thermal expansion, whilst the c axis behaves in the expected manner. The crystal structure of the Pbnm phase at 2 K is reported from an unconstrained Rietveld refinement for the first time.

Ultrafast optical snapshots of hybrid perovskites reveal the origin of multiband electronic transitions

Hybrid organic-inorganic perovskites are a promising class of materials for low-temperature solution-processed thin-film optoelectronic technologies. Here, the authors assign the visible and near-infrared optical transitions in large-grain thin films, using variable temperature transient absorption measurements that provide a sensitive probe of the electronic energy levels. These studies address an ongoing controversy concerning the nature of higher-order subband transitions. The strong visible absorption band is assigned to nearly degenerate transitions originating from different momentum states within the multivalley electronic band structure using symmetry analysis and the response of the optical resonances during the low-temperature structural phase transition.

Magnetic properties of La0.7Sr0.3MnO3 film on ferroelectric BaTiO3 substrate

Hybrid hetero-structure of ferromagnetic and ferroelectric materials provides a platform to make a multifunctional device with improved functionality. Epitaxial thin film of La0.7Sr0.3MnO3 was grown on ferroelectric BaTiO3 (001) substrate by pulse laser deposition. Here we report that the magnetic properties of ferromagnetic La0.7Sr0.3MnO3 were highly influenced by the strain induced by the successive low temperature structural phase transition of ferroelectric BaTiO3.

Micro Structural and Optical Properties of Ferrous Selenide Thin Films and Its Characterization

Ferrous Selenide (FeSe) thin films were prepared onto glass substrate by Electron beam evaporation technique. X-Ray Diffractometer analyses showed that highly c-axis oriented and high-quality films were obtained on various annealing temperatures of 100°C, 200°C and 300°C respectively. From Scanning Electron Microscopy and its Elemental analysis of all the films showed low-temperature structural phase transition at different annealing temperatures of RT, 100°C, 200°C and 300°C respectively. The optical studies of FeSe thin film on glass was found most different from all the others, and its bandgap decreases from 2.85 eV to 2.75 eV for different annealing temperatures of the thin films. Optical analysis is demonstrated that Ferrous (Fe) and Selenide (Se) are homogeneously distributed in the film. Atomic Force Microscopy (AFM) and its 2D and 3D analyses were found that microstructure of FeSe thin films.

Photoluminescence of Pr3+-doped calcium and strontium stannates

A series of Pr3+-doped CaSnO3, SrSnO3 and Ca2SnO4 samples were prepared by a conventional high temperature solid-state reaction route. All samples were characterized by powder X-ray diffraction (XRD) analysis, photoluminescence (PL), photoluminescence thermal quenching (TQ) and fluorescence lifetime (FL) measurements. Moreover, luminous efficacies (LE) and color points according to the CIE 1931 color space diagram were calculated and discussed. The incorporation of Pr3+ ions into CaSnO3 and SrSnO3 host materials results in intense white luminescence, which originates from the mixture of green, orange and red emissions. Pr3+-doped Ca2SnO4 phosphor emits intense red light upon excitation. Temperature dependent luminescence lifetime measurements of SrSnO3:Pr3+ phosphor showed abnormalities near 180 and 260K, which confirm the existence of a low temperature structural phase transitions in SrSnO3.

Electrical resistivity under extreme conditions in the Ce3Ir4Sn13 heavy fermion compound

We have performed measurements of temperature dependent electrical resistivity ρ(T) under pressures up to 27kbar and down to 0.1K on single crystals of the Ce3Ir4Sn13 heavy fermion compound. At ambient pressure (P=0) we have identified in the ρ(T) data interesting features associated with the presence of crystalline field effects, magnetic correlations, Kondo single impurity scattering and, possibly, a low temperature structural phase transition. All these features were mapped as a function of pressure which allowed us to construct a pressure–temperature phase diagram with these temperature scales. We have also carried out measurements of ρ(T) as a function of magnetic fields up to H=8T and the important temperature scales in ρ(T) were followed with field. Enlightened also by temperature dependent specific heat experiments we discuss the possible microscopic origins of the features found in our ρ(T) data.

Temperature dependence of the thermodynamic parameters of cobalt(II) clathrochelate: X-ray diffraction and calorimetric characteristics of the low temperature structural phase transition

The temperature dependence of heat capacity of the polycrystalline sample of cobalt(II) clathrochelate in a range of 6–300 K is studied. Based on the smoothed dependence C p(T), the entropy and enthalpy values in a temperature range of 8–300 K and their standard values at 298.15 K are calculated. In the C p(T) curve in a range of 50–70 K, a process is recorded whose entropy and enthalpy are 1.2 J·(K·mol−1) and 68 J·mol−1 respectively. A comparison of the results with the data of a multitemperature X-ray diffraction study makes it possible to attribute this process to the structural phase transition.

Electronic structure and magnetic ordering of the semiconducting chromium trihalidesCrCl3,CrBr3, andCrI3

We present results from an electronic structure investigation of the chromium halidesCrCl3,CrBr3, andCrI3, as obtained by the linearized augmented plane wave method of density functionaltheory. Our interest focuses on the chloride. While all three halides displaystrong ferromagnetic coupling within the halide–Cr–halide triple layers, ouremphasis is on differences in the interlayer magnetic coupling. In agreementwith experimental results, our calculations indicate ferromagnetic ordering forCrBr3 as well asCrI3. The antiferromagneticstate of CrCl3 can be reproduced by introducing an on-site electron–electron repulsion. However, we observe thatthe ground state depends critically on the specific approach used. Our results show that a lowtemperature structural phase transition from monoclinic to trigonal is energetically favourable forCrCl3.

Influence of deuteration on lithium acetate dihydrate studied by inelastic X-ray scattering, density functional theory, thermal expansion, elastic and thermodynamic measurements

The influence of deuteration on the properties of lithium acetate dihydrate has been investigated by thermal expansion measurements, ultrasound spectroscopy and calorimetry. Inelastic X-ray scattering has been employed to investigate if the low temperature structural phase transition can be detected by a change in the vibrational spectrum. Density functional theory, DFT, calculations have been employed to complement the experimental investigations. The thermal expansion coefficients and the specific heat of the deuterated compound differ significantly from the protonated form. The differences in the elastic stiffness coefficients are just above the detection limit of the technique employed here. Temperature dependent inelastic X-ray spectroscopic measurements show no significant change of the vibrational spectrum when crossing the transition temperature. The DFT calculations show that the methyl group dynamics are best described in the framework of coupled rotators of opposing methyl groups. One of the coupled rotational modes corresponds to a hindered rotator with a barrier of 15 meV, while the other is a free rotator.

Controlling the Decomposition Pathway of LiBH4 via Confinement in Highly Ordered Nanoporous Carbon

The wetting and decomposition behavior of LiBH4 has been investigated in the presence of highly ordered nanoporous hard carbon (NPC) with hexagonally packed 2 nm diameter columnar pores. Calorimetry, X-ray diffraction, and IR spectroscopy measurements confirm that the LiBH4 within the pores is amorphous. The confinement of LiBH4 in such small pores results in the disappearance of the low-temperature structural phase transition, the melting transition, and also the significant decrease of the onset desorption temperature from 460 to 220 °C with respect to bulk LiBH4, a lower temperature than observed in larger pore sizes in the literature. Most importantly, our results suggest that diborane release is suppressed or eliminated in the decomposition of noncrystalline LiBH4. Tight nanoconfinement may therefore mitigate both safety concerns and loss of active material in borohydride-based hydrogen storage systems.

Low-temperature fabrication of superconducting FeSe thin films by pulsed laser deposition

Superconducting FeSe thin films were prepared at a substrate temperature of 320 °C by pulsed laser deposition. X-ray diffraction and transmission electron microscopic analyses showed that highly c-axis-orientated and high-quality films were obtained on various substrate materials, including single-crystal MgO, LaAlO 3, SrTiO 3 and (100)-Si, and amorphous-SiO x, at such low temperature. From transport measurements all the films showed low-temperature structural phase transition at ∼ 60–90 K and superconducting transition at onset temperature varies from ∼ 7 K to < 2 K, depending on the substrate used. The transport property of FeSe film on Si was found most different from all the others, in spite of their similarity in structural analysis. Chemical analysis demonstrated that Fe and Se homogeneously distributed in the film and the stoichiometry of FeSe and the bonding states of Fe and Se are as well uniform along the film growth direction.

Low-temperature structural phase transition in deuterated and protonated lithium acetate dihydrate

Heat capacity measurements of protonated lithium acetate dihydrate show a structural phase transition at T = 12 K. This finding is in contrast to earlier work, where it was thought that only the deuterated compound undergoes a low temperature structural phase transition. This finding is confirmed by low temperature ultrasound spectroscopy, where the structural phase transition is associated with a velocity decrease of the ultrasonic waves, i.e. with an elastic softening. We compare the thermodynamic properties of the protonated and deuterated compounds and discuss two alternatives for the mechanism of the phase transition based on the thermal expansion measurements.