Temperature-dependent X-ray Diffraction Research Articles

Unique multiferroics with tunable ferroelastic transition in antiferromagnet Mn2V2O7

Multiferroics have received considerable interest over the last decade due to the fascinating fundamental phenomena and potential use in various applications, such as low-power electronics and spintronics. Among those, investigations have focused on the coexistence of ferroelectric and ferromagnetic materials. Here, we report the rare case that the para-to ferroelastic ordering transition in antiferromagnet Mn2V2O7 occurred at TS = 260–280 K, verified by temperature-dependent magnetization measurements, dielectric, differential scanning calorimetry, and macroscopic strain-stress hysteresis loops. Furthermore, this transition was accompanied by a structural transition from the high-temperature C2/m monoclinic phase (β-phase) to a low-temperature P1¯ triclinic phase (α-phase), as identified by temperature-dependent X-ray diffraction. Consequently, TS can be successfully increased by Co- and Ni-doping and decreased by Ca-doping. Thus, the phase diagram was established for the structural stability of (Mn1-xAx)2V2O7 (A = Co, Ni, and Ca). In addition, the physical and chemical pressure effects were applied on (Mn1-xCax)2V2O7 to correlate the ferroelastic (TS) and antiferromagnetic (TN) orderings. Consequently, the magnetoelastic coupling was revealed, and a unique multiferroic material (Mn2V2O7) with a ferroelastic and antiferromagnetic ordering was obtained.

The effects of Cu-substitution and high-pressure synthesis on phase transitions in Ni2MnGa Heusler alloys

The magnetic, structural, and thermal behaviors of the Cu-doped Heusler alloy Ni2Mn1−xCuxGa (0 ⩽ x ⩽ 0.4) were studied as a function of concentration x. As the Cu concentration increased, the structural transition temperatures increased, whereas the chemical order-disorder transitions and melting points decreased. The experimental results from temperature dependent X-ray diffraction reveal different crystal structures of the martensite phase at low temperatures for samples with different x, but all the samples ultimately crystallized in the L21 cubic crystal structure upon heating above their respective structural transitions. The experimental data were used to construct a comprehensive magnetic and structural phase diagram as a function of x from below their respective structural transition temperatures to their melting temperatures. The XRD analysis shows that the observed volume reduction is associated with the increasing structural transition temperature. Therefore, one of the samples was annealed under high pressure to permanently reduce its volume, and the correlation between the increasing structural transition temperatures and volumes was confirmed.

Studies of the Temperature Dependence of the Structure and Magnetism of a Hexagonal-Bipyramidal Dysprosium(III) Single-Molecule Magnet.

The hexagonal-bipyramidal lanthanide(III) complex [Dy(OtBu)Cl(18-C-6)][BPh4] (1; 18-C-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane ether) displays an energy barrier for magnetization reversal (Ueff) of ca. 1000 K in a zero direct-current field. Temperature-dependent X-ray diffraction studies of 1 down to 30 K reveal bending of the Cl-Ln-OtBu angle at low temperature. Using ab initio calculations, we show that significant bending of the O-Dy-Cl angle upon cooling from 273 to 100 K leads to a ca. 10% decrease in the energy of the excited electronic states. A thorough exploration of the temperature and field dependencies of the magnetic relaxation rate reveals that magnetic relaxation is dictated by five mechanisms in different regimes: Orbach, Raman-I, quantum tunnelling of magnetization, and Raman-II, in addition to the observation of a phonon bottleneck effect.

Spin-lattice coupling in an epitaxial NdFeO3 thin film

Rare-earth orthoferrite RFeO3 materials such as NdFeO3 are strongly studied because of their fascinating magnetic properties and their potential applications. Here, we show the successful epitaxial synthesis of parasitic-free NFO thin film by pulsed laser deposition on (001)-SrTiO3. High-resolution X-ray diffraction shows a coherent growth and a tetragonal-like structure of a tensile strained 80 nm thick NFO film in contrast with the bulk orthorhombic state. Room temperature magnetometry indicates a bulk-like antiferromagnetic state for the NFO film. Temperature-dependent X-ray diffraction and magnetometry highlight a significant spin-lattice coupling at the Néel Temperature while a new magneto-structural instability is discovered around 250 °C that needs further investigation.

Ferroelectric and magnetic properties in ε-Fe2O3 epitaxial film

The phase stability, ferroelectricity, and magnetism of highly crystalline epitaxial ε-Fe2O3 films deposited on SrTiO3(111) substrates are reported. Temperature-dependent x-ray diffraction measurements revealed that α-Fe2O3 appeared as a secondary phase after samples were annealed up to 1000 °C. A clear saturated and opened polarization–electric field hysteresis loop with the remnant polarization ∼2.6 μC cm−2 and coercive electric field ∼100 kV cm−1 was obtained at room temperature using a conventional ferroelectric measurement technique. The magnetic phase transition was observed at 155 K. Magnetization–magnetic field measurements revealed that magnetic softening via the phase transition accompanied a reduction in the coercive field from 10 to 6.8 kOe, which is smaller than the coercive field reduction previously observed for ε-Fe2O3 nanoparticles.

Probing the Role of Local Structure in Driving the Stability of Halide Perovskites CH3NH3PbX3

Organic–inorganic lead halide perovskites have rapidly emerged as one of the leading contenders in optoelectronic and photovoltaic technology, but they show stability issues. Among methylammonium (MA) lead bromide and MA lead iodide, the Br counterpart is reported to exhibit improved stability under ambient conditions. However, the underlying fundamentals of such an enhancement are not fully understood. Using temperature-dependent X-ray diffraction and extended X-ray absorption fine structure measurements, we find relatively reduced value of linear coefficient of thermal expansion, elevated Einstein temperature, and reduced dynamic disorder in MAPbBr3 compared to MAPbI3 which implies that a relatively rigid Pb–Br bond offers steric hindrance to the migration of bulky MA ion resulting in improved stability of MAPbBr3.

Thermal anomalies and phase transitions in Pb2Sc2Si2O9 and Pb2In2Si2O9

Abstract Pb2Sc2Si2O9 and Pb2In2Si2O9, respectively, the scandium and indium containing structural analogues of the mineral kentrolite are grown by spontaneous crystallization from a PbO flux. The corresponding polycrystalline powder samples are synthesized by conventional solid-state approach. The compounds are thoroughly characterized using temperature-dependent single crystal and powder X-ray diffraction, heat capacity measurements, second harmonic generation experiments and Raman spectroscopy. At ambient conditions, both compounds crystallize in the non-centrosymmetric Pna21 space group and undergo phase transitions to the centrosymmetric Pbcn space group at elevated temperatures. The Pbcn into Pna21 phase transitions are complemented by the signals of the temperature-dependent second harmonic generation. The specific heat capacity exhibits distinct cusp, supporting the λ-type second-order phase transition. The temperature dependency of some selective Raman modes further complements the findings, showing softening and hardening of the phonons across the phase transitions.

Cu9O2(SeO3)4Cl6 revisited: Crystal structure, Raman scattering and first-principles calculations

Chloromenite, Cu9O2(SeO3)4Cl6, is the product of volcanic activity. To conduct various physical studies both natural and synthetic samples of chloromenite were used which resulted in a long-standing controversy regarding its crystal structure. We carried out the temperature dependent single crystal X-ray diffraction study in the range 100 – 370 K and established that the reflection conditions are consistent with a primitive monoclinic cell with the only possible P21/n space group. Raman scattering evidences a complex interplay of crystal lattice and spin degrees of freedom. The observation of a sequence of short range fluctuation effects points to the existence of competing interactions or nearly degenerate energy scales. To gain microscopic understanding of the magnetic properties of Cu9O2(SeO3)4Cl6 the first principles density functional theory calculations have been performed. Basing on the new data on magnetization and the hierarchy of exchange interaction parameters a model of Cu9O2(SeO3)4Cl6 magnetic structure is suggested.

Observation of Exchange Bias in Antiferromagnetic Cr0.79Se Due to the Coexistence of Itinerant Weak Ferromagnetism at Low Temperatures.

We report on the structural, electrical transport, and magnetic properties of antiferromagnetic transition-metal monochalcogenide Cr0.79Se. Different from the existing off-stoichiometric compositions, Cr0.79Se is found to be synthesized into the same NiAs-type hexagonal crystal structure as that of CrSe. Resistivity data suggest Cr0.79Se to be a Fermi-liquid-type metal at low temperatures, while at intermediate temperatures, the resistivity depends sublinearly on the temperature. Eventually, at elevated temperatures, the rate of change of resistivity rapidly decreases with increasing temperature. Magnetic measurements suggest a transition from the paramagnetic phase to an antiferromagnetic phase at a Néel temperature of 225 K. Further reduction of the sample temperature results in the coexistence of weak ferromagnetism along with the antiferromagnetic phase below 100 K. As a result, below 100 K, we identify a significant exchange bias due to the interaction between the ferro- and antiferromagnetic phases. In addition, from temperature-dependent X-ray diffraction measurements, we observe that the NiAs-type structure is stable up to as high as 600 °C.

Oxygen vacancy migration and its lattice structural origin in A-site non-stoichiometric bismuth sodium titanate perovskites

Off-stoichiometry of perovskite structural Bi0.5Na0.5TiO3 (BNT) ferroelectrics can give rise to considerable oxide-ion conductivity. The inherent structural characteristics are urgent to be resolved due to its particular sensitivity of the conduction mechanism to the nominal composition and synthesis process. Herein, a thorough study of the temperature-dependent neutron, X-ray diffraction and Raman spectrum is carried out on a series of equivalently substituted A-site deficient non-stoichiometric and pristine BNT. Phase transition and defect association are systemically investigated in these dominated rhombohedral phases at room temperature, associated with well saturated ferroelectric states. Significant structural evolution identified by Rietveld refinements and the origin of the electrical performance are clarified at elevated temperatures, focusing on the subtle distortions of ionic displacements, oxygen octahedral tilts and local chemical environments for oxygen vacancies. The ion migration ability mediated by oxygen vacancies that are not energetically favorable in BNT mainly depends on the external substitutional disorder, and is strongly affected by the dopant concentration. Together with the lone pair substitution concept, superior oxide ionic conductivity is achieved, and an alternative strategy is provided in designing BNT based oxide ion conductors.

Anomalous charge density wave state evolution and dome-like superconductivity in CuIr2Te4−x Se x chalcogenides

We report the anomalous charge density wave (CDW) state evolution and dome-like superconductivity in CuIr2Te4−x Se x (0 ⩽ x ⩽ 0.5) series. Room temperature powder x-ray diffraction (XRD) results indicate that CuIr2Te4−x Se x (0 ⩽ x ⩽ 0.5) compounds retain the same structure as the host CuIr2Te4 and the unit cell constants a and c manifest a linear decline with increasing Se content. Magnetization, resistivity and heat capacity results suggest that superconducting transition temperature (T c) exhibits a weak dome-like variation as substituting Te by Se with the maximum T c = 2.83 K for x = 0.1 followed by suppression in T c and simultaneous decrease of the superconducting volume fraction. Unexpectedly, the CDW-like transition (T CDW) is suppressed with lower Se doping (0.025 ⩽ x ⩽ 0.2) but re-emerges at higher doping (0.25 ⩽ x ⩽ 0.5). Meanwhile, the temperature-dependent XRD measurements show that the trigonal structure is stable at 20 K, 100 K and 300 K for the host sample and the doping composition with x = 0.5, thus we propose that the behavior CDW-like transition arises from the disorder effect created by chemical doping and is not related to structural transition. The lower and the upper critical fields of these compounds are also addressed.

Coexisting Multiple Martensites in Ni57−xMn21+xGa22 Ferromagnetic Shape Memory Alloys: Crystal Structure and Phase Transition

A comprehensive study of the crystal structure and phase transition as a function of temperature and composition in Ni57−xMn21+xGa22 (x = 0, 2, 4, 5.5, 7, 8) (at. %) magnetic shape memory alloys was performed by a temperature-dependent synchrotron X-ray diffraction technique and transmission electron microscopy. A phase diagram of this Ni57−xMn21+xGa22 alloy system was constructed. The transition between coexisting multiple martensites with monoclinic and tetragonal structures during cooling was observed in the Ni51.5Mn26.5Ga22 (x = 5.5) alloy, and it was found that 5M + 7M multiple martensites coexist from 300 K to 160 K and that 5M + 7M + NM multiple martensites coexist between 150 K and 100 K. The magnetic-field-induced transformation from 7M martensite to NM martensite at 140 K where 5M + 7M + NM multiple martensites coexist before applying the magnetic field was observed by in situ neutron diffraction experiments. The present study is instructive for understanding the phase transition between coexisting multiple martensites under external fields and may shed light on the design of novel functional properties based on such phase transitions.

Ultrahigh thermal conductivity in three-dimensional covalent organic frameworks

Covalent organic frameworks (COFs) are a new, growing class of crystalline polymers with exceptional characteristics such as low density, mechanical strength, and flexibility. Taking three-dimensional (3D) COF-300 derivatives as an example, we apply molecular dynamics simulations to show that ultrahigh thermal conductivity (>15 W/(mK)) is achieved at a pore size of d = 0.63 nm. This is the first observation of such an outstanding intrinsic thermal conductivity of a 3D polymer in all directions. Further temperature-dependent thermal conductivity calculations and X-ray diffraction (XRD) patterns highlight the importance of maintaining microscopic order for a large thermal conductivity. We also demonstrate that the tunability and anisotropy in COFs provide sufficient room to engineer and obtain desired thermal conductivity. These findings provide a fundamental understanding of structure-thermal conductivity relation in COFs and shine a light on high-thermal-conductivity COFs for thermal management applications.

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.

Tuning the metal insulator transition of vanadium dioxide on oxide nanosheets

For practical applications, tuning the metal-insulator transition (MIT) behavior of high-quality vanadium dioxide (VO2) on arbitrary substrates, such as Si and glass, is desirable. Here, we demonstrate the ability to tune the MIT temperature (TMIT) of VO2 films by growing them on NbWO6 (NWO) nanosheets on arbitrary substrates and varying the film thicknesses. The oxidation and crystal structure of VO2 films are determined by x-ray photoelectron spectroscopy and temperature-dependent x-ray diffraction, respectively. It is observed that as the film thickness increases, the TMIT also increases to the bulk value, 341 K, because of the increase in the rutile c-axis of VO2. The strain effect accompanying with the film thickness variation on NWO nanosheets contribute to the shortening of the rutile cR axis in thin films and, hence, the lowering of TMIT of VO2. Furthermore, the arbitrary underlying substrates have negligible influence on the MIT behavior of VO2 on NWO nanosheets. These results open up the possibility to more freely choose a technical substrate material for functional VO2 films and tune its MIT.

Pressure-induced anomalies in the magnetic transitions of the exotic multiferroic material Tb2BaNiO5

We have studied the influence of external pressure up to 1 GPa on the magnetic transitions of the orthorhombic Haldane-spin chain compound ${\mathrm{Tb}}_{2}{\mathrm{BaNiO}}_{5}$, an exotic multiferroic material. This parent compound is known to undergo N\'eel ordering at ${T}_{N1}=63\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and another magnetic transition at ${T}_{N2}=25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at which ferroelectricity sets in, however, without any change in the magnetic symmetry, but with only a sharp change in the canting angle of Tb $4f$ and Ni $3d$ magnetic moments. There is a subtle difference in the antiferromagnetic state above and below ${T}_{N2}$, which is supported by the fact that there is a metamagnetic transition below ${T}_{N2}$ only (for 5 K, at \ensuremath{\sim}60 kOe). We report here that, with the application of external pressure, there is an upward shift of ${T}_{N1}$, while ${T}_{N2}$ shifts towards lower temperatures. It is interesting that the two magnetic transitions in the same compound behave differently under pressure and the opposite behavior at ${T}_{N2}$ is attributed to local distortion leading to ferroelectricity. The results are augmented by temperature dependent x-ray diffraction and positive chemical pressure studies. The chemical pressure caused by the isoelectronic doping at Ba site by Sr reduces both the transition temperatures. Clearly, the external pressure favors antiferromagnetic coupling (that is, leading to ${T}_{N1}$ enhancement), whereas the chemical pressure reduces ${T}_{N1}$, suggesting an important role of the changes in local hybridization induced by doping on magnetism in this material.

The Observation of Interchain Motion in Self-Assembled Crystalline Platinum(II) Complexes: An Exquisite Case but By No Means the Only One in Molecular Solids.

In organic and organometallic solids, upon electronic excitation, most intermolecular structural relaxations follow a pathway along the π-π stacking direction or metal-metal bond with significant coupling strength. Differently, we discovered that the self-assembled platinum(II) complexes, Pt(fppz)2, exhibit an unusual interchain contraction. The ground-state and excited-state multiple local minima were distinguished by temperature-dependent excitation/emission spectra, indicating the existence of multiple local minima. The time-resolved emission decay revealed the excited-state structural relaxation lifetime with τobs = 41 ns at 298 K. Temperature-dependent X-ray diffraction analysis showed that the packing geometries contract 0.6 Å along the interchain direction (a-axis) at 50 K compared to the geometries at 298 K. Such structural displacements render the slow internal conversion rate in the excited states. We thus demonstrate the correlation between the packing geometries and the excited-state dynamics of the self-assembled Pt(II) complexes, shedding light on the unique direction of interchain structural deformation of the molecular aggregates.

Role of A and B site doping on antiferrodistortive phase transition in sodium bismuth titanate

We report the results of temperature-dependent powder X-ray diffraction studies on lead-free piezoelectric sodium bismuth titanate (NBT) modified by partially substituting Ba at the A site or Nb at the B site. We observed the disappearance of certain reflections and a change in the main perovskite peaks of the rhombohedral phase suggesting a structural phase transition. Detailed Rietveld refinements revealed that both the ferroelectric and the antiferrodistortive phase transitions occur together at high temperature leading to a change in crystal structure from the low-temperature ferroelectric rhombohedral phase (space group: R3c) to the high-temperature paraelectric cubic phase (space group:Pm3¯m). This suggests the suppression of the intermediate tetragonal phase of pure NBT. We found that substituting Ba onto the A site enhances the lattice strain and lowers the antiferrodistortive phase transition temperature.

Systematic Exploration of the Synthetic Parameters for the Production of Dynamic VO2(M1).

Thermochromic dynamic cool materials present a reversible change of their properties wherein by increasing the temperature, the reflectance, conductivity, and transmittance change due to a reversible crystalline phase transition. In particular, vanadium (IV) dioxide shows a reversible phase transition, accompanied by a change in optical properties, from monoclinic VO2(M1) to tetragonal VO2(R). In this paper, we report on a systematic exploration of the parameters for the synthesis of vanadium dioxide VO2(M1) via an easy, sustainable, reproducible, fast, scalable, and low-cost hydrothermal route without hazardous chemicals, followed by an annealing treatment. The metastable phase VO2(B), obtained via a hydrothermal route, was converted into the stable VO2(M1), which shows a metal–insulator transition (MIT) at 68 °C that is useful for different applications, from energy-efficient smart windows to dynamic concrete. Within this scenario, a further functionalization of the oxide nanostructures with tetraethyl orthosilicate (TEOS), characterized by an extreme alkaline environment, was carried out to ensure compatibility with the concrete matrix. Structural properties of the synthesized vanadium dioxides were investigated using temperature-dependent X-ray Diffraction analysis (XRD), while compositional and morphological properties were assessed using Scanning Electron Microscopy, Energy Dispersive X-ray Analysis (SEM-EDX), and Transmission Electron Microscopy (TEM). Differential Scanning Calorimetry (DSC) analysis was used to investigate the thermal behavior.

Observation of structural anomaly and low-field magnetocaloric effect in Cu2OSeO3

We report here the detailed investigation on structural and magnetic behavior of the polycrystalline sample of Cu2OSeO3. The Mott insulator Cu2OSeO3 is a fascinating material showing skyrmionic state and magneto-electric coupling. We observe clear signature of thermal hysteresis between field-cooled and field-cooled heating data in the low-field magnetization versus temperature curve just below the magnetic ordering temperature of TC ~ 60 K. Temperature dependent powder x-ray diffraction data indicate the presence of lattice anomaly below TC. The sample also shows reasonable value of conventional magnetocaloric effect for small value of applied magnetic field, which can be related to the change in the magnetic structure with field.