Polycrystalline Samples Research Articles

On the magnetocaloric effect and the spin-phonon coupling in the multiferroic GdMn2O5

The spin-phonon coupling and the magnetocaloric effect (MCE) in polycrystalline GdMn2O5 are investigated by Raman spectroscopy and isothermal magnetization curves. The frequencies of 680 and 700 cm−1 modes show anomalous temperature variations suggesting a strong spin-phonon coupling below the Néel temperature TN ∼ 38 K. The frequency deviation from standard temperature dependence is quantitatively interpreted within a simplified theoretical model accounting for the modulation of the exchange interaction by phonon vibrations. A relatively large magnetic entropy change, ∆Smax = 9.8 J/(kg K), is also achieved with magnetic fields up to 70 kOe. We found that the refrigerant capacity values in GdMn2O5, at different magnetic fields, are quite similar to those of TbMn2O5 and HoMn2O5 in the polycrystalline average approximation. However, the low-cost fabrication of the polycrystalline samples of GdMn2O5 and their large electric polarisation makes this material more suitable for possible cooperative magneto-electrocaloric refrigeration.

Absence of magnetostriction and transition from rapid positive thermal expansion to negative thermal expansion at high temperatures in Sm0.5Y0.5Fe0.58Mn0.42O3 – A synchrotron X-ray diffraction study

X-ray photoemission spectroscopic (XPS) studies and temperature dependent synchrotron X-ray diffraction (SXRD) investigation have been conducted on polycrystalline samples of Sm0.5Y0.5Fe0.58Mn0.42O3 [SYFM (58-42)]. Room temperature XPS measurements revealed the mixed valance of the Fe and Mn ions along with the traces of oxygen vacancies in the material. The room temperature synchrotron X-ray diffraction (SXRD) of specimen shows the monophasic nature of the prepared composition, without the presence of any magnetic or non-magnetic impurity phases. SXRD measurements at variable temperatures between 298 and 873 K, indicates the absence of magnetostriction in the specimen at the magnetic transition temperatures TSR1 ≈ 348 K, TSR2 ≈ 300 K and TN ≈ 361 K. At much higher temperatures above TN, an anisotropic rapid positive thermal expansion (RPTE) to negative thermal expansion (NTE) of the a lattice parameter of the unit cell is further revealed for T > 573 K. The anomalous variation of phonons, are suggested to give rise to anomalous thermal variation in a lattice parameter of the specimen.

Crystal Structure, Raman Spectroscopy and Optical Property Study of Mg-Doped SnO2 Compounds for Optoelectronic Devices

Researchers have been consistently looking for new materials that can be integrated in optoelectronic and spintronic devices. In this research, we investigated the crystalline structure, Raman, and optical characteristics of Mg-doped SnO2 compounds. The solid-state reaction technique was utilized to produce polycrystalline samples of Sn1−xMgxO2 (0 ≤ x ≤ 0.10) for their potential use in optoelectronics devices. It was discovered that all the compounds were synthesized into a tetragonal rutile-type structure of SnO2. The analysis of these samples using Raman spectroscopy provided more evidence, supporting the creation of the tetragonal rutile phase of SnO2 and the successful integration of Mg ions in SnO2. The measurements of the optical properties, such as absorbance and transmittance, carried out with a UV-Vis spectrophotometer demonstrated that the optical band gap widened with the increase in the magnesium doping concentration in SnO2. In addition, it was noticed that increasing the quantity of magnesium doping concentration led to an increase in the transmittance value from 83% to 91%.

Design and Synthesis of a Metallic Polar Aurivillius Oxide Bi2ReO6

The number of reported metallic compounds with polar crystal structures is limited due to the incompatibility of the acentricity and metallicity. Particularly, metallic oxides with polar crystal structures are rare. In this work, we designed a single-layer Aurivillius oxide Bi2ReO6 to explore metallic polar oxides. Single crystals and polycrystalline samples of Bi2ReO6 are synthesized by high-pressure methods. Bi2ReO6 crystallizes in the tetragonal structure with the polar space group P42cm (No. 101). Bi2ReO6 consists of alternative stacking of (Bi2O2)2+ layers and perovskite-like [ReO4]2– layers. Theoretical calculations further support that Bi2ReO6 with the polar space group is more stable and the tilts of ReO6 octahedra stabilize the system and break the inversion symmetry. Our magnetic and electrical data and low-temperature specific heat data indicate that Bi2ReO6 is a Pauli metal. The calculated electronic structures for Bi2ReO6 support the metallic nature, and the density of states at the Fermi level is contributed mainly from Re 5d orbitals hybridized with O 2p orbitals.

Crystalline materials in art and conservation: verdigris pigments – what we know and what we still don’t know

Abstract Verdigris is a collective term for synthetic copper based green and blue pigments that have been used by mankind since the antiquity. As the term is only loosely defined it covers a great variety of chemical compounds. All of them contain copper but also acetate, formate, hydroxide, water and sometimes also chloride, carbonate and sulphate. This article focusses on the verdigris phases related to the ternary system Cu(CH3COO)2–Cu(OH)2–H2O, which are commonly denoted according to their chemical composition as x–y–z phases. Besides neutral verdigris (1–0–0 and 1–0–1 phase), several basic verdigris phases (2–1–5, 1–1–5, 1–2–0, 1–3–0, 1–3–2, 1–4–3, 7–1–4) have been reported. These compounds can be obtained either by intentional corrosion of copper metal by direct contact with acetic acid and its vapours, which represents the historic pigment manufacturing or by incomplete precipitation from copper(II) acetate solutions. Due to their slow crystallisation behaviour the synthesis of verdigris pigments usually leads to multiphase and polycrystalline samples, which impedes the phase characterisation, in particular of the basic verdigris samples. Hence, most crystal structures that have been published were solved ab-initio from X-ray powder diffraction data by applying global optimisation methods like simulated annealing or charge flipping . Up to now the crystal structures of the 1–0–0, 1–0–1, 1–3–2, 1–2–0 and 2–1–5 phases were determined, the spectral and physical properties of these compounds completely characterise and therefore their existence and their chemical composition unambiguously proven. Investigations on historic verdigris samples and systematic synthesis approaches, however, show that there are at least four additional, hitherto unknown pigment phases. Information on the thermodynamical stability and the solubility of the verdigris phases are also lacking.

PECULIARITIES OF THE FORMATION OF SCINCILLATION RESPONSE IN ORGANIC MATERIALS WITH STOCHASTIC CHARACTER OF LIGHT PROPAGATION

The paper examines the possibility of using an organic polycrystal as the “opaque” scintillator. Polycrystals are produced by pressing crystalline grains. When light propagates through a polycrystal, it is repeatedly reflected and refracted at the boundaries of the grains. This makes its propagation difficult. We studied the light output and optical transmittance of stilbene and p-terphenyl polycrystals with different fractions of crystalline grain: from 0.06…0.1 to 2.0…2.5 mm (the samples 20 mm in diameter and 2 mm in height) was conducted. Modelling of light propagation in polycrystalline samples of stilbene and p-terphenyl was carried out and the values of the light collection coefficients were calculated. It was found that in order to obtain the polycrystalline samples with sufficiently high light output and high efficiency of detection of local sites of interaction of ionizing radiations, grains in the range of 0.4…0.8 mm should be used.

Tunable magnetic and electronic transport properties of Co1+xV2-xO4

Here we present a study on the electronic and magnetic properties of the members of Co1+xV2-xO4 series. The solid-state reaction method was used to grow the polycrystalline samples. Using the localised electron relationship and the various hopping transport regimes, we analyse the electronic transport outcomes. Also, the results of magnetic measurements reveal the ferrimagnetic phase transition and non-collinear spin configuration of these vanadate samples. Additionally, our work illustrates the variation of electronic and magnetic properties with the Co-concentration values.

Field-Induced Single-Ion Magnet Behavior in Nickel(II) Complexes with Functionalized 2,2':6'-2″-Terpyridine Derivatives: Preparation and Magneto-Structural Study.

Two mononuclear nickel(II) complexes of the formula [Ni(terpyCOOH)2](ClO4)2∙4H2O (1) and [Ni(terpyepy)2](ClO4)2 MeOH (2) [terpyCOOH = 4'-carboxyl-2,2':6',2″-terpyridine and terpyepy = 4'-[(2-pyridin-4-yl)ethynyl]-2,2':6',2″-terpyridine] have been prepared and their structures determined by single-crystal X-ray diffraction. Complexes 1 and 2 are mononuclear compounds, where the nickel(II) ions are six-coordinate by the six nitrogen atoms from two tridentate terpy moieties. The mean values of the equatorial Ni-N bond distances [2.11(1) and 2.12(1) Å for Ni(1) at 1 and 2, respectively, are somewhat longer than the axial ones [2.008(6) and 2.003(6) Å (1)/2.000(1) and 1.999(1) Å (2)]. The values of the shortest intermolecular nickel-nickel separation are 9.422(1) (1) and 8.901(1) Å (2). Variable-temperature (1.9-200 K) direct current (dc) magnetic susceptibility measurements on polycrystalline samples of 1 and 2 reveal a Curie law behavior in the high-temperature range, which corresponds to magnetically isolated spin triplets, the downturn of the χMT product at lower temperatures being due to zero-field splitting effects (D). Values of D equal to -6.0 (1) and -4.7 cm-1 (2) were obtained through the joint analysis of the magnetic susceptibility data and the field dependence of the magnetization. These results from magnetometry were supported by theoretical calculations. Alternating current (ac) magnetic susceptibility measurements of 1 and 2 in the temperature range 2.0-5.5 K show the occurrence of incipient out-phase signals under applied dc fields, a phenomenon that is characteristic of field-induced Single-Molecule Magnet (SMM) behavior, which herein concerns the 2 mononuclear nickel(II) complexes. This slow relaxation of the magnetization in 1 and 2 has its origin in the axial compression of the octahedral surrounding at their nickel(II) ions that leads to negative values of D. A combination of an Orbach and a direct mechanism accounts for the field-dependent relation phenomena in 1 and 2.

Concentration anomalies of galvanomagnetic properties of (Bi1-хSbх)2Te3 solid solutions based on Sb2Te3 in the (Bi1-хSbх)2Te3 system

Today, solid solutions based on antimony and bismuth tellurides are among the most widely used materials for the p-legs of thermoelectric converters used at room temperature and below. This paper presents the results of a study of galvanomagnetic properties (electrical conductivity σ, Hall coefficient RH, Hall mobility of charge carriers μH) of thermoelectric solid solutions (Bi1-хSbх)2Te3 in the range of compositions close to pure antimony telluride (x = 1 – 0.96) at temperatures T = 80 K and T = 300 K. The study was carried out on cast polycrystalline samples obtained by the method of crystallization from the melt followed by long-term annealing in vacuum at a temperature of T = 650 K. Galvanomagnetic properties were measured by the standard dc-method, the experimental cell was cooled with liquid nitrogen. The measurement error of RH and σ did not exceed ± 5%. It is shown that the introduction of the first portions of Bi2Te3 leads to a sharp decrease in σ, which is associated with a high degree of disorder of the crystal lattice. In the range of compositions x = 0.99 – 0.9825, the concentration dependences of σ(x) and μH(x) revealed anomalous growth of σ and μH. The presence of concentration anomalies is associated with the transition from dilute to concentrated solid solutions. An assumption is made about the percolation nature of the phase transition. Within the framework of the problem of spheres of percolation theory, within the framework of the task of spheres of the theory of percolation the radius of the deformation sphere of an impurity atom (Bi) is estimated. The obtained value is consistent with the short-range potential of the impurity. It is shown that the position of the anomalies in the σ(x) and μH(x) dependences does not change with the decrease in temperature down to 80K. Thus, the observation of concentration anomalies of the properties for the solid solution (Bi1-xSbx)2Te3 is another confirmation of the hypothesis about the universal nature of the behavior of solid solutions at a low impurity concentration. The detected anomalies must be taken into account when developing methods for increasing the thermoelectric performance of materials by creating solid solutions and doping.

Differences in the resistive and thermodynamic properties of the single crystalline chiral superconductor candidate SrPtAs

The locally noncentrosymmetric superconductor SrPtAs is proposed to host a topological chiral $d$-wave state, but experimental reports have been limited to polycrystalline samples. Here we report the synthesis of single crystalline SrPtAs grown from Pb flux. SrPtAs crystallizes in the hexagonal space group $P{6}_{3}/mmc$ with lattice parameters $a=4.2445(4)$ \AA{} and $c=8.9513(18)$ \AA{}. Magnetic susceptibility and electrical resistivity measurements reveal a superconducting transition at ${T}_{c}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}2.2$ K, in agreement with previous reports on polycrystalline samples. Surprisingly, heat capacity data show only a small bulk transition at 0.7 K. We discuss the possible origins of the discrepancy between the various measurements.

Supercurrent diode effect in thin film Nb tracks

We demonstrate nonreciprocal critical current in 65 nm thick polycrystalline and epitaxial Nb thin films patterned into tracks. The nonreciprocal behavior gives a supercurrent diode effect, where the current passed in one direction is a supercurrent and the other direction is a normal state (resistive) current. We attribute fabrication artifacts to creating the supercurrent diode effect in our tracks. We study the variation of the diode effect with temperature and the magnetic field and find a dependence with the width of the Nb tracks from 2 to 10 μm. For both polycrystalline and epitaxial samples, we find that tracks of width 4 μm provide the largest supercurrent diode efficiency of up to ≈30%, with the effect reducing or disappearing in the widest tracks of 10 μm. We propose a model based on the limiting contributions to the critical current density to explain the track width dependence of the induced supercurrent diode effect. It is anticipated that the supercurrent diode will become a ubiquitous component of the superconducting computer.

Synthesis and anisotropic magnetic properties of LiCrTe single crystals with a triangular-lattice antiferromagnetic structure

We report on the synthesis of LiCrTe single crystals and on their anisotropic magnetic properties. We have obtained these single crystals by employing a Te/Li-flux synthesis method. We find LiCrTe to crystallize in a TlCdS -type structure with cell parameters of a = 3.9512(5) Å and c = 6.6196(7) Å at T = 175 K. The content of lithium in these crystals was determined to be neary stoichiometric by means of neutron diffraction. We find a pronounced magnetic transition at = 144 K and = 148 K, respectively. These transition temperatures are substantially higher than earlier reports on polycrystalline samples. We have performed neutron powder diffraction measurements that reveal that the long-range low-temperature magnetic structure of single crystalline LiCrTe is an A-type antiferromagnetic structure. Our DFT calculations are in good agreement with these experimental observations. We find the system to be easy axis with moments oriented along the c-direction experimentally as well as in our calculations. Thereby, the magnetic Hamiltonian can be written as with K (where ). We find LiCrTe to be highly anisotropic, with a pronounced metamagnetic transition for with a critical field of (5 K) ≈ 2.5 T. Using detailed orientation-dependent magnetization measurements, we have determined the magnetic phase diagram of this material. Our findings suggest that LiCrTe is a promising material for exploring the interplay between crystal structure and magnetism, and could have potential applications in spin-based 2D devices.

Effects of V addition on microstructure and pseudoelastic response in Fe–Mn–Al–Ni alloys

Avoiding quench cracks and controlling the formation process of NiAl precipitates are essential to regulate the microstructure and pseudoelastic properties of Fe–Mn–Al–Ni shape memory alloys (SMAs). In this study, we investigated the effect of 1.5 at.%V addition on the quench sensitivity of Fe–Mn–Al–Ni SMAs and the precipitation behavior of NiAl precipitates in air-cooled Fe–Mn–Al–Ni–V SMAs. It is found that the V addition significantly inhibits the precipitation process of non-transforming γ-phase. Polycrystalline sample of bamboo structure was prepared by abnormal grain growth and tested by incremental strain cycling tensile test at room temperature. The polycrystalline Fe–Mn–Al–Ni–V SMAs obtain fully reversible thermoelastic martensitic transformation without quenching, which also prevents the formation of quenching cracks. Single crystal samples were further prepared by directional recrystallization. High number-density and relatively large-size coherent β-NiAl precipitates were obtained to strengthen the matrix in the single crystal without aging, resulting in large stress hysteresis and irrecoverable strain. After aging Fe–Mn–Al–Ni–V single crystal for 3 h at 200 °C, both the stress hysteresis and irrecoverable strain decreased, thereby achieving good pseudoelasticity in Fe–Mn–Al–Ni–V SMAs.

Character and Distribution of Geometrically Necessary Dislocations in Polycrystalline Tantalum.

Geometrically necessary dislocations (GNDs) play a key role in accommodating strain incompatibility between neighboring grains in polycrystalline materials. One critical step toward accurately capturing GNDs in deformation models involves studying the microstructural features that promote GND accumulation and the resulting character of GND fields. This study utilizes high-resolution electron backscatter diffraction to map GND populations in a large polycrystalline sample of pure tantalum, under simple tension. A total of 1,989 grains, 3,518 grain boundaries (GBs), and 3,207 triple junctions (TJs) were examined in a subsurface region of the sample. Correlations between GND density and GB character, and to some extent, TJ character, are investigated. Statistical geometrical relationships between these entities are quantified, and also visualized, using a novel application of two-point statistics. The nature of GNDs across the sample is also visualized and assessed using a recently developed method of mapping the local net Burgers vectors. The different approaches to characterizing GND distribution are compared in terms of how they quantify the size of near boundary gradient zones.

Effect of Ca substitution on structural, magnetic and dielectric properties of Sr2MnTiO6 double perovskite

We report the structural, magnetic and dielectric properties of polycrystalline samples of double perovskites Sr2-xCaxMnTiO6 (x = 0 and 1) synthesized by the solid-state reaction method. The compound Sr2MnTiO6 (x = 0) could be indexed in both cubic structure (space group Pm3‾m) and tetragonal structure (space group I4/m); whereas by replacing Sr by Ca, i.e., SrCaMnTiO6 (x = 1) orthorhombic structure space group Pbnm is observed. Along with the differences in the crystal structure of these two compounds, noticeable differences in the physical properties were also observed. The magnetization data of Sr2MnTiO6 reveals magnetic anomaly at TC ⁓ 14 K, whereas the magnetization data of SrCaMnTiO6 exhibits antiferromagnetic ordering at TN = 7.5 K. The conductivity and dielectric behavior of these samples investigated over a wide frequency range at room temperature exhibits dipolar relaxation.

Grain boundary- and triple junction-induced martensitic transformations: A phase-field study of effects of grain boundary width and energy

An original thermodynamically consistent large strains-based general multiphase phase-field (PF) approach of Ginzburg–Landau type is developed for studying the grain boundary (GB)- and triple junction (TJ)-induced martensitic transformations (MTs) in polycrystalline materials at the nanoscale considering the structural stresses within the interfaces. In this general PF approach, N+1 order parameters are used for describing the austenite (A)↔martensite (M) transformations and N(>1) martensitic variants and other M order parameters are used to describe M(>1) grains in the polycrystalline samples. Neglecting the kinetics of the GBs and TJs and assuming a uniform temperature of the sample, the system of coupled mechanics and Ginzburg–Landau equations for the order parameters related to MTs is derived. Distinct energies for the GBs in austenitic and martensitic phases due to their structural rearrangement are considered by using variable energy for the GB regions as a function of the order parameter related to the A↔M transformation and misorientations. The strong effects of the austenitic GB width (size-effect), the difference of GB energies of two phases, and applied strains on heterogeneous nucleation of the phases and their subsequent growth are explored in bicrystals with a symmetric planar tilt GB during the forward and reverse transformations. A rich plot for the temperatures of transformations between A, premartensite (the intermediate states between A and M), and M in a bicrystal is plotted for varying austenitic GB width exhibiting a strong GB size effect. The TJ energy and the energy and width of the adjacent GBs are also shown to significantly influence the nucleation and microstructures using the tricrystals having three symmetric planar tilt GBs meeting at 120° dihedral angles. The elastic and structural stresses across the interfaces are plotted, which is essential for understanding the role of GBs and TJs in material failure. The plausible reasons for the nonintuitive behaviour of the GBs on martensite nucleation observed in experiments and atomistic studies are explained.

Chemical Switching of Ferroaxial and Nonferroaxial Structures Based on Second-Order Jahn–Teller Activity in (Na,K)2Hf(BO3)2

Ferroaxial order, which is characterized by a vortex of electric dipole moments, is attracting more attention in terms of a new category of ferroic order. Here we propose that the disodium hafnium borate, Na2Hf(BO3)2, shows ferroaxial order that is stabilized by the second-order Jahn–Teller (SOJT) effect. Furthermore, the dipotassium hafnium borate, K2Hf(BO3)2, the synthesis of which has never been reported previously, is predicted to have a nonferroaxial ground state from the perspective of the SOJT effect. This prediction is demonstrated through our structural refinements on polycrystalline samples and our first-principles calculations with representation theory. This study offers a chemical perspective for developing new ferroaxial materials and controlling their ferroaxial order.

Inelastic neutron scattering in the weakly coupled triangular spin tube candidate CsCrF4

We performed inelastic neutron scattering (INS) experiments to measure spin dynamics on a polycrystalline sample of a spin tube candidate CsCrF$_{4}$. The compound exhibits a successive phase transition from a paramagnetic phase through an intermediate temperature (IT) phase of a 120$^{\circ}$ structure to a low temperature (LT) phase of another 120$^{\circ}$ structure. Elaborate comparison between observed and calculated neutron spectra in LT phase reveals that the spin Hamiltonian is identified as antiferromagnetic spin tubes including perturbative terms of intertube interaction, Dzyaloshinskii-Moriya interaction, and single ion anisotropy. A phase diagram for the ground state is classically calculated. A set of parameters in the spin Hamiltonian obtained from the INS spectra measured in LT phase is quite close to a boundary to the phase of the 120$^{\circ}$ structure of IT phase. The INS spectra measured in IT phase is, surprisingly, the same as those in LT phase in the level of powder averaged spectra, even though the magnetic structures in IT and LT phases are different. Identical dynamical structures compatible with two different static structures are observed. No difference in the observed spectra indicates no change of the spin Hamiltonian with the temperature, suggesting that the origin of the successive phase transition being order-by-disorder mechanism.

Influence of Sr and Ce Codoping on Structural, Electrical and Thermoelectric Properties of CaMnO3

Polycrystalline samples of CaMnO3 and Ca1-x-ySrxCeyMnO3 with Ce/Sr double substitution (x = 0.02, 0.04, 0.06; y = 0.02) were prepared by citrate sol-gel route. The samples’ phase was confirmed using the X-ray diffraction (XRD) technique, revealing that they only contain a single phase with an orthorhombic perovskite structure and no impurity phases. Also, the increase in lattice parameters with Sr2+ doping was observed. Rietveld refinement was done on the X-ray pattern using Fullprof software to find parameters. Using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), analyses of morphology and elemental compositions were performed. On sintered samples, electrical resistivity measurements were made using DC four-probe techniques. And conduction mechanism was explained based on a small polaron hopping mechanism. Between 308 K and 673 K, thermoelectric characteristics were studied. Ca0.94Sr0.04Ce0.02MnO3 has the highest power factor of all the prepared co-doped Ce/Sr samples.

Greatly enhanced mechanical properties of thermoelectric SnSe through microstructure engineering

The outstanding thermoelectric materials SnSe is also known for its inferior mechanical properties, which bring great inconvenience for its application in thermoelectric devices. In this work, SnSe bulks were prepared via a sequential procedure of high-pressure synthesis, ball milling, and spark plasma sintering. The produced polycrystalline samples with a unique microstructure of tightly-bound quasi-equiaxed grains exhibited excellent mechanical properties. The Vickers hardness, compressive strength, and bending strength reached 1.1 GPa, 300 MPa, and 90 MPa, respectively, all of which are far superior to those of ordinary polycrystalline SnSe. Furthermore, the microstructures didn’t deteriorate the thermoelectric performance. This work demonstrated an effective procedure to prepare polycrystalline microstructure-engineered SnSe materials, which not only show advantages in device applications, but also shed light on properties enhancement for other layer-structured thermoelectric materials.