Magnetocaloric effect and critical behavior in arylamine-based copper chloride layered organic-inorganic perovskite

Layered organic-inorganic hybrid perovskites have been the focus of much research regarding their optoelectronic and multiferroic properties. Here, we demonstrate the presence of a large magnetocaloric effect in the ferromagnetic layered perovskite phenylmethylammonium copper chloride ((PMA)2CuCl4) below the Curie temperature of ∼9.5 K. We measure a magnetic entropy change ranging from 0.88 J/kg.K to 2.98 J/kg.K in applied fields of 10 kOe and 70 kOe, respectively. We also study the nature of the magnetic phase transition using critical isotherm analysis. The critical exponents are consistent with the 2D-XY spin model

Static and dynamic magnetic properties of K3CrO4

We report on the magnetic properties of geometrically frustrated K3CrO4, in which Cr5+ cations are arranged on a distorted pyrochlore lattice. The crystal structure, static and dynamic magnetic properties of the compound are investigated in detail. A combination of DC and AC magnetic susceptibility measurements together with thermoremanent magnetization decay measurements reveal several magnetic transitions: the onset of glassy canted antiferromagnetic order occurs at 36 K, followed by the appearance of ferromagnetic/ferrimagnetic cluster glass behavior below the freezing temperature of 20 K. Further field-induced, temperature-dependent transitions are observed in the range 3-10 K. The frequency dependence of the freezing temperature for the cluster glass state is analyzed on the basis of dynamic scaling laws including the critical slowing down formula and the Vogel-Fulcher law.Comment: A high-resolution version with supplementary material can be found at https://www.sciencedirect.com/science/article/pii/S0304885321004893?via%3Dihub. arXiv admin note: text overlap with arXiv:1912.0599

Polar Structure and Two-Dimensional Heisenberg Antiferromagnetic Properties of Arylamine-Based Manganese Chloride Layered Organic-Inorganic Perovskites

The breaking of inversion symmetry can enhance the multifunctional properties of layered hybrid organic-inorganic perovskites. However, the mechanisms by which inversion symmetry can be broken are not well-understood. Here, we study a series of MnCl4-based 2D perovskites with arylamine cations, namely, (C6H5CxH2xNH3)2MnCl4 (x = 0, 1, 2, 3), for which the x = 0, 1, and 3 members are reported for the first time. The compounds with x = 1, 2, and 3 adopt polar crystal structures to well above room temperature. We argue that the inversion symmetry breaking in these compounds is related to the rotational degree of freedom of the organic cations, which determine the hydrogen bonding pattern that links the organic and inorganic layers. We show that the tilting of MnCl6 octahedra is not the primary mechanism involved in inversion symmetry breaking in these materials. All four compounds show 2D Heisenberg antiferromagnetic behavior. A ferromagnetic component develops in each case below the long-range magnetic ordering temperature of μ42-46 K due to spin canting

Thermoelectric Performance of Single-Phase Tellurium-Reduced Quaternary (PbTe)(0.55)(PbS)(0.1)(PbSe)(0.35)

Lead chalcogenide quaternary systems have been shown to provide high thermoelectric (TE) efficiency superior to those of binary and ternary lead chalcogenides, arising from both altered electronic band structures and a reduction in lattice thermal conductivity. Here, we have synthesized single-phase samples of the quaternary compound (PbTe)0.55(PbS)0.1(PbSe)0.35 doped with Na and characterized their TE properties. We show that the dopant solubility is limited to 1 at. %. A very low lattice thermal conductivity of ∼0.6 W m–1 K–1 at 850 K is achieved at all dopant concentrations because of phonon scattering from point defects associated with solute atoms with high contrast atomic mass. As a result, a high TE figure of merit of approximately 1.5 is achieved at 823 K in heavily doped samples. Moreover, the figure of merit is greater than 1 over a wide temperature range above 675 K

Impact of two diammonium cations on the structure and photophysics of layered Sn-based perovskites

Layered metal-halide perovskites have shown great promise for applications in optoelectronic devices, where a large number of suitable organic cations give the opportunity to tune their structural and optical properties. However, especially for Sn-based perovskites, a detailed understanding of the impact of the cation on the crystalline structure is still missing. By employing two cations, 2,2′-oxybis(ethylammonium) (OBE) and 2,2′-(ethylenedioxy)bis(ethylammonium) (EDBE), we obtain a planar 〈100〉 and a corrugated 〈110〉-oriented perovskite, respectively, where the hydrogen bonding between the EDBE cations stabilises the corrugated structure. OBESnI4 exhibits a relatively narrow band gap and photoluminescence bands compared to EDBESnI4. In-depth analysis shows that the markedly different optical properties of the two compounds have an extrinsic origin. Interestingly, thin films of OBESnI4 can be obtained both in black and red colours. This effect is attributed to a second crystalline phase that can be obtained by processing the thin films at 100 °C. Our work highlights that the design of the crystal structure as obtained by ligand chemistry can be used to obtain the desired optical properties, whereas thin film engineering can result in multiple crystalline phases unique to Sn-based perovskites.</p

Elucidating the Structure and Photophysics of Layered Perovskites through Cation Fluorination

Optoelectronic devices based on layered perovskites containing fluorinated cations display a well-documented improved stability and enhanced performance over non-fluorinated cations. The effect of fluorination on the crystal structure and photophysics, however, has received limited attention up until now. Here, 3-fluorophenethylammonium lead iodide ((3-FPEA)(2)PbI4) single crystals are investigated and their properties to the non-fluorinated ((PEA)(2)PbI4) variant are compared. The bulkier 3-FPEA cation increases the distortion of the inorganic layers, resulting in a blue-shifted absorbance and photoluminescence. Temperature-dependent photoluminescence spectroscopy reveals an intricate exciton substructure in both cases. The fluorinated variant shows hot-exciton resonances separated by 12 to 15 meV, values that are much smaller than the 40 to 46 meV found for (PEA)(2)PbI4. In addition, high-resolution spectra show that the emission at lower energies consists of a substructure, previously thought to be a single line. With the analysis on the resolved photoluminescence, a vibronic progression is excluded as the origin of the emission at lower energies. Instead, part of the excitonic substructure is proposed to originate from bound excitons. This work furthers the understanding of the photophysics of layered perovskites that has been heavily debated lately

Double Perovskite Single-Crystal Photoluminescence Quenching and Resurge:The Role of Cu Doping on its Photophysics and Crystal Structure

Cs2AgBiBr6 is a potential lead-free double perovskite candidate for optoelectronic applications; however, its large and indirect band gap imposes limitations. Here, single crystals of Cs2AgBiBr6 are doped with Cu2+ cations to increase the absorption range from the visible region up to 0.5 eV in the near-infrared region. Inductively coupled plasma spectroscopy confirms the presence of 1.9% of copper in the Cs2AgBiBr6 structure. Structural and optical changes caused by Cu doping were studied by Raman spectroscopy combined with X-ray diffraction, heat capacity measurements, and low-temperature photoluminescence spectroscopy. Along with the 1.9 eV emission typical of the pristine Cs2AgBiBr6 single crystals, we report a novel low-energy emission at 0.9 eV related to deep defects. In the doped crystals, these peaks are quenched, and a new emission band at 1.3 eV is visible. This new emission band appears only above 120 K, showing that thermal energy is necessary to trigger the copper-related emission

Ratio effect of salt fluxes on structure, dielectric and magnetic properties of La,Mn-doped PbBi2Nb2O9 Aurivillius phase

The double-layer Aurivillius phase Pb0.4Bi2.1La0.5Nb1.7Mn0.3O9 was synthesized by a molten salt method using a K2SO4/Na2SO4 flux. The effect on the crystal structure, morphology, dielectric and magnetic properties of varying the molar ratio of the oxide precursors to salt flux was investigated. Single-phase products with an orthorhombic structure were obtained for oxide to salt ratios of between 1:5 and 1:9, whereas for lower concentrations of salt a pyrochlore impurity phase is found in the products. SEM showed anisotropic plate-like grains, the size of which increases for larger salt ratios. An investigation of the magnetic properties showed the presence of mixed Mn3+ and Mn4+; the unit cell volume of the single-phase products decreases as the proportion of salt increases, which implies a higher proportion of smaller Mn4+ cations. This can be explained by the oxide ion donating properties (oxobasicity) of the molten salt mixture, which produces an oxidizing environment during synthesis. The best dielectric properties are obtained for an oxide to salt ratio of 1:7, exhibiting relaxor ferroelectric behavior. This is also the ratio at which the most pronounced ferromagnetic properties are observed, resulting from double-exchange interactions between Mn3+ and Mn4+ the proportions of which are approximately equal. Pb(0.4)Bi(2.1)La(0.5)Nb(1.7)Mn(0.3)O(9 )synthesized under these conditions thus exhibits optimal multiferroic properties

Structure-property relationships in the lanthanide-substituted PbBi₂Nb₂O₉ Aurivillius phase synthesized by the molten salt method

Samples of PbBi2Nb2O9, PbBi1.5La0.5Nb2O9, and PbBi1.5Nd0.5Nb2O9 have been prepared by the molten salt method. The structure, morphology, and electrical properties were investigated. All samples are single-phase and crystallize in an orthorhombic structure with A21am symmetry. Neutron diffraction data indicate that the Ln3+ cations prefer to occupy the perovskite A-site, whereas Pb/Bi occupy the perovskite A-site and the Bi2O2 layer. Changes in unit cell volume are observed on substitution and are attributed to the ionic radii of the Ln3+ cations and also correlated to changes in the B-O bond distances in the BO6 octahedra, which are also observed in IR spectra. SEM images reveal anisotropic plate-like grains, which increase in size with the presence of Ln3+ ions. The ferroelectric transition temperature (Tc) decreases with decreasing degree of BO6 distortion as the influence of the 6s2 lone pair of Bi3+ is diminished. Relaxor ferroelectric behavior is observed with Ln3+ substitution, driven by the disorder of the A-site cations. The room temperature ferroelectric polarization increases with Ln3+ substitution, ascribed to the decreased dielectric loss

Construction of new 1D and 2D coordination polymers generated from rigid N,N′-bis(4-pyridylmethylene)-1,5-naphthalenediamine ligand:Syntheses, crystal structures and luminescence properties

Treatment of N,N′-bis(4-pyridylmethylene)-1,5-naphthalenediamine (L) with Pb(OAc)2/KBr, Cu(acac)2, and Cu(OAc)2 afforded three new coordination polymers [Pb(μ-L)(μ-Br)2]n (1), [Cu(μ-L)(acac)2]n (2), and [Cu2(μ-L)(μ-OAc)4]n (3). These coordination polymers have been structurally characterized by single crystal X-ray diffraction. Compound 1 has a 2D sheet structure in which the lead(II) centers are bridged by both L and bromide ligands. Compound 2 adopts a 1D neutral coordination chain and consists of bis(2,4-pentanedionato) copper(II) units connected by rigid bridging L ligands. The structure of compound 3 also adopts a 1D neutral coordination chain in which two copper centers are connected through four acetate groups to form a Cu(OAc)4Cu paddle-wheel-type cage between two bridging L ligands. The FT-IR spectra, thermal behavior and photoluminescence properties of these coordination polymers have also been investigated.</p