Search

Abstract Plastic crystals as barocaloric materials exhibit the large entropy change rivalling freon, however, the limited pressure-sensitivity and large hysteresis of phase transition hinder the colossal barocaloric effect accomplished reversibly at low pressure. Here we report reversible colossal barocaloric effect at low pressure in two-dimensional van-der-Waals alkylammonium halides. Via introducing long carbon chains in ammonium halide plastic crystals, two-dimensional structure forms in (CH3–(CH2)n-1)2NH2X (X: halogen element) with weak interlayer van-der-Waals force, which dictates interlayer expansion as large as 13% and consequently volume change as much as 12% during phase transition. Such anisotropic expansion provides sufficient space for carbon chains to undergo dramatic conformation disordering, which induces colossal entropy change with large pressure-sensitivity and small hysteresis. The record reversible colossal barocaloric effect with entropy change ΔSr ~ 400 J kg−1 K−1 at 0.08 GPa and adiabatic temperature change ΔTr ~ 11 K at 0.1 GPa highlights the design of novel barocaloric materials by engineering the dimensionality of plastic crystals.

Materials with zero thermal expansion (ZTE) or precisely tailored thermal expansion are in urgent demand of modern industries. However, the overwhelming majority of materials show positive thermal expansion. To develop ZTE or negative thermal expansion (NTE) materials as compensators has become an important challenge. Here, we present the evidence for the realization of ultra-low thermal expansion in Mn–Co–Ge–In particles. The bulk with the Ni2In-type hexagonal structure undergoes giant NTE owing to a martensitic magnetostructural transition. The major finding is that the thermal expansion behavior can be totally controlled by modulating the crystallinity degree and phase transition from atomic scale. Self-compensation effect leads to ultra-low thermal expansion with a linear expansion coefficient as small as +0.68 × 10−6/K over a wide temperature range around room temperature. The present study opens an avenue to reach ZTE particularly from the large class of giant NTE materials based on phase transition.

Structural characterization is developed to determine crystallographic site-specific metal disorder. Anomalous scattering at the Zn and Cu edges is used for determining that there is no Zn occupation of the intralayer Cu sites within the kagome layer, however there is Cu present on the Zn intersite, which has led to a structural formula of ([Zn.sub.0.85][Cu.sub.0.15])[Cu.sub.3][(OH).sub.6][Cl.sub.2].

MnCl2(urea)2 is a new linear chain coordination polymer that exhibits slightly counter-rotated Mn2Cl2 rhomboids along the chain-axis. The material crystallizes in the noncentrosymmetric orthorhombic space group Iba2, with each Mn(II) ion equatorially surrounded by four Cl- that lead to bibridged ribbons. Urea ligands coordinate via O atoms in the axial positions. Hydrogen bonds of the Cl···H-N and O···H-N type link the chains into a quasi-3D network. Magnetic susceptibility data reveal a broad maximum at 9 K that is consistent with short-range magnetic order. Pulsed-field magnetization measurements conducted at 0.6 K show that a fully polarized magnetic state is achieved at Bsat = 19.6 T with another field-induced phase transition occurring at 2.8 T. Zero-field neutron diffraction studies made on a powdered sample of MnCl2(urea)2 reveal that long-range magnetic order occurs below TN = 3.2(1) K. Additional Bragg peaks due to antiferromagnetic (AFM) ordering can be indexed according to the Ib′a2′ magnetic space group and propagation vector τ = [0, 0, 0]. Rietveld profile analysis of these data revealed a Néel-type collinear ordering of Mn(II) ions with an ordered magnetic moment of 4.06(6) μB (5 μB is expected for isotropic S = 5/2) oriented along the b-axis, i.e., perpendicular to the chain-axis that runs along the c-direction. Owing to the potential for spatial exchange anisotropy and the pitfalls in modeling bulk magnetic data, we analyzed inelastic neutron scattering data to retrieve the exchange constants: Jc = 2.22 K (intrachain), Ja = −0.10 K (interchain), and D = −0.14 K with J > 0 assigned to AFM coupling. This J configuration is most unusual and contrasts the more commonly observed AFM interchain coupling of 1D chains. [ABSTRACT FROM AUTHOR]

Phase separation in Sr doped BiMnO3 (Bi1−xSrxMnO3, x = 0.4−0.6) was studied by means of temperature-dependent high-resolution neutron powder diffraction (NPD), high resolution X-ray powder diffraction (XRD), and physical property measurements. All the experiments indicate that a phase separation occurs at the temperature coinciding with the reported charge ordering temperature (TCO) in the literature. Below the reported TCO, both the phases resulting from the phase separation crystallize in the orthorhombically distorted perovskite structure with space group Imma. At lower temperature, these two phases order in the CE-type antiferromagnetic structure and the A-type antiferromagnetic structure, respectively. However, a scrutiny of the high-resolution NPD and XRD data at different temperatures and the electron diffraction experiment at 300 K did not manifest any evidence of a long-range charge ordering (CO) in our investigated samples, suggesting that the anomalies of physical properties such as magnetization, electric transport, and lattice parameters at the TCO might be caused by the phase separation rather than by a CO transition. [ABSTRACT FROM AUTHOR]

Examines the long-range magnetic properties of Magnesium [N(CN)[sub 2]] [sub 2](pyz) {pyz = pyrazine}. Use of direct current magnetization, alternating current susceptibility, specific heat and neutron diffraction measurements; Properties of the crystallographic and antiferromagnetic structures; Evidence of spin-flop to paramagnetic transition in magnesium ion.