We use neutron scattering to demonstrate the presence of lattice polarons and their short-range correlations for several samples of La1-xCaxMnO3 in the Ca doping range 0.15</=x</=0.3. We establish the doping dependence of the orientation, commensuration, and coherence length of the polaron correlations and show that the populations of correlated and uncorrelated polarons are intimately related to the transport properties of the materials.

Short-range polaron correlations in the ferromagnetic La_1-xCa_xMnO_3

Functional dynamic molecular crystals have drawn increasing interest in exploring next-generation flexible and smart materials. Molecular rotors, as a typical type of dynamic material, are good candidates that can exhibit bulk properties and functionalities. Herein, we report a molecular rotor crystal as a model system to show a unique structural phase transition-related ferroelasticity. The molecular rotor is dumbbell shaped containing a freely rotating axial rotator and multiple peripheral tert-butyl groups on the two plates with restricted motions. The crystal undergoes a ferroelastic structural phase transition at 263 K with unconventional inverse temperature symmetry breaking (ITSB), i.e., a higher-symmetric low-temperature paraelectric phase (point group mmm) vs. a lower-symmetric high-temperature ferroelastic phase (point group 2/m). Combined crystallographic and NMR spectroscopy studies reveal that unequal motions of the peripheral tert-butyl rotators and anisotropic steric repulsion among the molecules are the key cooperative intermolecular interactions to drive a concerted molecular movement to result in the unique ferroelastic phase transition with ITSB. Our study may open avenues for designing and exploring new types of dynamic functional materials.

A ferroelastic molecular rotor crystal showing inverse temperature symmetry breaking

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High- and low-temperature dual ferroelasticity in a new hybrid crystal: (Me3NCH2CH2OH)4[Ni(NCS)6]

Molecular dielectrics with temperature-tuned dipolar motion can function as thermosensitive electropolarized materials in smart devices. Up to now, most research was limited to the dipolar motion of simple and symmetric structures, yet more complicated dielectric responses for low-symmetry dipoles in deformable space were scarcely known but quite meaningful. Here we report such a crystalline dielectric with Me2EtNH+ as a low-symmetry dipolar cation enclosed by host [Ni(NCS)6]4− anions, namely (Me2EtNH)4[Ni(NCS)6]. Upon heating, it exhibits a rarely observed four-step thermosensitive dielectric response accompanying multiple structural phase transitions, owing to gradually enhanced rotation/jumping of the Me2EtNH+ ions confined in deformable supramolecular cages. The motion dynamics of this low-symmetry cation is anisotropic and strongly affected by the flexible deformability of the host supramolecular cages, suggesting a complexity of molecular dynamics in such a simple crystalline salt.

Four-step thermosensitive dielectric response arising from motionable low-symmetry ammonium confined in deformable supramolecular cages

Ferroic phase transition molecular crystals (FPTMCs), i.e., ferroelectrics and ferroelastics, are an important family of functional molecular materials, having merits of easy synthesis, structural tunability and flexibility, and biocompatibility. Both...

Ferroic phase transition molecular crystals

Multi-step structural phase transitions with novel symmetry breaking and inverse symmetry breaking characteristics in a [Ag4I6]2- cluster hybrid crystal.

Stimuli-responsive smart materials have applications in a range of technologies. Herein, we present a hybrid (1), built from Me3EtN+ organic cations and {Cu2I3-}∞ inorganic chains with Cu⋯Cu metal⋯metal interactions. The two-step phase transition undergone in 1 on the first heating and the phase transition at a lower temperature show symmetry-broken features, leading to switchable dielectrics; the one at a higher temperature displays isomorphic characteristics. Besides the switchable dielectrics, 1 exhibited other multi-stimuli-responsive functionalities, including thermochromism and piezochromism. Combining temperature-dependent powder and single crystal X-ray diffraction, as well as variable-temperature UV-visible absorption spectrum and EPR spectrum analyses, it is demonstrated that the thermochromism is due to the synergy of anharmonic fluctuations with electron-phonon couplings, and the piezochromism arises from compression inducing a lattice distortion in 1. Our study provides insight into understanding the thermochromic and piezochromic mechanisms of cuprous halide-based hybrids, and paves a pathway for designing new multi-stimuli-responsive hybrid materials.

A {Cu2I3-}∞ chain hybrid with two-step phase transition, switchable dielectrics, thermochromism and piezochromism.

Phase transition and ionic conduction enhancement induced by co-doping of LiI and MnI2 in a one-dimensional lead iodide perovskite.

Two haloargentate hybrids, [Me-dabco]Ag2X3 (Me-dabco = 1-methyl-1,4-diazabicyclo-[2.2.2]octan-1-ium, X = I (1) or Br (2)), with the same formula but different structures have been synthesized by a slow evaporation method and characterized by microanalysis, infrared spectroscopy, thermogravimetric, and powder X-ray diffraction techniques. Hybrid 1 consists of completely isolated [Ag4I6]2- clusters, while hybrid 2 exhibits a complicated one-dimensional (1D) chain structure formed by four different configurations of neutral chains and two dissimilar configurations of anionic chains. Hybrid 2 undergoes two reversible order-disorder phase transitions, while hybrid 1 displays one reversible and one irreversible structural phase transition. Both 1 and 2 displayed step-like dielectric anomalies in the vicinity of the phase transition temperature. The corresponding dielectric constants in the high dielectric states are approximately 13 and 6 times higher than those in the low dielectric states for 1 and 2, respectively. Interestingly, the subtle change of halides from I- to Br- significantly affects the aggregated structure of haloargentate, the phase transition, and dielectric behaviors, revealing the typical 'butterfly effect' with the ion radii of halides in these two haloargentate hybrids.

Organic-inorganic haloargentate hybrids of [Me-dabco]Ag2X3 (X = I or Br) with halide ions manipulating the crystal structure, phase transition, and dielectric behavior.

Xue-Wei Pan

Papers

Multi-step structural phase transitions with novel symmetry breaking and inverse symmetry breaking characteristics in a [Ag4I6]2- cluster hybrid crystal.

A {Cu2I3-}∞ chain hybrid with two-step phase transition, switchable dielectrics, thermochromism and piezochromism.

Phase transition and ionic conduction enhancement induced by co-doping of LiI and MnI2 in a one-dimensional lead iodide perovskite.

Organic-inorganic haloargentate hybrids of [Me-dabco]Ag2X3 (X = I or Br) with halide ions manipulating the crystal structure, phase transition, and dielectric behavior.

Comprehensively understanding the natures of crystal structure and magnetic behavior of a radical salt with regular mixed anion-cation stacks