Designing dynamic coordination bonds in polar hybrid crystals for a high-temperature ferroelastic transition

Ferroelastic materials have gained widespread attention as promising candidates for mechanical switches, shape memory, and information processing. Their phase-transition mechanisms usually originate from conventional order-disorder and/or displacive types, while those involving dynamic coordination bonds are still scarce. Herein, based on a strategic molecular design of organic cations, we report three new polar hybrid crystals with a generic formula of AA'RbBiCl6 (A = A' = Me3SO+ for 1; A = Me3SO+ and A' = Me4N+ for 2; A = A' = Me3NNH2+ for 3). Their A-site cations link to the [RbBiCl6]n2n- inorganic framework with lon topology through Rb-O/N coordination bonds, while their significantly different interactions between A'-site cations and inorganic frameworks provide distinct phase-transition behaviour. In detail, the strongly coordinative A'-site Me3SO+ cations prevent 1 from a structural phase transition, while coordinatively free A'-site Me4N+ cations trigger a conventional order-disorder ferroelastic transition at 247 K in 2, accompanied by a latent heat of 0.63 J g-1 and a usual "high → low" second-harmonic-generation (SHG) switch. Interestingly, the A'-site Me3NNH2+ cations in 3 reveal unusual dynamic coordination bonds, driving a high-temperature ferroelastic transition at 369 K with a large latent heat of 18.34 J g-1 and an unusual "low → high" SHG-switching behaviour. This work provides an effective molecular assembly strategy to establish dynamic coordination bonds in a new type of host-guest model and opens an avenue for designing advanced ferroelastic multifunctional materials.

Pauli-paramagnetic and metallic properties of high pressure polymorphs of BaRhO3 oxides containing Rh2O9 dimers

From our material exploration study in wide pressure and temperature conditions, we found a new 6H polymorph of BaRhO₃ was stabilised under high pressure conditions from 14 to 22 GPa. The material crystallised in the monoclinic 6H hexagonal perovskite structure in space group C2/c. The 4H BaRhO₃ polymorph was stabilised at lower pressures, but the 3C cubic BaRhO₃ likely requires pressures greater than 22 GPa. Both 6H and 4H polymorphs contain Rh₂O₉ dimers and the large 4d Rh orbital spatial diffusivity in these dimers leads to Pauli paramagnetic and metallic ground states, which are also supported by first-principles electronic structure calculations. High Wilson ratios of approximately 2 for either compound indicate strong electron correlation.

Layered Hexagonal Perovskite Oxides 21R Ba7Fe5Ge2O20 and 12H Ba6Fe3Ge3O17

Two hexagonal-perovskite-structure oxides, 21R Ba₇Fe₅Ge₂O₂₀ and 12H Ba₆Fe₃Ge₃O₁₇, were obtained by synthesis with a high-pressure and high-temperature technique. The Fe-containing hexagonal-perovskite-structure units are sandwiched by nonmagnetic GeO₄ tetrahedral layers in the structures, and thus both compounds show two-dimensional ferrimagnetic behaviors due to intra- and interunit magnetic interactions. 21R Ba₇Fe₅Ge₂O₂₀ has the ionic formula Ba₇Fe1₂³⁺Fe2⁴⁺Fe3₂⁴⁺Ge4₂⁴⁺O₂₀ at room temperature, and unusually high valence Fe⁴⁺ in the trimers undergoes charge disproportionation, Fe2⁴⁺ + 2Fe3⁴⁺ → Fe2^(4+2δ)+ + 2Fe3^(4-δ)+, at low temperatures. In contrast, 12H Ba₆Fe₃Ge₃O₁₇ with ionic formula Ba₆Fe1₂³⁺(Fe2_0.5⁴⁺Ge2_0.5⁴⁺)₂Ge3₂⁴⁺O₁₇ does not show a charge transition.

Thermal expansion in BaRuO3 perovskites – an unusual case of bond strengthening at high temperatures

The temperature dependences of the structures of three polytypes of BaRuO₃ have been investigated between room temperature and 1000 °C using high resolution synchrotron X-ray diffraction.

Structure-Band Gap Relationships in Hexagonal Polytypes and Low-Dimensional Structures of Hybrid Tin Iodide Perovskites

The present study deals with the structural characterization and classification of the novel compounds 1-8 into perovskite subclasses and proceeds in extracting the structure-band gap relationships between them. The compounds were obtained from the employment of small, 3-5-atom-wide organic ammonium ions seeking to discover new perovskite-like compounds. The compounds reported here adopt unique or rare structure types akin to the prototype structure perovskite. When trimethylammonium (TMA) was employed, we obtained TMASnI₃ (1), which is our reference compound for a "perovskitoid" structure of face-sharing octahedra. The compounds EASnI₃ (2b), GASnI₃ (3a), ACASnI₃ (4), and IMSnI₃ (5) obtained from the use of ethylammonium (EA), guanidinium (GA), acetamidinium (ACA), and imidazolium (IM) cations, respectively, represent the first entries of the so-called "hexagonal perovskite polytypes" in the hybrid halide perovskite library. The hexagonal perovskites define a new family of hybrid halide perovskites with a crystal structure that emerges from a blend of corner- and face-sharing octahedral connections in various proportions. The small organic cations can also stabilize a second structural type characterized by a crystal lattice with reduced dimensionality. These compounds include the two-dimensional (2D) perovskites GA₂SnI₄ (3b) and IPA₃Sn₂I₇ (6b) and the one-dimensional (1D) perovskite IPA₃SnI₅ (6a). The known 2D perovskite BA₂MASn₂I₇ (7) and the related all-inorganic 1D perovskite "RbSnF₂I" (8) have also been synthesized. All compounds have been identified as medium-to-wide-band-gap semiconductors in the range of E_g = 1.90-2.40 eV, with the band gap progressively decreasing with increased corner-sharing functionality and increased torsion angle in the octahedral connectivity.

High stability of electro-transport and magnetism against the A-site cation disorder in SrRuO3

It is known that the electro-transport and magnetism of perovskite alkaline-earth ruthenate oxides are sensitive to the lattice distortion associated with the A-site cation size. Orthorhombic CaRuO3 and cubic BaRuO3 exhibit distinctly different electro-transport and magnetic properties from orthorhombic SrRuO3. It has been suggested that SrRuO3 can be robust against some intrinsic/external perturbations but fragile against some others in terms of electro-transport and magnetism, and it is our motivation to explore such stability against the local site cation disorder. In this work, we prepare a set of SrRuO3-based samples with identical averaged A-site size but different A-site cation disorder (size mismatch) by Ca and Ba co-substitution of Sr. It is revealed that the electro-transport and magnetism of SrRuO3 demonstrate relatively high stability against this A-site cation disorder, characterized by the relatively invariable electrical and magnetic properties in comparison with those of SrRuO3 itself. A simple electro-transport network model is proposed to explain quantitatively the measured behaviors. The present work suggests that SrRuO3 as an itinerant electron ferromagnetic metal possesses relatively high robustness against local lattice distortion and cation occupation disorder.

Ba4Ru3O10.2(OH)1.8: a new member of the layered hexagonal perovskite family crystallised from water

Two chemical reagents in water at 200 °C yield a complex barium ruthenate with a new 8H perovskite stacking sequence.

Structural and magnetic properties of Ba0.7Sr0.3Ru(1-x)Mn(x)O3 hexagonal perovskites

The structures and properties of Ba(0.7)Sr(0.3)Ru(1-x)Mn(x)O(3) perovskites have been investigated in samples prepared at 1300 degrees C in air. Two polytypes are found, a 6H phase for 0.2 < or = x < or = 0.4 and a 4H type for 0.6 < or = x < or = 1. The cell parameters and volume vary linearly in both solid solution ranges. Spin-freezing transitions up to 60 K are observed for the 6H samples, but the 4H materials show high temperature Néel transitions at 210 to 280 K.

High-pressure synthesis of the cubic perovskite BaRuO3 and evolution of ferromagnetism in ARuO3 (A = Ca, Sr, Ba) ruthenates

The cubic perovskite BaRuO(3) has been synthesized under 18 GPa at 1,000 degrees C. Rietveld refinement indicates that the new compound has a stretched Ru-O bond. The cubic perovskite BaRuO(3) remains metallic to 4 K and exhibits a ferromagnetic transition at T(c) = 60 K, which is significantly lower than the T(c) approximately = 160 K for SrRuO(3). The availability of cubic perovskite BaRuO(3) not only makes it possible to map out the evolution of magnetism in the whole series of ARuO(3) (A = Ca, Sr, Ba) as a function of the ionic size of the A-site r(A,) but also completes the polytypes of BaRuO(3). Extension of the plot of T(c) versus r(A) in perovskites ARuO(3) (A = Ca, Sr, Ba) shows that T(c) does not increase as the cubic structure is approached, but has a maximum for orthorhombic SrRuO(3). Suppressing T(c) by Ca and Ba doping in SrRuO(3) is distinguished by sharply different magnetic susceptibilities chi(T) of the paramagnetic phase. This distinction has been interpreted in the context of a Griffiths' phase on the (Ca Sr)RuO(3) side and bandwidth broadening on the (Sr,Ba)RuO(3) side.