The ferroelectric oxide LaTaO4, with a layered perovskite-related structure, exhibits a reversible temperature-dependent phase transition between its low-temperature paraelectric and high-temperature ferroelectric states. Herein, using in situ three-dimensional electron diffraction (3D ED) with atomic-resolution, we resolve the structural evolution of LaTaO4 nanocrystals across four distinct phases during sequential heating treatment (300-500 K). Beyond ab initio structure determination of the known monoclinic (m-LaTaO4) and orthorhombic (o-LaTaO4) phases, we unravel: (i) a, previously undetected, intermediate phase (m'-LaTaO4) with a shorter c-axis transiently stabilised at around 372 K, arising from tilting of TaO6 octahedra; (ii) an incommensurately modulated phase (IC-o-LaTaO4) with a modulation vector q ≈ (0.4669, 0, 0) through a combination of 3D ED and high-resolution electron microscopy imaging. The real-time tracking reveals three first-order phase transitions governed by synergetic Ta-O bond distortion dynamics. These findings establish a hierarchical phase transition model that reconciles previous studies and these single nanocrystal observations.

Azetidinium (Az+)-based antimony and bismuth organic-inorganic hybrid halide B-site-deficient perovskite analogues Az3B2X9 (B3+ = Sb, Bi; X- = Cl, Br, I) were systematically studied. All Az3B2X9 stoichiometries adopt hexagonal close-packed perovskite structures with the 6H (hcc)2 stacking sequence, differing only in the positions of the ordered B-site vacancies. In Az3Sb2Cl9 and Az3Sb2Br9 ordering of B-site vacancies in a single face-sharing octahedral layer leads to the formation of an unusual 2D layered polar structure with the P63mc space group. Variable-temperature single-crystal and powder XRD, DSC, DTA, and dielectric spectroscopy showed several successive phase transitions at low temperatures associated with distortions of the octahedral framework and order/disorder of the Az+ cation. In contrast, in Az3Sb2I9, Az3Bi2Br9, and Az3Bi2I9, the preferred arrangement of vacancies at corner-sharing octahedral sites generates a 0D "dimer" structure. This work highlights the flexibility for structural variations based on particular configurations of vacancy ordering in B-site-deficient halide perovskites.

In this paper, we re-explore a simple textbook Landau model describing improper ferroelectricity and show that in the limit where both proper and improper instabilities exist and compete, improper ferroelectrics can display switching between multiple polarisation states. Using first principles calculations we highlight how the hexagonal tungsten bronze materials may be an archetypal case, with the possibility to switch between improper and proper phases. The resulting functional characteristics are akin to "ferrielectrics", with switching behaviour in the form of a triple hysteresis loop. Such functionality could be ideal for creating non-volatile multistate systems for use in memory devices or as a backbone for neuromorphic computing.

Vacancy-ordered halide perovskites have received great interest in optoelectronic applications. In this work, we report the novel inorganic halide Cs10MnSb6Cl30 with a distinctive 10H (10-layer hexagonal) perovskite polytype structure with (hcccc)2 stacking. Cs10MnSb6Cl30 has 30% B-site vacancies ordered at both corner- and face-sharing sites, resulting in [MnSb6Cl30]10-n columns, i.e., a reduction of octahedral connectivity to 1D. This results in enhanced photoluminescence in comparison to the previously reported 25% vacancy-ordered 3C polytype Cs4MnSb2Cl12 with 2D connectivity. This demonstrates not only the existence of the 10H perovskite structure in halides but also demonstrates the degree of B-site deficiency and stacking sequence variation as a direction to tune the optical properties of perovskite polytypes via vacancy rearrangements.

We present the influence of positional isomerism on the crystal structure of fluorobenzylammonium copper(II) chloride perovskites A2CuCl4 by incorporating ortho-, meta-, and para-fluorine substitution in the benzylamine structure. Two-dimensional (2D) polar ferromagnet (3-FbaH)2CuCl4 (3-FbaH+ = 3-fluorobenzylammonium) is successfully obtained, which crystallizes in a polar orthorhombic space group Pca21 at room temperature. In contrast, both (2-FbaH)2CuCl4 (2-FbaH+ = 2-fluorobenzylammonium) and (4-FbaH)2CuCl4 (4-FbaH+ = 4-fluorobenzylammonium) crystallize in centrosymmetric space groups P21/c and Pnma at room temperature, respectively, displaying significant differences in crystal structures. These differences indicate that the position of the fluorine atom is a driver for the polar behavior in (3-FbaH)2CuCl4. Preliminary magnetic measurements confirm that these three perovskites possess dominant ferromagnetic interactions within the inorganic [CuCl4]∞ layers. Therefore, (3-FbaH)2CuCl4 is a polar ferromagnet, with potential as a type I multiferroic. This work is expected to promote further development of high-performance 2D copper(II) halide perovskite multiferroic materials.

Mixed halide azetidinium lead perovskites AzPbBr3-xXx (X = Cl or I) were obtained by mechanosynthesis. With varying halide composition from Cl- to Br- to I-; the chloride and bromide analogs both form in the hexagonal 6H polytype while the iodide adopts the 9R polytype. An intermediate 4H polytype is observed for mixed Br/I compositions. Overall the structure progresses from 6H to 4H to 9R perovskite polytype with varying halide composition. Rietveld refinement of the powder X-ray diffraction patterns revealed a linear variation in unit cell volume as a function of the average radius of the anion, which is not only observed within the solid solution of each polytype (according to Vegard’s law) but extends uniformly across all three polytypes. This is correlated with a progressive (linear) tuning of the bandgap from 3.41 to 2.00 eV. Regardless of halide, the family of azetidinium halide perovskite polytypes are highly stable, with no discernible change in properties over more than 6 months under ambient conditions

A family of Ruddlesden-Popper (n = 1) layered perovskite-related phases, Az2PbClxBr4-x with composition 0 ≤ x ≤ 4 were obtained using mechanosynthesis. A linear variation in unit cell volume as a function of anion average radius is observed. A tunable bandgap is achieved, ranging from 2.81 to 3.43 eV, and the bandgap varies in a second order polynomial relationship with the halide composition.

The formation and study of partial solid solutions in Az(1-x)FA(x)PbBr(3), using reportedly similar-sized cations azetidinium (Az(+)) and formamidinium (FA(+)), were explored via mechanosynthesis and precipitation synthesis. The compositions and lattice parameters of samples from both syntheses were analyzed by H-1 NMR and Rietveld refinement of powder X-ray diffraction. A clear mismatch in the composition of the perovskite was found between the precipitated samples and the corresponding solutions. Such a mismatch was not observed for samples obtained via mechanosynthesis. The discrepancy suggests that products are kinetically controlled during precipitation compared to thermodynamically controlled mechanosynthesis. Furthermore, the cell volume as a function of composition in both hexagonal, 6H (Az-rich), and cubic, 3C (FA-rich), perovskite solid solutions suggests that FA(+) is actually smaller than Az(+), contradicting the literature. In the 3C (Az-poor) solid solutions, the extent of Az(1-x)FA(x)PbBr(3) is unexpectedly smaller than that in the corresponding methylammonium (MA(+)) system, Az(1-x)FA(x)PbBr(3), which suggests that the extent of solid-solution formation in these halide perovskites is predominantly dependent on the average A-cation size while the size mismatch plays a lesser role in comparison to oxides.

The recent report of an intermediate incommensurately modulated orthorhombic phase in $\mathrm{LaTa}{\mathrm{O}}_{4}$ has prompted a re-examination of the phase transition sequence in $\mathrm{LaTa}{\mathrm{O}}_{4}$ as a function of temperature. With falling temperature, the sequence of phases examined is (orthorhombic) $Cmc{2}_{1}(\mathrm{C})\ensuremath{\leftrightarrow}Cmc{2}_{1}(\mathrm{IC})\ensuremath{\leftrightarrow}(\mathrm{monoclinic})P{2}_{1}/c$, with C and IC denoting commensurate and incommensurate phases, respectively. The orthorhombic to monoclinic transition, ${T}_{\mathrm{m}\ensuremath{-}\mathrm{o}}$, is a first order reconstructive transition occurring at 440 K and ${T}_{\mathrm{IC}\ensuremath{-}\mathrm{C}}$ is a first-order displacive transition occurring at 500--530 K. Strain and elasticity data confirm a first-order transition between the basic and modulated $Cmc{2}_{1}$ phases, with similarities to the isostructural fluoride $\mathrm{BaMn}{\mathrm{F}}_{4}$. A Raman spectroscopic study of the $\mathrm{LaTa}{\mathrm{O}}_{4}$ phase transition indicates that the IC-C phase transition is driven by a soft zone-boundary phonon (unstable) of the commensurate orthorhombic ($Cmc{2}_{1}$) phase. The soft phonon is found to appear (underdamped) above 443 K and vanishes (overdamped) around 528 K. A large supercell of the monoclinic phase below ${T}_{\mathrm{m}\ensuremath{-}\mathrm{o}}$ is proposed based on the Raman spectroscopic results.

The improper ferroelectric CsNbW2O9 has recently been highlighted as the first material outside the manganite family to exhibit a similar meandering, sixfold domain structure to that responsible for enhanced and diminished conduction at charged domain walls in the rare earth manganites. While there is no current evidence for variation in domain wall conduction relative to bulk in CsNbW2O9, the similarities in microstructure strongly suggest that charged domain walls are present in this material. Herein, we report a comprehensive study of the domain microstructure of CsNbW2O9 by both piezoresponse force microscopy and transmission electron microscopy to reveal that there are, in fact, clear distinctions in the domain structure of the two systems. Constraints arising from the crystal structure of CsNbW2O9, namely, the connectivity of the BO6 polyhedra and atomic displacements occurring purely along the c axis, mean that domain walls preferentially run parallel to the c direction (the polar axis of the material) and thus remain uncharged. The characteristic cloverleaf domain structure reminiscent of the manganites is still present; however, the structure meanders predominantly in the ab plane and, therefore, appears differently depending on the projection direction from which it is viewed. As a result of this microstructural constraint, charged domain walls are not prevalent in this material.

The thermal evolution of the phase behaviour and crystal structure of the polar hexagonal tungsten bronze RbNbW 2 O 9 has been characterised using powder neutron diffraction supported by symmetry-mode analysis and dielectric measurements. • The phase transition sequence is driven by loss of an octahedral tilting mode, A 6 + . • Followed by loss of a second octahedral tilting mode, A 3 + . • Significantly different phase transition sequence between related Rb and Cs compositions.

The thermal evolution of the phase behaviour and crystal structure of the polar hexagonal tungsten bronze RbNbW2O9 has been characterised using powder neutron diffraction supported by symmetry-mode analysis and dielectric measurements.Image 1

The layered perovskite LaTaO(4)has been synthesized to be stable in both (polar) orthorhombic and (nonpolar) monoclinic polymorphs at ambient conditions. Although the structural transition between monoclinic and orthorhombic phases has been well established, there is some controversy regarding a further, unidentified transition around 500 K. Here this is identified as an incommensurate-commensurate first-order transition between incommensurate Cmc2(1)(alpha 00)0s0 and commensurate Cmc2(1)orthorhombic phases. Transmission electron microscopy indicates partially ordered stacking of different structural units ina, identifying the local cause for the modulation, whereas variable temperature powder neutron diffraction has shown the overall macroscopic modulation vector,q approximate to (0.456, 0, 0)-roughly a 2.2 x expansion ina, corresponding to an approximate 11acommensurate superunit cell dimension. The modulation shows a continuous temperature dependence until transitioning to the basic (commensurate) cell atT(IC-C). Doping the interlayer La sites with smaller Nd cations stabilizes the incommensuration to higher temperature, suggesting the modulation is geometrically driven at the A site.

The "empty" tetragonal tungsten bronze Ba4La0.67 rectangle 1.33Nb10O30 displays both relaxor-like and normal dielectric anomalies as a function of temperature; the former is associated with loss of ferroelectricity and was proposed to originate from anion disordering [Chem. Mater. 2016, 28, 4616-4627]. Here we present total neutron scattering and pair distribution function (PDF) analysis, which shows an increase in the distribution of oxygen-oxygen distances at the relaxor transition and which supports the proposed anion disordering mechanism. The disordering process can be destabilized by reducing the average A-cation size (i.e., Nd-doping: Ba-4(La1-xNdx)(0.67)Nb10O30); this introduces a more strongly propagating tilt system in line with the previously reported crystal-chemical framework model [Chem. Mater. 2015, 27, 3250-3261]. Mechanical loss data obtained using resonant ultrasound spectroscopy also indicate destabilization of the disordering process with increasing Nd-substitution.

Ca2Mn3O8 forms a delafossite-related layered structure, which crystallises with monoclinic C-2/m sym-metry. Compared with the delafossite-structure, the MnO6 layers in Ca2Mn3O8 exhibit an ordered cation void which forms a magnetic 'bow-tie' like connectivity of Mn4+ ion layers separated by Ca2+ ions. In-situ variable temperature diffraction data demonstrates that the structure is robust up to a temperature of approximately 1173 K before the material decomposes into the perovskite, CaMnO3 and marokite, CaMn2O4 phases. Simultaneous thermal analysis suggests that a very small amount of water remains within the layers post synthesis. Impedance spectroscopy indicates that Ca2Mn3O8 is an electronic conductor in the range similar to 400-700 K with an activation energy of 0.50 +/- 0.01 eV. (C) 2020 Elsevier B.V. All rights reserved.

The hexagonal tungsten bronze RbNbW2O9 is shown, by variable-temperature powder neutron diffraction and symmetry-mode analysis, to display a significantly different phase transition sequence compared to the related CsNbW2O9 composition. At ambient temperature, RbNbW2O9 adopts the polar orthorhombic space group Cmc2(1). Upon heating, the thermal evolution of the crystal structure proceeds via two transitions. These correspond to sequential loss of two distinct octahedral tilting modes, leading to space group P6(3)mc at around 655K, and space group P6mm near 700 K. The polar distortion is retained up to the highest temperature studied here. The differences in structural behaviour between the proper ferroelectric RbNbW2O9 and the improper ferroelectric CsNbW2O9 emphasises the need for careful crystallographic analyses of materials of this type.

The rapidly emerging field of domain wall nanoelectronics exploits the different functionality of ferroelectric domain walls relative to the bulk. In article number 1903620, Finlay D. Morrison and co-workers present a new improper ferroelectric with the hexagonal tungsten bronze structure. This material displays the same complex labyrinthine domain structure, including six-fold “cloverleaf” domain vertices, as the widely studied hexagonal manganites. These domain structures have generated much interest due to their rich functionality associated with a high density of charged domain walls.

We report inelastic light scattering from underdamped plasmons in azetidinium lead bromide (AzPbBr3). The plasmons are very strongly temperature dependent and serve as a soft mode for the semiconductor-insulator phase transition near TC » 150 K, demonstrating a continuous decrease in hole concentration np(T) by at least a factor of four and implying a nearly tricritical transition. The plasmon frequency and linewidth agree with independent measurements, and the impedance analysis reveals a frequency dependence (modelled by a constant phase element, CPE) that can be identified as due to electron-phonon coupling. The dependence of plasmon frequency upon (TC-T) is analogous to that for magnons in magnetic insulators or soft transverse optical phonons in ferroelectrics and ferroelastics, or for phasons in incommensurately modulated insulators.