Structural chemistry of layered lead halide perovskites containing single octahedral layers

Funding: Leverhulme Trust (award No. RPG-2018-065).We present a comprehensive review of the structural chemistry of hybrid lead halides of stoichiometry APbX4, A2PbX4 or AAʹPbX4, where A and Aʹ are organic ammonium cations and X = Cl, Br or I. These compounds may be considered as layered perovskites, containing isolated, infinite layers of corner-sharing PbX4 octahedra separated by the organic species. We first extract over 250 crystal structures from the CCDC and classify them in terms of unit cell metrics and crystal symmetry. Symmetry mode analysis is then used to identify the nature of key structural distortions of the [PbX4]∞ layers. Two generic types of distortion are prevalent in this family: tilting of the octahedral units and shifts of the inorganic layers relative to each other. Although the octahedral tilting modes are well-known in the crystallography of purely inorganic perovskites, the additional layer shift modes are shown to enrich enormously the structural options available in layered hybrid perovskites. Some examples and trends are discussed in more detail in order to show how the nature of the interlayer organic species can influence the overall structural architecture, although the main aim of the paper is to encourage workers in the field to make use of the systematic crystallographic methods used here to further understand and rationalise their own compounds, and perhaps to be able to design-in particular structural features in future work.Publisher PDFPeer reviewe

Variable dimensionality in 'hollow' hybrid tin iodide perovskites

We acknowledge support from the University of St Andrews and the Leverhulme trust (RPG-2018-065).Two ‘hollow’ B-site deficient perovskites, (TzH)11(H3PO2)Sn6I23 and (TzH)3Sn2I7 (TzH+ = 1,2,4-triazolium, H3PO2 = hypohosphorous acid), have been prepared. (TzH)11(H3PO2)Sn6I23 is the first example of a 2D layered structure of this type. Leaving the same reaction mixture for an extended time also affords the 3D derivative (TzH)3Sn2I7.Publisher PDFPeer reviewe

Unprecedented phase transition sequence in the perovskite Li0.2Na0.8NbO3

Funding for this research was provided by: EPSRC (award No. EP/L505079/1).We demonstrate the existence of a novel phase transition sequence in the perovskite Li0.2Na0.8NbO3 at elevated temperature. The rare Glazer tilt system, a+a+c-, is observed in both polar and centrosymmetric polymorphs. The perovskite Li0.2Na0.8NbO3 is shown, by powder neutron diffraction, to display a unique sequence of phase transitions at elevated temperature. The ambient temperature polar phase (rhombohedral, space group R3c) transforms via a first-order transition to a polar tetragonal phase (space group P42mc) in the region 150 – 300 °C; these two phases correspond to Glazer tilt systems a-a-a- and a+a+c-, respectively. At 500 °C a ferroelectric – paraelectric transition takes place from P42mc to P42/nmc, retaining the a+a+c- tilt. Transformation to a single-tilt system, a0a0c+ (space group P4/mbm), occurs at 750 °C, with the final transition to the aristotype cubic phase at 850 °C. The P42mc and P42/nmc phases have each been seen only once previously in perovskite crystallography, in both cases in compositions prepared at high pressure.Publisher PDFPeer reviewe

Octahedral tilting in the polar hexagonal tungsten bronzes RbNbW2O9 and KNbW2O9

We thank the Science and Technology Facilities Council (STFC) for the provision of neutron diffraction facilities at ISIS (HRPD experiment RB1710021, doi: 10.5286/ISIS.E.RB1710021) and the School of Chemistry, University of St Andrews for funding of a studentship to JAM through the EPSRC doctoral training grant (EP/K503162/1).The ambient temperature structures (orthorhombic, space group, Cmc21) of the polar hexagonal tungsten bronzes RbNbW2O9 and KNbW2O9 have been determined by high-resolution powder neutron diffraction. Displacement of the A-site cation in the polar c-axis, with concomitant octahedral tilting, occurs to optimise the A-cation bonding environment reducing the coordination from 18 to 16. This effect is more evident in KNbW2O9 due to decreased A-cation size. The octahedral tilting in both compositions results in a doubling of the c-axis that has not previously been reported, highlighting the importance of neutron diffraction as a complementary technique for structural determination of such systems.Publisher PDFPeer reviewe

Local structure and order-disorder transitions in "empty" ferroelectric tetragonal tungsten bronzes

JAM would like to acknowledge the School of Chemistry, University of St Andrews for the allocation of a PhD studentship through the EPSRC doctoral training grant (EP/K503162/1). AR would like to acknowledge support through the Strategic Grant POSDRU/159/1.5/S/133255, Project ID 133255 (2014), co-financed by the European Social Fund within the Sectorial Operational Program Human Resources Development 2007–2013 and also the University of Craiova and University of Cambridge for the mobility grant “Resonant ultrasound spectroscopy. (RUS) characterization of dielectric and ferroelectric tetragonal tungsten bronzes”. The work carried out at the University of St Andrews and University of Cambridge is part of an EPSRC- funded collaboration (EP/P02453X/1 and EP/P024904/1).The ‘empty’ tetragonal tungsten bronze Ba4La0.67 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 destabilised by reducing the average A-cation size (i.e. Nd-doping: Ba4(La1-xNdx)0.67Nb10O30); 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 destabilisation of the disordering process with increasing Nd-substitution.PostprintPeer reviewe

Structural diversity in layered hybrid perovskites, A2PbBr4 or AA′PbBr4, templated by small disc-shaped amines

Y.-Y.G. and T.L. acknowledge the University of St Andrews and China Scholarship Council for funding of studentships (201603780005) and (201606280032), respectively. J.A.M. and P.L. acknowledge financial support from the Leverhulme trust (RPG-2018-065). S.B., K.S. and F.P. acknowledge financial support from the German National Science Foundation via the Projects 423895689, KO 3973/2-1 and PA 3373/3-1 and further acknowledge support by the Bavarian State Ministry of Science, Research, and the Arts for the Collaborative Research Network ‘‘Solar Technologies go Hybrid’’.We present three new hybrid layered lead(II) bromide perovskites of generic composition A2PbBr4 or AA′PbBr4, which exhibit three distinct structure types. [TzH]2PbBr4, ([TzH+]= 1,2,4-triazolium), adopts a (001)-oriented layer structure, [AaH]2PbBr4, ([AaH+] =acetamidinium), adopts a (110)-oriented type, whereas [ImH][TzH]PbBr4, ([ImH+] =imidazolium),adopts a rare (110)-oriented structure with enhanced corrugation (i.e. ‘3 ×3’ type). The crystal structures of each are discussed in terms of the differing natureof the templating molecular species. Photoluminescent spectra for each are reported and the behaviours discussed in relation to the different structure of each composition.PostprintPeer reviewe

Polar ferromagnet induced by fluorine positioning in isomeric layered copper halide perovskites

We acknowledge the University of St Andrews and the China Scholarship Council (studentship to CH) and the University of St Andrews (studentship to AJB).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 behaviour 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.Publisher PDFPeer reviewe

Structure-directing effects in (110)-layered hybrid perovskites containing two distinct organic moieties

We acknowledge support from the University of St Andrews, the China Scholarship Council (studentship to YYG) and the Leverhulme trust (RPG-2018-065).The hybrid perovskites (ImH)(GuH)PbBr4 and (TzH)(GuH)PbBr4 (ImH+ = imidazolium, GuH+ = guanidinium, TzH+ = 1,2,4-triazolium) both adopt (110)-oriented layer structures. However, the GuH+ cation adopts differing crystallographic sites in the two structures (intra-layer versus inter-layer); this is discussed in terms of the sizes of the organic cations and their hydrogen-bonding preferences.PostprintPeer reviewe

Phase transition behavior of the layered perovskite CsBi0.6La0.4Nb2O7 : a hybrid improper ferroelectric

We thank EPSRC for a PhD studentship to CALD (EP/P505097/1).The phase behavior of the layered perovskite CsBi0.6La0.4Nb2O7, of the Dion-Jacobson family, has been studied by high-resolution powder neutron diffraction between temperatures 25 < T < 850 °C. At ambient temperature this material adopts the polar space group P21am; this represents an example of hybrid improper ferroelectricity caused by the interaction of two distinct octahedral tilt modes. Within the limits of our data resolution the thermal evolution of the crystal structure is consistent with a first-order transition between 725 and 750 °C, with both tilt modes vanishing simultaneously, leading to the aristotype space group P4/mmm. This apparent ‘avalanche transition’ behavior resembles that seen in the related Aurivillius phase SrBi2Nb2O9.Publisher PDFOtherPeer reviewe

Techstyle Haus

Preliminary design work for the Solar Decathlon 2014 entry Techstyle Haus completed in a wintersession 2013 RISD design studio in Erfurt, Germany taught by Jonathan Knowles. The Solar Decathlon competition challenges twenty collegiate teams to design and build sustainable homes that are powered exclusively by solar energy and incorporate sustainable architecture and design. Techstyle Haus is an international Brown University, RISD and University of Applied Sciences Erfurt,Germany collaboration designing a solar passivehaus out of high performance textiles