Structural chemistry of layered lead halide perovskites containing single octahedral layers

Abstract

We present a comprehensive review of the structural chemistry of hybrid lead halides of stoichiometry APbX 4, A 2PbX4 or A A′PbX 4, where A and A′ 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 PbX 4 octahedra separated by the organic species. First, over 250 crystal structures were extracted from the CCDC and classified in terms of unit-cell metrics and crystal symmetry. Symmetry mode analysis was then used to identify the nature of key structural distortions of the [PbX 4]∞ 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 enormously enrich 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 rationalize their own compounds, and perhaps to be able to design-in particular structural features in future work.

Polar Ferromagnet Induced by Fluorine Positioning in Isomeric Layered Copper Halide Perovskites

“…In addition, in copper halide perovskites, there is also the J–T mode, which leads to alternating long and short in-plane Cu–Cl bonds. In the case of (3-FbaH) 2 CuCl 4 , use of the online tool ISODISTORT 29 reveals that the key “standard” distortion modes 7 are the layer shift mode, together with two distinct octahedral tilt modes (i.e., both rotation around the c -axis and out-of-plane tilting, leading to the common Glazer-like tilt system a – a – c /–( a – a – c )). The additional significant modes are the J–T mode and a polar mode, acting along the c -axis.…”

“…There is a third, intermediate case displaying a (1/2,0) shift, which has been designated DJ2. 7 However, in the case of hybrid perovskites, the huge variation of interlayer ion sizes and shapes seriously affects the final structure, resulting in a much more complex array of possible “shifts” of adjacent layers, often resulting in adjacent layers being neither “fully eclipsed” (DJ-like) nor “fully staggered” (RP-like). 8 − 11 Therefore, recent papers have suggested a more flexible structural classification of hybrid layered perovskites, beyond the simple “DJ or RP” types, according to the relative shifts of neighboring inorganic layers and regardless of the stoichiometries ABX 4 and A 2 BX 4 .…”

Pressure Driven Rotational Isomerism in 2D Hybrid Perovskites

“…Compression results in the layer-to-layer shift of the bi-layer inorganic slabs in n = 2 RPP along -a and +b axes, as represented by the blue and grey arrows in Fig. 2a (right side), which is related to a 𝑀𝑀 5 − (𝑎𝑎, 𝑏𝑏) mode 23 . Such an interlayer shift maintains n = 2 RPP in the orthorhombic symmetry, consistent with the conformational change of BA molecules without tilting under pressure.…”

“…The molecular tilting drives the adjacent inorganic layer to shift in the same direction along +b axis, as represented by the grey arrows in Fig. 2a (left side), which is related to a Γ 5 + mode 23 . The layer shift decreases the initial stacking offset and therefore the crystal undergoes a phase transition to the monoclinic class 24 .…”

Polarity and Ferromagnetism in Two-Dimensional Hybrid Copper Perovskites with Chlorinated Aromatic Spacers

“…The tolerance factor constraint that occurs in 3D perovskites can be considerably relaxed in 2D hybrid perovskites, endowing them with striking structural flexibility and novel functionalities. 7 − 9 As a result, the crystal structures and physical properties of 2D hybrid perovskites can be modulated by much larger and more complex organic molecules with various sizes and functional groups. 10 − 12 A halogen substitution strategy, especially introducing fluorine atoms into the organic molecules, has been proved to be a powerful approach to enhance the ferroelectric performance in 2D lead-based halide perovskites.…”