FA₂PbBr₄: Synthesis, Structure, and Unusual Optical Properties of Two Polymorphs of Formamidinium-Based Layered (110) Hybrid Perovskite

Abstract

Small cations such as guanidinium and cesium can act as templating cations to form low dimensional phases (2D, 1D, 0D) in the case of excess of organic halides. However, such phases with the widely used formamidinium (FA + ) cation have not been reported so far. In this study, we discovered two novel low dimensional phases with composition of FA2PbBr4 and investigated the prerequisites of their formation upon crystallization of FABr-excessive solutions of FAPbBr3. We found that both phases have the structure of (110) layered perovskite but is represented by two different polymorphs with "eclipsed" and "staggered" arrangement of adjacent layers. It was shown that FA2PbBr4 phases usually exist in a labile equilibrium with FAPbBr3 3D perovskite and can form composites with it. The optical properties of both polymorphs were comprehensively studied by means of absorption spectroscopy, diffuse reflection spectroscopy and photoluminescence spectroscopy. DFT calculations were applied to investigate the band structure of the FA2PbBr4 and to corroborate the conclusions on their optoelectronic properties. As a result, we found that FA2PbBr4 phases irradiated by UV can exhibit effective green photoluminescence due to a transfer of excitation energy to defective states or 3D perovskite inclusions.

New Variants of (110)‐Oriented Layered Lead Bromide Perovskites, Templated by Formamidinium or Pyrazolium

“…The Tauc-Plot reveals a band gap of 2.90 eV for γ-(FA) 2 PbBr 4 , as shown in the inset, Figure 6a. The band gap is similar to the very recently reported polymorphs (t-(FA) 2 PbBr 4 and m-(FA) 2 PbBr 4 ), [11] which have band gaps of 2.82 eV and 3.00 eV, respectively. Fateev et al carried out a more detailed study of the band gaps in (110)-cut LHPs and found that t-(FA) 2 PbBr 4 has the lowest band gap amongst (110)-cut lead bromides.…”

“…Indeed, this is lower than any previously reported (110)-cut lead bromides. [11] It shows relatively small Δd parameters, in the range 4 -6 × 10 À 4 , but a shortest interlayer BrÀ Br contact that is much longer than the three (FA) 2 PbBr 4 polymorphs, at 4.50 Å. The sharp excitonic absorption bands at~400 nm for both γ-(FA) 2 PbBr 4 and (FA) 1.5 (AA) 0.5 PbBr 4 , can be attributed to the dielectric mismatch between organic and inorganic layers leading to strongly bound excitons.…”

“…Very recently, two polymorphs of FA 2 PbBr 4 have also been shown to adopt (110)-cut structure types, differing in the degree of relative staggering of the [PbBr 4 ] 1 layers. [11] It should be noted that compositions of the type APbBr 4 , A 2 PbBr 4 or AA'PbBr 4 have, in principle, a choice between adopting (100)-cut structure types (i. e. Ruddlesden-Popper or Dion-Jacobson families) or the (110)-cut family. A recent comprehensive review of the (100)-cut families reported more than 250 examples, [12] whereas recent works, including an on-line database, [13] suggest only around 20 examples of the (110)-cut family.…”

“…For example, the compositions (IM)(GA)PbBr 4 (IM = imidazolium), (1,2,4-TZ)(GA)PbBr 4 (1,2,4-TZ = 1,2,4-triazolium) and (AA) 2 PbBr 4 (AA = acetamidinium), all show variants of the 'standard' (110)cut structure types, originally reported in purely inorganic systems, such as BiReO 4 [17] or NdBaScO 4 [18] (Figure 1). There are a handful of examples of (110)-derived compositions in hybrid systems with differing inorganic-layer architectures, such as extended "n × n" or "n × m" step-like structures [10,[19][20][21] or "eclipsed" layers [11,22] which may lead to 3 : 1 ordering of the interlayer species (Figure 1). In addition to the further exploration of small, rigid cyclic amines to template such systems, the use of FA in related systems is clearly a worthy avenue.…”

“…There is a complex network of H-bonds linking the FA moieties to the framework (see ESI). Fateev et al [11] crystallised their two distinct polymorphs of (FA) 2 PbBr 4 from DMF solution, and suggested that the process is kinetically controlled, with the thermodynamic stability of the two polymorphs being similar. In our case, the distinctly different synthetic conditions (moderate heating in conc.…”