The concept of tolerance factors is applied quantitatively to hybrid inorganic–organic materials that adopt perovskite-like architectures.

Tolerance Factors of possible hybrid perovskites are calculated for over 2500 amine-metal-anion permutations of the periodic table.

The targeted incorporation of defects into crystalline matter allows for the manipulation of many properties and has led to relevant discoveries for optimized and even novel technological applications of materials. It is therefore exciting to see that defects are now recognized to be similarly useful in tailoring properties of metal-organic frameworks (MOFs). For instance, heterogeneous catalysis crucially depends on the number of active catalytic sites as well as on diffusion limitations. By the incorporation of missing linker and missing node defects into MOFs, both parameters can be accessed, improving the catalytic properties. Furthermore, the creation of defects allows for adding properties such as electronic conductivity, which are inherently absent in the parent MOFs. Herein, progress of the rapidly evolving field of the past two years is overviewed, putting a focus on properties that are altered by the incorporation and even tailoring of defects in MOFs. A brief account is also given on the emerging quantitative understanding of defects and heterogeneity in MOFs based on scale-bridging computational modeling and simulations.

We report an experimental study of the mechanical properties of the organic-inorganic halide perovskites, CH3NH3PbX3 (X=I, Br and Cl). Nanoidentation on single crystals was used to obtain Young's moduli (E) and hardnesses (H) of this class of hybrid materials, which have attracted considerable attention for photovoltaic applications. The measured Young's moduli of this family lie in the range 10-20 GPa and a trend of ECl > EBr > EI is observed. The physical properties are consistent with the underlying crystal structure. In particular, the results are in reasonable agreement with recent calculations using density functional theory and align with expectations based upon bond energy, packing, and hydrogen-bonding considerations. The anisotropy in these systems is quite small, with E100 > E110 for the cubic bromide and chloride cases and E112 ≈ E100 for the tetragonal iodide perovskites. Interestingly, CH3NH3PbI3 is harder than the Br-and Cl-based perovskites.

The discovery of lead-free hybrid double perovskites provides a viable approach in the search for stable and environmentally benign photovoltaic materials as alternatives to lead-containing systems such as MAPbX 3 (X = Cl, Br, or I). Following our recent reports of (MA) 2 KBiCl 6 and (MA) 2 TlBiBr 6 , we have now synthesized a hybrid double perovskite, (MA) 2 AgBiBr 6 , that has a low band gap of 2.02 eV and is relatively stable and nontoxic. Its electronic structure and mechanical and optical properties are investigated with a combination of experimental studies and density functional theory calculations.

IN A SEARCH FOR LEAD-FREE MATERIALS THAT COULD BE USED AS ALTERNATIVES TO THE HYBRID PEROVSKITES, (MA)PbX 3 , in photovoltaic applications, we have discovered a hybrid double perovskite, (MA) 2 KBiCl 6 , which shows striking similarities to the lead analogues. Spectroscopic measurements and nanoindentation studies are combined with density functional calculations to reveal the properties of this interesting system.The light-harvesting, semiconducting hybrid inorganic-organic perovskites (HIOPs) have recently attracted a great deal of attention in the photovoltaic community, with their solar cell efficiencies rising from ~4% to over 20% in just six years. 1, 2 The most extensively studied materials are the lead-containing systems, APbX 3 , where A is alkyl ammonium cation (e.g. CH 3 NH 3 + (methylammonium, MA) or NH 2 CHNH 2 + (formamidinium, FA)) and X is Cl -, Bror I -. However, the toxicity of lead to the environment could become a major drawback in their commercialization and the quest for lead-free alternatives is therefore attracting a lot of attention. Other group IV metals such as Ge and Sn are being explored, but the chemical instability of Sn 2+ and Ge 2+ presents challenges for their practical utilization. 3,4 Alternatively, the replacement of Pb 2+ by isoelectronic ions also seems attractive because the strong light absorption and long carrier life-times exhibited by MAPbX 3 are believed to be related to the 6s 2 6p 0 electronic configuration of Pb 2+ . 5 While Tl + is also toxic, Bi 3+ is an interesting option because coordination complexes of bismuth are used in over-the-counter medicines such as Pepto-Bismol. 6 However, this strategy poses challenges because Bi 3+ has a different valence state from Pb 2+ and cannot therefore be simply substituted into phases such as (MA)PbX 3 . In the present work, we show that the incorporation of Bi 3+ into a HIOP can be achieved by synthesizing a hybrid double perovskite of general formula (MA) 2 M I M III X 6 .There has been significant recent progress in the incorporation of Bi 3+ into hybrid perovskiterelated halides. For example, (MA) 3 Bi 2 I 9 can be readily obtained by using a synthetic route analogous to that used for MAPbI 3 , 7 and an ammonium bismuth iodide phase, (NH 4 ) 3 Bi 2 I 9 , was recently reported to show a bandgap of 2.04eV. 8 A number of alkali metal systems of

Density functional theory screening of the hybrid double perovskites (MA) 2 B I BiX 6 (B I =K,Cu,Ag,Tl; X=Cl,Br,I) shows that systems with band gaps similar to those of the MAPbX 3 lead compounds can be expected for B I =Cu,Ag,Tl. Motivated by these findings, (MA) 2 TlBiBr 6 , isoelectronic with MAPbBr 3 , was synthesised and found to have a band gap of ~2.0eV. The remarkable performance of hybrid perovskite-based solar cells has launched a new paradigm in the area of photovoltaic research.synthesis and optical properties of a new double hybrid perovskite, (MA) 2 TlBiBr 6 , which is isoelectronic with MAPbBr 3 and has a much narrower band-gap than (MA) 2 KBiCl 6 .Exploring the properties of lead-free hybrid double perovskites using a combined computational-experimental approach Computational MethodologyThe DFT calculations were performed using the Vienna ab initio Simulation Package (VASP). 1,2 Projected augmented wave (PAW) 3 pseudopotentials were employed with the following electrons treated explicitly: H (1s 1 ), C (2s 2 2p 2 ), N (2s 2 2p 3 ), K (3s 2 3p 6 4s 1 ), Bi (5d 10 6s 2 6p 3 ), Tl (5d 10 6s 2 6p 1 ), Cu (3p 6 3d 10 4s 1 ), Ag (4p 6 4d 10 5s 1 ), Pb(5d 10 6s 2 6p 2 ), Cl (3s 2 3p 5 ), Br (4s 2 4p 5 ) and I (5s 2 5p 5 ).The non-local van der Waals density functional (vdW-DF) 4 was used with the exchange-correlation energy calculated as , where the exchange energy is obtained from the generalized gradient approximation (GGA) using the optB86b functional, the local correlation energy from the local density approximation (LDA) and the non-local correlation energy from double space integration. K-points were sampled in the first Brillouin zone using a 4×4×2 Monkhorst-Pack 5 mesh, and for electronic density of states (DOS) calculations, a finer 8×8×3 mesh was used. A 500 eV plane-wave kinetic energy cutoff was employed for all calculations. The effect of relativistic spin-orbit coupling (SOC) was included inthe DOS and electronic band structure calculations. The experimentally synthesized double perovskite (MA) 2 KBiCl 6 was used as a basis for constructing all the structures which were then relaxed until the interatomic forces were less than 0.01 eV/Å while maintaining the same rhombohedral symmetry as (MA) 2 KBiCl 6 .

Hybrid organic–inorganic perovskites represent a special class of metal–organic framework where a molecular cation is encased in an anionic cage. The molecule–cage interaction influences phase stability, phase transformations, and the molecular dynamics. We examine the hydrogen bonding in four AmBX3 formate perovskites: [Am]Zn(HCOO)3, with Am+ = hydrazinium (NH2NH3+), guanidinium (C(NH2)3+), dimethylammonium (CH3)2NH2+, and azetidinium (CH2)3NH2+. We develop a scheme to quantify the strength of hydrogen bonding in these systems from first-principles, which separates the electrostatic interactions between the amine (Am+) and the BX3– cage. The hydrogen-bonding strengths of formate perovskites range from 0.36 to 1.40 eV/cation (8–32 kcalmol–1). Complementary solid-state nuclear magnetic resonance spectroscopy confirms that strong hydrogen bonding hinders cation mobility. Application of the procedure to hybrid lead halide perovskites (X = Cl, Br, I, Am+ = CH3NH3+, CH(NH2)2+) shows that these compounds have significantly weaker hydrogen-bonding energies of 0.09 to 0.27 eV/cation (2–6 kcalmol–1), correlating with lower order–disorder transition temperatures.