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 .

The synthesis of four novel crystalline zeolitic imidazolate framework (ZIF) structures using a mixed-ligand approach is reported. The inclusion of both imidazolate and halogenated benzimidazolate-derived linkers leads to glass-forming behavior by all four structures. Melting temperatures are observed to depend on both electronic and steric effects. Solid-state NMR and terahertz (THz)/Far-IR demonstrate the presence of a Zn-F bond for fluorinated ZIF glasses. In situ THz/Far-IR spectroscopic techniques reveal the dynamic structural properties of crystal, glass and liquid phases of the halogenated ZIFs, linking the melting behavior of ZIFs to the propensity of the ZnN4 tetrahedra to undergo thermally-induced deformation. The inclusion of halogenated ligands within MOFglasses improves their gas uptake properties.

We demonstrate the presence of a symbiotic stability reinforcement effect between bioentities and crystalline ZIFs, where the ZIF protects biomolecules from denaturation and the biomolecules improve the acid resistance of the ZIF framework. The strategy provides a potential route for stabilizing MOFs for diverse technological and industrial applications.