In the fast-evolving field of halide perovskite semiconductors, the 2D perovskites (A′)2(A)n−1MnX3n+1 [where A = Cs+, CH3NH3+, HC(NH2)2+; A′ = ammonium cation acting as spacer; M = Ge2+, Sn2+, Pb2+; and X = Cl−, Br−, I−] have recently made a critical entry. The n value defines the thickness of the 2D layers, which controls the optical and electronic properties. The 2D perovskites have demonstrated preliminary optoelectronic device lifetime superior to their 3D counterparts. They have also attracted fundamental interest as solution-processed quantum wells with structural and physical properties tunable via chemical composition, notably by the n value defining the perovskite layer thickness. The higher members (n > 5) have not been documented, and there are important scientific questions underlying fundamental limits for n. To develop and utilize these materials in technology, it is imperative to understand their thermodynamic stability, fundamental synthetic limitations, and the derived structure–function relationships. We report the effective synthesis of the highest iodide n-members yet, namely (CH3(CH2)2NH3)2(CH3NH3)5Pb6I19 (n = 6) and (CH3(CH2)2NH3)2(CH3NH3)6Pb7I22 (n = 7), and confirm the crystal structure with single-crystal X-ray diffraction, and provide indirect evidence for “(CH3(CH2)2NH3)2(CH3NH3)8Pb9I28” (“n = 9”). Direct HCl solution calorimetric measurements show the compounds with n > 7 have unfavorable enthalpies of formation (ΔHf), suggesting the formation of higher homologs to be challenging. Finally, we report preliminary n-dependent solar cell efficiency in the range of 9–12.6% in these higher n-members, highlighting the strong promise of these materials for high-performance devices.

Hybrid perovskites, especially methylammonium lead iodide (MAPbI 3 ), exhibit excellent solar power conversion efficiencies. However, their application is plagued by poor chemical and structural stability. Using direct calorimetric measurement of heats of formation, MAPbI 3 is shown to be thermodynamically unstable with respect to decomposition to lead iodide and methylammonium iodide, even in the absence of ambient air or light or heat-induced defects, thus limiting its long-term use in devices. The formation enthalpy from binary halide components becomes less favorable in the order MAPbCl 3 , MAPbBr 3 , MAPbI 3 , with only the chloride having a negative heat of formation. Optimizing the geometric match of constituents as measured by the Goldschmidt tolerance factor provides a potentially quantifiable thermodynamic guide for seeking chemical substitutions to enhance stability.hybrid halide perovskites | thermodynamic instability | solar cells | tolerance factor | enthalpy of formation O rganic-inorganic hybrid materials which adopt the perovskite structure have gained increasing attention due to their excellent solar to electric power conversion efficiencies (PCEs). Methylammonium lead halide (MAPbX 3 ) perovskite leads the race with a PCE of 20%. Although these perovskites were discovered in 1978 by Weber, who described their structural and physical properties (1, 2), their solar cell application was explored only in 2009 by Miyasaka and coworkers. (3). Recently a very impressive PCE of 20% has been claimed for these materials (4). The extraordinary performance of hybrid perovskites has been attributed to their large absorption efficiency, favorable band gap, high charge mobility, and long-range electron hole transport (5-8). The AMX 3 perovskite structure provides great flexibility for modifications where, in principle, a range of similar compounds with different energy gaps can be synthesized by variation and/or partial substitution of the organic moiety (A = methylammonium, ethylammonium, formamidinium, or other possible organic cations), the metal (M = Pb, Sn and possible others), or the halide (X = I, Br, Cl). All these factors make organic metal halide perovskites frontrunners compared with other conventional solar cell materials (9).

Bastnasite is a fluoro-carbonate mineral that is the largest source of rare earth elements (REEs) such as Y, La, and Ce. With increasing demand for REE in many emerging technologies, there is an urgent need for improving the efficiency of ore beneficiation by froth flotation. To design improved flotation agents that can selectively bind to the mineral surface, a fundamental understanding of the bulk and surface properties of bastnasite is essential. Unexpectedly, density functional theory (DFT) calculations using the PBEsol exchange correlation functional and the DFT-D3 dispersion correction reveal that the most stable form of La-bastnasite is isomorphic to the structure of Ce-bastnasite belonging to the P6̅ 2c space group, whereas the common structure listed in the Inorganic Crystal Structure Database structure belonging to the P6̅ 2m space group is ca. 11.3 kJ/mol higher in energy per LaFCO 3 formula unit. We report powder X-ray diffraction measurements on synthetic La-bastnasite to support these theoretical findings. Six different surfaces are studied by DFT, namely, [101̅ 0], [0001], [101̅ 1], [101̅ 2], [101̅ 4], and [112̅ 2]. Among these, the [101̅ 0] surface is the most stable with a surface energy of 0.73 J/m 2 in vacuum and 0.45 J/m 2 in aqueous solution. The shape of a La-bastnasite nanoparticle is predicted via thermodynamic Wulff construction to be a hexagonal prism with [101̅ 0] and [0001] facets, chiseled at its ends by the [101̅ 1] and [101̅ 2] facets. The average surface energy of the nanoparticle in the gas phase is estimated to be 0.86 J/m 2 , in good agreement with a value of 1.11 J/m 2 measured by calorimetry. The calculated adsorption energy of a water molecule varies widely with the surface plane and specific adsorption sites within each facet. The first layer of water molecules is predicted to adsorb strongly on the La-bastnasite surface, in agreement with water adsorption calorimetry experiments. Our work provides an important step toward a detailed atomistic understanding of the bastnasite−water interface and designing collector molecules that can bind specifically to bastnasite.

Solid–solid inter‐polytype transformations are observed during the thermal dehydration of sulfate‐containing layered double hydroxides (LDHs). The metal hydroxide layer behaves as a “structural synthon” and the interconversion of polytypes of rhombohedral and hexagonal symmetries takes place by rigid translations of successive layers by (± 1/3, ± 2/3) relative to one another in the ab plane. These translations are selected among the many possible, as they preserve the coincidence of the symmetry elements of the individual layers and thereby conserve the threefold symmetry of the crystal across the inter‐polytype conversions. As a result, these transformations are enthalpically not expensive. These translations are facilitated at near ambient temperatures (30–60 °C) by the reversible dehydration of the LDH, which involves the deinsertion/insertion of water molecules within the restricted space of the interlayer region.