Cesium lead iodide (CsPbI3) perovskite has shown great potential as a light absorbing material for solar cell applications. Despite intense research leading to increasing power conversion efficiency, a major problem concerning CsPbI3 lies in the long term stability and interconversion between different CsPbI3 polymorphs, a subject barely studied from the thermodynamic perspective. We report the formation enthalpies of two CsPbI3 polymorphs, α and δ CsPbI3, using a combination of room temperature solution calorimetry in dimethyl sulfoxide (DMSO) and differential scanning calorimetry. We show that both polymorphs are stable with respect to their binary halides and confirm that the α-phase is a high temperature polymorph, metastable under ambient conditions. This work sheds light on patterns in polymorphism, possible decomposition reactions, materials stability, and compatibility within halide perovskites and related systems. Thermodynamic instability near ambient temperature of functional perovskites may be a general phenomenon related to their vibrational density of states.

We report the first systematic experimental and theoretical study of the relationship between the linker functionalization and the thermodynamic stability of metal–organic frameworks (MOFs) using a model set of eight isostructural zeolitic imidazolate frameworks (ZIFs) based on 2-substituted imidazolate linkers. The frameworks exhibit a significant (30 kJ·mol–1) variation in the enthalpy of formation depending on the choice of substituent, which is accompanied by only a small change in molar volume. These energetics were readily reproduced by density functional theory (DFT) calculations. We show that these variations in the enthalpy of MOF formation are in linear correlation to the readily accessible properties of the linker substituent, such as the Hammett σ-constant or electrostatic surface potential. These results provide the first quantifiable relationship between the MOF thermodynamics and the linker structure, suggesting a route to design and tune MOF stability.

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The stability of functional materials in watercontaining environments is critical for their industrial applications. A wide variety of metal−organic frameworks (MOFs) synthesized in the past decade have strikingly different apparent stabilities in contact with liquid or gaseous H 2 O, ranging from rapid hydrolysis to persistence over days to months. Here, we show using newly determined thermochemical data obtained by high-temperature drop combustion calorimetry that these differences are thermodynamically driven rather than primarily kinetically controlled. The formation reaction of a MOF from metal oxide (MO) and a linker generally liberates water by the reaction MO + linker = MOF + H 2 O. Newly measured enthalpies of formation of Mg-MOF-74 (s) + H 2 O (l) and Ni-MOF-74 (s) + H 2 O (l) from their crystalline dense components, namely, the divalent MO (MgO or NiO) and 2,5dihydroxyterephthalic acid, are 303.9 ± 17.2 kJ/mol of Mg for Mg-MOF-74 and 264.4 ± 19.4 kJ/mol of Ni for Ni-MOF-74. These strongly endothermic enthalpies of formation indicate that the reverse reaction, namely, the hydrolysis of these MOFs, is highly exothermic, strongly suggesting that this large thermodynamic driving force for hydrolysis is the reason why the MOF-74 family cannot be synthesized via hydrothermal routes and why these MOFs decompose on contact with moist air or water even at room temperature. In contrast, other MOFs studied previously, namely, zeolitic imidazolate frameworks (ZIF-zni, ZIF-1, ZIF-4, Zn(CF 3 Im) 2 , and ZIF-8), show enthalpies of formation in the range 20−40 kJ per mole of metal atom. These modest endothermic enthalpies of formation can be partially compensated by positive entropy terms arising from water release, and these materials do not react appreciably with H 2 O under ambient conditions. Thus, these differences in reactivity with water are thermodynamically controlled and energetics of formation, either measured or predicted, can be used to assess the extent of water sensitivity for different possible MOFs.

Metal–organic framework minerals stepanovite and zhemchuzhnikovite can exhibit high proton conduction and structure retention on dehydration.