RESUMEN
In the present study, the molar heat capacity of solid formamidinium lead iodide (CH5N2PbI3) was measured over the temperature range from 5 to 357 K using a precise automated adiabatic calorimeter. In the above temperature interval, three distinct phase transitions were found in ranges from 49 to 56 K, from 110 to 178 K, and from 264 to 277 K. The standard thermodynamic functions of the studied perovskite, namely the heat capacity C°p(T), enthalpy [H0(T) - H0(0)], entropy S0(T), and [G°(T) - H°(0)]/T, were calculated for the temperature range from 0 to 345 K based on the experimental data. Herein, the results are discussed and compared with those available in the literature as measured by nonclassical methods.
RESUMEN
Organoammonium lead halide perovskites, especially methylammonium lead iodide CH3NH3PbI3, are promising photovoltaic materials, but they are far from commercial applications due in particular to their thermal instability and moisture sensitivity. Here, we present a multitechnique study aimed at investigating the kinetic and thermodynamic stability of the simplest quaternary ammonium lead iodide, tetramethylammonium lead iodide N(CH3)4PbI3. The kinetics of thermal decomposition was studied by X-ray powder diffraction of samples treated in air at different temperatures combined with Rietveld quantitative phase analysis, and by the isoconversional analysis of differential thermal analysis measurements. Evidence for first order kinetics was obtained, with an activation energy of 280-290 kJ mol-1, suggesting that the breaking of the C-N bond is the rate determining step. The composition of the gas phase released under heating was investigated by Knudsen Effusion Mass Spectrometry, giving evidence for the occurrence of the process N(CH3)4PbI3(s) = PbI2(s) + N(CH3)3(g) + CH3I(g), consistent with the kinetic results. Decomposition pressures and thermodynamic properties were derived by Knudsen effusion mass loss experiments, obtaining values of 391.5 ± 2.0 kJ mol-1 and -577.4 ± 4.0 kJ mol-1 for the decomposition and formation enthalpies at 298 K, respectively. The reactivity towards water of N(CH3)4PbI3 was checked by XRD after total and prolonged immersion in water at room temperature. Overall, N(CH3)4PbI3 was found to be thermally much more stable than CH3NH3PbI3, both kinetically and thermodynamically, and much less prone to water-induced degradation, suggesting that the use of a quaternary ammonium cation may be an effective strategy in order to produce more stable materials.
RESUMEN
We report the results of a multi-technique study on the thermodynamics and kinetics of formamidinium lead iodide (FAPI) thermal decomposition. Thermodynamics was investigated by means of Knudsen effusion techniques. Kinetics was studied either by temperature-controlled powder X-ray diffraction or by two isoconversional treatments of differential scanning calorimetry data. FAPI appears to be much more thermally stable compared to methylammonium lead iodide, as predictable from the lower acidity of the formamidinium cation compared to methylammonium. The chemical processes responsible for its thermal degradation appear to be quite complex as highlighted by the composition of the gaseous phase evolved during the process. The apparent activation energy values of the decomposition obtained from X-ray diffraction (XRD) (112 ± 9 kJ/mol) and differential scanning calorimetry (DSC) measurements (205 ± 20 and 410 ± 20 kJ/mol, respectively, for the first and second decomposition steps identified by the deconvolution procedure) reflect the different steps of the process observed by the two techniques. The thermodynamic properties of the more important decomposition channels and the enthalpy of formation of FAPI were estimated by combining the results of Knudsen effusion measurements.