Publication date: 30th October 2021
Hybrid Organic-Inorganic Perovskites (HOIPs) have received unprecedented levels of attention in view of their promising optoelectronic features, which could make them suitable for photovoltaic applications. Moreover, their structural characterization continues to pose significant challenges to crystallography at low temperature and ambient conditions even in the case of quintessential systems, like methylammonium lead iodide (MAPbI3) [1,2,3].
The present work builds up from our previous research on MAPbI3 [4,5], focusing on its low-temperature phase by means of experimental, Inelastic Neutron Scattering (INS) and Specific Heat (Cp), and theoretical, Density Functional Theory (DFT), tools in order to scrutinize its orthorhombic Pnma and pseudo-orthorhombic P1 structural models further.
First, theoretical reproduction of high-resolution INS patterns of MAPbI3 evinces that orthorhombic Pnma does not allow for superb description of the vibrational dynamics of the organic cation as the one achieved by pseudo-orthorhombic P1. Second, experimental Cp measurements expressed in the Debye-reduced representation, Cp/RT3 as a function of T, revealed the presence of a hump which had been reported earlier in the literature [1,6]. We were able to discern the origin of this anomaly to sub-THz optical phonons. Third, a quantitative analysis of the thermophysical data predictions using mean absolute errors (MAE) showed that pseudo-orthorhombic P1 model performs almost 1.6 times better as Cp predictor than the orthorhombic Pnma model.
In summary, we provide a joint experimental and computational protocol allowing for a quantitative validation of the structural models of HOIPs at low temperatures in a spectrometer-free manner. According to our findings, pseudo-orthorhombic P1 model exhibits a more satisfactory overall performance than orthorhombic Pnma in reproducing experimental data of the low-temperature phase of MAPbI3.
Financial support from the Basque Government (PIBA-2021-0026 and IKUR) and the Spanish Ministry of Science and Innovation (PID2020-114506GB-I00) is gratefully acknowledged. The UK Science & Technology Facilities Council is acknowledged for access to beam time and laboratory equipment at the ISIS Facility. This work has been supported by PL-Grid Infrastructure and the PROMETHEUS facility. We kindly thank Prof. Philippe Bourges, Université Paris-Saclay, CNRS, CEA, for sharing the previously-published S(Q,ω) data for MAPbI3, measured on IN5 at the Institut Laue-Langevin (ILL, France).
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