Unraveling the Ordered Phase of the Quintessential Hybrid Perovskite MAPbI₃─Thermophysics to the Rescue

Abstract

Hybrid perovskites continue to attract an enormous amount of attention, yet a robust microscopic picture of their different phases as well as the extent and nature of the disorder present remains elusive. Using specific-heat data along with high-resolution inelastic neutron scattering and ab initio modeling, we address this ongoing challenge for the case of the ordered phase of the quintessential hybrid-perovskite MAPbI 3 . At low temperatures, the specific heat of MAPbI 3 reveals strong deviations from the Debye limit, a common feature of pure hybrid perovskites and their mixtures. Our thermophysical analysis demonstrates that the (otherwise ordered) structure around the organic moiety is characterized by a substantial lowering of the local symmetry relative to what can be inferred from crystallographic studies. The physical origin of the observed thermophysical anomalies is unequivocally linked to excitations of sub-terahertz optical phonons responsible for translational–librational distortions of the octahedral units.

Cation Dynamics as Structure Explorer in Hybrid Perovskites─The Case of MAPbI3

“…Afterward, we benchmarked semilocal DFA approximations for the whole configuration space, following the selection of exchange−correlation functionals presented in our previous works. 15,19,21 PBEsol with Tkatchenko−Schefler corrections was included to check the performance of soft-GGA in conjunction with semiempirical dispersion corrections. The results of these tests are presented in Figure 5.…”

“…In situ neutron powder diffraction (NPD) measurements were performed on the OSIRIS backscattering instrument at the ISIS Neutron & Muon Spallation Source (STFC, Didcot, UK), − using the same hydrogenous sample of MAPbI 3 (purity >99%; Xi’an Polymer Light Technologies) that had been extensively characterized in our previous works. , Raw time-of-flight data were reduced to d -spacing using the Mantid software package, roughly covering the range 3–6.5 Å. The whole pattern fitting of the NPD data was performed using the GSAS-ii software .…”

“…The vibrational density of states [VDoS, D(E)], defined as the number of modes per unit of energy, is obtained by integrating over all q-points. 15 Using the VDOS, the thermodynamic quantities under interest can be expressed as follows 67 Ä…”

“…As a result of the above, average crystal structures inferred from standard diffraction experiments often depart from the perovskite local structure, and a consistent picture of the short- and long-range structures remains a challenge to both experiment and computation . This situation holds even within the “cation-ordered” phases observed at low temperatures, where the effects of dynamical disorder are minimized . These limitations call for using local structure probes rather than conventional crystallographic techniques, the sensitivity of which is limited to long-range order.…”

“…These limitations call for using local structure probes rather than conventional crystallographic techniques, the sensitivity of which is limited to long-range order. In this regard, nuclear magnetic resonance (NMR) and radiation-scattering techniques using photons and neutrons can be deployed. , In such a context, vibrational spectroscopy techniques, such as optical terahertz (THz) spectroscopy and inelastic neutron scattering (INS), can be considered valuable alternatives. THz spectroscopy combined with first-principles calculations allows insight into the local structure and the interactions responsible for preferred short-range ordering under temperature–pressure conditions that are often hardly accessible to other spectroscopy techniques.…”