The Vibrational and Thermodynamic Properties of CsPbI₃ Polymorphs: An Improved Description Based on the SCAN meta-GGA Functional

Abstract

We report the vibrational and thermodynamic properties of four known CsPbI 3 polymorphs in the framework of the density functional theory. We compare the recently introduced strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation (meta-GGA) with the local density approximation (LDA). We found that the SCAN, compared to the LDA, could explain discrepancies between theoretical and experimental results. Evaluating the Helmholtz free energy as a function of temperature, we found that within the SCAN (a) all polymorphs had negative formation enthalpies at the room temperature and (b) CsPbI 3 underwent the phase transition from the δ- to α-phase at 480 K. This is not true for the LDA. In contrast to the previous reports based on the LDA, we did not find the ferroelectric instability in the phonon spectra of the cubic and tetragonal phases at the meta-GGA level. This result agrees with the lack of observation of the ferroelectricity in CsPbI 3 .

First-Principles Molecular Dynamics in Metal-Halide Perovskites: Contrasting Generalized Gradient Approximation and Hybrid Functionals

“…Previously, Marronnier et al used the local density approximation (LDA) functional to obtain a double-well potential with an energy barrier of 7.3 meV for the cubic α-phase of CsPbI 3 from density functional perturbation theory calculations for the soft optical mode at Γ . Using the LDA, a recent study reported a double-well instability with a 5.1 meV barrier height in the α-CsPbI 3 , while the strongly constrained and appropriately normed (SCAN) meta-GGA functional did not show any anharmonicity at Γ but produced an accurate lattice constant . While the SCAN meta-GGA is known to yield accurate structural properties of MHPs, the absence of the double-well potential suggests that it cannot fully capture the octahedral tilting and the iodine displacement in MHPs.…”

“…29 Using the LDA, a recent study reported a double-well instability with a 5.1 meV barrier height in the α-CsPbI 3 , while the strongly constrained and appropriately normed (SCAN) meta-GGA functional did not show any anharmonicity at Γ but produced an accurate lattice constant. 58 While the SCAN meta-GGA is known to yield accurate structural properties of MHPs, 35 the absence of the double-well potential suggests that it cannot fully capture the octahedral tilting and the iodine displacement in MHPs. Previous studies did analyze further the octahedral tilting by the LDA and GGA functionals with different flavors of dispersion correction schemes, 59 but without quantifying the anharmonicity.…”

The structural complexity of perovskites

“…34 Comparing the complexities of the metastable polymorphs with the complexity of δ -CsPbI 3 ( I G,conf ( δ , Pnma ) = 3.240 bit pos −1 arity −1 ), it can be concluded that the thermodynamic stability of the δ -phase originates from enthalpic interactions which agrees with results from DFT calculations. 53 Nonetheless, it has recently been noted that CsPbI 3 shows a rather complicated temperature-dependent crystal chemistry and a more thorough analysis involves the treatment of partially occupied sites as it has been previously demonstrated. 32 For other examples of how complexities evolve along a polymorphic series, we would like to refer to some previous complexity studies on CaCO 3 's polymorphs 28 and the displacive phase transitions of [(MoO) 2 P 4 O 13 ].…”

First-principles study of the defect-activity and optical properties of FAPbCl3

“…Subsequently, single point energy and property evaluation are done using a more recently (2015) introduced meta-GGA functional, namely the Strongly Constrained and Appropriately Normed (SCAN) functional of Sun et al 38 The latter functional is particularly useful for thermodynamic calculations since it was recently demonstrated that this functional is better than GGA or local density approximation (LDA) functionals at predicting the lattice parameters and formation enthalpies of CsPbI 3 phases; it also did not show ferroelectric instability in the phonon spectra, unlike LDA which erroneously reported the material as ferroelectric. 39 SCAN has also been shown to yield reliable results for semiconductor bandgaps comparable to the widely used HSE hybrid functional 40 but at reduced computational cost. [41][42][43][44] By analyzing the formation energies of the various electron donating and accepting defects as a function of the Fermi energy, we found antisite defects, which are not previously studied, are in fact thermodynamically quite significant.…”