Dimensionality-Driven Polymorphism in Ruddlesden-Popper Phases of Ba-Zr-S Perovskites

Chalcogenide perovskites, in particular BaZrS₃, has gained a lot of popularity in the last few years due to its great potential as an alternative lead-free photovoltaic absorber material. This is due to promising optoelectronic properties such as defect tolerance, strong dielectric screening, and light absorption [1]. In our previous work, we demonstrated that phase pure synthesis of this material is challenging due to the coexistence of competing Ba_n+1Zr_nS_3n+1Ruddlesden-Popper (RP) phases [2]. The properties of these competing RP phases remain understudied, especially given that they are expected to affect the photovoltaic performance. For the n=2 phase, a high-temperature <i>I4/mmm</i> and a low-temperature <i>P4₂/mnm </i>phase has been reported [3]. We have also shown that the high-temperature <i>I4/mmm</i> phases of n=1,2,3 RP phase have imaginary phonon modes at the X-point in reciprocal space, which indicates the existence of a stable lower symmetry structure [4].<br/><br/>In this work, we present our machine learning potential model trained on n=1 to n=6 RP phases and the n= perovskite phase with the neuroevolution potential method [5]. These models have been trained on properties derived from the HSE06 hybrid exchange-correlation functional, which is shown to provide more reliable phase transition temperatures for the perovskite phase compared to the traditionally used generalized gradient-based functionals [6].<br/><br/>Using molecular dynamics we study phase transitions in RP phases across a variety of n-values. Whilst cooling the high-temperature RP phases, octahedral tilting along the X-mode leads to a second-order phase transition. We consider 48 unique tilted structures to systematically investigate all possible ground-state structures. We observe long-range ordering of octahedral tiling in the perovskite layer and show the dependence of the phase transition temperature with increasing n-value. We find that small-n RP phases adopt a different long-range ordering pattern compared to that of large-n RP phases. We demonstrate a limit where the large-n RP phase transitions coincide with the transition of the n= ∞ perovskite.<br/><br/>References:<br/>[1] Sopiha et al, Adv. Opt. Mater. (2022) 10 2101704.<br/>[2] Kayastha et al, Solar RRL (2023) 7 2201078.<br/>[3] Niu et al, J Mater. Res. (2019) 22 3819.<br/>[4] Pallikara et al, Electron. Struct. (2022) 4 033002.<br/>[5] Fan et al, J Chem. Phys. (2022) 157 114801.<br/>[6] Kayastha et al, in preparation.