Design of metastable oxychalcogenide phases by topochemical (de)intercalation of sulfur in La2O2S2

Designing and synthesising new metastable compounds is a major challenge of today’s material science. While exploration of metastable oxides has seen decades-long advancement thanks to the topochemical deintercalation of oxygen as recently spotlighted with the discovery of nickelate superconductor, such unique synthetic pathway has not yet been found for chalcogenide compounds. Here we combine an original soft chemistry approach, structure prediction calculations and advanced electron microscopy techniques to demonstrate the topochemical deintercalation/reintercalation of sulfur in a layered oxychalcogenide leading to the design of novel metastable phases. We demonstrate that La2O2S2 may react with monovalent metals to produce sulfur-deintercalated metastable phases La2O2S1.5 and oA-La2O2S whose lamellar structures were predicted thanks to an evolutionary structure-prediction algorithm. This study paves the way to unexplored topochemistry of mobile chalcogen anions.

reciprocal lattice vectors. All structures are optimized until the net forces on atoms are below 1 meV/Å, and all forces on atoms are converged to less than 0.005 eV/Å.
The series of structures predicted by USPEX algorithm fell into three structure types after the relaxation at the PBE level of theory but one of these three structure with monoclinic P21/m space group was found to be dynamically unstable with negative phonon frequency (vide infra for calculation procedure). Therefore, this monoclinic structure was excluded from further considerations. Other two dynamically stable structures were subject to further optimization with the strongly constrained and appropriately normed (SCAN) meta-GGA functional 4 combined with the revised Vydrov-van Voorhis nonlocal correlation (rVV10). [5][6] SCAN+rVV10 functional allows to treat the weak dispersion forces of layered structures and therefore it is suitable to estimate the interlayer spacing as well as intra-layer lattice parameters of La2O2S whose predicted structures are lamellar. The optimization at SCAN+rVV10 level of theory increased the difference in the relative enthalpy between each structure type (Table S2). On the other hand, their structural parameters were hardly varied by the inclusion of van der Waals effects. Thus, the structures given at the PBE level of theory were used for other calculations (vide infra) and the comparison with our experimental data.
For better estimation of the optical band gap (Eg) of La2O2S phases, the single-point energy calculation using the optimized structure was performed at the Heyd−Scuseria−Ernzerhof (HSE06) hybrid functional level of theory.

Phonon calculations
In this work, first principles phonon calculations using the Density functional perturbation theory (DFPT) at a quasi-harmonic level are done using the open source package PHONOPY. 7 Supercell structures with or without displacements are created from a reference La2O2S unit cell considering all possible crystal symmetry operations. To avoid unphysical imaginary frequencies, a 2x2x2 or a 3x3x2 supercell are used. Force constants are calculated using the optimized structure (VASP). The Brillouin zone and the associated k-path of each structure are computed using the Bilbao crystallographic server tools. 8

Ab Initio molecular dynamics (AIMD)
Ab initio molecular dynamics (AIMD) with canonical ensemble using the Nosé heat bath scheme were performed to evaluate the thermal stability of specific phases up to 1500 K for 10 ps, for a timestep of 1 fs. The simulations based on DFT are also carried out using VASP code to examine the candidate phases (PBE level of theory). In such AIMD simulations, the Brillouin zone integration is restricted to the Γ point of the supercell, due to a high calculation cost. A 2x2x2 supercell is used (up to 160 atoms par repeat unit). A metastable crystalline structure is definitively termed viable if no structural transformations are observed in the La-S-O network, i.e., an absence of La-O, and La-S bond breaking after a long simulation (10 ps), indicating the existence of a substantial kinetic barrier that facilitates formation and trapping of viable (meta)stable structures. Table S1. Structural parameters and space groups of the two structure types predicted for the fixed composition La2O2S (distances in Å, angle in °).

Crystal and energy parameters of investigated stable and metastable structures
+0.00 +0.00 oA-La2O2S +0.072 +0.396 Figure S1. The lowest-energy structure hP-La2O2S predicted by the evolutionary algorithm USPEX (See Table S1 for its structural parameters). These images are viewed along (left) a-axis,   Table S1 for its structural parameters). These images are viewed along (left) a-axis, Anisotropic strain (Å -2 ) 2 S400 = 11.7(9); S040 = 8.60 (1) Table S4). b The La2O2S2 phase was refined using the crystallographic data reported by Ostoréro et al. 11 Its atomic parameters were not refined and fixed to the same values with the reported ones. a Site-occupancy factors of all atoms were fixed to full occupancy. Isotropic thermal parameters were fixed to the same values as obtained by the PEDT analysis (See Table S4). b The La2O2S2 phase was refined using the crystallographic data reported by Ostoréro et al. 11 Its atomic parameters were not refined and fixed to the same values with the reported ones. c The α-NiS phase was refined using the crystallographic data reported by Trahan et al. 12 Its atomic parameters were not refined and fixed to the same values with the reported ones. Figure S18. The most stable structures predicted for La2O2S1.5 composition by USPEX calculation.
Those with the same structure type (and space groups) was grouped by each colour and the more stable one in each group was displayed. Each structure type was compared with respect to its enthalpy [eV/atom] relative to the most stable (1st in the rank) structure. Compared to the most stable structure type (1st in the rank), the second and third most stable structure types (3rd and 5 th in the rank, respectively) are higher in energy by 31 and 65 meV/atom, respectively.