Guodong Ren¹,Boyang Zhao²,Hongyan Mei³,Arashdeep Thind¹,Jad Salman³,Nan Wang²,Tengfei Cao¹,John Cavin⁴,Han Wang²,Mikhail Kats³,Miaofang Chi⁵,Jayakanth Ravichandran²,Rohan Mishra¹

Washington University in St.Louis¹,University of Southern California²,University of Wisconsin–Madison³,Washington University in St. Louis⁴,Oak Ridge National Laboratory⁵

Abstract

Guodong Ren¹,Boyang Zhao²,Hongyan Mei³,Arashdeep Thind¹,Jad Salman³,Nan Wang²,Tengfei Cao¹,John Cavin⁴,Han Wang²,Mikhail Kats³,Miaofang Chi⁵,Jayakanth Ravichandran²,Rohan Mishra¹

Washington University in St.Louis¹,University of Southern California²,University of Wisconsin–Madison³,Washington University in St. Louis⁴,Oak Ridge National Laboratory⁵

Hexagonal perovskite sulfides of the form A1+xBS3 (A, B = metals) have a quasi-one-dimensional crystal structure with face-shared (BS6) octahedral chains. Giant optical anisotropy has been reported in one such compound, BaTiS3.1 Replacing Ba2+ with isovalent, but smaller sized cation, Sr2+, leads to large dichroism in Sr1+xTiS3 (dichroic ratio > 20).2 Furthermore, Sr1+xTiS3 displays periodic structural modulations, where the periodicity x can be changed by controlling the stoichiometry of the compound.3,4 However, the origins of giant optical anisotropy in these compounds and their connection with structural modulations remain unresolved. Here, we combine density-functional-theory (DFT) calculations with structural characterization to reveal that the subtle periodic modulations in Sr1+xTiS3 structures result in selective occupancy of Ti-dz2 states, which when combined with the anisotropic crystal structure of these quasi-1D chalcogenides enables their record-breaking optical anisotropy. We construct Sr1+xTiS3 structures with different modulation periodicities by varying the chemical ratio of Sr from 0.2 (x=0) to 0.27 (x=0.5). These modulated structures consist of blocks of face-shared octahedra that are separated by trigonal prismatic TiS6 units along the c axis. Using DFT total energy calculations, we show that modulated structures with x < 0.2 lie on the convex hull, and are thermodynamically stable against decomposition, as opposed to stoichiometric SrTiS3, which is not. Electronic structure calculations reveal that the electrons introduced by excess Sr2+ cations selectively occupy 3dz2 states of Ti atoms present at the modulations. This results in a Mott band gap between the occupied 3dz2 states and the remaining unoccupied Ti 3d states in the modulated Sr1+xTiS3 structures — as opposed to S 3p states and Ti 3d in SrTiS3, which is a regular band insulator. The Mott band gap when combined with 1D chains of TiS6 polyhedra results in a large dielectric response within the chains, instead of, between neighboring chains. We predict the giant optical anisotropy shown by modulated Sr1+xTiS3 structures by computing their frequency-dependent dielectric functions. Experimentally, we confirm the structural modulation in Sr1+xTiS3 structures by performing structural refinement on X-ray diffraction results that show superlattice peaks. Our optical measurements on Sr1+xTiS3 also support our theoretical results. We will further discuss design rules to materials with giant optical anisotropy. This work was supported by W911NF-21-1-0327 and NSF through DMR-1806147, DMR-2122070, and DMR-2122071. 1 Niu, S. et al. Giant optical anisotropy in a quasi-one-dimensional crystal. Nature Photonics 12, 392-396 (2018). 2 Niu, S. et al. Mid-wave and Long-Wave Infrared Linear Dichroism in a Hexagonal Perovskite Chalcogenide. Chemistry of Materials 30, 4897-4901 (2018). 3 Onoda, et al. Structure refinement of the incommensurate composite crystal Sr1.145TiS3 through the Rietveld analysis process. Acta Crystallographica Section B 49, 929-936 (1993). 4 Gourdon, et al. Synthesis, structure determination, and twinning of two new composite compounds in the hexagonal perovskite-like sulfide family: Eu8/7TiS3 and Sr8/7TiS3. Zeitschrift für Kristallographie-Crystalline Materials 216, 541-555 (2001).