Crystal growth and structural analysis of perovskite chalcogenide BaZrS3_3 and Ruddlesden-Popper phase Ba3_3Zr2_2S7_7

Perovskite chalcogenides are gaining substantial interest as an emerging class of semiconductors for optoelectronic applications. High quality samples are of vital importance to examine their inherent physical properties. We report the successful crystal growth of the model system, BaZrS$_3$ and its Ruddlesden-Popper phase Ba$_3$Zr$_2$S$_7$ by flux method. X-ray diffraction analyses showed space group of $Pnma$ with lattice constants of $a$ = 7.056(3) \AA\/, $b$ = 9.962(4) \AA\/, $c$ = 6.996(3) \AA\/ for BaZrS$_3$ and $P4_2/mnm$ with $a$ = 7.071(2) \AA\/, $b$ = 7.071(2) \AA\/, $c$ = 25.418(5) \AA\/ for Ba$_3$Zr$_2$S$_7$. Rocking curves with full-width-at-half-maximum of 0.011$^\circ$ for BaZrS$_3$ and 0.027$^\circ$ for Ba$_3$Zr$_2$S$_7$ were observed. Pole figure analysis, scanning transmission electron microscopy images and electron diffraction patterns also establish high quality of grown crystals. The octahedra tilting in the corner-sharing octahedra network are analyzed by extracting the torsion angles.Comment: 4 Figures, 2 Table

Ideal Bandgap in a 2D Ruddlesden-Popper Perovskite Chalcogenide for Single-junction Solar Cells

Transition metal perovskite chalcogenides (TMPCs) are explored as stable, environmentally friendly semiconductors for solar energy conversion. They can be viewed as the inorganic alternatives to hybrid halide perovskites, and chalcogenide counterparts of perovskite oxides with desirable optoelectronic properties in the visible and infrared part of the electromagnetic spectrum. Past theoretical studies have predicted large absorption coefficient, desirable defect characteristics, and bulk photovoltaic effect in TMPCs. Despite recent progresses in polycrystalline synthesis and measurements of their optical properties, it is necessary to grow these materials in high crystalline quality to develop a fundamental understanding of their optical properties and evaluate their suitability for photovoltaic application. Here, we report the growth of single crystals of a two-dimensional (2D) perovskite chalcogenide, Ba3Zr2S7, with a natural superlattice-like structure of alternating double-layer perovskite blocks and single-layer rock salt structure. The material demonstrated a bright photoluminescence peak at 1.28 eV with a large external luminescence efficiency of up to 0.15%. We performed time-resolved photoluminescence spectroscopy on these crystals and obtained an effective recombination time of ~65 ns. These results clearly show that 2D Ruddlesden-Popper phases of perovskite chalcogenides are promising materials to achieve single-junction solar cells.Comment: 4 Figure

Discovery of highly polarizable semiconductors BaZrS₃ and Ba₃Zr₂S₇

There are few known semiconductors exhibiting both strong optical response and large dielectric polarizability. Inorganic materials with large dielectric polarizability tend to be wide-band gap complex oxides. Semiconductors with a strong photoresponse to visible and infrared light tend to be weakly polarizable. Interesting exceptions to these trends are halide perovskites and phase-change chalcogenides. Here we introduce complex chalcogenides in the Ba-Zr-S system in perovskite and Ruddlesden-Popper structures as a family of highly polarizable semiconductors. We report the results of impedance spectroscopy on single crystals that establish BaZrS₃ and Ba₃Zr₂S₇ as semiconductors with a low-frequency relative dielectric constant ɛ0 in the range 50–100 and band gap in the range 1.3–1.8 eV. Our electronic structure calculations indicate that the enhanced dielectric response in perovskite BaZrS₃ versus Ruddlesden-Popper Ba₃Zr₂S₇ is primarily due to enhanced IR mode-effective charges and variations in phonon frequencies along 〈001〉; differences in the Born effective charges and the lattice stiffness are of secondary importance. This combination of covalent bonding in crystal structures more common to complex oxides, but comprising sulfur, results in a sizable Fröhlich coupling constant, which suggests that charge carriers are large polarons

Colossal optical anisotropy from atomic-scale modulations

In modern optics, materials with large birefringence ({\Delta}n, where n is the refractive index) are sought after for polarization control (e.g. in wave plates, polarizing beam splitters, etc.), nonlinear optics and quantum optics (e.g. for phase matching and production of entangled photons), micromanipulation, and as a platform for unconventional light-matter coupling, such as Dyakonov-like surface polaritons and hyperbolic phonon polaritons. Layered "van der Waals" materials, with strong intra-layer bonding and weak inter-layer bonding, can feature some of the largest optical anisotropy; however, their use in most optical systems is limited because their optic axis is out of the plane of the layers and the layers are weakly attached, making the anisotropy hard to access. Here, we demonstrate that a bulk crystal with subtle periodic modulations in its structure -- Sr9/8TiS3 -- is transparent and positive-uniaxial, with extraordinary index n_e = 4.5 and ordinary index n_o = 2.4 in the mid- to far-infrared. The excess Sr, compared to stoichiometric SrTiS3, results in the formation of TiS6 trigonal-prismatic units that break the infinite chains of face-shared TiS6 octahedra in SrTiS3 into periodic blocks of five TiS6 octahedral units. The additional electrons introduced by the excess Sr subsequently occupy the TiS6 octahedral blocks to form highly oriented and polarizable electron clouds, which selectively boost the extraordinary index n_e and result in record birefringence ({\Delta}n > 2.1 with low loss). The connection between subtle structural modulations and large changes in refractive index suggests new categories of anisotropic materials and also tunable optical materials with large refractive-index modulation and low optical losses.Comment: Main text + supplementar

High frequency atomic tunneling yields ultralow and glass-like thermal conductivity in chalcogenide single crystals

Crystalline solids exhibiting glass-like thermal conductivity have attracted substantial attention both for fundamental interest and applications such as thermoelectrics. In most crystals, the competition of phonon scattering by anharmonic interactions and crystalline imperfections leads to a non-monotonic trend of thermal conductivity with temperature. Defect-free crystals that exhibit the glassy trend of low thermal conductivity with a monotonic increase with temperature are desirable because they are intrinsically thermally insulating while retaining useful properties of perfect crystals. However, this behavior is rare, and its microscopic origin remains unclear. Here, we report the observation of ultralow and glass-like thermal conductivity in a hexagonal perovskite chalcogenide single crystal, BaTiS₃, despite its highly symmetric and simple primitive cell. Elastic and inelastic scattering measurements reveal the quantum mechanical origin of this unusual trend. A two-level atomic tunneling system exists in a shallow double-well potential of the Ti atom and is of sufficiently high frequency to scatter heat-carrying phonons up to room temperature. While atomic tunneling has been invoked to explain the low-temperature thermal conductivity of solids for decades, our study establishes the presence of sub-THz frequency tunneling systems even in high-quality, electrically insulating single crystals, leading to anomalous transport properties well above cryogenic temperatures