A Gold(I) Oxide Double Perovskite: Ba2AuIO6.

Oxide perovskites offer improved stability compared to halide perovskite compounds for optoelectronic applications. Here, we report the first gold-containing double perovskite, Ba2AuIO6, and compare it to Ba2AgIO6 and Ba2NaIO6. Ba2AuIO6 and Ba2AgIO6 exhibit a monoclinic distortion from the cubic perovskite structure possessed by Ba2NaIO6 and have similar lattice constants despite the nominally larger size of Au+ compared to Ag+. Ba2AgIO6 shows photoluminescence (PL) at 2.10 eV, and Ba2AuIO6 exhibits PL at 1.30 and 1.47 eV. As prepared, both compounds appear stable under visible light at room temperature but decompose when subjected to gentle heating followed by illumination. Our data suggest that this behavior is due to the presence of -OH defects in the crystal structures. This discovery provides a new route to semiconductors with a near-IR band gap and identifies engineering challenges that must be addressed to use oxide perovskites for optoelectronic devices.

Twisting of 2D Kagomé Sheets in Layered Intermetallics.

Chemical bonding in 2D layered materials and van der Waals solids is central to understanding and harnessing their unique electronic, magnetic, optical, thermal, and superconducting properties. Here, we report the discovery of spontaneous, bidirectional, bilayer twisting (twist angle ∼4.5°) in the metallic kagomé MgCo6Ge6 at T = 100(2) K via X-ray diffraction measurements, enabled by the preparation of single crystals by the Laser Bridgman method. Despite the appearance of static twisting on cooling from T ∼300 to 100 K, no evidence for a phase transition was found in physical property measurements. Combined with the presence of an Einstein phonon mode contribution in the specific heat, this implies that the twisting exists at all temperatures but is thermally fluctuating at room temperature. Crystal Orbital Hamilton Population analysis demonstrates that the cooperative twisting between layers stabilizes the Co-kagomé network when coupled to strongly bonded and rigid (Ge2) dimers that connect adjacent layers. Further modeling of the displacive disorder in the crystal structure shows the presence of a second, Mg-deficient, stacking sequence. This alternative stacking sequence also exhibits interlayer twisting, but with a different pattern, consistent with the change in electron count due to the removal of Mg. Magnetization, resistivity, and low-temperature specific heat measurements are all consistent with a Pauli paramagnetic, strongly correlated metal. Our results provide crucial insight into how chemical concepts lead to interesting electronic structures and behaviors in layered materials.

Oxygen-Induced Ordering in Bulk Polycrystalline Cu2ZnSnS4 by Sn Removal.

Solid-state nuclear magnetic resonance spectroscopy, X-ray diffraction, and Raman spectroscopy were used to show that Cu2ZnSnS4 (CZTS) bulk solids grown in the presence of oxygen had improved cation ordering compared to bulk solids grown without oxygen. Oxygen was shown to have negligible solubility in the CZTS phase. The addition of oxygen resulted in the formation of SnO2, leading to Sn-deficient CZTS. At the highest oxygen levels, other phases such as Cu9S5 and ZnS were observed. Beneficial ordering was only observed in samples produced with more than 2 at. % oxygen in the precursor materials but did not occur in samples designed with excess Sn and O. Thus, it is the removal of Sn and formation of Sn-deficient CZTS that improves ordering rather than the presence of SnO2 or O alone. These results indicate that using oxygen or air annealing to tailor the Sn content of CZTS followed by an etching step to remove SnO2 may significantly improve the properties of CZTS.