Stereochemically active lone pairs of electrons play an important role in a diverse range of physical phenomena in many materials, ranging from semiconducting halide perovskites to thermochromic inorganic−organic hybrids. In this paper, we demonstrate the importance of the 6s 2 lone pair of Pb on the reversible thermochromic transition in the mixed-anion inorganic compound, PbVO 3 Cl. This 6s 2 stereochemically active lone pair results in subtle structural distortions upon heating while maintaining its overall orthorhombic structure. These distortions result in competing interactions with the Pb 6s 2 lone pair and ultimately, a pronounced change between yellow and red at ∼200 °C. X-ray diffraction analyses of PbVO 3 Cl demonstrate two-dimensional features in contrast to the three-dimensional network in isostructural BaVO 3 Cl. X-ray and neutron pair distribution function experiments reveal that Pb−O interatomic distances decrease upon heating, while Pb−Cl distances are only affected by thermal motion. X-ray photoelectron spectroscopy measurements provide experimental evidence of the presence of the 6s 2 lone pair at the valence band maximum, which are corroborated by first-principles calculations. The results demonstrate a broadly generalizable mechanism for using repulsions between lone-pair electrons of p-block cations to drive discontinuous changes of local symmetry and electronic structure.

Mixed-anion systems have garnered much attention in the past decade with attractive properties for diverse applications such as energy conversion, electronics, and catalysis. The discovery of new materials through mixed-cation and single-anion systems proved highly successful in the previous century, but solid-state chemists are now embracing an exciting design opportunity by incorporating multiple anions in compounds such as oxychalcogenides. Materials containing rare-earth ions are arguably a cornerstone of modern technology, and herein, we review recent advances in rare-earth oxychalcogenides. We discuss ternary rare-earth oxychalcogenides whose layered structures illustrate the characters and bonding preferences of oxide and chalcogenide anions. We then review quaternary compounds which combine anionic and cationic design strategies toward materials discovery and describe their structural diversity. Finally, we emphasize the progression from layered two-dimensional compounds to three-dimensional networks and the unique synthetic approaches which enable this advancement.

No abstract

This study focuses on a solid solution series, Ca­(La1–x Ce x )2S4 (0 ≤ x ≤ 1), where the f electron density is absent in CaLa2S4 and is progressively increased until it is maximized in CaCe2S4. Correspondingly, these samples, synthesized by a sealed ampule method, showed progressive variations in color ranging from gray for CaLa2S4 to orange-red for CaCe2S4. The crystal structural nuances of both the end members and three solid solutions with x = 0.25, 0.50, and 0.75 were established with the complementary use of synchrotron X-ray diffraction and neutron scattering. Interestingly, these data were consistent with a two-phase composition centered around each nominal solid solution stoichiometry. Optical characterization via diffuse reflectance spectroscopy and Tauc analyses showed a shrinking of the energy band gap (from the UV to vis range) when Ce was progressively introduced into the host CaLa2S4 structure. These data were in concert with electronic band structure calculations, using density functional theory, which showed the progressive formation of an intermediate f band when Ce was introduced intro the structure. Photoelectrochemical measurements in an aqueous redox electrolyte, as well as surface photovoltage and Kelvin probe measurements, revealed all samples to be n-type semiconductors. The valence and conduction band edge positions of the end members and the three solid solutions could be mapped, on both the redox and vacuum reference energy scales, by combining these measurements with the optical data.

Heteroanionic compounds continue to gain interest in materials design because the expanded composition space provides opportunities to discover new phases and tune physical properties. Among heteroanionic materials, oxytellurides comprised of oxygen and tellurium anions are relatively underexplored despite the significant role of tellurium in emerging technologies. Herein, we present synthetic strategies toward oxytelluride Ln 2 O 2 Te (Ln = La−Pr), whose layered structure features square nets of Te 2− anions. Upon heating in H 2 or air, we find a reversible phase transition between the oxytelluride and tellurate Ln 2 TeO 6 (Ln = La, Pr), wherein Te is octahedrally coordinated and a 6+ oxidation state is corroborated by bond valence analysis. We use X-ray diffraction along with thermogravimetric analyses to confirm the presence of oxytelluride and tellurate phases and emphasize key structural distinctions. In contrast, we find that Ce 2 O 2 Te decomposes to form CeO 2 and demonstrate the instability of Ce 2 O 2 Te in ambient conditions by timelapse X-ray diffraction and diffuse-reflectance spectroscopy experiments. Band gaps of Ln 2 O 2 Te (Ln = La−Pr) were estimated from diffuse-reflectance spectroscopy in the semiconducting range ∼2.1−2.7 eV, while band gaps for La 2 TeO 6 and Pr 2 TeO 6 were much larger at ∼4.3 and ∼3.7 eV, respectively.