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  3. Two multinary selenides, Ba8Hf2Se11(Se2) and Ba9Hf3Se14(Se2), with unprecedented structure types have been prepared using high-temperature synthesis techniques and represent the first known compounds in the Ba-Hf-Se system. Their structures were determined from single crystal X-ray diffraction (XRD) data. The Ba8Hf2Se11(Se2) compound crystallizes in the monoclinic C2/c space group with a = 12.3962(15) Å, b = 12.8928(15) Å, c = 18.1768(17) Å, and β = 90.685(4)°, while Ba9Hf3Se14(Se2) forms in the rhombohedral R space group with a = b = 19.4907(6) Å and c = 23.6407(11) Å. Both have pseudo-zero-dimensional structures with homoatomic Se–Se bonding in the form of (Se2)2− at distances of 2.400–2.402 Å. The structure of Ba8Hf2Se11(Se2) is comprised of [Hf2Se11]14−, Ba2+, and (Se2)2− dimers. Conversely, the Ba9Hf3Se14(Se2) structure contains a novel perovskite-type cluster constructed from eight octahedrally-coordinated Hf cations, i.e., [Hf8Se36]40−, and isolated [HfSe6]8− units which are separated by (Se2)2− dimers and Ba2+ cations. Polycrystalline Ba8Hf2Se11(Se2) is synthesized at 1073 K using a two-step solid-state synthesis method, with the co-formation of a small amount of a BaSe secondary phase. A direct bandgap of 2.2(2) eV is obtained for the polycrystalline sample of Ba8Hf2Se11(Se2), which is consistent with its yellow color. Density functional theory calculations reveal their bandgap transitions stem from predominantly filled Se-4p to empty Hf-5d at the edges of the valence bands (VB) and conduction bands (CB), respectively. The optical absorption coefficients are calculated to be relatively large, exceeding ∼105 cm−1 at about >2.0 eV with effective masses in the CB varying from ∼0.5 me (Γ → A) in Ba8Hf2Se11(Se2) to ∼1.0 me (Γ → L) in Ba9Hf3Se14(Se2). Thus, their optoelectronic properties are shown to be competitive with existing perovskite-type chalcogenides that have been a focus of recent research efforts. 
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    Free, publicly-accessible full text available January 1, 2025
  4. Traditional synthetic efforts to prepare Eu(II)-containing oxides have principally involved the use of high temperature reactions starting from EuO or a controlled, highly-reducing, atmosphere. Conversely, chimie douce approaches that are more amenable to the targeted syntheses of new, and potentially metastable, Eu(II)-oxides have yet to be explored. Herein, a cation-exchange route to new Eu(II)-containing oxides, e.g., EuTa4-xO11 (x = 0.04), has been discovered and its structure determined by powder X-ray diffraction (Space group P6322 (#182), a = 6.2539(2) Å; c = 12.3417(2) Å). The compound derives from the cation exchange of Na2Ta4O11, via a reaction with EuBr2 at 1173 K, and replacement by half the number of divalent Eu cations. Rietveld refinements show preferential ordering of the Eu cations over one of the two possible cation sites, i.e., Wyckoff site 2d (~94%; Eu1) versus 2b (~6%; Eu2). Total energy calculations confirm an energetic preference of the Eu cation in the 2d site. Tantalum vacancies of ~1% occur within the layer of Eu cations and TaO6 octahedra, and ~20% partial oxidation of Eu(II) to Eu(III) cations from charge balance considerations. 151Eu M¨ossbauer spectroscopy measured at 78 K found a Eu(II):Eu(III) ratio of 69:31, with a relatively broad line width of the former signal of Γ = 7.6(2) mm s–1. Also, the temperature-dependent magnetic susceptibility could be fitted to a Curie Weiss expression, giving a μeff = 6.2 μB and θCW = 10 K and confirming a mixture of Eu(II)/Eu(III) cations. The optical bandgap of EuTa4-xO11 was found to be ~1.5 eV (indirect), significantly redshifted as compared to ~4.1 eV for Na2Ta4O11. Spin-polarized electronic structure calculations show that this redshift stems from the addition of Eu 4f7 states as a higher-energy valence band. Thus, these results demonstrate a new cation-exchange approach that represents a useful synthetic pathway to new Eu(II)-containing ox- ides for tunable magnetic and optical properties. 
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    Free, publicly-accessible full text available December 1, 2024
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  8. Sn(II)-based perovskite oxides, being the subject of longstanding theoretical interest for the past two decades, have been synthesized for the first time in the form of nano eggshell particle morphologies. All past reported synthetic attempts have been unsuccessful owing to their metastable nature, i.e. , by their thermodynamic instability towards decomposition to their constituent oxides. A new approach was discovered that finally provides an effective solution to surmounting this intractable synthetic barrier and which can be the key to unlocking the door to many other predicted metastable oxides. A low-melting KSn2Cl5 salt was utilized to achieve a soft topotactic exchange of Sn(II) cations into a Ba-containing perovskite, i.e., BaHfO3 with particle sizes of ∼350 nm, at a low reaction temperature of 200 °C. The resulting particles exhibit nanoshell-over-nanoshell morphologies, i.e., with SnHfO3 forming as ∼20 nm thick shells over the surfaces of the BaHfO3 eggshell particles. Formation of the metastable SnHfO3 is found to be thermodynamically driven by the co-production of the highly stable BaCl2 and KCl side products. Despite this, total energy calculations show that Sn(II) distorts from the A-site asymmetrically and randomly and the interdiffusion has a negligible impact on the energy of the system (i.e., layered vs. solid solution). Additionally, nano eggshell particle morphologies of BaHfO3 were found to yield highly pure SnHfO3 for the first time, thus circumventing the intrinsic ion-diffusion limits occurring at this low reaction temperature. In summary, these results demonstrate that the metastability of many theoretically predicted Sn(II)-perovskites can be overcome by leveraging the high cohesive energies of the reactants, the exothermic formation of a stable salt side product, and a shortened diffusion pathway for the Sn(II) cations. 
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