Alexander George Squires

Lithium-conducting fluoride garnets exhibit ionic conductivities of ∼10–10 S cm–1, far below those of their oxide counterparts. Here, we investigate the origin of this poor transport in Na3Ga2Li3F12 and Na3Al2Li3F12 through a combination of structural characterization and first-principles calculations. Both compounds are highly ordered, with no significant antisite disorder, and exhibit intrinsic ionic conductivities of ∼2 × 10–11 S cm–1 at 100 °C. Nudged elastic band calculations reveal low Li-ion migration barriers (0.16 to 0.29 eV), indicating that the fluoride garnet framework does not inherently restrict Li-ion diffusion. However, the formation energies for lithium Frenkel and Schottky defect pairs are high (1.9 to 2.4 eV), indicating that the intrinsic concentrations of mobile Li-ion defects are negligibly small. These results establish that the low conductivity of stoichiometric fluoride garnets arises from high defect-formation energies, rather than high migration barriers.

Dimension-driven design offers a powerful strategy for tuning the electronic structure of multinary chalcogenides, yet its application to solar absorbers remains limited. Previously explored systems lack candidates with optimal band gaps, while the time-consuming nature of solid-state synthesis impedes rapid synthesis and characterization of high-purity materials. Here, we report a colloidal soft-chemistry approach to systematically explore the Cu3–yAyVS4–xSex (A = K, Rb, Cs; y = 0, 1, 2; 0 ≤ x ≤ 3.5) compositional space, enabling simultaneous control over structural dimensionality (y), alkali-metal composition (A+), and chalcogen ratio (x) within a unified synthetic framework. By varying only precursor ratios, we access a broad family of phases spanning three-dimensional (3D), two-dimensional (2D), and one-dimensional (1D) [Cu–V–S] frameworks, as well as highly complex septenary solid solutions such as Cu(K, Rb, Cs)2VS2Se2. The resulting materials exhibit x-, y-, and A+-dependent optoelectronic properties, with band gaps tunable within the solar-relevant range of 1.40–1.67 eV, favorable band-edge alignment with CdS buffer layers, and intrinsic photocurrent responses. Theoretical calculations further elucidate the energy landscape and band structures of the understudied 1D series. Temperature- and time-resolved aliquot studies reveal pathway-dependent formation mechanisms, including direct nucleation of sulvanite Cu3VS4 and a two-step alkali-metal incorporation process governing the formation of alkali-metal-containing structures, providing key insights for related multinary chalcogenide systems. Overall, this work establishes a versatile platform for efficiently screening composition–structure–property relationships in complex chalcogenides for optoelectronic applications.

Surface‐enhanced IR absorption (SEIRA) and surface‐enhanced Raman spectroscopy (SERS) are complementary techniques that allow for highly sensitive chemical fingerprinting. Non‐invasive optical sensing would be significantly improved by a robust implementation of a reusable substrate that combines these techniques. Here, we present an electrochemically cleanable metamaterial that enables flow‐compatible real‐time SEIRA and SERS. This metamaterial facilitates the study of surface‐adsorbed species and diffusion layers, elicits spectral shifts from changes in nanogap refractive index of 1400 nm/RIU, and delivers sensitive analyte detection. Combining SERS and SEIRA clarifies molecular (electro)chemical transformations and tracks changes in selection rules and symmetry breaking at the analyte‐electrode interface. This development in enhanced multimodal spectro‐electrochemistry is suited for multiple domains, including understanding charge transport mechanisms and interfacial dynamics at electrodes, and is capable of real‐time flow monitoring for a wide range of molecular processes.

Defect engineering of the earth-abundant layered phosphide BaCuP: La doping tunes the intrinsic hole concentration towards the optimum, giving a peak zT of 0.69 at 800 K, the highest among 1-1-1 phosphides.

The development of earth-abundant transparent conducting oxides (TCOs) requires materials that combine high carrier concentrations with minimal degradation of electron mobility. Sb(V)-based oxides have recently emerged as promising candidates owing to their highly dispersive Sb 5s-derived conduction bands and deep valence-band positions. While degenerate conductivity in Sb2O5 has previously been predicted through anion-site F substitution, the viability of resonant donor engineering on the cation sublattice remains unexplored. Here, using hybrid density functional theory and rigorous defect thermodynamics, we show that W acts as an effective resonant donor dopant when substituted on Sb sites in Sb2O5, whereas Mo exhibits only weak donor behavior due to the stabilization of the localized electronic state for the neutral charge state. Both WSb and MoSb exhibit low formation energies under n-type conditions, with W-doped Sb2O5 yielding an electron concentration exceeding 1 × 1019 cm–3 while Mo-doping results in a moderate value of around 1 × 1018 cm–3. Unfolded band structure analysis reveals that the W 5d and Mo 4d donor states reside well above the conduction band minimum and exhibit negligible hybridization with the Sb 5s states that govern transport. This preservation of the dispersive conduction band enables degenerate doping without significant perturbation of band curvature. These results establish resonant cation-site doping as a transferable design strategy in Sb(V)-based oxides and broaden the chemical space for high-mobility, earth-abundant TCOs, while recognizing monoclinic Sb2O5 as an experimentally realized but synthetically challenging host material.

In functional multinary nitrides, intrinsic cation disorder typically has severe electronic consequences, introducing gap states and driving band gap reduction through local bonding irregularities. CaAlSiN3 (CASN), a benchmark nitride phosphor host for phosphor-converted light-emitting diodes, therefore presents a paradox: despite substantial Al/Si disorder generated during high-temperature synthesis, it maintains its exceptional optical performance. Understanding the fundamental effects of intrinsic disorder in this system could therefore provide broader insight into both its performance as a phosphor and the electronic structure of disorder-tolerant nitrides. Experiments typically treat the Al/Si disorder in CASN as random, whereas prior theoretical studies have been limited to a narrow set of ordered configurations, leaving its thermodynamic origin and electronic consequences unresolved. Here, we combine cluster-expansion Monte Carlo simulations with hybrid density functional theory to characterize the Al/Si disorder in CASN across a range of temperatures. Despite the emergence of disorder, the electronic structure remains resilient, with a maximum band gap reduction of only 3.8% and minimal carrier localization. This contrasts with related ternary nitrides, such as ZnGeN2 and ZnSnN2, in which less correlated cation disorder directly perturbs both band edges, driving pronounced band gap collapse and carrier localization. We attribute this tolerance to two features of CASN’s crystal chemistry: a Ca d-dominated conduction-band minimum decoupled from the disordered sublattice and correlated disorder that suppresses the most electronically damaging local environments. These results establish CASN as a model system for disorder tolerance in multinary nitrides and suggest broader design principles for electronically robust phosphor hosts.

Transparent conducting oxides (TCOs) play an indispensable role in modern optoelectronic technologies. However, industry primarily relies on a limited range of n-type oxides, severely constraining the design flexibility and optimization of optoelectronic device architectures. Expanding the range of known TCOs would significantly broaden the available electron affinities, facilitating precise energetic matching and thereby enhancing device performance. Recently, Sb(V)-based oxides have emerged as promising candidates, introducing an unexplored class of potential TCOs. In this study, we performed a computational screening of the ASb₂O₆ series (A = Mg, Ca, Sr, Ba, or Cd) to identify promising TCO candidates. Our analysis reveals that Ga-doped MgSb₂O₆ has a moderate carrier concentration of around 10¹⁷ cm^–3, which can be further enhanced with higher annealing temperatures, while Y-doped CdSb₂O₆ is predicted to be a degenerate TCO. Experimental characterization of solid-state synthesized Y-doped CdSb₂O₆ powders corroborates these computational predictions, where Y incorporation leads to a Burstein–Moss widening of the optical band gap and a 0.1 eV shift of XPS core levels to higher binding energies, confirming a Fermi-level rise due to n-type doping. In addition, the resonant nature of Y_Cd preserves high mobility and reinforces the material’s strong potential for practical applications in next-generation optoelectronic devices.

Simple features in complex hybrid inorganic-organic crystalline materials provide opportunities for targeted discovery of materials with desired optoelectronic properties. In this study, we report the structure and optoelectronic properties of isostructural (NH₃(CH₂)₇NH₃)₂Bi₂I₁₀ and (NH₃(CH₂)₇NH₃)₂Sb₂I₁₀. The crystal structures are characterized by corner-connected metal-iodide octahedral chains that form a cubic close-packed iodine inorganic framework. Variable temperature UV-visible diffuse reflectance spectroscopy reveals stark contrasts in the onset of absorption and color changes between the [MX₆]^3- based structures, due to differences in the interaction of the (NH₃(CH₂)₇NH₃)²⁺ organic ammonium cation and the iodine packing of the inorganic framework. Density functional theory (DFT) calculations reveal flat bands reflective of the pseudo-1D crystal structure. Dark microwave conductivity (DMC) and time-resolved microwave conductivity (TRMC) reveal an excitonic character with long carrier lifetimes, consistent with the electronically confined octahedral chains. Comparison of the structural features with those of other diammonium-containing crystals reveals that diammoniumheptane can substitute into structures, displacing inorganic octahedra while retaining a close-packed anion framework. This provides a means for targeting new hybrid materials in which "vacancy-ordering" provides a crystal chemical approach for targeting desirable optoelectronic properties.

Transparent conducting oxides (TCOs) play an indispensable role in modern optoelectronic technologies. However, industry primarily relies on a limited range of n-type oxides, severely constraining the design flexibility and optimization of optoelectronic device architectures. Expanding the range of known TCOs would significantly broaden the available electron affinities, facilitating precise energetic matching and thereby enhancing device performance. Recently, Sb(V)-based oxides have emerged as promising candidates, introducing a new class of potential TCOs. In this study, we performed a computational screening of the ASb ₂ O ₆ series (A = Mg, Ca, Sr, Ba, Cd) to identify promising TCO candidates. Our analysis reveals that Ga-doped MgSb ₂ O ₆ has a moderate carrier concentration of around 10 ¹⁷ cm ⁻³ , which can be further enhanced with higher annealing temperatures, while Y-doped CdSb ₂ O ₆ is predicted to be a degenerate TCO. Experimental characterisation of solid-state synthesised Y-doped CdSb ₂ O ₆ powders corroborates these computational predictions, where Y incorporation leads to a Burstein-Moss widening of the optical band gap and a 0.1 eV shift of XPS core levels to higher binding energies, confirming a Fermi-level rise due to n-type doping. In addition, the resonant nature of Y _Cd preserves high mobility and reinforces the material’s strong potential for practical applications in next-generation optoelectronic devices.

Thermoelectric (TE) materials enable direct conversion between heat and electricity, allowing efficient recovery of waste heat, which accounts for nearly 50% of global energy consumption. Therefore, TE materials hold great potential for applications in waste heat recovery and sustainable energy technologies. Owing to the composition of earth-abundant and low-toxicity elements, as well as the presence of relatively heavy elements and mixed-anion characteristics, SrZn₂S₂O is considered a promising, environmentally friendly TE material. In this study, the TE performance of SrZn₂S₂O was investigated based on density functional theory (DFT) and compared with that of the prototypical mixed-anion oxide BiCuSeO. The calculated results show that SrZn₂S₂O exhibits a higher optimal average p-type power factor than that of BiCuSeO at 900 K, reaching 1150 μW m^–1 K^–2 compared with 770 μW m^–1 K^–2 for BiCuSeO. In addition, nanostructuring strategies can reduce the lattice thermal conductivity of SrZn₂S₂O by 40% or more in all crystallographic directions. This leads to a maximum n-type ZT value of 0.65 along the b direction and a maximum p-type ZT value of 0.77 along the c direction for SrZn₂S₂O.