Prospects for Engineering Thermoelectric Properties in La1/3NbO3 Ceramics Revealed via Atomic-Level Characterization and Modeling

Second-order Jahn-Teller effect induced high-temperature ferroelectricity in two-dimensional NbO2X (X = I, Br)

Based on the first-principles calculations, we investigated the ferroelectric properties of two-dimensional (2D) materials NbO₂X (X = I, Br). Our cleavage energy analysis shows that exfoliating one NbO₂I monolayer from its existing bulk counterpart is feasible. The phonon spectrum and molecular dynamics simulations confirm the dynamic and thermal stability of the monolayer structures for both NbO₂I and NbO₂Br. Total energy calculations show that the ferroelectric phase is the ground state for both materials, with the calculated in-plane ferroelectric polarizations being 384.5 pC m^-1 and 375.2 pC m^-1 for monolayers NbO₂I and NbO₂Br, respectively. Moreover, the intrinsic Curie temperature T_C of monolayer NbO₂I (NbO₂Br) is as high as 1700 K (1500 K) from Monte Carlo simulation. Furthermore, with the orbital selective external potential method, the origin of ferroelectricity in NbO₂X is revealed as the second-order Jahn-Teller effect. Our findings suggest that monolayers NbO₂I and NbO₂Br are promising candidate materials for practical ferroelectric applications.

Unraveling the Impact of Graphene Addition to Thermoelectric SrTiO3 and La-Doped SrTiO3 Materials: A Density Functional Theory Study

We present a detailed theoretical investigation of the interaction of graphene with the SrO-terminated (001) surface of pristine and La-doped SrTiO₃. The adsorption of graphene is thermodynamically favorable with interfacial adsorption energies of -0.08 and -0.32 J/m² to pristine SrTiO₃ and La-doped SrTiO₃ surfaces, respectively. We find that graphene introduces C 2p states at the Fermi level, rendering the composite semimetallic, and thus the electrical properties are predicted to be highly sensitive to the amount and quality of the graphene. An investigation of the lattice dynamics predicts that graphene adsorption may lead to a 60-90% reduction in the thermal conductivity due to a reduction in the phonon group velocities, accounting for the reduced thermal conductivity of the composite materials observed experimentally. This effect is enhanced by La doping. We also find evidence that both La dopant ions and adsorbed graphene introduce low-frequency modes that may scatter heat-carrying acoustic phonons, and that, if present, these effects likely arise from stronger phonon-phonon interactions.

Enhancing the thermoelectric properties of Sr1- xPr2 x/3□ x/3TiO3± δ through control of crystal structure and microstructure

A-site deficient perovskites are among the most important n-type thermoelectric oxides. Ceramics of Sr_{1- x}Pr_{2 x/3}□ _x/3TiO₃ ( x = 0.1-1.0) were prepared by solid-state reaction at 1700-1723 K using highly reducing atmospheres. Samples with the highest Sr content had a cubic crystal structure [Formula: see text]; incorporating Pr with A-site vacancies led to a reduction in symmetry to tetragonal ( I4/mcm) and then orthorhombic ( Cmmm) crystal structures. HRTEM showed Pr_2/3TiO₃ had a layered structure with alternating fully and partially occupied A-sites and a short-range order along the (100) direction. Electrical conductivity was highest in samples of high symmetry ( x ≤ 0.40), where the microstructures featured core-shell and domain structures. This enabled a very high power factor of approximately 1.75 × 10^-3 W m^-1 K^-2 at 425 K. By contrast, at high Pr content, structural distortion led to reduced electron transport; enhanced phonon scattering (from mass contrast, local strain and cation-vacancy ordering) led to reduced, glass-like, thermal conductivity. Carbon burial sintering increased the oxygen deficiency leading to increased carrier concentration, a maximum power factor of approximately 1.80 × 10^-3 W m^-1 K^-2 at 350 K and thermoelectric figure of merit of 0.26 at 865 K. The paper demonstrates the importance of controlling both crystal structure and microstructure to enhance thermoelectric performance. This article is part of a discussion meeting issue 'Energy materials for a low carbon future'.

The impact of tilt grain boundaries on the thermal transport in perovskite SrTiO3 layered nanostructures. A computational study

Thermal management at solid interfaces presents a technological challenge for modern thermoelectric power generation. Here, we define a computational protocol to identify nanoscale structural features that can facilitate thermal transport in technologically important nanostructured materials. We consider the highly promising thermoelectric material, SrTiO3, where tilt grain boundaries lower thermal conductivity. The magnitude of the reduction is shown to depend on compositional and structural arrangements at the solid interface. Quantitative analysis indicates that layered nanostructures less than 10 nm will be required to significantly reduce the thermal conductivity below the bulk value, and it will be virtually independent of temperature for films less than 2 nm depending on the orientation with a reduction of thermal transport up to 75%. At the nanoscale, the vibrational response of nanostructures shows concerted vibrations between the grain boundary and inter-boundary regions. As the grain boundary acts markedly as a phonon quencher, we predict that any manipulation of nanostructures to further reduce thermal conductivity will be more beneficial if applied to the inter-boundary region. Our findings may be applied more widely to benefit other technological applications where efficient thermal transport is important.