Diffused Lattice Vibration and Ultralow Thermal Conductivity in the Binary Ln-Nb-O Oxide System

Realization of Fine-Tuning the Lattice Thermal Conductivity and Anharmonicity in Layered Semiconductors via Entropy Engineering

Entropy engineering is widely proven to be effective in achieving ultra-low thermal conductivity for well-performed thermoelectric and heat management applications. However, no strong correlation between entropy and lattice thermal conductivity is found until now, and the fine-tuning of thermal conductivity continuously via entropy-engineering in a wide entropy range is still lacking. Here, a series of high-entropy layered semiconductors, Ni1- x(Fe0.25Co0.25Mn0.25Zn0.25)xPS3, where 0 ≤ x < 1, with low mass/size disorder is designed. High-purity samples with mixing configuration entropy of metal atomic site in a wide range of 0-1.61R are achieved. Umklapp phonon-phonon scattering is found to be the dominating phonon scattering mechanism, as revealed by the linear T-1 dependence of thermal conductivity. Meanwhile, fine tuning of the lattice thermal conductivity via continuous entropy engineering at metal atomic sites is achieved, in an almost linear dependence in middle-/high- entropy range. Moreover, the slope of the κ - T-1 curve reduces with the increase in entropy, and a linear response of the reduced Grüneisen parameter is revealed. This work provides an entropy engineering strategy by choosing multiple metal elements with low mass/size disorder to achieve the fine tuning of the lattice thermal conductivity and the anharmonic effect.

Designing rare earth-free high entropy oxides with a tungsten bronze structure for thermoelectric applications

In recent years, forming high entropy oxides has emerged as one of the promising approaches to designing oxide thermoelectrics. Entropy engineering is an excellent strategy to improve thermoelectric performance by minimizing the thermal conductivity arising from enhanced multi-phonon scattering. In the present work, we have successfully synthesized a rare-earth-free single phase solid solution of novel high entropy niobate (Sr_0.2Ba_0.2Li_0.2K_0.2Na_0.2)Nb₂O₆, with a tungsten bronze structure. This is the first report on the thermoelectric properties of high entropy tungsten bronze-type structures. We have obtained a maximum Seebeck coefficient of -370 μV K^-1 at 1150 K, which is the highest among tungsten bronze-type oxide thermoelectrics. The minimum thermal conductivity of 0.8 W m^-1 K^-1 is obtained at 330 K, which is so far the lowest reported value among rare-earth-free high entropy oxide thermoelectrics. This synergistic combination of large Seebeck and record low thermal conductivity gives rise to a maximum ZT of 0.23 which is so far the highest among rare-earth free high entropy oxide-based thermoelectrics.

Glassy thermal conductivity in Cs3Bi2I6Cl3 single crystal

As the periodic atomic arrangement of a crystal is made to a disorder or glassy-amorphous system by destroying the long-range order, lattice thermal conductivity, κ_L, decreases, and its fundamental characteristics changes. The realization of ultralow and unusual glass-like κ_L in a crystalline material is challenging but crucial to many applications like thermoelectrics and thermal barrier coatings. Herein, we demonstrate an ultralow (~0.20 W/m·K at room temperature) and glass-like temperature dependence (2–400 K) of κ_L in a single crystal of layered halide perovskite, Cs₃Bi₂I₆Cl₃. Acoustic phonons with low cut-off frequency (20 cm⁻¹) are responsible for the low sound velocity in Cs₃Bi₂I₆Cl₃ and make the structure elastically soft. While a strong anharmonicity originates from the low energy and localized rattling-like vibration of Cs atoms, synchrotron X-ray pair-distribution function evidence a local structural distortion in the Bi-halide octahedra and Cl vacancy. The hierarchical chemical bonding and soft vibrations from selective sublattice leading to low κ_L is intriguing from lattice dynamical perspective as well as have potential applications.

The investigation of thermal conductivity is crucial to the success of many modern technologies. Here the authors have reported an unusual glass-like thermal conductivity in a single crystal of layered halide perovskite, Cs₃Bi₂I₆Cl₃.

Toward Optimal Heat Transfer of 2D-3D Heterostructures via van der Waals Binding Effects

Two-dimensional (2D) materials and their heterogeneous integration have enabled promising electronic and photonic applications. However, significant thermal challenges arise due to numerous van der Waals (vdW) interfaces limiting the dissipation of heat generated in the device. In this work, we investigate the vdW binding effect on heat transport through a MoS₂-amorphous silica heterostructure. We show using atomistic simulations that the cross-plane thermal conductance starts to saturate with the increase of vdW binding energy, which is attributed to substrate-induced localized phonons. With these atomistic insights, we perform device-level heat transfer optimizations. Accordingly, we identify a regime, characterized by the coupling of in-plane and cross-plane heat transport mediated by vdW binding energy, where maximal heat dissipation in the device is achieved. These results elucidate fundamental heat transport through the vdW heterostructure and provide a pathway toward optimizing thermal management in 2D nanoscale devices.

Discovery of a Low Thermal Conductivity Oxide Guided by Probe Structure Prediction and Machine Learning

We report the aperiodic titanate Ba10 Y6 Ti4 O27 with a room-temperature thermal conductivity that equals the lowest reported for an oxide. The structure is characterised by discontinuous occupancy modulation of each of the sites and can be considered as a quasicrystal. The resulting localisation of lattice vibrations suppresses phonon transport of heat. This new lead material for low-thermal-conductivity oxides is metastable and located within a quaternary phase field that has been previously explored. Its isolation thus requires a precisely defined synthetic protocol. The necessary narrowing of the search space for experimental investigation was achieved by evaluation of titanate crystal chemistry, prediction of unexplored structural motifs that would favour synthetically accessible new compositions, and assessment of their properties with machine-learning models.