Antiperovskites have attracted significant interest in the field of energy conversion in recent years. While extensive research has focused on the magnetism, ionic conductivity and superconductivity of antiperovskites, their thermal properties including lattice anharmonicity and thermal transport remain less explored compared to their well-studied perovskite counterparts. Recently, nitrohalide double antiperovskites have been successfully synthesized. In this work, we investigate the thermal transport properties of nitrohalide double antiperovskites Li₆NII₂ and Li₆NBrBr₂ using first-principles machine-learning potentials. Our results reveal that within the perturbation theory framework, imaginary phonons appear throughout the entire Brillouin zone in both the harmonic regime and at elevated temperatures. Atomic vibrational analysis indicates that stochastic Li-ion movements confined within a single conventional unit cell are responsible for the presence of these imaginary phonons. Furthermore, homogeneous nonequilibrium molecular dynamics and equilibrium molecular dynamics simulations demonstrate that Li₆NII₂ and Li₆NBrBr₂ exhibit ultralow glass-like lattice thermal conductivities. Spectral thermal conductivity analysis shows that the dominant contributions arise from phonons with frequencies below 5 THz and around 11 THz. The substantial phonon contribution near 11 THz is attributed to the confined stochastic motions of Li ions. This work uncovers the unconventional microscopic cation dynamics and strong lattice anharmonicity in double antiperovskites Li₆NII₂ and Li₆NBrBr₂, thereby advancing the understanding of phonon transport in these materials.

With the continuous miniaturization of micro-devices and the rapid advancement of novel nanomaterials, thermal characterization techniques tailored for two-dimensional (2D) structures (films and coatings) and one-dimensional (1D) architectures (wires and fibers) have become essential for elucidating structure-property relationships and optimizing material performance. This review provides an in-depth analysis of the Transient Electro-Thermal (TET) technique, a recently developed method for measuring the thermal diffusivity and conductivity of 1D and 2D materials, including dielectric, metallic, and semiconductive films, coatings, and wires/fibers. We discuss the fundamental principles of TET operation, the associated physical and mathematical models for data reduction, and critical methodologies for data fitting, uncertainty analysis, and stray heat transfer mitigation to ensure high repeatability and accuracy. In addition, the latest developments and applications of TET are highlighted, including its extension to atomic-scale thickness, in-situ dynamic thermal property measurements during structural evolution, and the zero-temperature-rise limit method. The outstanding agreement (within ~0.6%) between the measured and reference thermal diffusivity of a Pt wire, validated through extensive experiments and zero-temperature-rise extrapolation, demonstrates the robustness and reliability of the TET technique. Owing to its simplicity in principles, experimental implementation, and data analysis, TET offers significant advantages in uncertainty control, measurement accuracy, and throughput.

Antiperovskites have attracted significant interest in the field of energy conversion in recent years. While extensive research has focused on the magnetism, ionic conductivity and superconductivity of antiperovskites, their thermal properties including lattice anharmonicity and thermal transport remain less explored compared to their well-studied perovskite counterparts. Recently, nitrohalide double antiperovskites have been successfully synthesized. In this work, we investigate the thermal transport properties of nitrohalide double antiperovskites Li₆NII₂ and Li₆NBrBr₂ using first-principles machine-learning potentials. Our results reveal that within the perturbation theory framework, imaginary phonons appear throughout the entire Brillouin zone in both the harmonic regime and at elevated temperatures. Atomic vibrational analysis indicates that stochastic Li-ion movements confined within a single conventional unit cell are responsible for the presence of these imaginary phonons. Furthermore, homogeneous nonequilibrium molecular dynamics and equilibrium molecular dynamics simulations demonstrate that Li₆NII₂ and Li₆NBrBr₂ exhibit ultralow glass-like lattice thermal conductivities. Spectral thermal conductivity analysis shows that the dominant contributions arise from phonons with frequencies below 5 THz and around 11 THz. The substantial phonon contribution near 11 THz is attributed to the confined stochastic motions of Li ions. This work uncovers the unconventional microscopic cation dynamics and strong lattice anharmonicity in double antiperovskites Li₆NII₂ and Li₆NBrBr₂, thereby advancing the understanding of phonon transport in these materials.

With the continuous miniaturization of micro-devices and the rapid advancement of novel nanomaterials, thermal characterization techniques tailored for two-dimensional (2D) structures (films and coatings) and one-dimensional (1D) architectures (wires and fibers) have become essential for elucidating structure-property relationships and optimizing material performance. This review provides an in-depth analysis of the Transient Electro-Thermal (TET) technique, a recently developed method for measuring the thermal diffusivity and conductivity of 1D and 2D materials, including dielectric, metallic, and semiconductive films, coatings, and wires/fibers. We discuss the fundamental principles of TET operation, the associated physical and mathematical models for data reduction, and critical methodologies for data fitting, uncertainty analysis, and stray heat transfer mitigation to ensure high repeatability and accuracy. In addition, the latest developments and applications of TET are highlighted, including its extension to atomic-scale thickness, in-situ dynamic thermal property measurements during structural evolution, and the zero-temperature-rise limit method. The outstanding agreement (within ~0.6%) between the measured and reference thermal diffusivity of a Pt wire, validated through extensive experiments and zero-temperature-rise extrapolation, demonstrates the robustness and reliability of the TET technique. Owing to its simplicity in principles, experimental implementation, and data analysis, TET offers significant advantages in uncertainty control, measurement accuracy, and throughput.