Traian Dumitrică

LiCoO2, a layered crystalline material characterized by pronounced bonding heterogeneity between stiff covalent CoO2 layers and soft ionic Li layers, remains a dominant cathode material for lithium-ion batteries. Using a theoretical framework that combines ab initio anharmonic lattice dynamics with equilibrium molecular dynamics based on machine-learning interatomic potentials, we reveal a dual thermal role of the Li layers: while contributing negligibly to heat transport directly, they fundamentally reshape the mechanism of cross-plane vibrational energy transfer. First, the ab initio calculations uncover a giant lattice thermal conductivity anisotropy, with κL∥/κL⊥ ≈ 21.6 at room temperature. This disparity is accompanied by a striking contrast in temperature dependence: the in-plane conductivity follows a conventional crystalline trend (κL∥ ∝ T–0.89), characteristic of particle-like phonon transport limited by anharmonic scattering, whereas the cross-plane conductivity exhibits a much weaker temperature dependence (κL⊥ ∝ T–0.22). The soft Li layers strongly suppress the particle-like phonon across the layers while preserving sufficient interlayer vibrational coupling for wave-like heat transfer through intermode coherences to become significant. Second, the role of Li-ion diffusion is evaluated using molecular dynamics simulations with a developed Deep Potential model. After validating the model in the diffusive regime, the thermal contributions of Li-ion diffusion and its coupling with lattice vibrations are quantified using the Green–Kubo formalism. Heat transport remains dominated by lattice vibrations, demonstrating that high Li-ion mobility does not necessarily translate into a significant ionic contribution to thermal transport. The negligible ion–phonon cross-correlation further demonstrates that Li-ion motion does not form a coupled heat-transport channel with lattice vibrations.

Using machine-learned molecular dynamics and ab initio anharmonic lattice dynamics, we reveal the temperature-dependent thermal transport mechanisms in Li ₃ OCl, including a non-negligible ionic heat transport above ∼1100 K.

Two‐dimensional (2D) transition metal carbides and nitrides, known as MXenes, possess unique physical and chemical properties, enabling diverse applications in fields ranging from energy storage to communication, catalysis, sensing, healthcare, and beyond. Despite extensive research and notable advancements, a fundamental understanding of MXenes’ phase diversity and its connection to their hierarchical precursors, including the intermediate MAX phases and the ancestral bulk phases, remains limited. Here, it is hypothesized that the atomic coordination environments adopted by transition metal and nonmetallic atoms in their three‐dimensional (3D) bulk precursors may persist in 2D MXenes to govern their phase diversity. Using high‐throughput modeling based on first‐principles density functional theory, a wide range of MXene phases is unveiled and comprehensively evaluate their relative stabilities across a large chemical space. The key to the approach lies in considering various atomic coordination environments drawn from four types of ancestral bulk phases. Through this comprehensive structural library of MXenes, general guiding principles are uncovered, such as a close alignment between the phase stability of MXenes and that of their 3D precursors. These findings introduce a new design strategy in which the atomic coordination environments in bulk phases can serve as reliable predictors for accessing the diverse structural landscape of MXenes.

The early concept of a "phonon-glass electron-crystal" for enhancing the thermoelectric figure of merit (ZT) is explored theoretically in layered Ge-Se crystals, where phonon transport exhibits glass-like behavior. Ab initio lattice dynamics and the rigid electronic band method project an ultrahigh ZT = 4.04 at 1000 K along the a axis in the high-temperature GeSe₂ phase at an electron doping concentration of 10²⁰ cm^-3. Meanwhile, the low-temperature Ge₄Se₉ phase achieves a high ZT = 2.19 at 600 K along the a axis with an electron doping concentration of 6 × 10¹⁹ cm^-3. These maximal values reflect the ultralow lattice thermal conductivity, 0.168 W m^-1 K^-1 (GeSe₂, 1000 K) and 0.289 W m^-1 K^-1 (Ge₄Se₉, 600 K), and high power factor at optimized carrier concentrations along the a axis. Our calculations indicate a promising pathway for approaching the early concept of maximizing ZT, by tailoring carrier doping in layered crystals with glass-like phononic transport.

Because of their rapid ionic diffusion, Ag -based argyrodites have great potential as solid-state electrolytes. The fundamental thermal transport in these materials is important for the overall all-solid-state battery design. Using an accurate framework and a unified heat-transport theory, we compute both the particle-like ( κ p ) and the glass-like ( κ c ) components of the lattice thermal conductivity ( κ L ) of Ag 8 Sn Se 6 in the orthorhombic phase. Despite being a crystal, Ag 8 Sn Se 6 exhibits an exceptionally low room-temperature κ L = κ p + κ c of 0.274 W/mK, which is nearly temperature independent over the 100–350 K range. This unusual behavior is linked to the lonesome Ag atoms, which, in this temperature range, exhibit intense vibrations while remaining weakly bonded to the Sn - Se framework. While their bonding anharmonicity significantly reduces κ p , the Ag -dominated extended vibrations make κ c the primary heat-transfer mode. By unveiling the role of Ag in the heat transport of Ag 8 Sn Se 6 , we support the notion that the bonding and structural characteristics responsible for fast ionic conduction can also help identify extreme materials with glass-like thermal transport.

We use ReaxFF molecular dynamics (MD) to investigate the relationship between structural and mechanical properties in bulk and nanostructured amorphous carbon (a-C). The liquid-quench MD method is used to generate isotropic bulk samples with mass densities ranging from 0.96 to 3.29 g/cm3. Structural analysis identifies two types of structures with distinct short- and medium-range order: lower-density sp2-dominated a-C, which is characterized by a bimodal ring-size distribution, and higher-density sp3-dominated tetrahedral amorphous carbon (ta-C), exhibiting a unimodal ring-size distribution. Stress–strain MD simulations and analysis reveal how an atomistic structure impacts elastic properties and post-yield atomic rearrangements. All stretched structures demonstrate elastic isotropy and plasticity driven by a ring-size expansion mechanism reflected in changes in ring statistics. The plastic region is substantially larger in ta-C than in a-C due to the post-yield shift from sp3 to sp2 C dominant bonding. In both a-C and ta-C, ultimate failure occurs when a reactive crack, traversed by long sp chains, forms and propagates predominantly perpendicular to the direction of the applied strain. Oxygen infiltration into the fractured region significantly reduces stress resistance, primarily through the early rupture of long sp chains. MD simulations and analysis are extended to a-C slabs, a-C nanotubes, and partially a-C nanotubes. The latter nanostructure highlights the differences between the elastically isotropic a-C walls, which develop circumferential cracking, and the crystalline walls, which tear along crystallographic directions. These results provide a strong foundation for further computational characterization of a-C materials.

Crystalline Bi₄O₄SeCl₂ exhibits record-low 0.1 W/mK lattice thermal conductivity (κ_L), but the underlying transport mechanism is not yet understood. Using a theoretical framework which incorporates first-principles anharmonic lattice dynamics into a unified heat transport theory, we compute both the particle-like and glass-like components of κ_L in crystalline and pellet Bi₄O₄SeCl₂ forms. The model includes intrinsic three- and four-phonon scattering processes and extrinsic defect and extended defect scattering contributing to the phonon lifetime, as well as temperature-dependent interatomic force constants linked to phonon frequency shifts and anharmonicity. Bi₄O₄SeCl₂ displays strongly anisotropic complex crystal behavior with dominant glass-like transport along the cross-plane direction. The uncovered origin of κ_L underscores an intrinsic approach for designing extremely low κ_L materials.