Phase Stability, Strong Four-Phonon Scattering, and Low Lattice Thermal Conductivity in Superatom-Based Superionic Conductor Na₃OBH₄

Abstract

Superatom-based superionic conductors are of current interest due to their promising applications in solid-state electrolytes for rechargeable batteries. However, much less attention has been paid to their thermal properties, which are vital for safety and performance. Motivated by the recent synthesis of superatom-based superionic conductor Na3OBH4 consisting of superhalogen cluster BH4, we systematically investigate its lattice dynamics and thermal conductivity using the density functional theory combined with a self-consistent phonon approach. We reveal the bonding hierarchy features by studying the electron localization function and potential energy surface and further unveil the rattling effect of the BH4 superatom, which introduces strong quartic anharmonicity and induces soft phonon modes in low temperatures by assisting Na displacements, thus calling for the necessity of quartic renormalization and four-phonon scattering in calculating the lattice thermal conductivity. We find that the contribution of four-phonon processes to the lattice thermal conductivity increases from 13 to 32% when the temperature rises from 200 to 400 K. At room temperature (300 K), the phonon scattering phase space is enlarged by 133% due to the four-phonon interactions, and the lattice thermal conductivity is evaluated to be 5.34 W/mK, reduced by 24% as compared with a value of 6.99 W/mK involving three-phonon scattering only. These findings provide a better understanding of the lattice stability and thermal transport properties of superionic conductor Na3OBH4, shedding light on the role of strong quartic anharmonicity played in superatom-based materials.

Unlocking the potential of coinage-based quaternary chalcogenides for thermoelectricity

“…73 However, among non-CST materials, and even compared to previously reported Ag-based thermoelectrics 71,[76][77][78] Pb-containing Ag 2 PbGeS 4 stands out with a k L value of approximately 0.15 W m −1 K −1 at 300 K. This value is lower than that of Cu 4 TiSe 4 (k L z 0.19 W m −1 K −1 ), which is getting close to the amorphous limit of lattice thermal conductivity. 79 The inclusion of additional four-phonon scattering rates 80 and considering various other phonon scattering mechanisms may further reduce the calculated k L within this family of compounds. Therefore, the amorphous k L and anharmonic nature of these materials necessitate a particle-like phonon treatment, 81 an exciting topic for future research.…”

Modulation of Graphene Interlayer Spacing by Superatoms

“…As stable clusters, superatoms have atomic-like electronic structures, which can be qualitatively described by the Jellium model, and the electrons are located in the superatomic molecular orbitals (SAMOs) called 1S, 1P, 1D, 2S, etc. − Over the past decades, a large number of superatoms have been discovered, predicted, and even synthesized, ,, suggesting that superatoms, as artificial units, have an important prospect of replacing chemical elements and overcoming some limits of natural elements. , Furthermore, superatoms have been gradually prepared on a large scale, which is an important milestone for superatoms acting as new building blocks of functional materials. , These advantages of superatoms different from natural atoms and nanoparticles are important for us to tune the interlayer spacing of graphene at the atomic level. Particularly for metal clusters, small-sized superatoms have received widespread attention owing to their nonbulk-like and size-specific properties. − Among them, a representative example is Au 20 superatom with high structural and chemical stability. It is a pyramid structure with excellent stability owing to its close electron shells. , Previous studies have shown that Au 20 superatoms can be prepared on a large scale. , Therefore, it is very promising to tune the interlayer spacing of graphene to meet functional requirements using the advantages of coinage metal superatoms.…”

Type-1 Pentagonal Tiling Realized in 2D Penta-SrP2 Sheet

“…The results are plotted in Figure c, where imaginary acoustic phonon modes are found around the high-symmetry point Y at the harmonic level, which resemble the soft phonon modes in the paraelectric phase of ferroelectric materials like SrTiO 3 , and SnSe, indicating that penta-SrP 2 is dynamically unstable at 0 K. However, by considering the renormalization of anharmonic effects within the scheme of SCP theory, the imaginary phonon modes are suppressed at finite temperature, and they vanish at temperature above 500 K. It is worth mentioning that there are some minor imaginary phonon modes in the vicinity of the high-symmetry point Γ at each finite temperature from 100 to 800 K. These imaginary phonon frequencies of the out-of-plane acoustic branches represent the rippling of the penta-SrP 2 sheet rather than dynamical instability or numerical error, as is the case in hexagonal nitride monolayers . Thus, similar to other reported systems, , the penta-SrP 2 sheet with the type-1 pentagonal tiling geometry is dynamically stable at high temperature.…”