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Post-annealing effects on (00l) texture, Cl/Se ratio, and electrical and glass-like thermal transport in Bi₄O₄SeCl₂
(2026-04-25)
Somdock, Nuttakrit
;
Theekhasuk, Nattharika
;
Voraud, Athorn
;
Limsuwan, Pichet
;
Naemchanthara, Kittisakchai
Bi₄O₄SeCl₂ is a heteroanionic layered material with intrinsically low lattice thermal conductivity and anisotropic charge transport. In this work, the effects of post-annealing temperature on the crystallographic texture, anion chemistry, defect evolution, and transport properties of Bi₄O₄SeCl₂ were systematically investigated. Polycrystalline Bi₄O₄SeCl₂, synthesized by solid-state reaction combined with high-energy ball milling, was post-annealed at 400–700 °C. X-ray diffraction and electron microscopy revealed that post-annealing eliminated the residual BiOCl precursor phase, enhanced the (00 l) preferred orientation, and promoted grain growth up to 600 °C, followed by partial texture degradation at 700 °C due to recrystallization. Energy-dispersive spectroscopy showed progressive Se and Cl volatilization during annealing, leading to an increased Cl/Se ratio. The carrier mobility and electrical conductivity reached maximum values at 600 °C, consistent with improved texture and layered-domain connectivity. Thermal transport remained lattice-dominated and only weakly temperature-dependent. The phonon mean free path, estimated using kinetic theory, was in the sub-nanometer range (∼0.25–0.57 nm), comparable to the interatomic spacing, indicating glass-like phonon transport. Representative HRTEM observations also revealed dislocation-related lattice defects and locally distorted regions, suggesting that vacancy disorder and local strain fields may provide additional phonon scattering. These results demonstrate that post-annealing optimizes electrical transport through phase purification, texture development, and defect-mediated carrier regulation, while the lattice thermal conductivity remains fundamentally limited by intrinsic glass-like phonon transport in Bi₄O₄SeCl₂.
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Post-ball-milling-assisted solid-state synthesis of Bi4O4SeCl2: A low thermal conductivity material
(2025-02-01)
Theekhasuk, Nattharika
;
Somdock, Nuttakrit
;
Voraud, Athorn
;
Ounrit, Iyarat
;
Limsuwan, Pichet
This study investigates the synthesis of Bi<inf>4</inf>O<inf>4</inf>SeCl<inf>2</inf> through a cost-effective ball-milling-assisted solid-state reaction method. The as-grown samples predominantly consisted of the Bi<inf>12</inf>O<inf>15</inf>Cl<inf>6</inf> phase, with minor contributions from BiOCl and Bi<inf>4</inf>O<inf>4</inf>SeCl<inf>2</inf>. A systematic post-ball-milling process was applied to enhance the formation of the Bi<inf>4</inf>O<inf>4</inf>SeCl<inf>2</inf> phase. Prolonged milling time led to the progressive dominance of the Bi<inf>4</inf>O<inf>4</inf>SeCl<inf>2</inf> phase, resulting in significant improvements in electrical conductivity and reductions in thermal conductivity. After 30 min of milling, the carrier concentration increased notably from −2.23 × 1016 cm−3 (as-grown) to −1.01 × 1018 cm−3, while electrical conductivity rose from 0.14 S/cm (as-grown) to 2.26 S/cm. Simultaneously, thermal conductivity decreased from 0.65 W m−1 K−1 (as-grown) to 0.35 W m−1 K−1. These findings demonstrate that post-ball-milling is a scalable and economical method for synthesizing Bi<inf>4</inf>O<inf>4</inf>SeCl<inf>2</inf> with low thermal conductivity, highlighting its potential as a promising material for thermal barrier coatings and thermoelectric applications.

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