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<p>This study presents a comprehensive first-principles investigation of the structural, electronic, optical, elastic, and thermoelectric properties of the double perovskites Cs<sub>2</sub>TlXI<sub>6</sub> (X = Ag, Hg) using density functional theory. Structural optimization in the cubic phase (Fm3̅m) yields lattice constants of 12.14 Å and 12.36 Å, with corresponding unit cell volumes of 2992.88 a.u.<sup>3</sup> and 3170.94 a.u.<sup>3</sup> for Cs<sub>2</sub>TlAgI<sub>6</sub> and Cs<sub>2</sub>TlHgI<sub>6</sub>, respectively. The calculated bulk moduli (15.31 GPa and 15.14 GPa) indicate that both compounds are mechanically soft. Electronic structure analysis reveals semiconducting behavior with valence bands dominated by I-p states and conduction bands primarily composed of Tl-p orbitals. Optical properties demonstrate a strong contrast between the two systems, where Cs<sub>2</sub>TlAgI<sub>6</sub> exhibits a high static dielectric constant (Ɛ<sub>1</sub>(0) = 20) and refractive index (<i>n</i> = 5), compared to Cs<sub>2</sub>TlHgI<sub>6</sub> (Ɛ<sub>1</sub>(0) = 5, <i>n</i> = 2.26). Both materials show strong ultraviolet absorption, with maximum absorption coefficients of 1.57 × 10<sup>6</sup> cm<sup>−1</sup> at 13.53&#xa0;eV for Cs<sub>2</sub>TlAgI<sub>6</sub> and 1.39 × 10<sup>6</sup> cm<sup>−1</sup> at 13.08&#xa0;eV for Cs<sub>2</sub>TlHgI<sub>6</sub>, along with peak optical conductivities of 4690 Ω<sup>−1</sup>cm<sup>−1</sup> and 4544 Ω<sup>−1</sup>cm<sup>−1</sup>, respectively. Elastic constants confirm mechanical stability with values of C<sub>11</sub> = 37.27 GPa, C<sub>12</sub> = 5.15 GPa, C<sub>44</sub> = 4.66 GPa for Cs<sub>2</sub>TlAgI<sub>6</sub> and 33.27 GPa, 1.14 GPa, 5.55 GPa for Cs<sub>2</sub>TlHgI<sub>6</sub>. Derived moduli indicate that Cs<sub>2</sub>TlAgI<sub>6</sub> is relatively ductile (G/B = 0.50, ν = 0.287), whereas Cs<sub>2</sub>TlHgI<sub>6</sub> exhibits more brittle behavior (G/B = 0.73, ν = 0.206). Thermoelectric analysis shows temperature-dependent enhancement, with Cs<sub>2</sub>TlHgI<sub>6</sub> achieving a higher figure of merit of ZT ≈ 0.0115 at 1500&#xa0;K compared to ZT ≈ 0.0095 for Cs<sub>2</sub>TlAgI<sub>6</sub>. Additionally, low lattice thermal conductivity (0.2–0.4 Wm<sup>−1</sup>K<sup>−1</sup> at high temperature) supports thermoelectric efficiency. Overall, Cs<sub>2</sub>TlAgI<sub>6</sub> demonstrates superior optical response and strong light–matter interaction, making it suitable for ultraviolet optoelectronic applications, while Cs<sub>2</sub>TlHgI<sub>6</sub> exhibits improved thermoelectric performance and thermal transport properties, highlighting its potential for energy conversion applications.</p>

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Exploring Cs2TlAgI6 and Cs2TlHgI6 double perovskites for ultraviolet optoelectronic and thermoelectric applications: a DFT study

  • 期刊论文

  • 发表日期：

    2026年5月26日

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  • Jaffer Saddique,
  • Ali H. Reshak,
  • Amir Ullah,
  • Nasir Rahman,
  • Eman M. Alshehri,
  • Saleha Qissi,
  • Vineet Tirth,
  • Ali Algahtani,
  • Essam A. Al-Ammar,
  • Q. Humayun,
  • Muneeb Ur Rahamn,
  • Dania Ali

摘要

This study presents a comprehensive first-principles investigation of the structural, electronic, optical, elastic, and thermoelectric properties of the double perovskites Cs2TlXI6 (X = Ag, Hg) using density functional theory. Structural optimization in the cubic phase (Fm3̅m) yields lattice constants of 12.14 Å and 12.36 Å, with corresponding unit cell volumes of 2992.88 a.u.3 and 3170.94 a.u.3 for Cs2TlAgI6 and Cs2TlHgI6, respectively. The calculated bulk moduli (15.31 GPa and 15.14 GPa) indicate that both compounds are mechanically soft. Electronic structure analysis reveals semiconducting behavior with valence bands dominated by I-p states and conduction bands primarily composed of Tl-p orbitals. Optical properties demonstrate a strong contrast between the two systems, where Cs2TlAgI6 exhibits a high static dielectric constant (Ɛ1(0) = 20) and refractive index (n = 5), compared to Cs2TlHgI6 (Ɛ1(0) = 5, n = 2.26). Both materials show strong ultraviolet absorption, with maximum absorption coefficients of 1.57 × 106 cm−1 at 13.53 eV for Cs2TlAgI6 and 1.39 × 106 cm−1 at 13.08 eV for Cs2TlHgI6, along with peak optical conductivities of 4690 Ω−1cm−1 and 4544 Ω−1cm−1, respectively. Elastic constants confirm mechanical stability with values of C11 = 37.27 GPa, C12 = 5.15 GPa, C44 = 4.66 GPa for Cs2TlAgI6 and 33.27 GPa, 1.14 GPa, 5.55 GPa for Cs2TlHgI6. Derived moduli indicate that Cs2TlAgI6 is relatively ductile (G/B = 0.50, ν = 0.287), whereas Cs2TlHgI6 exhibits more brittle behavior (G/B = 0.73, ν = 0.206). Thermoelectric analysis shows temperature-dependent enhancement, with Cs2TlHgI6 achieving a higher figure of merit of ZT ≈ 0.0115 at 1500 K compared to ZT ≈ 0.0095 for Cs2TlAgI6. Additionally, low lattice thermal conductivity (0.2–0.4 Wm−1K−1 at high temperature) supports thermoelectric efficiency. Overall, Cs2TlAgI6 demonstrates superior optical response and strong light–matter interaction, making it suitable for ultraviolet optoelectronic applications, while Cs2TlHgI6 exhibits improved thermoelectric performance and thermal transport properties, highlighting its potential for energy conversion applications.