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<p>This comprehensive study systematically investigates the structural, spectroscopic, and electrical properties of LiH2PO<sub>4</sub> single crystals using multiple characterisation techniques. X-ray diffraction and FTIR analysis confirm a highly ordered crystalline structure with strong hydrogen-bonding networks, as evidenced by distinct P–O–H vibrational modes between 2739 and 1650&#xa0;cm<sup>−1</sup>. Optical characterisation reveals a wide direct bandgap of 5.03&#xa0;eV and excellent transparency (&gt; 80%) in the visible region, making it suitable for UV-optoelectronic applications. Dielectric spectroscopy demonstrates an exceptionally high static dielectric constant (~ 10<sup>3</sup>) and broad relaxation behaviour, characteristic of proton-coupled polarisation dynamics. Impedance spectroscopy and electric modulus analysis reveal bulk-dominated proton conduction with a relaxation time of 10⁻<sup>4</sup>–10⁻<sup>5</sup>&#xa0;s, consistent with a Grotthuss-type transport mechanism with distributed energy barriers. These findings establish LiH2PO<sub>4</sub> as a promising candidate for solid-state proton conductors and dielectric applications. This integrated study examines the structural, vibrational, and thermodynamic properties of&#xa0;<b>LiH</b><sub><b>2</b></sub><b>PO</b><sub><b>4</b></sub>&#xa0;through phonon dispersion, Debye temperature analysis, heat capacity, enthalpy, free energy, and Raman spectroscopy. Phonon density of states (DOS) reveals acoustic and optical modes dominated by rigid PO<sub>4</sub> tetrahedra and hydrogen bonding, with van Hove singularities influencing thermal transport. The Debye temperature (θ<sub>D</sub>) declines sharply with decreasing temperature (2500–1000&#xa0;K), indicating lattice softening and anharmonicity. Thermodynamic profiles show heat capacity anomalies (~ 200–100&#xa0;cal/(cell·K)) and rising enthalpy/thermal energy (up to 7&#xa0;eV), linked to Li⁺ mobility and phase stability. Raman spectra (0–3500&#xa0;cm<sup>−1</sup>) confirm PO<sub>4</sub> vibrations (ν₁, ν<sub>2</sub>/ν<sub>4</sub>), weak O–H stretching, and Li⁺ dynamics, underscoring covalent rigidity and proton-mediated ionic conduction. These properties position LiH<sub>2</sub>PO<sub>4</sub> as a promising solid-state electrolyte, balancing structural integrity with ion transport, while the risk of thermal degradation at high temperatures necessitates material optimisation.</p>

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Structural, vibrational, and proton-conducting properties of LiH2PO4 single crystals: a multifunctional wide-bandgap solid-state electrolyte

  • 期刊论文

  • 发表日期：

    2026年4月7日

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  • Mitesh B. Solanki

摘要

This comprehensive study systematically investigates the structural, spectroscopic, and electrical properties of LiH2PO4 single crystals using multiple characterisation techniques. X-ray diffraction and FTIR analysis confirm a highly ordered crystalline structure with strong hydrogen-bonding networks, as evidenced by distinct P–O–H vibrational modes between 2739 and 1650 cm−1. Optical characterisation reveals a wide direct bandgap of 5.03 eV and excellent transparency (> 80%) in the visible region, making it suitable for UV-optoelectronic applications. Dielectric spectroscopy demonstrates an exceptionally high static dielectric constant (~ 103) and broad relaxation behaviour, characteristic of proton-coupled polarisation dynamics. Impedance spectroscopy and electric modulus analysis reveal bulk-dominated proton conduction with a relaxation time of 10⁻4–10⁻5 s, consistent with a Grotthuss-type transport mechanism with distributed energy barriers. These findings establish LiH2PO4 as a promising candidate for solid-state proton conductors and dielectric applications. This integrated study examines the structural, vibrational, and thermodynamic properties of LiH2PO4 through phonon dispersion, Debye temperature analysis, heat capacity, enthalpy, free energy, and Raman spectroscopy. Phonon density of states (DOS) reveals acoustic and optical modes dominated by rigid PO4 tetrahedra and hydrogen bonding, with van Hove singularities influencing thermal transport. The Debye temperature (θD) declines sharply with decreasing temperature (2500–1000 K), indicating lattice softening and anharmonicity. Thermodynamic profiles show heat capacity anomalies (~ 200–100 cal/(cell·K)) and rising enthalpy/thermal energy (up to 7 eV), linked to Li⁺ mobility and phase stability. Raman spectra (0–3500 cm−1) confirm PO4 vibrations (ν₁, ν2/ν4), weak O–H stretching, and Li⁺ dynamics, underscoring covalent rigidity and proton-mediated ionic conduction. These properties position LiH2PO4 as a promising solid-state electrolyte, balancing structural integrity with ion transport, while the risk of thermal degradation at high temperatures necessitates material optimisation.