Structural, electronic, optical, mechanical and piezoelectric properties of AIO3 (A = Na, K, and Rb) for energy harvesting materials: an ab initio approach

Abstract

This work presents a comprehensive investigation of AIO₃ (A = Na, K, and Rb) compounds as promising candidates for energy harvesting applications based on density functional theory (DFT) calculations. Structural optimization confirmed that all the compounds crystallized in both R3m and R3c symmetries. Thermodynamic stability was validated through negative formation energies, further supported by molecular dynamics simulations and tolerance factors consistent with the perovskite structure. The materials exhibited direct band gaps ranging from 2.27 to 2.78 eV within the GGA approximation, which were moderately reduced to 2.09–2.61 eV upon the inclusion of spin–orbit coupling (SOC) while retaining their direct bandgap nature. Additionally, the implementation of HSE06 revealed a bandgap within the range of 3.32 to 3.9 eV, which was the closest estimate to the experimental bandgaps. The R3c phase, owing to its higher atomic packing density, exhibited a greater density of states near the Fermi level and a comparatively low charge transfer. The inclusion of spin–orbit coupling (SOC) in the optical property calculations resulted in a noticeable shift in the onset of optical transitions, thereby enhancing their activity within the visible spectrum. The optical properties were re-examined using HSE06, resulting in stronger absorption in the UV region. Among the compounds, KIO₃ in the R3m phase showed the highest static dielectric constant, static refractive index, and static reflectivity, whereas NaIO₃ demonstrated the strongest optical absorption. The materials exhibited strong optical absorption (∼10⁵ cm⁻¹) and relatively low reflectivity, lower than 31% across the visible to ultraviolet spectra. When using the HSE06 functional, the reflectivity further dropped down to 28% in the R3m phase and 25% in the R3c phase, highlighting their suitability for UV optoelectronics. Mechanically, all the compounds fulfilled the stability criteria, exhibiting high elastic moduli despite inherent brittleness. Piezoelectric analysis further revealed that KIO₃ (R3m) possessed the highest piezoelectric response (−1.346 C m⁻² along the e₃₃ direction), while NaIO₃ (R3m) achieved the highest piezoelectric efficiency. The Born effective charges were analysed in relation to the piezoelectric responses. In summary, this systematic investigation highlights the dual-functional potential of these alkaline iodate derivatives for future applications in both UV optoelectronics and piezoelectric energy harvesting technologies.