Unveiling the structural, electronic, and optical properties of ATaO3 (A = K, Rb, Cs) perovskites via DFT calculations

Alkali-metal tantalates ATaO3 (A = K, Rb, Cs) are cubic perovskite oxides whose properties can be tuned by varying the A-site cation. Using density functional theory with Perdew–Burke–Ernzerhof generalized gradient approximation and validated by the hybrid HSE06 functional, we systematically investigated their structural, electronic, and optical behavior. Structural optimization shows that increasing cation size from K+ to Rb+ to Cs+ expands lattice parameters, elongates Ta–O bonds, and reduces stiffness, with RbTaO3 exhibiting the highest bulk modulus due to optimal bonding. Band structure analysis reveals a progressive narrowing of bandgaps, with CsTaO3 displaying a direct bandgap, making it promising for optoelectronic applications. Density of states highlights strong hybridization between O-2p and Ta-5d orbitals, further enhanced by A-site contributions, particularly Cs-6s states. Effective mass calculations indicate heavy holes and light electrons in KTaO3 and RbTaO3, while CsTaO3 shows lighter, more balanced carriers, suggesting improved charge transport. Optical properties, including dielectric functions, absorption spectra, and loss functions, confirm dominant O-2p to Ta-5d transitions responsible for visible and UV absorption. These results establish clear correlations between ionic radius, lattice geometry, and optical response, offering pathways to tailor ATaO3 perovskites for energy conversion and optoelectronic applications.