First-principles study of the structural and optoelectronic properties of ANbO3 (A = Na, K and Rb) in four crystal phases

In the present work structural and optoelectronic properties of alkali niobates ANbO3 (A = Na, K and Rb) are investigated in different crystal structures e.g. cubic, tetragonal, orthorhombic and rhombohedral using generalized gradient approximation (GGA), GGA with Hubbard potential (GGA + U) and modified Becke-Johnson (mBJ) approach. Structural properties show that ferroelectric rhombohedral crystal structure is the most stable structure of these compounds. The anti-ferroelectric and paraelectric are their higher energy crystal structures. The obtained results of the electronic band gaps show that mBJ is the appropriate exchange-correlation approximation for the calculations of electronic and optical properties of these niobates. They are indirect band gap semiconductors, except the ferroelectric rhombohedral and antiferroelectric orthorhombic NaNbO3. The valence band is predominantly contributed by O 2p orbitals and conduction band is made of Nb 4d orbitals. Real and imaginary parts of dielectric function and other optical parameters such as energy loss function, sum rule and refractive index are also investigated.