Electronic and Optical Properties of Sodium Niobate: A Density Functional Theory Study

Abstract

In recent years, much effort has been devoted to replace the most commonly used piezoelectric ceramic lead zirconate titanate Pb[Zr x Ti 1−x ]O 3 (PZT) with a suitable lead-free alternative for memory or piezoelectric applications. One possible alternative to PZT is sodium niobate as it exhibits electrical and mechanical properties that make it an interesting material for technological applications. e high-temperature simple cubic perovskite structure undergoes a series of structural phase transitions with decreasing temperature. However, particularly the phases at room temperature and below are not yet fully characterised and understood. Here, we perform density functional theory calculations for the possible phases at room temperature and below and report on the structural, electronic, and optical properties of the different phases in comparison to experimental findings.

Isolating the Two Room-Temperature Polymorphs of NaNbO3: Structural Features, Optical Band Gap, and Reactivity

“…Several authors mention the occurrence of at least seven allotropic forms of NaNbO 3 as a function of temperature from 930 to 15 K, corresponding to cubic, tetragonal, orthorhombic, and rhombohedral space groups related to the perovskite network. − The high- and room-temperature phase transitions are mainly consisting in the tilting of the [NbO 6 ] octahedra, whereas the low-temperature transitions are associated with the off-centered displacements of niobium atoms as well as the distortion of the [NbO 6 ] octahedra. It must be pointed out that similar Madelung energies or free energies of all these varieties account for competing structural instabilities, giving rise to all these allotropic forms. , However, as far as the accurate structure determination is concerned, the main difficulties arise from the slight differences between diffraction patterns, especially for the phases that stabilize around room temperature. , To find the best model for structural analysis of NaNbO 3 at room temperature, complementary experimental techniques are necessary. The combination of structural refinements from X-ray powder diffraction (XRD) and 23 Na solid-state nuclear magnetic resonance (NMR) measurements to probe the local environments of sodium ions allows solving these ambiguities.…”

“…It must be pointed out that similar Madelung energies or free energies of all these varieties account for competing structural instabilities, giving rise to all these allotropic forms. 28,29 However, as far as the accurate structure determination is concerned, the main difficulties arise from the slight differences between diffraction patterns, especially for the phases that stabilize around room temperature. 26,30 To find the best model for structural analysis of NaNbO 3 at room temperature, complementary experimental techniques are necessary.…”

Estudio teórico fundamental comparativo de perovskitas: NaNbO3 y SrTiO3

“…La energía de Fermi (punto medio del band gap, que separa las bandas de valencia, de las bandas de conducción), para el Niobato de Sodio se encuentra más cerca de la banda de conducción. Ésto permite definir que su comportamiento podría ser de un semiconductor extrínseco, sin embargo sería necesario que estuviera dopado, por lo que sólo indica una alta cantidad de huecos positivos y la posibilidad de que un electrón de conducción sin alguna fuerza externa de la banda de conducción descienda (Grosso, 1988), lo que diverge de los resultados de Fritsch, que se pueden observar en la Tabla 7, aunque tienen parámetros de red de 3.975, 3.944, y 3.947 Å, parecidos a los que se obtuvieron en el presente estudio, tiene una energía de Fermi cercana al punto máximo de la banda de valencia y no al punto mínimo de la banda de conducción (Fritsch, 2018) por lo que difieren en el comportamiento semiconductor.…”

Revisiting the Cu-Zn Disorder in Kesterite Type Cu2ZnSnSe4 Employing a Novel Approach to Hybrid Functional Calculations

“…So-called hybrid functionals, where a particular fraction of Hartree-Fock exact exchange is mixed into an underlying (semi)local functional, capture the electronic and optical properties of semiconducting materials in much better agreement with experimental results, however, this accuracy has to be paid for by an increase in computational resources. While hybrid functional calculations have become standard for the calculation of structural, electronic, and optical properties of bulk materials [22][23][24][25][26][27][28][29], their widespread application for problems that require a large number of calculations has been hindered by the computational demands.…”

“…Lastly, for each of the 10 symmetry inequivalent cation arrangements we also calculated the optical properties, i.e., the real and imaginary parts of the dielectric functions, as described in some earlier works [25,26]. However, they have been used to calculate the absorption coefficient α, which is depicted in Figure 5b for the ordered (black lines) and disordered (red lines) kesterite crystal structures.…”