Structural, electronic and optical properties of unstrained and strained LiNbO3: A first-principles study

The exchange-correlation functions of local density approximation and generalized gradient approximation using density functional theory were performed to calculate electronic and optical properties of LiNbO3 crystal. To improve the underestimated value of band gap, a reverse scissor correction procedure was performed based on the experimental refractive index value because it is more suitable for compound with polar crystal. It is found that underestimated electronic band gap ~4.76 eV was generated due to nature of generalized gradient approximation exchange correlation. Based on density of states calculation, Li ion tends to diffuse into substrate as it possesses pure ionic character. Thus, the chemical bonding in LiNbO3 crystal has a mixed covalent-ionic character. The effect of strain on a-axis and c-axis of the crystal toward electronic and optical properties were investigated. It is found that the a-axis of the crystal is more sensitive towards the strain compared to c-axis as the band gap largely changes upon the applied strain. The optical properties such as dielectric function, refractive index, extinction coefficient and absorption were calculated and explained in detail. The dielectric constant and refractive index increased to higher frequency with positive strain and vice versa. Meanwhile for the absorption decreased to lower frequency with positive strain and vice versa. These results would assist to provide the fundamental explanation about the effect of strain on the properties of LiNbO3 crystal.