Ab initio study of elasticity, piezoelectricity, and nonlinear optical performance in monoclinic NaAsSe2

Monoclinic NaAsSe2 (γ-NaAsSe2) has been reported to be a potential IR nonlinear optical material due to the very strong second harmonic generation (SHG). Here, we present a systematic density functional theory (DFT) study on the elasticity, SHG response, linear electro-optical (EO) effect, and piezoelectricity of this compound and analyze the microscopic origins of these properties. We find that γ-NaAsSe2 has smaller values of elastic moduli than those of AgGaSe2, and behaves in a ductile manner. The strong SHG response observed experimentally is well predicted by the 2n + 1 theorem and the “sum over states” expressions based on the DFT. As with the strong SHG response, γ-NaAsSe2 also presents large linear EO coefficient (r11 = 32.7 pm/V, as large as that of LiNbO3), and piezoelectric coefficient (e11 = 1.27 C/m2, about seven times larger than that of α-quartz). The highly distorted structure and the strong hybridizations between As and Se sp orbitals are identified as the chief sources of these unique properties. Our results demonstrate the potential applications of γ-NaAsSe2 in piezoelectric, electro-optical, and nonlinear optical devices.