Effect of ordered B-site cations on the structure, elastic and thermodynamic properties of KTa_0.5Nb_0.5O₃ crystal

BO₆ oxygen octahedral was considered as the key part in ABO₃ perovskite structure, and the electro-optical, elastic and thermodynamic properties of potassium tantalate niobate (KTa_0.5Nb_0.5O₃, abbreviated as KTN) were closely depended on the B-site Ta/Nb ratio and ordering. The effect of [100]_NT, [110]_NT, and [111]_NT B-site cations ordering (N means a pure Nb layer parallel to (h, k, l), T means a pure Ta layer parallel to (h, k, l)) on structure, elastic properties and Debye temperatures properties of KTN were investigated based on density functional theory (DFT). KTN with [111]_NT B-site ordering presents an cubic phase structure with excellent stability from the view of lattice properties. The elastic properties include elastic stiffness coefficients C_ij, bulk modulus B, shear modulus G, Young's modulus E and Poisson' ratio ν were calculated. The elastic stiffness coefficients C₁₁ of KTN with B-site ordering have approached to maximum 485.506 GPa, indicating that KTN materials have better deformation ability along x axis compared with other perovskite materials. The calculated results of bulk modulus B and the shear modulus G show that KTN with [100 ]_NT B-site ordering has stronger ability to resist fracture and plastic deformation. And the criteria B/G <1.75 suggests that KTN should be classified as a brittle material. The KTN with [100 ]_NT B-site has excellent ductility properties compared with any other B-site arrangements. Debye temperatures of KTN with [100 ]_NT, [110 ]_NT, [111 ]_NT are about 650 K, and KTN with [100 ]_NT B-site has best thermodynamic stability. (orig.)