Local Lattice Distortion Around V^2-_Pb in PbWO₄ and the Origin of the 420nm Luminescence Band

The lattice structures around one lead vacancy V^2-_Pb in PbWO₄ are studied within the framework of the full-relativistic density functional theory. Using the conjugate gradient method, we optimize the geometry of the ions around V^2-_Pb by moving the ions within the sub-cell until specified tolerance is satisfied. We obtain the geometric positions of ions around V^2-_Pb after optimization. The calculated results indicate that there would be local lattice distortion caused by the existence of V^2-_Pb. The electronic structures of both the optimized and pre-optimized PWO containing V^2-_Pb have been calculated. The density of states of the distorted PWO indicates that the energy gap between the 2p state of O^2- and the 5d state of W⁶⁺ is 3.90eV, which shows that the distorted W-O tetrahedron may be the 420nm luminescence centre.