Investigation of defect structure of impurity-doped lithium niobate by combining thermodynamic constraints with lattice constant variations

The defect structures of impurity-doped congruent lithium niobates (c-LN) were determined for impurities with various valences, including divalent, trivalent, and tetravalent impurities, in a concentration range where antisite niobium (Nb_Li) exists. On the basis of the “Li site vacancy model,” six kinds of defect structures in impurity-doped c-LN are possible. Using thermodynamic constraints, these can be narrowed down to two kinds. The first structure is that in which impurities, vacancies and Nb exist as defects in the Li site and no defects exist in the Nb site (structure A), described as ([Li_Li] _1-5x-jy[Nb_Li]_x[M_Li]_y[V_Li]_4x+(j-1)y)[Nb_Nb][O_O] ₃ (V: vacancy, M: impurity, j: valence of impurity, x, y: compositional variable (≠0), Li/Nb = congruent ratio). ([Li_Li^×] _1-5x-2y[Nb_Li^••••]_x[M_Li^•]_y[V_Li^′]_4x+y)[Nb_Nb^×][O_O^×] ₃ is an example by the Kröger-Vink notation for divalent M. In the second structure, vacancies and Nb exist as defects in the Li site and impurities exist as defects in the Nb site (structure B), described as ([Li_Li] _1-5x-(j-5)y[Nb_Li]_x[V_Li]_4x+(j-5)y)([Nb_Nb] _1-y[M_Nb]_y)[O_O] ₃. ([Li_Li^×] _1-5x+y[Nb_Li^••••]_x[V_Li^′]_4x-y)([Nb_Nb^×] _1-y[M_Nb^′]_y)[O_O^×] ₃ is an example for tetravalent M. Since the relationship between impurity concentration and lattice constants for structures A and B differs, the defect structures can be differentiated by analyzing lattice constant variations as a function of impurity concentration. The results show that the defect structure of divalent and trivalent impurity-doped c-LN is structure A and that of tetravalent impurity-doped c-LN is structure B. The Nb_Li concentration increased with increasing tetravalent impurity concentration. In contrast, the Nb_Li concentration decreased with increasing divalent and trivalent impurities, leading to suppression of optical damage. The valence of an impurity determines whether the impurity is located in the Li site or Nb site in c-LN, consequently determining whether Nb_Li decreases or increases when the population of the impurity changes