Ionic transport in LiNbO 3
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I. INTRODUCTION
6LiLi + Nb Nb
LiNbO3 is currently the most widely used electrooptic material. Many of its properties are significantly affected by the presence of impurities, whether naturally occurring or deliberately added. Before the effects of impurities can be thoroughly understood, it is necessary to understand the ionic and electronic defects created by intrinsic disorder and nonstoichiometry. Information on transport behavior can contribute to such an understanding. In this study, the high temperature ionic conductivity of LiNbO3 has been determined by two independent techniques that are in good agreement with each other. The LiNbO3 phase extends from the nominally ideal composition, with Li/Nb = 1, to a substantial amount of Li2O deficiency. The congruently melting composition, from which almost all single crystals are grown, contains only 48.6 mol% Li2O. It was initially proposed that this deficiency of Li2O is accommodated in the crystal by the creation of corresponding amounts of lithium and oxygen vacancies, i.e., that lattice sites are conserved.1 2LiLi + O o Li2O (1) However, it was soon determined that the crystal density of LiNbO 3 increases with increasing Li2Odeficiency, and an alternative model was proposed that involves only cation defects, lithium vacancies and antisite niobium.2 In this model, a stoichiometric set of lattice sites is lost for every three Li2O that leave the crystal.
a)
Current address: BOC Group, Inc., 100 Mountain Avenue, Murray Hill, New Jersey 07974. J. Mater. Res., Vol. 6, No. 4, Apr 1991
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3Li2O
3OO
(2)
A more recent highly precise x-ray diffraction study suggests that the Li vacancies have been eliminated by occupation by Nb atoms, leaving Nb vacancies3 6LiLi + 5NbNb + 3OO ^ ^ 3Li2O + 5Nb& + 4V&,
(3)
It is unlikely that a set of such highly charged individual defects would be the lowest energy solution to the accommodation of a loss of Li2O. This can be relieved by assuming that each Nb vacancy is located adjacent to an antisite Nb to give a singly charged defect complex4'5 6LiLi + 5NbNb + 3OO ^ ^ 3Li2O + 4(Nb4,V£b)' + Nb 4 , (4) The complex may be simply derived from a Li-vacancy by the movement of an adjacent lattice Nb (5) (NbkV&b)' =?=^ NbNb + Vu The complex defect corresponds to a Li-vacancy in a local sequence of cations that corresponds to the ilmenite structure.4'5 An oxygen loss reaction that is consistent with this defect model for Li2O-deficient LiNbO3 involves the consumption of the complexes 2(NbHiV£b)' + 3OO ^=± 3/2O2 + NbNb + Nb& + 6e' (6) This reaction also results in the loss of a stoichiometric set of lattice sites for each three oxygen atoms that leave the crystal. The mass-action expression for this reaction is r^2 = KredJP(O2)
-3/2
© 1991 Materials Research Society
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A. Mehta, E. K. Chang, and D. M. Smyth: Ionic transport in LiNbO3
As long as the reduction does not result in a significant change in the concentration of lattice defects, the dependence
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