MgO-doped lithium niobate that is simultaneously congruent and stoichiometric, denoted cs-MgO:LN (Li2O:Nb2O5MgO=45.3:50.0:4.7) Ill has a partition coefficient of unity for each of its constituent species, including ionic species. As such, this material exhibits no segregation of ions during crystal growth. However, a crystallization electromotive force (c-EMF) is observed during growth by the micropulling down (mu-PD) method, due to segregation of the cs-MgO:LN ionic species. This arises from a steep temperature gradient at the solid liquid interface that generates an electric field due to the Seebeck effect. In this case, the equilibrium partition coefficient, k(o), must be modified to km to take into account the effect of an electric field on the partitioning of ionic species. A coefficient of km rather than k(o) has a value of non unity and therefore can lead to generation of a c-EMF. An electric current was injected into the melt in such a way that the Seebeck effect was canceled, and this demonstrated that the value of k(o) is unity for all constituent ionic species of cs-MgO:LN. It has thus been confirmed that an injected electric current reduces the c-EMF. Only cs-MgO:LN attained a zero c-EME value at a specific current that was valid at all growth rates, whereas s-MgO(2.5 mol%):LN and ZnO-doped LN required velocity -dependent currents to reduce their c-EMEs to zero. The observation of a zero c-EMF at all growth rates upon removing the Seebeck field effect indicates that the value of k(o) is unity for all constituent species of cs-MgOIN in both the melt and crystal phases, including ionic species. Therefore, the activity of all components of cs-MgO:LN in both phases is unity and this compound is simultaneously stoichiometric and congruent. (C) 2014 Elsevier B.V. All rights reserved.