To es-mate the varia-on of the Gibbs free energy curve under stress based on this equa-on, we first make a simplifying assump-on that α chem along the b-axis, α22 chem , is equal for each phase. In this case, the Gibbs free energy curve of each phase varies under compressive stress (σ22 < 0) as shown in Fig. . Here, the reference Li content was set to x = 0.5. Moreover, we assumed that the elas-c compliance of each FMO phase was equal and independent of the Li content for simplicity. Under these assump-ons, the second term in Eq. ( ) decreases the Gibbs free energy curve of each phase by a constant value. In addi-on to this, the third term modifies the free energy curves by adding a linear term to each curve. The common tangent lines of the free energy curves under stress and without stress are represented by the black solid and grey dashed lines in Fig. , respec-vely. As shown in this figure, although the free energy curves of both phases and the common tangent line were varied under stress, the posi-ons of the contact points between the curves and the common tangent line, i.e., the upper and lower limits of the two-phase coexistence region, remain unchanged in the case of equal α chem for each phase. This result is natural, as equal α chem for both phases simply leads to the addi-on of an iden-cal linear term to the curves of both phases. From these discussions, we can conclude that if α chem is equal for both phases, the stress will not thermodynamically change the two-phase region, regardless of its magnitude. Next, we consider the case where the α chem differs for each phase, which is more likely to be a case of FMO. It is reported that the change in the laoce constant in the b-axis during lithia-on is smaller for the Li-poor phase compared to the Li-rich phase , indica-ng that the α chem along the b-axis is smaller for the Li-poor phase than the Li-rich phase. This is also indirectly supported by our XRD measurements of the FMO thin film.