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Sodium niobate (NaNbO
3
) is a potential material for lead-free dielectric ceramic capacitors for energy storage applications because of its antipolar ordering. In principle, a reversible phase transition between antiferroelectric (AFE) and ferroelectric (FE) phases can be induced by an application of electric field (
E
) and provides a large recoverable energy density. However, an irreversible phase transition from the AFE to the FE phase usually takes place and an AFE-derived polarization feature, a double polarization (
P
)-
E
hysteresis loop, does not appear. In this study, we investigate the impact of chemically induced hydrostatic pressure (
p
chem
) on the phase stability and polarization characteristics of NaNbO
3
-based ceramics. We reveal that the cell volume of Ca-modified NaNbO
3
[(Ca
x
Na
1−2
x
V
x
)NbO
3
], where
V
is A-site vacancy, decreases with increasing
x
by a positive
p
chem
. Structural analysis using micro-X-ray diffraction measurements shows that a reversible AFE–FE phase transition leads to a double
P
-
E
hysteresis loop for the sample with
x
= 0.10. DFT calculations support that a positive
p
chem
stabilizes the AFE phase even after the electrical poling and provides the reversible phase transition. Our study demonstrates that an application of positive
p
chem
is effective in delivering the double
P
-
E
loop in the NaNbO
3
system for energy storage applications.