  Density functional calculations are performed to study the effect of
epitaxial strain on PbZrO3. We find a remarkably small energy difference
between the epitaxially strained polar R3c and nonpolar Pbam structures over
the full range of experimentally accessible epitaxial strains -3% < \eta < 4%.
While ferroelectricity is favored for all compressive strains, for tensile
strains the small energy difference between the nonpolar ground state and the
alternative polar phase yields a robust antiferroelectric ground state. The
coexistence of ferroelectricity and antiferroelectricity observed in thin films
is attributed to a combination of strain and depolarization field effects.
