PbTiO$_3$ is a simple but very important ferroelectric oxide that has been extensively studied and widely used in various technological applications. However, most previous studies and applications were based on the bulk material or the conventional [$001$]-orientated films. There are few studies on PbTiO$_3$ films grown along other crystalline axes. In this study, a first-principles calculation was performed to compute the polarization of PbTiO$_3$ films strained by SrTiO$_3$ and LaAlO$_3$ substrates. Our results show that the polarization of PbTiO$_3$ films strongly depends on the growth orientation as well as the monoclinic angles. Further, it is suggested that the ferroelectricity of PbTiO$_3$ mainly depends on the tetragonality of the lattice, instead of the simple strain.
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