  Epitaxial strain can be used to modify the properties of ferroelectric thin
films. From the experimental viewpoint, the challenge is to fine-tune the
magnitude of the strain. We illustrate how, by using a suitable combination of
composition and substrate, the magnitude of the epitaxial strain can be
controlled in a continuous manner. The phase diagram of PbxSr1-xTiO3 films
grown epitaxially on (110)-DyScO3 is calculated using a Devonshire-Landau
approach. A boundary between in-plane and out-of-plane oriented ferroelectric
phases is predicted to take place at $x \approx$ 0.8. A series of PbxSr1-xTiO3
films grown by MBE show good agreement with the proposed phase diagram
