| Summary: | Coherent epitaxial growth allows us to produce strained crystalline films with structures that are unstable in the bulk. Thereby, the overlayer lattice parameters in the interface plane, ( a , b ), determine the minimum-energy out-of-plane lattice parameter, ${{c}_{{\rm min} }}(a,b)$ . We show by means of density-functional total energy calculations that this dependence can be discontinuous and predict related first-order phase transitions in strained tetragonal films of the elements V, Nb, Ru, La, Os, and Ir. The abrupt change of ${{c}_{{\rm min} }}$ can be exploited to switch properties specific to the overlayer material. This is demonstrated for the example of the superconducting critical temperature of a vanadium film which we predict to jump by 20% at a discontinuity of ${{c}_{{\rm min} }}$ .
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