The strain energy density in epitaxial overlayers depends strongly on the substrate's surface orientation. The affect of a growth orientation dependent strain energy contribution to the phase stability was studied by growing overlayers of metastable, semiconducting a-Sn films on the (100) and (111) surfaces of Cd0.8Zn0.2Te substrates. On the (100) surface the α-phase of Sn remains stable for coverages up to and beyond 125 ML. On the (111) surface though, the Sn overlayer shows metallic behavior from a partial α→β phase separation for coverages lower than 100 ML. Static calculations using the stress-strain relationship bear out these experimental results by showing that the strain energy density for the (111) growth direction is 38% larger than for the (100) direction.