Thermal expansion of tetragonal ZnP2 and CdP2 single crystals grown fr
om the vapor phase was studied by dilatometric measurements. The relat
ive size change in the [001] and [100] directions exhibits hysteretic
behavior. A change in temperature-scan rate results in a kink in the D
elta L/L(0) = f(T) curves. The obtained data are discussed in terms of
the narrow homogeneity range of ZnP2 and CdP2.