PbTiO3-Bi(Zn1/2Ti1/2)O-3 and related solid solutions exhibit an unusually negative thermal expansion with considerably enhanced tetragonality and Curie temperature. A zero thermal expansion is found over a wide temperature range of room temperature to 500 degrees C. Direct experimental evidence is given for the fact that the negative thermal expansion of PbTiO3-based solid solutions is strongly correlated with the temperature dependence of the ferroelectricity, which is a new negative thermal expansion mechanism.