Enthalpy of transformation of a high-pressure polymorph of titanium dioxide to the rutile modification.

The enthalpy of transformation of a high-pressure form of titanium dioxide, which has the alpha lead dioxide structure, to the rutile modification was measured by the method of "transposed temperature-drop calorimetry." For the reaction, titanium dioxide (alpha lead dioxide form) transforming to rutile, the change in the heat content is -0.76 +/- 0.17 kilocalorie per mole. From this value and the volume change (+ 2.8 percent) associated with the transformation, we estimate the equilibrium pressure at 294 degrees K to be 60 +/- 20 kilobars.