  We studied the high-pressure effects on the crystalline structure of
monoclinic HfTiO4 and ZrTiO4. We found that the compressibility of these
ceramics is highly non-isotropic, being the b-axis the most compressible one.
In addition, the a-axis is found to have a small and negative compressibility.
At 2.7 GPa (10.7 GPa) we discovered the onset of an structural phase transition
in HfTiO4 (ZrTiO4), coexisting the low- and high-pressure phases in a broad
pressure range. The new high-pressure phase has a monoclinic structure which
involves an increase in the Ti-O coordination and a collapse of the cell
volume. The equation of state for the low-pressure phase is also determined.
