  A pressure-induced phase transition, associated with an increase of the
coordination number of In and Ta, is detected beyond 13 GPa in InTaO4 by
combining synchrotron x-ray diffraction and Raman measurements in a diamond
anvil cell with ab-initio calculations. High-pressure optical-absorption
measurements were also carried out. The high-pressure phase has a monoclinic
structure which shares the same space group with the low-pressure phase (P2/c).
The structure of the high-pressure phase can be considered as a slight
distortion of an orthorhombic structure described by space group Pcna. The
phase transition occurs together with a unit-cell volume collapse and an
electronic bandgap collapse observed by experiments and calculations.
Additionally, a band crossing is found to occur in the low-pressure phase near
7 GPa. The pressure dependence of all the Raman-active modes is reported for
both phases as well as the pressure dependence of unit-cell parameters and the
equations of state. Calculations also provide information on IR-active phonons
and bond distances. These findings provide insights into the effects of
pressure on the physical properties of InTaO4.
