  In this paper we perform a high-pressure study of fergusonite-type HoNbO4.
Powder x-ray diffraction experiments and ab initio density-functional theory
simulations provide evidence of a phase transition at 18.9(1.1) GPa from the
monoclinic fergusonite-type structure (space group I2/a) to another monoclinic
polymorph described by space group P21/c. The phase transition is reversible
and the high-pressure structural behavior is different than the one previously
observed in related niobates. The high-pressure phase remains stable up to 29
GPa. The observed transition involves a change in the Nb coordination number
from 4 to 6, and it is driven by mechanical instabilities. We have determined
the pressure dependence of unit-cell parameters of both phases and calculated
their room-temperature equation of state. For the fergusonite-phase we have
also obtained the isothermal compressibility tensor. In addition to the
high-pressure studies, we report ambient-pressure Raman and infrared
spectroscopy measurements. We have been able to identify all the active modes
of fergusonite-type HoNbO4, which have been assigned based upon
density-functional theory calculations. These simulations also provide the
elastic constants of the different structures and the pressure dependence of
the Raman and infrared modes of the two phases of HoNbO4.
