  In this article we report the structural stability of Er$_2$Ti$_2$O$_7$ cubic
pyrochlore with pressure using x-ray diffraction, Raman spectroscopy,
photoluminescence, x-ray absorption and ab-initio calculations. Our studies
establish a phase transformation in Er$_2$Ti$_2$O$_7$ from ambient cubic phase
to high-pressure orthorhombic (cotunnite) phase, initiated at ~40 GPa. The
transformation is sluggish and it does not complete even at the highest
measured pressure in our study i.e. ~60.0 GPa. This is further supported by the
first principle calculations which reveal that cotunnite phase is energetically
more stable than the ambient phase above ~53 GPa. After complete release of
pressure, the high-pressure cotunnite phase is retained while the fraction of
untransformed pyrochlore phase becomes amorphous. Furthermore, the EXAFS data
of the recovered sample at L3 edge of Er3+ ion show an increase in the
coordination number of cations from eight at ambient to nine in the
high-pressure phase. The mechanism of structural transformation is explained in
terms of accumulation of cation antisite defects and subsequent disordering of
cations and anions in their respective sublattice. The amorphization of the
pyrochlore phase upon release is interpreted as the inability of accommodating
the point defects at ambient conditions, which are formed in the pyrochlore
lattice under compression.
