  We study pressure-induced structural evolution of vanadium diselenide (VSe2),
a 1T polymorphic member of the transition metal di-chalcogenide (TMD) family
using synchrotron-based powder X-ray diffraction (PXRD) and first-principles
density functional theory (DFT). Our XRD results reveal anomalies at P ~4 GPa
in c/a ratio, V-Se bond length and Se-V-Se bond angle signalling an
isostructural transition. This is followed by a first order structural
transition from 1T (space group P-3m1) phase to a 3R (space group R-3m) phase
at P ~11 GPa due to sliding of adjacent Se-V-Se layers. We present various
scenarios to understand the experimental results within DFT and find that the
1T to 3R transition can be captured only after inclusion of enthalpic
correction associated with errors in cell volume with underestimated transition
pressure. The abrupt increase in the Debye-Waller factors of Se atoms by a
factor of ~4 and hence the anharmonic effects across the structural transition
pressure are hitherto not reported so far and hint a possible way to understand
the mismatch between the experimental and theoretical transition pressure
values.
