  We report a comprehensive single-crystal neutron diffraction investigation of
the Kitaev quantum magnet $\alpha$-RuCl$_{3}$ under hydrostatic pressure.
Utilizing a He-gas pressure cell, we successfully applied an ideal hydrostatic
pressure in situ at low temperatures, which allows to effectively eliminate any
possible influences from the structural transition occurring between 200 K and
50 K under ambient conditions. Our experiments reveal a gradual suppression of
the ziagzag antiferromagnetic order as hydrostatic pressure increases.
Furthermore, a reversible pressure-induced structural transition occurs at a
critical pressure of $P_d$ = 0.15 GPa at 30 K, as evidenced by the absence of
magnetic order and non-uniform changes in lattice constants. The decrease in
magnetic transition temperature is discussed in relation to a pressure-induced
change in the trigonal distortion of the Ru-Cl octahedra in this compound. Our
findings emphasize the significance of the trigonal distortion in Kitaev
materials, and provide a new perspective on the role of hydrostatic pressures
in the realization of the Kitaev quantum spin liquid state in
$\alpha$-RuCl$_{3}$.
