Pressure induced structural modifications in NaFe$_{1.99}$Co$_{0.01}$As$_2$ superconductor

NaFe$_{1.99}$Co$_{0.01}$As$_2$ superconductor with the tetragonal ThCr$_2$Si$_2$-type structure ($I4/mmm$) has been studied using x-ray diffraction and Raman spectroscopy up to 25 GPa (at RT). Recently, it was found that, for this compound, $T_c$ increases with pressure to a maximum of 32 K at 2.5 GPa. With further compression $T_c$ decreases up to 6 GPa, the highest pressure superconductivity has been detected. We report that, although NaFe$_{1.99}$Co$_{0.01}$As$_2$ remains in the ambient pressure phase, the lattice parameters evolution with pressure shows distinct behavior below and above a critical pressure Pc=2.5 GPa. This is accompanied by a subtle change of Raman spectra at Pc. Below Pc, a-axis increases while both the c-axis and the c/a axial ratio decrease. In contrast above Pc, both axes show a normal decrease and c/a remains almost constant. The different behavior of c-axis, below and above Pc, can be viewed as a modification of the initial tetragonal phase (T) to a collapsed tetragonal (CT) one. This is in line with previous studies on 122 iron-based superconductors. We conclude that the high compressibility of c-axis, in the T phase, enhances superconductivity since layers are brought together. Above Pc, compression of CT phase seems to have the opposite effect.