Atomic coordinates of CeNiC2 under pressure: Switching of the Ce-Ce first nearest neighbor direction

When pressurized, the heavy fermion compound ${\mathrm{CeNiC}}_{2}$ reveals a rich electronic phase diagram and shows unconventional superconductivity with a transition temperature ${T}_{c}\ensuremath{\sim}3.7$ K, the highest among Ce-based heavy fermion superconductors [Katano et al., Phys. Rev. B 99, 100501(R) (2019)]. Understanding of this appearance of superconductivity in the vicinity of a magnetic quantum critical point is still lacking. Given that physical properties of ${\mathrm{CeNiC}}_{2}$ are sensitive to subtle changes in the interatomic distances, information on atomic coordinates may offer essential insights into the local lattice arrangements, and thus the mechanisms behind the exotic phases and phase transitions. However, extraction of precise information on the atomic coordinates under pressure remains a challenge. To find a correlation between the local lattice environments and exotic physical properties in ${\mathrm{CeNiC}}_{2}$, we investigate its crystal structure from ambient pressure to 18.6 GPa via single crystal x-ray diffraction. The pressure dependence of unit-cell parameters reveals anisotropic linear compressibility $k$, following the relationship $|{k}_{a}|(3.70\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.16em}{0ex}}\mathrm{G}{\mathrm{Pa}}^{\ensuremath{-}1})>|{k}_{c}|(1.97\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.16em}{0ex}}\mathrm{G}{\mathrm{Pa}}^{\ensuremath{-}1})>|{k}_{\mathrm{b}}|(1.39\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.16em}{0ex}}\mathrm{G}{\mathrm{Pa}}^{\ensuremath{-}1})$, and a large bulk modulus, ${B}_{0}\ensuremath{\sim}134\ifmmode\pm\else\textpm\fi{}3$ GPa. Although the atomic coordinates between Ce and Ni remain unchanged under applied pressure, the directions of the first nearest and the second nearest neighbors between both the Ce-Ce and Ni-Ni atoms switch at $\ensuremath{\sim}7$ GPa. Notably, near this pressure the antiferromagnetic ordering temperature reaches maximum in the pressure-temperature phase diagram of ${\mathrm{CeNiC}}_{2}$. Our results suggest that the direction of the nearest neighbors interchange might play a key role in the suppression of magnetic order and the enhancement of the Kondo effect.