Pressure-temperature phase diagram of BaNi₂As₂

The normal state of the superconductor BaNi2As2 (Tc=0.6K) exhibits a variety of unconventional charge-ordered phases that have recently attracted much attention. At ambient conditions BaNi2As2 shows the same ThCr2Si2 structure type (SG: I4/mmm) as the iron-based superconductor BaFe2As2. An incommensurate charge density wave (IC-CDW) with ordering wavevector q=(0.28 0 0)tet forms upon cooling and the system undergoes a second order transition to an orthorhombic I/mmm phase below T=142K. This transition is shortly followed (T=137K) by a first order one into a triclinic phase (SG: P-1) in which a commensurate CDW (C-CDW) appears (q=(1/3 0 1/3)tet) replacing the IC-CDW. [1-3]
We report a detailed high-resolution x-ray diffraction study of the crystal structure and the various charge-ordered phases as function of temperature and hydrostatic pressure (up to 13 GPa), revealing a cascade of new CDW phases.