Pressure-Driven Structural Transition in the Charge Density Wave Material SrAl4

SrAl₄, which possesses a BaAl₄-type tetragonal structure (I4/mmm, No. 139), exhibits both a charge density wave (CDW) order and a topological semimetal state at ambient pressure. Here, the electronic and structural properties of SrAl₄ were systematically investigated with pressure up to 49.4 GPa through electrical transport, X-ray diffraction (XRD), and Raman scattering measurements, as well as theoretical calculations. With increasing pressure, the T_CDW is monotonically decreased, and the CDW state is eventually suppressed to zero temperature at ∼10 GPa based on the linear extrapolation. At ambient pressure, three Raman vibrational modes are identified, which are assigned to B_1g (230.9 cm^–1), E2g$E_g^2$$E_g^2$ (302.7 cm^–1), and A_1g (357.4 cm^–1), respectively. Upon compression to P_C = 19.0 GPa, the original Raman modes all disappear, and simultaneously, four new peaks emerge, which indicate the occurrence of a structural transition. Combined with XRD and theoretical calculations, the C2/m phase is believed to be the most plausible crystal structure of SrAl₄ above P_C. In addition, the residual resistance ratio as well as magnetoresistance shows abrupt changes across P_C, which further manifest the structural transition (I4/mmm → C2/m) of SrAl₄ under high pressure.