First-order structural phase transition at low temperature in GaPt5P and its rapid enhancement with pressure

Single crystals of XPt₅P (X = Al, Ga, and In), belonging to the 1-5-1 family of compounds, were grown from a Pt-P solution at high temperatures, and measurements of the ambient pressure, temperature-dependent magnetization, resistivity, and x-ray diffraction were made. Additionally, the ambient-pressure Hall resistivity and temperature-dependent resistance under pressure were measured on GaPt₅P. All three compounds have a tetragonal $P4/mmm$ crystal structure at room temperature with metallic transport and weak diamagnetism over the 2--300 K temperature range. Surprisingly, at ambient pressure, both the transport and magnetization measurements on GaPt₅P show a steplike feature in the 70--90 K region, suggesting a possible structural phase transition. Neither AlPt₅P nor InPt₅P have any signatures of a phase transition in their temperature-dependent electrical resistance and magnetization data. Both the hysteretic nature and sharpness of the features in the GaPt₅P data suggest that the transition is first-order. Single-crystal x-ray diffraction measurements provided further details of the structural transition with a possibility of a crystal symmetry different from $P4/mmm$ below the transition temperature. The transition is characterized by anisotropic changes in the lattice parameters and a volume collapse with respect to the high-temperature tetragonal crystal structure. Furthermore, satellite peaks are observed at two distinct and nonequivalent wave vectors (0, 0, 0.5) and (0.5, 0.5, 0.5), and density functional theory calculations present phonon softening, especially at (0.5, 0.5, 0.5), as a possible driving mechanism. Additionally, we find that the structural transition temperature increases rapidly with increasing pressure, reaching room temperature by ~2.2 GPa, highlighting the high degree of pressure sensitivity of GaPt₅P and fragile nature of its room-temperature structure. Even though the volume collapse and extreme pressure sensitivity suggest chemical pressure should drive a similar structural change in AlPt₅P, where both unit-cell dimensions and volume are smaller, its structure is found to be the same as that of the room-temperature GaPt₅P. Overall, GaPt₅P stands out as a sole member of the 1-5-1 family of compounds for which a temperature-driven structural change has been observed.