The 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) were grown from a Pt-P solution at high temperatures, and ambient-pressure measurements of temperature-dependent magnetization, resistivity, and X-ray diffraction were made. Also, the ambient-pressure Hall resistivity and temperature-dependent resistance under pressure were measured on GaPt₅P. All three compounds have 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 step-like feature in $70-90$~K region suggesting a possible structural phase transition, and no such features were observed in (Al/In)Pt₅P. Both the hysteretic nature and sharpness of the feature suggest the first-order transition, and single-crystal X-ray diffraction measurements provided further details of the structural transition with a crystal symmetry likely different than $P4/mmm$ below transition. The transition is characterized by anisotropic changes in the lattice parameters, a volume collapse, and satellite peaks at two distinct wave-vectors. Density functional theory calculations present phonon softening as a possible driving mechanism. Additionally, the structural transition temperature increases rapidly with increasing pressure, reaching room temperature by ~ 2.2~GPa, highlighting the high degree of pressure sensitivity and fragile nature of GaPt₅P room-temperature structure. Although the volume collapse and extreme pressure sensitivity suggest chemical pressure should drive a similar structural change in AlPt₅P, with smaller unit cell dimensions and volume, its structure is found to be $P4/mmm$ as well. Overall, GaPt₅P stands out as a sole member of the 1-5-1 family of compounds with a temperature-driven structural change.