  Single crystals of XPt$_{5}$P (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 GaPt$_{5}$P. 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 GaPt$_{5}$P 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)Pt$_{5}$P. 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 $\sim 2.2$~GPa,
highlighting the high degree of pressure sensitivity and fragile nature of
GaPt$_{5}$P room-temperature structure. Although the volume collapse and
extreme pressure sensitivity suggest chemical pressure should drive a similar
structural change in AlPt$_{5}$P, with smaller unit cell dimensions and volume,
its structure is found to be $P4/mmm$ as well. Overall, GaPt$_{5}$P stands out
as a sole member of the 1-5-1 family of compounds with a temperature-driven
structural change.
