P.*

Recent discoveries of novel magnetism in the TMPt₅P and TMPd₅P (TM = 3d transition metal) based families [1-3] has brought renewed interest to the wider class of M(Pt/Pd)₅X (M = TM, IIIA elements, and rare earths; X = P, As) materials. IIIA element-based M(Pt/Pd)₅X, where M = Al, Ga, and In, are non-magnetic analog of magnetic TM(Pt/Pd)₅X and other than structure, are relatively un-characterized. Here, we present detail studies of GaPt₅P single crystals using ambient and high pressure resistivity measurements as well as ambient pressure magnetization and X-ray diffraction measurements. At ambient pressure, a step-like feature at about T ~ 80 K in both resistivity and magnetization measurements indicates a possible structural change. The hysteretic nature suggests the transition to be the first-order type. Furthermore, X-ray diffraction measurements confirm the first-order structural transition to a lower symmetry structure with a volume collapses. Hydrostatic pressure increases the transition temperature bringing it to room temperature by ~ 2.3 GPa. [1] X. Gui et. al., Appl. Electron. Matter. 3, 3501 (2021). [2] X. Gui et. al., Chem. Mater. 32, 3922 (2020). [3] T. J. Slade and P. C. Canfield Z. Anorg. Allg. Chem. 648, e202200145, 2022.