Coupled structural collapse and evolution of magnetic phases inSr(Co0.88Ni0.12)2P2under pressure.

We report high-pressure x-ray diffraction and electrical resistance measurements on Sr(Co0.88Ni0.12)2P2, an itinerant antiferromagnet with the highest Néel temperature (TN= 36 K) among Sr(Co1-xNix)2P2series. Room-temperature diffraction data reveal a pressure-induced transition from an uncollapsed tetragonal (ucT) to a collapsed tetragonal (cT) structure, beginning near 8.4 GPa and completing before 12.4 GPa. This transition is characterized by an expansion of the in-plane a-lattice parameter and a contraction in the c-lattice parameter, resulting in a reduction of the axial ratio c/a. Resistance measurements show a low-pressure anomaly near 38 K, consistent with the reported antiferromagnetic transition. As the room temperature value of the applied pressure exceeds 8 GPa, signs of a partial cT transition appear on cooling with the full cT transition occurring for pressures exceeding 10 GPa. Associated with the cT phase is a much higher temperature ferromagnetic transition with Curie temperature reaching ∼ 190 K before slowly decreasing for higher applied pressures. These results demonstrate strong coupling between the lattice collapse and magnetic behavior in Sr(Co0.88Ni0.12)2P2.