Coupled structural collapse and evolution of magnetic phases in Sr(Co_0.88Ni_0.12)₂P₂under pressure

We report high-pressure x-ray diffraction and electrical resistance measurements on Sr(CoNi)P, an itinerant antiferromagnet with the highest Néel temperature (= 36 K) among Sr(CoNi)Pseries. Room-temperature diffraction data reveal a pressure-induced transition from an uncollapsed tetragonal 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-lattice parameter and a contraction in the-lattice parameter, resulting in a reduction of the axial ratio. 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. The cT phase is accompanied by a much higher-temperature transport anomaly that resembles the pressure-induced ferromagnetic transition reported for SrCoP, reaching approximately 190 K before gradually decreasing at higher pressures. These results demonstrate strong coupling between the lattice collapse and magnetic behavior in Sr(CoNi)P.