Compressibility of BiCu₂PO₆: Polymorphism against S = ¹/₂ Magnetic Spin Ladders

BiCu₂PO₆ is a unique example of a S = ¹/₂ ladder where the magnetic exchanges are mainly confined in 1D _∞[BiCu₂O₂]³⁺ cationic ribbons, although the shortest Cu-Cu separation between them exists. Its original magnetic topology gives the most representative example of a frustrated quantum ladder to investigate the complex physics behind it. Herein, we report the synthesis and characterization of one high-pressure polymorph. In this new phase, the preservation of 1D _∞[BiCu₂O₂]³⁺ units somewhat restacked leads to the preservation of its gapped magnetic ground state and ladder topology. The comparison of both compounds highlights the start of a thermodynamic conjuncture, where both the stable ambient-pressure (AP) and metastable high-pressure (HP) forms display the same equilibrium volume and superposed volume dependence of the energy, leading to a first-order AP → HP transition undetected by differential thermal analysis.