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

BiCu₂PO₆ is a unique example of a <i>S</i> = 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

Structural Study of a Doubly Ordered Perovskite Family NaLnCoWO₆ (Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb): Hybrid Improper Ferroelectricity in Nine New Members

The compounds of the doubly ordered perovskite family NaLnCoWO₆ (Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb) were synthesized by solid-state reaction, nine of which (Ln = Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb) are new phases prepared under high-temperature and high-pressure conditions. Their structural properties were investigated at room temperature by synchrotron X-ray powder diffraction and neutron powder diffraction. All of them crystallize in monoclinic structures, especially the nine new compounds have the polar space group <i>P</i>2₁ symmetry, as confirmed by second harmonic generation measurements. The <i>P</i>2₁ polar structures were decomposed and refined in terms of symmetry modes, demonstrating that the polar mode is induced by two nonpolar modes in a manner of Hybrid Improper Ferroelectricity. The amplitudes of these three major modes all increase with decreasing the Ln cation size. The spontaneous ferroelectric polarization is estimated from the neutron diffraction data of three samples (Ln = Y, Tb, and Ho) and can be as large as ∼20 μC/cm²