The making of a monolith: Amorphous calcium carbonate is a hard material that is difficult to make into large, clear blocks. It is also sensitive to heating, and compacting the starting nanoparticles too much tends to lead to crystallization. Mu et al. determined the optimal amount of water in amorphous calcium carbonate to create clear, solid monoliths through compression. The key is to regulate the amount of diffusion in the system so that particle boundaries fuse without triggering sample-wide crystallization. This fusion strategy may also work for similar amorphous inorganic ionic compounds. Science , abg1915, this issue p. 1466

Transition metal oxides are promising candidates for the next generation of spintronic devices due to their fascinating properties that can be effectively engineered by strain, defects, and microstructure. An excellent example can be found in ferroelastic LaCoO₃ with paramagnetism in bulk. In contrast, unexpected ferromagnetism is observed in tensile-strained LaCoO₃ films, however, its origin remains controversial. Here we simultaneously reveal the formation of ordered oxygen vacancies and previously unreported long-range suppression of CoO₆ octahedral rotations throughout LaCoO₃ films. Supported by density functional theory calculations, we find that the strong modification of Co 3d-O 2p hybridization associated with the increase of both Co-O-Co bond angle and Co-O bond length weakens the crystal-field splitting and facilitates an ordered high-spin state of Co ions, inducing an emergent ferromagnetic-insulating state. Our work provides unique insights into underlying mechanisms driving the ferromagnetic-insulating state in tensile-strained ferroelastic LaCoO₃ films while suggesting potential applications toward low-power spintronic devices.

Negative pressure has emerged as a powerful tool to tailor the physical properties of functional materials. However, a negative pressure control of spin-phonon coupling for engineering magnetism and multiferroicity has not been explored to date. Here, using uniform three-dimensional strain-induced negative pressure in nanocomposite films of (EuTiO₃)_0.5:(MgO)_0.5, we demonstrate an emergent multiferroicity with magnetodielectric coupling in EuTiO₃, matching exactly with density functional theory calculations. Density functional theory calculations are further used to explore the underlying physics of antiferromagnetic-paraelectric to ferromagnetic-ferroelectric phase transitions, the spin-phonon coupling, and its correlation with negative pressures. The observation of magnetodielectric coupling in the EuTiO₃ reveals that an enhanced spin-phonon coupling originates from a negative pressure induced by uniform three-dimensional strain. Our work provides a route to creating multiferroicity and magnetoelectric coupling in single-phase oxides using a negative pressure approach.

Ferroelectric lead-free ${\mathrm{K}}_x{\mathrm{Na}}_{1-x}{\mathrm{NbO}}_3$ (KNN) perovskite, as the representative of the preferred oxides with good dielectric and piezoelectric behaviors, has attracted broad attentions in recent years. Here we systematically investigate the complete process of temperature dependent structural evolution in manganese (Mn)-doped ${\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}{\mathrm{NbO}}_3\text{−}{\mathrm{LiBiO}}_3$ single crystals, presenting the ultrahigh piezoelectric coefficient of about 1050 pC ${\mathrm{N}}^{-1}$ and the excellent dielectric and ferroelectric performances, by demonstrating phonon thermodynamics associated with all Raman active modes at a wide temperature range of 80–800 K. All-round unpolarized and polarized scattering characteristics reflecting various phonon and structure properties are specified in detail. Symmetry difference, symmetry breaking, and structure rearrangement among the molecular vibrations have been proven to be related to a multiphase coexistence and the discontinuity of first-order phase transition. In comparison with the lattice dynamics, a complete phase transition ordering and the shift of transition point have been observed in $\mathrm{KNN}\text{−}{\mathrm{LiBiO}}_3$ crystals with the different Mn contents. This work aims at deeply revealing the details of good ferroelectric/dielectric performance, structure, and phonon thermodynamics, as well as understanding the mechanisms of first-order phase transition under the doping manipulation in KNN systems.

As one of the classic antiferroelectrics, high complexity of a NaNbO₃ structure sequence attracts great attention in the ferroelectric physics field. Here, temperature-pressure phase diagrams as a function of a CaSnO₃ content for antiferroelectric (1-x)NaNbO₃-xCaSnO₃ ceramics have been improved by Raman spectroscopy. We clarify structural order of phase transitions on CaSnO₃-modified NaNbO₃ ceramics within the temperature range of 80–840 K by discussing the anomalies of lattice and phonon dynamics. The doping effect of CaSnO₃ on the P-R phase transition has been summarized from the decreased critical temperature from 660 to 580 K. The intermediate phase at 480 K was recognized as an incommensurate phase. In addition, the anomalous pressure with respect to phonon frequency at the stress field of 0–25 GPa also provides the evidence of structural transformations at 6.55 and 10.05 GPa. Upon increasing the CaSnO₃ content, phase transition moves to a lower pressure range. This work would provide the powerful supplement of phase transitions for the broad NaNbO₃-based crystalline family with Raman scattering.