Electric-Field-Induced Reorientation of the Magnetic Easy Plane in a Co-Substituted BiFeO₃ Single Crystal

Single crystals of BiFe_0.9Co_0.1O₃ and BiFe_0.892Mn_0.008Co_0.1O₃, room temperature ferroelectric ferromagnets, were successfully grown by a flux method at a high pressure of 3 GPa. Remanent magnetization measurements along 18 crystallographic directions revealed the existence of a magnetic easy plane perpendicular to the electric polarization. Reorientation of the magnetic easy plane occurred in connection with 71° ferroelectric switching by applying an electric field. This is the first demonstration of an electric field affecting the local magnetic moment of Co-substituted BiFeO₃

Giant Polarization and High Temperature Monoclinic Phase in a Lead-Free Perovskite of Bi(Zn_0.5Ti_0.5)O₃‑BiFeO₃

Lead-free piezoelectrics have attracted increasing attention because of the awareness of lead toxicity to the environment. Here, a new bismuth-based lead-free perovskite, (1 – <i>x</i>)­Bi­(Zn_0.5Ti_0.5)­O₃-<i>x</i>BiFeO₃, has been synthesized via a high-pressure and high-temperature method. It exhibits interesting properties of giant polarization, morphotropic phase boundary (MPB), and monoclinic phase. In particular, large tetragonality (<i>c</i>/<i>a</i> = 1.228) and giant spontaneous polarization of 110 μC/cm² has been obtained in 0.6 Bi­(Zn_0.5Ti_0.5)­O₃-0.4BiFeO₃, which is much higher than most available lead-free materials and conventional Pb­(Zr,Ti)­O₃. MPB is clearly identified to be constituted of tetragonal and monoclinic phases at <i>x</i> = 0.5. Notably, a single monoclinic phase has been observed at <i>x</i> = 0.6, which exhibits an intriguing high-temperature property. The present results are helpful to explore new lead-free MPB systems in bismuth-based compounds

Colossal Volume Contraction in Strong Polar Perovskites of Pb(Ti,V)O₃

The unique physical property of negative thermal expansion (NTE) is not only interesting for scientific research but also important for practical applications. Chemical modification generally tends to weaken NTE. It remains a challenge to obtain enhanced NTE from currently available materials. Herein, we successfully achieve enhanced NTE in Pb­(Ti_1–<i>x</i>V_<i>x</i>)­O₃ by improving its ferroelectricity. With the chemical substitution of vanadium, lattice tetragonality (<i>c</i>/<i>a</i>) is highly promoted, which is attributed to strong spontaneous polarization, evidenced by the enhanced covalent interaction in the V/Ti–O and Pb–O2 bonds from first-principles calculations. As a consequence, Pb­(Ti_0.9V_0.1)­O₃ exhibits a nonlinear and much stronger NTE over a wide temperature range with a volumetric coefficient of thermal expansion α_V = −3.76 × 10^–5/°C (25–550 °C). Interestingly, an intrinsic giant volume contraction (∼3.7%) was obtained at the composition of Pb­(Ti_0.7V_0.3)­O₃ during the ferroelectric-to-paraelectric phase transition, which represents the highest value ever reported. Such volume contraction is well correlated to the effect of spontaneous volume ferro­electro­striction. The present study extends the scope of the NTE family and provides an effective approach to explore new materials with large NTE, such as through adjusting the NTE-related ferroelectric property in the family of ferroelectrics

A‑Site and B‑Site Charge Orderings in an <i>s–d</i> Level Controlled Perovskite Oxide PbCoO₃

Perovskite PbCoO₃ synthesized at 12 GPa was found to have an unusual charge distribution of Pb²⁺Pb⁴⁺₃Co²⁺₂Co³⁺₂O₁₂ with charge orderings in both the A and B sites of perovskite ABO₃. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to Pb²⁺Pb⁴⁺₃Co²⁺₂Co³⁺₂O₁₂ quadruple perovskite structure. It is shown that the average valence distribution of Pb^3.5+Co^2.5+O₃ between Pb³⁺Cr³⁺O₃ and Pb⁴⁺Ni²⁺O₃ can be stabilized by tuning the energy levels of Pb 6<i>s</i> and transition metal 3<i>d</i> orbitals