New PbTiO₃‑Type Giant Tetragonal Compound Bi₂ZnVO₆ and Its Stability under Pressure

A new PbTiO₃-type compound, Bi₂ZnVO₆, with a giant tetragonal distortion of <i>c/a</i> = 1.26 (<i>a</i> = 3.7869(3) Å, <i>c</i> = 4.7660(7) Å) was synthesized under high pressure–high temperature conditions (9 GPa and 1373 K). A point charge model calculation based on the atomic positions refined by Rietveld analysis of synchrotron X-ray diffraction (SXRD) data gave an electrical ionic polarization of 126 μC/cm², the largest value among PbTiO₃-type perovskite compounds. The tetragonality (<i>c/a</i>) decreased with increasing temperature from 100 to 570 K without any trace of a phase transition. Instead, a pressure-induced transition from a polar tetragonal structure to a paraelectric GdFeO₃ one accompanied by a 2.4% volume collapse was observed at 6.01 GPa. Bi₂ZnVO₆ showed paramagnetic behavior with <i>S</i> = 1/2 because of the random distribution of nonmagnetic Zn²⁺ and magnetic V⁴⁺ ions. Transport measurements indicated semiconductivity with an activation energy of 0.43 eV

In Situ Observing and Tuning the Crystal Orientation of Two-Dimensional Layered Perovskite via the Chlorine Additive

Precise control of crystal orientation in two-dimensional (2D) layered perovskites (LPs) is vital for their optoelectronic applications due to the structure-induced anisotropy in optical and electrical properties. Herein, we directly observe and control the crystal orientation of the butylammonium-based 2D LP films. Employing the synchrotron-based in situ grazing-incidence X-ray diffraction technique, we reveal the orientation modulation mechanism of the Cl additive by following the crystallization dynamics and chemical conversion pathways during film formation. Two new Cl-related intermediates are identified which serve as templates directing the orientational growth of the 2D LP films. We fine-tune the crystal orientation of 2D LP films through the Cl additive and incorporate the films with the requisite crystal orientations in solar cells and photodetectors. The optoelectronic performances of the devices show a strong correlation with the crystal orientation of the 2D LP films

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

Melting of Pb Charge Glass and Simultaneous Pb–Cr Charge Transfer in PbCrO₃ as the Origin of Volume Collapse

A metal to insulator transition in integer or half integer charge systems can be regarded as crystallization of charges. The insulating state tends to have a glassy nature when randomness or geometrical frustration exists. We report that the charge glass state is realized in a perovskite compound PbCrO₃, which has been known for almost 50 years, without any obvious inhomogeneity or triangular arrangement in the charge system. PbCrO₃ has a valence state of Pb²⁺_0.5Pb⁴⁺_0.5Cr³⁺O₃ with Pb²⁺–Pb⁴⁺ correlation length of three lattice-spacings at ambient condition. A pressure induced melting of charge glass and simultaneous Pb–Cr charge transfer causes an insulator to metal transition and ∼10% volume collapse

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