Ultrahigh dielectric permittivity in oxide ceramics by hydrogenation.

Boosting dielectric permittivity representing electrical polarizability of dielectric materials has been considered a keystone for achieving scientific breakthroughs as well as technological advances in various multifunctional devices. Here, we demonstrate sizable enhancements of low-frequency dielectric responses in oxygen-deficient oxide ceramics through specific treatments under humid environments. Ultrahigh dielectric permittivity (~5.2 × 106 at 1 Hz) is achieved by hydrogenation, when Ni-substituted BaTiO3 ceramics are exposed to high humidity. Intriguingly, thermal annealing can restore the dielectric on-state (exhibiting huge polarizability in the treated ceramics) to the initial dielectric off-state (displaying low polarizability of ~103 in the pristine ceramics after sintering). The conversion between these two dielectric states via the ambient environment-mediated treatments and the successive application of external stimuli allows us to realize reversible control of dielectric relaxation characteristics in oxide ceramics. Conceptually, our findings are of practical interest for applications to highly efficient dielectric-based humidity sensors.

Multiple charge density wave phases of monolayer VSe2manifested by graphene substrates.

A combined study of scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) is conducted to understand the multiple charge density wave (CDW) phases of monolayer (ML) VSe2films manifested by graphene substrates. Submonolayer (∼0.8 ML) VSe2films are prepared on two different substrates of single-layer graphene (SLG) and bi-layer graphene (BLG) on a 6H-SiC(0001). We find that ML VSe2films are less coupled to the SLG substrate compared to that of ML VSe2/BLG. Then, ML VSe2grown on SLG and BLG substrates reveals a very different topography in STM. While ML VSe2/BLG shows one unidirectional modulation of √3 × 2 and √3 × âˆš7 CDW in topography, ML VSe2/SLG presents a clear modulation of 4 × 1 CDW interfering with √3 × 2 and √3 × âˆš7 CDW which has not been previously observed. We explicitly show that the reciprocal vector of 4 × 1 CDW fits perfectly into the long parallel sections of cigar-shaped Fermi surfaces near the M point in ML VSe2, satisfying Fermi surface nesting. Since bulk VSe2is also well-known for the 4 × 4 × 3 CDW formed by Fermi surface nesting, the 4 × 1 CDW in ML VSe2/SLG is attributed to the planar projection of 4 × 4 × 3 CDW in bulk. Our result clarifies the nature of the 4 × 1 CDW in ML VSe2system and is a good example demonstrating the essential role of substrates in two-dimensional transition metal dichalcogenides.

Experimental and theoretical studies on induced ferromagnetism of new (1 - x)Na0.5Bi0.5TiO3 + xBaFeO3-δ solid solution.

New solid solution of Na0.5Bi0.5TiO3 with BaFeO3-δ materials were fabricated by sol-gel method. Analysis of X-ray diffraction patterns indicated that BaFeO3-δ materials existed as a well solid solution and resulted in distortion the structure of host Na0.5Bi0.5TiO3 materials. The randomly incorporated Fe and Ba cations in the host Na0.5Bi0.5TiO3 crystal decreased the optical band gap from 3.11 to 2.48 eV, and induced the room-temperature ferromagnetism. Our density-functional theory calculations further suggested that both Ba for Bi/Na-site and Fe dopant, regardless of the substitutional sites, in Na0.5Bi0.5TiO3 lead to the induced magnetism, which is illustrated in terms of the exchange splitting between spin subbands through the crystal field theory and Jahn-Teller distortion effects. Our work proposes a simple method for fabricating lead-free ferroelectric materials with ferromagnetism property for multifunctional applications in smart electronic devices.

Structural, Optical and Magnetic Properties of (1-x)Bi0.5Na0.5TiO3+xBaCoO3-δ Solid Solution Systems Prepared by the Sol-Gel Method.

A new solid solution, (1-x)Bi0.5Na0.5TiO3+xBaCoO3-δ materials, was fabricated using the sol-gel method. X-ray diffraction showed that the crystal structure of the compound exhibited rhombohedral symmetry and is similar to the crystal structures of host Bi0.5Na0.5TiO3 materials. Distortions in the structures and reduction in the optical band gaps of the Bi0.5Na0.5TiO3 materials were possibly due to the random incorporation of Ba and Co cations into host lattice materials. The magnetic properties of the Bi0.5Na0.5TiO3 materials were tuned by controlling the concentrations of BaCoO3-δ as the solid solution. We expect that our work will provide valuable information on current methods for integrating ferromagnetic properties into lead-free ferroelectric materials for the development of multiferroic materials.

Author Correction: Structure, optical and magnetic properties of new Bi0.5Na0.5TiO3- SrMnO3-δ solid solution materials.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Structure, optical and magnetic properties of new Bi0.5Na0.5TiO3- SrMnO3-δ solid solution materials.

The new Bi0.5Na0.5TiO3-SrMnO3-δ solid solution materials were fabricated via sol-gel method. The random incorporation of Sr and Mn cations into host lattice of Bi0.5Na0.5TiO3 resulted in structural distortion and influenced on the reduction of the optical band gap from 3.07 eV to 1.81 eV for pure Bi0.5Na0.5TiO3 and 9 mol% SrMnO3-δ solid solution into Bi0.5Na0.5TiO3. The magnetic properties of Bi0.5Na0.5TiO3 materials at room temperature were tuned via compensation of diamagnetic material with weak-ferromagnetism to ferromagnetism with low SrMnO3-δ content and combination of paramagnetism/antiferromagnetism-like and ferromagnetism with higher SrMnO3-δ content solid solution in Bi0.5Na0.5TiO3. The tunable magnetic and optical properties of lead-free ferroelectric materials was promising for their application to green electronic devices.