Crystal and magnetic structure transitions in bimno3+δ ceramics driven by cation vacancies and temperature

The crystal structure of BiMnO$_{3+δ}$ ceramics has been studied as a function of nominal oxygen excess and temperature using synchrotron and neutron powder diffraction, magnetometry and differential scanning calorimetry. Increase in oxygen excess leads to the structural transformations from the monoclinic structure (C2/c) to another monoclinic (P2$_{1}$/c), and then to the orthorhombic (Pnma) structure through the two-phase regions. The sequence of the structural transformations is accompanied by a modification of the orbital ordering followed by its disruption. Modification of the orbital order leads to a rearrangement of the magnetic structure of the compounds from the long-range ferromagnetic to a mixed magnetic state with antiferromagnetic clusters coexistent in a ferromagnetic matrix followed by a frustration of the long-range magnetic order. Temperature increase causes the structural transition to the nonpolar orthorhombic phase regardless of the structural state at room temperature; the orbital order is destroyed in compounds BiMnO$_{3+δ}$ (δ ≤ 0.14) at temperatures above 470 °C

Investigation of Local Conduction Mechanisms in Ca and Ti-Doped BiFeO3 Using Scanning Probe Microscopy Approach

In this work we demonstrate the role of grain boundaries and domain walls in the local transport properties of n- and p-doped bismuth ferrites, including the influence of these singularities on the space charge imbalance of the energy band structure. This is mainly due to the charge accumulation at domain walls, which is recognized as the main mechanism responsible for the electrical conductivity in polar thin films and single crystals, while there is an obvious gap in the understanding of the precise mechanism of conductivity in ferroelectric ceramics. The conductivity of the Bi0.95Ca0.05Fe1-xTixO3-δ (x = 0, 0.05, 0.1; δ = (0.05 - x)/2) samples was studied using a scanning probe microscopy approach at the nanoscale level as a function of bias voltage and chemical composition. The obtained results reveal a distinct correlation between electrical properties and the type of charged defects when the anion-deficient (x = 0) compound exhibits a three order of magnitude increase in conductivity as compared with the charge-balanced (x = 0.05) and cation-deficient (x = 0.1) samples, which is well described within the band diagram representation. The data provide an approach to control the transport properties of multiferroic bismuth ferrites through aliovalent chemical substitution.The reported study was funded by RFBR according to the research project No. 18-38-20020 mol_a_ved. Experimental investigations done at the CFisUC were supported by Fundação para a Ciência e a Tecnologia (FCT), through the projects CENTRO-01-0145-FEDER-000014, IF/00819/2014/CP1223/CT0011, and UID/04564/2020 (co-funded by FEDER/COMPETE). Access to TAIL-UC facility funded under QREN-Mais Centro project ICT_2009_02_012_1890 is gratefully acknowledged. M.V.S. acknowledges Russian academic excellence project “5-100” for Sechenov University. D.V.K., M.V.S., C.B, M.S. acknowledge the support of the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 778070

Nanoengineered Nickel/reduced graphene oxide composites: control of interfacial nanostructure for tunable electrophysical properties

Here we present a novel solvothermal synthesis approach for the accurate control over the structural features of nickel/reduced graphene oxide (Ni/rGO) nanocomposites for tuneable electrical properties. We discovered that the dynamic chemical structure of GO during reaction, acts as an active template for the controlled nanostructured growth of nickel nanoparticles (Ni NPs). Therefore, the precise control of reaction time offered the possibility to modulate nucleation and coalescence phenomena of Ni NPs, allowing in this way to precisely adjust their size, density and NiO@Ni structure on the final Ni/rGO nanocomposites. The electrophysical properties (work function and conductivity) of different Ni/rGO nanocomposites were determined and found to be directly dependent on the Ni NPs radius and also on the NiO buffer layer width. We confirmed a crucial role of the NiO buffer layer thickness at the Pt-NiO-Ni-NiO-rGO interface changing the conductivity from metallic to those specific to a Schottky contact or to a p-n heterojunction. These new findings reveal a relevant potential for using Ni/rGO nanocomposites as a versatile and promising material for micro-, nano- and optoelectronics as well as for energy storage technologies.publishe

Crystal and Magnetic Structure Transitions in BiMnO3+δ Ceramics Driven by Cation Vacancies and Temperature

The crystal structure of BiMnO3+δ ceramics has been studied as a function of nominal oxygen excess and temperature using synchrotron and neutron powder diffraction, magnetometry and differential scanning calorimetry. Increase in oxygen excess leads to the structural transformations from the monoclinic structure (C2/c) to another monoclinic (P21/c), and then to the orthorhombic (Pnma) structure through the two-phase regions. The sequence of the structural transformations is accompanied by a modification of the orbital ordering followed by its disruption. Modification of the orbital order leads to a rearrangement of the magnetic structure of the compounds from the long-range ferromagnetic to a mixed magnetic state with antiferromagnetic clusters coexistent in a ferromagnetic matrix followed by a frustration of the long-range magnetic order. Temperature increase causes the structural transition to the nonpolar orthorhombic phase regardless of the structural state at room temperature; the orbital order is destroyed in compounds BiMnO3+δ (δ ≤ 0.14) at temperatures above 470 °C