Bi2V1-x (Mg0.25Cu0.25Ni0.25Zn0.25) x O5.5-3x/2: A high entropy dopant BIMEVOX

A high entropy dopant approach has been used to prepare a new BIMEVOX ceramic system, Bi2V1-x(Mg0.25Cu0.25Ni0.25Zn0.25)xO5.5-3x/2. Structures were investigated using a combination of X-ray and neutron powder diffraction, with electrical characterisation by A.C. impedance spectroscopy. A γ-type phase is observed at room temperature over the compositional range 0.10 ≤ x ≤ 0.30, the upper limit of which is beyond that seen for all the single substituted systems based on these substituents, apart from BIMGVOX. No stabilisation of the fully disordered γ-phase is seen at room temperature over this compositional range, with only the incommensurately ordered γ'-phase evident below around 450 °C. Changes in defect structure are used to explain an apparent transition in the compositional variation of lattice parameters. The HE dopant approach has no detrimental effect on ionic conductivity, with values comparable to those of the single substituted systems based on the component oxides

Local structure and conductivity behaviour in Bi7WO13.5

Total neutron scattering analysis reveals details of cation coordination and vacancy distribution in Bi7WO13.5.</p

Self-reduction of the native TiO2(110) surface during cooling after thermal annealing - in-operando investigations

We investigate the thermal reduction of TiO2 in ultra-high vacuum. Contrary to what is usually assumed, we observe that the maximal surface reduction occurs not during the heating, but during the cooling of the sample back to room temperature. We describe the self-reduction, which occurs as a result of differences in the energies of defect formation in the bulk and surface regions. The findings presented are based on X-ray photoelectron spectroscopy carried out in-operando during the heating and cooling steps. The presented conclusions, concerning the course of redox processes, are especially important when considering oxides for resistive switching and neuromorphic applications and also when describing the mechanisms related to the basics of operation of solid oxide fuel cells

Dopant clustering and vacancy ordering in neodymium doped ceria

Lanthanide doped cerias, show fast oxide ion conduction and have applications as electrolytes in intermediate temperature solid oxide fuel cells. Here, we examine the long- and short-range structures of Ce1−xNdxO2−x/2 (0.05 ≤ x ≤ 0.30, NDC) using reverse Monte Carlo modelling of total neutron scattering data, supported by measurements of electrical behaviour using a.c. impedance spectroscopy. Three distinct features are evident in the local structure of NDC, viz.: clustering of Nd3+ cations, preferred Nd3+-oxide ion vacancy association and oxide ion vacancy clustering with preferential alignment in the 〈100〉 direction. Interestingly, the presence of preferential dopant cation-oxide ion vacancy association is also observed at 600 °C, although diminished compared to the level at room temperature. This suggests a continued contribution of defect association enthalpy to activation energy at elevated temperatures and is reflected in similar compositional variation of high- and low-temperature activation energies

Analytical Approach to the Local Contact Potential Difference on (001) Ionic Surfaces: Implications for Kelvin Probe Force Microscopy

An analytical model of the electrostatic force between the tip of a non-contact Atomic Force Microscope (nc-AFM) and the (001) surface of an ionic crystal is reported. The model is able to account for the atomic contrast of the local contact potential difference (CPD) observed while nc-AFM-based Kelvin Probe Force Microscopy (KPFM) experiments. With the goal in mind to put in evidence this short-range electrostatic force, the Madelung potential arising at the surface of the ionic crystal is primarily derived. The expression of the force which is deduced can be split into two major contributions: the first stands for the coupling between the microscopic structure of the tip apex and the capacitor formed between the tip, the ionic crystal and the counter-electrode; the second term depicts the influence of the Madelung surface potential on the mesoscopic part of the tip, independently from its microscopic structure. These short-range electrostatic forces are in the range of ten pico-Newtons. When explicitly considering the crystal polarization, an analytical expression of the bias voltage to be applied on the tip to compensate for the local CPD, i.e. to cancel the short-range electrostatic force, is derived. The compensated CPD has the lateral periodicity of the Madelung surface potential. However, the strong dependence on the tip geometry, the applied modulation voltage as well as the tip-sample distance, which can even lead to an overestimation of the real surface potential, makes quantitative KPFM measurements of the local CPD extremely difficult

Origin of Polarization in Bismuth Sodium Titanate-Based Ceramics.

The classical view of the structural changes that occur at the ferroelectric transition in perovskite-structured systems, such as BaTiO3, is that polarization occurs due to the off-center displacement of the B-site cations. Here, we show that in the bismuth sodium titanate (BNT)-based composition 0.2(Ba0.4Sr0.6TiO3)-0.8(Bi0.5Na0.5TiO3), this model does not accurately describe the structural situation. Such BNT-based systems are of interest as lead-free alternatives to currently used materials in a variety of piezo-/ferroelectric applications. A combination of high-resolution powder neutron diffraction, impedance spectroscopy, and ab initio calculations reveals that Ti4+ contributes less than a third in magnitude to the overall polarization and that the displacements of the O2- ions and the A-site cations, particularly Bi3+, are very significant. The detailed examination of the ferroelectric transition in this system offers insights applicable to the understanding of such transitions in other ferroelectric perovskites, particularly those containing lone pair elements