High-Pressure Structural and Vibrational Study of PbZr0.40Ti0.60O3

Synchrotron PXRD deconvolutes nickel particle and support changes in Ni/ZrO2 methanation catalysts

Operando synchrotron powder X-ray diffraction deconvolutes support and metal nanoparticle changes during thermal deactivation of Ni/ZrO2 methanation catalysts.

Thermodynamic analyses of the orthorhombic-to-tetragonal phase transition in Pr2-xNdxNiO4+δ under controlled oxygen partial pressures

The behavior of the structural orthorhombic-tetragonal phase transition of Pr_2-xNd_xNiO_4+δ, a candidate material for solid oxide fuel cells and oxygen permeation membranes, was investigated by differential scanning calorimetry and thermogravimetry (TG), under controlled oxygen partial pressures, P(O₂). The structural phase transition temperature, T_P, of Pr_2-xNd_xNiO_4+δ increased with increasing Nd content, x, or increasing P(O₂). The phase transitions of all compositions involved discrete variations in the oxygen content, Δδ, which were observed in the TG curves under various P(O₂) values. Δδ of Pr_2-xNd_xNiO_4+δ with 0.5 ≤x≤ 1.5 were between those of Nd₂NiO_4+δ and Pr₂NiO_4+δ, regardless of P(O₂), and were slightly increased with decreasing P(O₂). It was proposed that the effect of the valence change of the Pr ion on Δδ was decreased with increasing Nd content. The standard enthalpy change, ΔH°, and entropy change, ΔS°, of the phase transition were estimated from the Ellingham diagrams and van't Hoff plots, which were prepared from the relationship between P(O₂) and T_P using an ideal solution model. ΔS° was decreased with increasing Nd content for the specimens with 0.0 ≤x≤ 1.5. The ΔH° of Pr_2-xNd_xNiO_4+δ with 0.0 ≤x≤ 1.5 was almost constant for all Nd contents. The increase in the phase transition temperature of Pr_2-xNd_xNiO_4+δ with increasing x from 0.0 to 1.5 was successfully explained using the calculated values of ΔH° and ΔS°.

Structural dynamics in Ni–Fe catalysts during CO2 methanation – role of iron oxide clusters

Operando XAS coupled with MES supported by DFT unravel the highly dynamic nature of Ni–Fe catalysts during CO₂ methanation and beneficial formation of iron oxide clusters.

Mixing thermodynamics and electronic structure of the Pt1−xNix (0 ≤ x ≤ 1) bimetallic alloy

Density functional theory simulations complemented by force-field based calculations show that the bimetallic Pt_0.5Ni_0.5 equilibrium composition is highly ordered at room conditions.

A survey of the structural models proposed for PbZr1−x Ti x O3 using mode analysis

The numerous structures that have been reported for the different phases of the lead zirconate titanate system, PbZr1−x Ti x O3 (PZT), are analysed by means of a systematic symmetry-mode analysis. The distortion corresponding to the order parameter has been separated out and expressed in all phases in a comparable form. The fact that the physical origin of the PZT phases is an unstable threefold degenerate polar mode, plus in some cases an unstable octahedral tilting mode, produces structural correlations between the different phases. These correlations had remained unnoticed until now but are directly observable in a mode parameterization. They can be used both to characterize the evolution of the order parameters through the phase diagram and as a stringent test of the reported structural models. It is further shown that the activity of a single polar mode yields a specific feature in the mode decomposition of the monoclinic phases. This single-mode signature can be observed in the majority of the monoclinic structures proposed, making the others questionable. In fact, this internal constraint is satisfied by PZT to such a high degree that it drastically reduces the number of effective structural degrees of freedom. It is conjectured that this type of structural constraint beyond space-group symmetry can be a rather general property of low-symmetry distorted structures. As shown here, its existence can be detected and assessed by a symmetry-mode analysis, if considered in relation to the single underlying multidimensional order parameter.

Local ordering in lead-based relaxor ferroelectrics

Lead-based ferroelectric materials are both well-studied and widely used and have a wide range of applications from ultrasonics to energy harvesting and beyond. However, the use of Pb-containing materials is environmentally undesirable, due to the toxicity of lead. This is particularly highlighted by the disposal of Pb-based devices when their lifespan is through. Because of this large drawback, chemists have been searching for Pb-free ferroic materials that can replace PZN (PbZn1/3Nb2/3O3), PMN (PbMg1/3Nb2/3O3), PZT (PbZr1-xTixO3), and all their derivatives. Underlying much of materials chemistry is the idea that function arises from structure, so if we can determine the structure of a material, we can understand how its useful properties arise. This understanding can then lead to the tuning of these properties and the development of new materials. However, the question arises: What is meant by structure? Conventionally, structure is determined by X-ray or neutron diffraction, in which the Bragg peak intensities are measured and a unit cell is determined. In many materials, local ordering, order that persists only for few unit cells or nanometers, is important in determining the physical properties. This is very much the case in the relaxor ferroelectrics, an important class of functional oxides. Indeed, disorder, randomness, and short-range order (SRO) are all invoked to help explain many of the key properties. The local order in Pb-based ferroelectrics has been extensively studied, with the most definitive probe being single-crystal diffuse scattering. In this Account, I outline the current debate on the nature of the local order and explore how this information can inform the search for lead-free materials. Local order, as distinct from the overall average order revealed by conventional techniques, relates more closely to the crystal chemistry of the individual ions and so appears to give a better insight into how the crystal chemistry leads to the ferroelectric properties.