Max Avdeev

Does local disorder occur in the pyrochlore zirconates

The zirconates Ln(2)Zr(2)O(7) (Ln = lanthanoid) have been studied using a combination of Zr L-edge X-ray absorption near edge structure (XANES) and synchrotron X-ray and neutron powder diffraction methods. These studies demonstrate that as the size of the lanthanoid cation decreases, the local structure evolves smoothly from the ideal pyrochlore toward the defect fluorite rather than undergoing an abrupt transformation. The Zr L-edge spectrum is found to be extremely sensitive to changes in the local coordination environment and demonstrates an increase in local disorder across the pyrochlore oxides. The sensitivity of the XANES measurements enables us to identify the progressive nature of the transition that could not be detected using bulk diffraction techniques.

The Structure of C-type Gd2O3. A Powder Neutron Diffraction Study using Enriched 160Gd

The structure of the cubic C-type phase of Gd2O3 has been refined using high-resolution powder neutron diffraction data. The sample was enriched in 160Gd to avoid the high neutron absorption of naturally occurring Gd. The refined structure is in excellent agreement with that estimated using perturbed angular correlation spectroscopy.

Anion Disorder in Lanthanoid Zirconates Gd2–xTbxZr2O7

The pyrochlore-defect fluorite order-disorder transition has been studied for a series of oxides of the type Gd(2-x)Tb(x)Zr2O7 by a combination of diffraction and spectroscopy techniques. Synchrotron X-ray diffraction data suggest an abrupt transition from the coexistence of pyrochlore and defect fluorite phases to a single defect fluorite phase with increasing Tb content. However neutron diffraction data, obtained at λ ≈ 0.497 Å for all Gd-containing samples to minimize absorption, not only provide evidence for independent ordering of the anion and cation sublattices but also suggest that the disorder transition across the pyrochlore-defect fluorite boundary of Ln2Zr2O7 is rather gradual. Such disorder was also evident in X-ray absorption measurements at the Zr L3-edge, which showed a gradual increase in the effective coordination number of the Zr from near 6-coordinate in the pyrochlore rich samples to near 7-coordinate in the Tb rich defect fluorites. These results indicate the presence of ordered domains throughout the defect fluorite region, and demonstrate the gradual nature of the order-disorder transition across the Gd(2-x)Tb(x)Zr2O7 series.

Perovskites in low dimensional multi-layer structure types

This study introduces examples of structure property relationships within the multi-layered Sillen-Aurivillius family (shown in Figure) and aims to investigate the effect of chemical doping and lattice matching effects. The first example involves doping 1/3 of the n = 3 ferroelectric perovskite layers with magnetic transition metal cations in Bi5PbTi3O14Cl [1] with charge balancing by removing Pb2+ for Bi3+. A statistical 1:2 distribution of M3+ and Ti4+ across all three perovskite layers was found in Bi6Ti2MO14Cl, M = Cr3+, Mn3+, Fe3+, resulting in highly strained structures (enhancing the ferroelectricity compared to Bi5PbTi3O14Cl) and pronounced spin-glass behavior below Tirr(0) = 4.46 K. Ferroelectric transitions were observed at high temperature for each of the new compounds. Ferroelectric properties were also measured on Bi6Ti2FeO14Cl using piezoresponse force microscopy showing hysteretic phase behavior. A new n = 2 Sillen-Aurivillius compound Bi3Sr2Nb2O11Br, based on Bi3Pb2Nb2O11Cl [2], was synthesized by simultaneously replacing Pb2+ with Sr2+ and Cl− with Br−. Inter-layer mismatch prevented the formation of Bi3Sr2Nb2O11Cl and Bi3Pb2Nb2O11Br. Sr2+ doping reduces the impact of the stereochemically active 6s2 lone pair found on Pb2+ and Bi3+, resulting in a stacking contraction in the lattice parameters by 1.22 % and an expansion of the a-b plane by 0.25 %, improving inter-layer compatibility with Br−. X-ray Absorption Near Edge Structure spectra analysis shows that the ferroelectric distortion of the B-site cation is less apparent in Bi3Sr2Nb2O11Br compared to Bi3Pb2Nb2O11Cl. Variable-temperature neutron diffraction data show no evidence for a ferroelectric distortion.

Neutron diffraction studies of the ferroelectric phase of CdTiO3

Temperatured-programmed X-ray diffraction technology is employed to study phase transitions of modified ZSM-5 catalysts for C4-olefin cracking reactions to produce propylene. The crystal phase transitions of the fresh, used and regenerated catalysts are investigated respectively with temperature increasing and decreasing under vacuum and air conditions. The samples were prepared with following steps. HZSM-5 zeolite and Al2O3 (as binder) were fully mixed, kneaded and then molded by extruder. Elements of alkaline-earth metals and phosphorus were introduced by impregnation. After drying and calcination, the fresh ZSM-5-based catalyst was obtained. The catalyst for C4 cracking was evaluated in a fixed-bed reactor. The deactivated catalyst was regenerated through combustion with the mixture of air and nitrogen on line. The crystal structures of the samples were recorded by X-ray powder diffraction analysis on Bruker AXS D8 Advance SSS Xray diffractometer equipped with a graphite monochromator and scinitillation counter, and using CuKα radiation (40KV and 300mA). Anton Parr XRK 900 reaction chamber was equipped, which was used to heat samples from room temperature to 900°C and provide certain experimental conditions. The research results show the crystal phases of all these catalyst are changed into orthorhombic structure when the temperature is increased to a certain degree no matter what kind of their initial crystal structure (monoclinic or orthorhombic)(shown in fig.1). The experimental conditions whether under vacuum or air prove further the water has nothing with phase transitions of these catalysts.