A detailed analysis of defect mediated changes on structural, optical and. Pdf an electron and xray powder diffraction study of. Gradual structural evolution from pyrochlore to defect. Many compounds, notably the common mineral fluorite caf 2, adopt this structure. For the degree of doctor of philosophy september 2009 school of biological and chemical sciences queen mary, university of london united kingdom. Above 15 gpa, a highpressure hp phase forms that has a distorted defect fluoritestructure of lower sym metry. While the diffraction data indicate a clear phase transition from ordered pyrochlore to disordered defect fluorite at x. The addition of nb stabilizes this phase and all of the materials we examine will have this structure. The relationship to the fluorite structure is as follows. Theoretical technique to reconcile inconsistencies between crystallographic. Where an electron is supposed to be in an atoms orbit, the electron has been disturbed by.
The parameter for a completely disordered defectx fluorite structure is 0. Fast ion conductivity in strained defectfluorite structure created by. Electrical and thermal conductivities of congruently melting single crystals of isovalent m 1. Defects and transport in fluorite related materials. Abstract fluorite deposits occur in a variety of geologic environments throughout the globe. S1 a defectfluorite or disordered pyrochlore structure with a corresponding formula of abo 3. Fluorite structured ceo 2 due to increased oxygen vacancies displays better optical and ferromagnetic properties 19,20. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon the mohs scale of mineral hardness, based on scratch hardness comparison, defines value 4 as fluorite pure fluorite is transparent, both in visible. Defect structure in yttrium, niobium and lead substituted. Given that there is a single cation site in defect fluorite, the. Electrical and thermal conductivities of congruently. Defect equilibria in fluorite structure crystals aip publishing. Firstprinciples study on cationantisite defects of. Emergence of defect fluorite structure in nanosized thoria through doping with some divalent transition metal ions.
In this master thesis, defects and transport properties in fluorite related materials ln2ce2o7 lnnd, gd were investigated by electrical measurements and thermogravimetry. This makes the materials more suitable for biomedical and photocatalytic applications with enhanced functionality 17,21,22. Crystal growth and defect crystal structure of cdf2. Rajni magotra, stanzin namga, pawan singh, neha arora, p. All the samples share a pure defectfluorite phase before irradiation, and such structure is wellretained after irradiation.
Highpressure structural changes in the gd2zr2o7 pyrochlore. Crystal structure and defect property predictions in ceramic materials. Cubes, octahedra, rarely dodecahedra, or combinations, with many other forms. In solid state chemistry, the fluorite structure refers to a common motif for compounds with the formula mx 2.
Read defect fluorite structures in the birich part of the system bi 2 o 3 re 2 o 7, solid state ionics on deepdyve, the largest online rental service for scholarly research with thousands of academic publications available at your fingertips. Defect structure, ionic conductivity, and diffusion in. Fluorite crystalline structures understanding defect impact. Effects of stoichiometry on the defect clustering in. Ordered arrays of defect aggregates in the 111 planes of natural. Figure 1 illustrates the differences between the pyrochlore and defectfluorite structures. Pyrochlore a2b2o7 compounds are related to the fluorite structure and the nature of. An electron and xray powder diffraction study of the defect fluorite structure of mnsub 0. This orderdisorder transition from the pyrochlore to the defect fluorite structure is a very rare example of simultaneous disordering of both anions and cations 9. Calcium fluoride the aim of this chapter is to make the reader familiar with the ionic crystal calcium fluoride, and to emphasize some specific features of this material that are important in the context of this work. View the article pdf and any associated supplements and figures for a period of 48 hours. For a given rare earth the lattice parameters of compounds prepared with either niobia or tantala are substantially the same. Pdf intricate disorder in defect fluoritepyrochlore.
In the first part, the geometrical and electronic structure of the pure material is introduced. Magnussonite, manganese arsenite, a fluorite derivative structure peul b. Defect structure, ionic conductivity, and diffusion in yttria stabilized zirconia and related oxide electrolytes with fluorite structure to cite this article. Gradual structural evolution from pyrochlore to defectfluorite in. New defectcrystalchemical approach to nonvegardianity.
Lab 1 the defect structure of cubic bi2o3nb2o5 solid. A previously described solid solution in the system bi 2 o 3nb 2 o 5 based on the. The structure obtained from our predictive method presented here shows excellent agreement with the experimental structure. In nonionic crystals it refers to lattice valency defects in ionic crystals, this type of point defect forms when oppositely charged ions leave their lattice sites, creating vacancies. The structure and ionconducting properties of the defectfluorite ring structure formed around amorphous iontracks by swift heavy ion. It is an empty space in the electron structure of a fluorite molefluorite, primorskiy kray, russia b.
Lanthanum and neodymium oxides form compounds with the orthorhombic structure, whereas with samarium, gadolinium, dysprosium, yttrium, and scandium oxides the compounds are cubic. These are identified as a cubic defect fluorite or pyrochlore. Pyrochlore, an ordered derivative of the defect fluorite structure, shows complex disordering behavior as a function of composition, temperature, pressure, and radiation damage. Defectfluorite gd zr o ceramics under helium irradiation.
Cerium also forms ceriumiii oxide, ce 2 o 3, which is unstable and will oxidize to ceriumiv oxide. The structure and ionconducting properties of the defectfluorite ring structure formed around amorphous iontracks by swift heavy ion irradiation of. Effects of stoichiometry on the defect clustering in uranium dioxide. Previously, different defect structures were observed for ba0. Through these studies, the intermediatepyrochlore pbased local structure nature of the socalled stabilizedzirconia sz and stabilizedhafnia shtype defectfluorite df. Ca, sr, cd, pb in relation to their defect fluorite structure. A dissertation submitted to the university of london for the degree of doctor of. Fluorite caf2 c 20012005 mineral data publishing, version 1 crystal data. Britt collection fluorite, smith county, tennessee b. Formation of defectfluorite structured ndnioxhy epitaxial. F i and fluoride vacancy v f and between paired f 1 trivalent cation and. Read new defectcrystalchemical approach to nonvegardianity and complex defect structure of fluoritebased mo 2 lno 1. The structure and ionconducting properties of the defectfluorite ring structure formed around amorphous iontracks by swift heavy ion irradiation of gd 2 ti 2 o 7 pyrochlore are investigated. Srivastava economic geology research group, department of geology, university of jammu, jammu, india.
The x ions occupy the eight tetrahedral interstitial sites whereas m ions occupy the regular sites of an fcc structure. We have found that this compound adopts a disordered defect fluorite structure with diffuse features evident in the neutrondiffraction pattern. Further, the coordination numbers of the cations in both the defectfluorite and pyrochlore structures are. While the diffraction data indicate a clear phase transition from ordered pyrochlore to disordered defectfluorite at x. High angle annular dark field imaging complemented with iontrack molecular dynamics simulations show that the atoms in the ring structure are disordered, and have relatively larger. There exists a fairly extensive crystal chemistry based on the ordered, defect fluorite structure of stoichiometry ln 3 mo 7, where ln is a rare earth and m is a pentavalent metal ion, usually from the 4d or 5d transition series, but the phase is also known for msb. Fluorite also called fluorspar is the mineral form of calcium fluoride, caf 2. Structure and magnetism in pr3reo7 and nd3reo7materials. These vacancies are formed in stoichiometric units, to maintain an overall neutral charge in the ionic solid. A schottky defect is a type of point defect in a crystal lattice named after walter h. The dissolution of nb 2 o 5 occurs at elevated temperatures and under these conditions, the cubic fluoritelike. Materials possessing a defectfluorite pyrochlore structure can have a range of useful properties that are sought after, which include their radiation tolerance, nuclear waste immobilization, and phase stability at elevated temperatures. Ceriumiv oxide is formed by the calcination of cerium oxalate or cerium hydroxide.
Defect mediated changes in structural, optical and. Experimental methods epitaxial ndnio 3 thin films were deposited as precursors onto srtio 3 sto 100 substrates shinkosha co. Four distinct phases have been found, all of which are based upon defect fluorite structures, and the structural principles of the phase with the greatest compositional range have been. This orderdisorder transition from the pyrochlore to the defect fluorite structure is a very rare example of simultaneous disordering of both anions and cations9. With the objective of incorporating some divalent transition.
Eels spectrum of cubic zro 2, the archetype of the defect fluorite structure, clearly displays a single peak at 2234 ev in the l 3 edge green. The properties of the materials are affected by defects e. Moreover, the growth orientation of the defectfluorite phase changed as a function of the reaction temperature with cah 2. The number and type of defects depend on several factors e. Theoretical technique to reconcile inconsistencies between. Crystal structure and defect property predictions in. Bi 2 o 3 structure has been reinvestigated by electron diffraction and high resolution electron microscopy.