Anion Deficiency Research Articles

In-SituEELS Analysis of Cobalt Valence in La1-xSrxCoO3-y

Abstract Transition and rare earth metal elements have played a vital role in synthesis of functional and smart materials because of their unique electronic structures and mixed valences. Experimental measurement of the valence states is thus important for understanding the mechanism which drives the functional properties. Lai1.xSrxCoO3-y(LSCO), for example, is an ionic conductor with potential applications in fuel cells and many other fields. In this type of materials, the anion deficiency is directly associated with the ratio of Co2+/Co3+ (or Co3+/Co4+) present in the specimen. It is known that the anion deficiency of the material depends on the operation temperature, but a direct measurement of anion deficiency is a challenge to existing microscopy techniques. In this paper, we present the in-situ analysis of Co valence in LSCO using electron energy-loss spectroscopy (EELS) in a transmission electron microscope. LSCO thin films (with x = 0.5) were grown on polished MgO(00l) substrate by liquid source metal-organic chemical vapor deposition (MOCVD) method [1].

The Humic Lake Acidification Experiment (HUMEX): Main physico-chemical results after five years of artificial acidification

The HUMEX-project is a whole catchment manipulation experiment where the effects of the addition of H 2SO 4 and NH 4NO 3 to a humic-rich lake, Lake Skjervatjern, and its catchment were studied. The lake was physically divided into an experimental lake (Basin A) and a control lake (Basin B). Two years after the division, Basin A and its catchment were artificially acidified. Hydrological data, meteorology, precipitation, and runoff chemistry collected during a 2-y preacidification period and during 5 y of acidification were evaluated. Randomized intervention analysis (RIA) was used to evaluate statistical significant differences between runoff chemistry from the two basins before and after the acidification. RIA showed significantly higher concentrations of SO 4 2−, H +, NH 4 +, NO 3 −, Al n+, Ca 2+, Mg 2+, total reactive Al (RAL), and labile Al (LAL) in Basin A after treatment compared with the control basin. After the treatment, the acid neutralizing capacity (ANC) in Basin A was significantly lower than in Basin B. However, the average ANC is substantially higher in the control basin after acidification compared with the two years before acidification, while unchanged in the manipulated catchment. The main reason for this is the long lasting effect of Na leakage after seasalt-episodes. No significant changes were observed regarding the amount of total organic carbon (TOC), water color, or UV-absorbency after the treatment, but the anion deficiency (A −) was significantly lower in the treated basin. This indicates that the organic acids are more protonated in the treated basin compared with the control basin. After a cold winter in 1993/94, an extreme NH 4 + increase was observed in runoff water from Basin A. This increase was accompanied by increases in water color and UV-absorbency, but without any increase in TOC.

The Renland ice core. A Northern Hemisphere record of aerosol composition over 120,000 years

The Renland ice core from East Greenland covers a full glacial cycle from the Holocene into the previous Eem interglacial. The first northern hemispheric record of both anions (Cl - , NO 3 - , SO 4 2- , CH 3 SO 3 - ), cations (Na + , NH 4 + , K + , Mg 2+ , Ca 2+ ), total amount of insoluble dust and acidity (measured as solid electrical conductivity) is extracted from this ice core in continuous profiles between 10 and 120 ka b.p. and from the 19th century. The ions measured are in balance during the interglacials but there is a large deficiency of anions during the glacial period. The ice is alkaline during most parts of the glacial period and Ca 2+ is the totally dominating ion therefore the deficiency of anions is probably due to carbonates (not measured). The concentration of most impurities is higher during glacial stages than the interglacials. The largest increase is found for crustally derived impurities (up to a factor of 10) while the sea salt elements are only slightly enhanced (less than a factor of 2). The concentration of impurities with a strong biogenic component is generally lower during glacial stages than the interglacials. A simple model tests the effect of changes in the physical conditions of the atmosphere influencing transport and deposition processes while keeping the source area and production rate constant. The model results show that the increased glacial concentrations observed for some impurities can be explained entirely by changes in transport and deposition, while the decrease in others may indicate a change in source emissions. The total content of impurities in the glacial atmosphere is higher if physical changes in the atmosphere rather than source emission changes are responsible for the variations observed in the Renland ice core. Thus, physical changes in the atmosphere can have a large impact on the radiative properties of the atmosphere and a climate forcing mechanism may be found in the dynamics of the atmosphere. However, a better understanding of the hydrological cycle and the general circulation of the atmosphere during glacial conditions has to be attained to be able to interpret influences on the global climate of a changed composition of the atmosphere from ice core data. DOI: 10.1034/j.1600-0889.1994.t01-4-00005.x

A Single-Crystal X-Ray Diffraction Study of Lithium Zirconate, Li 6Zr 2O 7, a Solid-State Ionic Conductor

The structure of lithium zirconate (Li 6Zr 2O 7) has been solved by the single-crystal X-ray method. Li 6Zr 2O 7 is monoclinic, space group C2/ c and cell parameters a = 10.440(4), b = 5.991(1), c = 10.204(2) Å, β = 100.25(3)°, Z = 8 (for the asymmetric unit Li 3ZrO 7/2). A least-squares refinement gave conventional and weighted R factors of 0.041 and 0.052, respectively. The structure is of the NaCl type and is characterized by an ordered anionic deficiency. As a consequence the coordination number of the Li atoms is 5, with the coordination geometry of a distorted square pyramid. Also, the oxygen octahedron around each Zr atom is significantly distorted. A comparison between Li 6Zr 2O 7 and Li 2ZrO 3, another phase of the NaCl type, shows that the structure of the former compound is more "open" than that of Li 2ZrO 3 and, for this reason, a higher mobility of both the Li + and O 2- ions can be expected for the former phase. These results are in agreement with conductivity measurements.

The Pentagonal Column and the ReO3‐Type Structure

AbstractThe pentagonal column (PC), consisting of MX7 bipyramids sharing their equatorial edges with five MX6 octahedra, is a structural feature frequently occurring in association with frameworks of the ReO3‐type, where it enables accomodation of an anion deficiency. The present article discusses the geometry of the PC and its compatibility with the ReO3‐type structure, which naturally emerges from the finding that a PC may result from twinning of an ReO3‐type structure on the unit cell level. A new symbolism is presented which facilitates visualization of a large variety of crystal and defect structures of MX3–x stoichiometry. This symbolism may also serve as a convenient aid in deriving models of possible new structures that may be of value in the interpretation of HREM images.

Anion deficiency in strontium titanate

FRANCO and REGI's1 diffraction patterns and lattice images of anion-deficient strontium titanate, SrTiO3−x, show a sixfold superlattice along [111] and associated lattice images with irregular fringe spacings. The possibility of anion vacancy ordering was induced with reference to the systems (Ba,Sr) MnO3–x (ref. 2) and BaFeO3–x (ref. 3). These results contrasted with earlier data on SrTiO3–x (0⩽x⩽0.5) which indicated4,5 on the basis of powder X-ray diffraction, a simple cubic cell with virtually unchanging lattice constant over the whole range of x. Here we draw attention to two points concerning the nonstoichiometry of strontium titanate. First, unlike the manganate and ferrate of refs 2 and 3, SrTiO3 is a simple cubic perovskite with cubic stacking of SrO3 layers. Second, in contrast to the impression given, oxygen vacancy ordering had not been convincingly established at all until recently in any anion-deficient cubic-stacked perovskite. Examples of such nonstoichiometric systems are SrTiO2.5–3.0 (refs 4, 5), SrVO2.5–3.0 (ref. 4), SrFeO2.78–3.0 (ref. 6) and Sr2LiMoO5.5 (ref. 7). I started to study the three strontium-transition metal oxide systems in the expectation that application of the more powerful diffraction techniques such as electron diffraction would reveal anion ordering processes possibly overlooked by X rays. This proved to be the case for the SrFeO3–x system where an ordered phase, SrFeO2.75, with superlattice reflections clearly apparent by electron diffraction, but not by powder X-ray diffraction, was found8.

Anion deficiency in strontium titanate

STUDIES of the anion deficient phases of titanium and vanadium perovskites showed that the perovskite SrTiO3−x has a wide range of composition extending from x = 0 to x = 0.5 (ref. 1). The nonstoichiometry was inferred from the monophasic behaviour of the solid within this compositional range as obtained by means of powder X-ray diffraction. The same cubic structure was found for both Sr Ti O2.5 and Sr Ti O3. Perovskites have been widely studied recently in view of the very interesting and useful properties of these oxides2. The physiochemical properties of Sr Ti O3 include semiconductivity3,4 and superconductivity5 as well as peculiar phase transitions6. Of particular interest in this, and other7, systems is the presence of point defects and their influence on those properties. It seems that, at small degrees of reduction, doubly ionised oxygen vacancies are the main defects present on Sr Ti O3−x (ref. 8). Although in that case, the oxygen vacancies, being at small numbers, are probably distributed at random, the presence of so many empty oxygen positions as the stoichiometry of SrTiO2.5 implies—one in every six oxygens missing—automatically suggests the possibility of ordering. This possibility seems to be realised under the synthesis conditions that we have used and we report here some of the results so far obtained in the system SrO–Ti2O3–TiO2.

Electrical resistivity of Gd and La monochalcogenides as a function of stoichiometry

The electrical resistivity of GdS, GdSe, GdTe, LaS, LaSe and LaTe single crystals has been measured between 4.2 K and 300 K for various stoichiometries. The diamagnetic La chalcogenides show normal metallic resistivities with d ϱ/d T being positive for metal excess or deficiency. The antiferromagnetic Gd chalcogenides exhibit a peak in the resistivity at T N when being stoichiometric. Sufficient anion deficiency results in d ϱ/d T being positive over the whole investigated temperature interval and sufficient metal deficiency leads to a steady increase of the resistivity below T N. For this anomalous behavior a model based on the exchange interaction between conduction electrons and a defect induced ferromagnetic cluster is proposed.

Structural theory for non-stoicheiometry. Part I. Defect fluorite-type structures: lanthanoid oxides MOxwith 1·7 ⩽x⩽ 2·0

The concept of an octahedrally co-ordinated anion vacancy □O6 is introduced to develop a rational structural theory for the known series of ordered non-stoicheiometric lanthanoid oxides of formula MnO2n–2(n= 7–∞). The resulting co-ordination defect (c.d.) has the composition M7//2□O, and it is suggested that this retains its structural integrity in all the homologues. Topological analysis has been employed to establish the fundamental superstructures of the fluorite lattice which characterize these oxide phases. It emerges that the anion deficiency is accommodated by point defects gathered in oblique {213} planes each being separated by (n– 1) oxide-intact {213} planes. Effectively, all these phases are based on regularly spaced intergrowths of the progenitors ι-Pr7O12 and PrO2. It is shown that the homologues are structurally coherent and that irregularity in spacing of intergrown phases provides a logical description for the grossly non-stoicheiometric α-phase which overlays the region MO1·7–MO2·0 at higher temperatures. The extended {213} sheets of defects can be circumscribed coherently by PrO2 to generate finite discs of Pr7//2□O6. Order–disorder transformations are considered in terms of the interchangeability of Pr7//2□O6 and Pr7//2□O7 structural sub-units which are topologically identical. The non-existence of the homologue Pr8O14 and a possible structure for the β′-Pr12O22 phase are examined in terms of the cubic pyrochlore (E81) structure.

Electron microscopy of solid solutions and crystallographic shear structures. II. Fe 2O 3TiO 2 and Ga 2O 3TiO 2 systems

Replacement of Ti 4+ in TiO 2 by Fe 3+ or Ga 3+ gives a limited range of solid solutions with anion-deficient rutile structure. Anion deficiency is accommodated by eliminating a sheet of O 2− sites by a new type of planar boundary, on 〈210〉 (rutile), with the extinction characteristics of an α boundary and apparent displacement vector 1 2 [0, 1, 0] . Structural considerations suggest, however, that this is neither a true α boundary nor a crystallographic shear plane as hitherto defined. In Fe 2O 3TiO 2 solid solutions, rutile with isolated 〈210〉 boundaries coexists with Fe 2TiO 5. In Ga 2O 3TiO 2, parallel recurrent [210] boundaries generate a new homologous series of phases Ga 4Ti n−4 O 2 n−2 (15 < n < 23, n odd) which were characterized in an imperfectly ordered state by electron diffraction and direct lattice imaging.

The system TiO 2Cr 2O 3: Electron microscopy of solid solutions and crystallographic shear structures

The TiO 2-rich end of the system TiO 2CrO 1.5 has been studied by electron diffraction and electron microscopy, to examine the effects of controlled anion deficiency upon rutile-based structures. Materials equilibrated at 1330 K to 1570 K fall into three ranges. With > 15 mol % CrO 1.5, compounds of the known series Cr 2Ti n−2 O 2 n−1 , with (121) crystallographic shear planes are formed. With < 5 mol % CrO 1.5, rutile structure-defect solid solutions are found, with no shear planes. It appears that the anion deficiency is accommodated by point defects or small defect clusters. The limiting composition of this solid-solution range is strongly temperature dependent, and the rutile solid solutions are readily reduced, with the introduction of shear planes on (132), as for TiO 2 itself. Between these limiting ranges, well-ordered shear phases are formed, in which the slabs of rutile structure (the normal spacing between recurrent shear planes) remain constant but the orientation of the shear planes pivots round progressively from (121) to (132) as the anion : cation ratio is changed.

Oxidation kinetics of solid solutions of urania-yttria

Solid solutions of uranium oxide and yttrium oxide react with oxygen changing the average valence of the uranium. The amount and rate of oxygen uptake varies with the yttria content. The uptake kinetics are influenced by the type of structure of the solid solution and the degree of anion deficiency in the lattice. A detailed oxidation study of the 50 mole percent Y 2O 3 composition from −196° C to 110° C revealed that the chemisorption characteristics were similar to that of UO 2 except the uptake beyond the first layer occurred at a much lower temperature. Oxidation beyond the first layer occurred readily above −43° C and the kinetics followed a one-fourth power time dependence if the intitial oxygen to uranium ratio was 2.27. If the average oxygen to uranium ratio was 2.58 initially, then the oxidation kinetics were parabolic with time but the rate was decreased by two orders of magnitude. Possible mechanisms compatible with the data are proposed.

Crystal Chemistry of Pyrochlore

Analyses of natural minerals suggest the wide variety of both cationic and anionic substitutions possible in the pyrochlore structure. Limits on the possibilities of ionic substitution in pyrochlore have been determined by the preparation of a large number of compounds. An important feature found is that substitution of up to two of seven anions is possible but apparently no more. Many ion-deficient structures can be made, especially those with one large cation absent. Anion deficiency to the same extent or even in smaller amounts leads to the formation of perovskite structures. Quantitative data on solid solubility among three related structures, pyrochlore, perovskite, and columbite, have been determined for the case of Cd2Nb2O7-NaNbO3-CdNb2O6. An attempt has been made to study the relation of pyrochlore and C-type rare earth to the fluorite structure as an example of the ordering of defects.

Electrical Conductivity of Single Crystals of MgO

The electrical conductivity of magnesium oxide at temperatures in the region of 1300°C is observed to depend upon the partial pressure of oxygen surrounding the sample. The conductivity increases at oxygen pressures both higher and lower than 10—5 atmospheres. At this pressure the conductivity is a minimum. This effect is increased as the iron content is increased and is almost absent in the purest samples. The conductivity is electronic rather than ionic and the number of charge carriers is controlled by the number of lattice vacancies. The dependence of conductivity on oxygen pressure may be satisfactorily explained by changes in stoichiometry and thus lattice defects in magnesium oxide. These changes in stoichiometry are larger when the magnesium oxide is contaminated with a variable valence impurity like iron than when it is pure. If an explanation based on iron changing valance state is accepted, then it may be demonstrated by an analysis of the chemical equilibria involved that anion deficiencies must appear at low oxygen pressures, cation deficiences at high oxygen pressures, and that all defects must lie close to the variable valence impurity ions.