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Lattice defects in oxides. Correlations between defects, properties and crystal structures

Lattice defects in oxides. Correlations between defects, properties and crystal structures. Binary oxides (ZnO). Ternary oxides (ZnAl 2 O 4 ). Defects in oxides. Lattice or point defects: Vacancies (oxygen, cation) Interstitial ions

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Lattice defects in oxides. Correlations between defects, properties and crystal structures

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  1. Lattice defects in oxides. Correlations between defects, properties and crystal structures

  2. Binary oxides (ZnO) Ternary oxides (ZnAl2O4) Defects in oxides • Lattice or point defects: • Vacancies (oxygen, cation) • Interstitial ions • Foreign atoms at regular sites (doping/solid solutions) • Defect pairs and clusters • Electronic defects (free electrons and holes) • Extended defects: • Crystallographic shear • Dislocations • Grain boundaries Defect chemistry. Lattice defect can be treated as chemical entities (energy of formation) using defect reactions (mass-action law, equilibrium constant). Kroger-Vink notation for defects. Defect charge is referred to the perfect crystal.

  3. Frenkel disorder 1eV = 96.5 kJ/mol Defects in oxides • Rules for defect reactions: • Site relation. The number of sites must be in the correct proportion (MaXb: M/X = a/b). • Sites can be created or destroyed taking into account the site relation. • Mass balance. • Electroneutrality condition. Schottky disorder

  4. Thermodynamic probability for distinguishable particles N0: total number of atoms ni: number of atoms on the i-th energy state - - - - Defects and entropy N atoms arranged at (N+n) sites with n vacancies

  5. Fe1-yO Defects and nonstoichiometry in binary oxides Oxygen nonstoichiometry (TiO2, CeO2, Nb2O5, V2O5) Metal nonstoichiometry (FeO, NiO, MnO)

  6. CeO2-x Defect ordering Formation of subphases Defect complexes Isolated defects Defects and nonstoichiometry in binary oxides Equilibrium constant Electroneutrality n = 15-18 x: fraction of vacant sites in CeO2-x n = 4 -GO2 (kcal/mol) In general: n = 5 n = 6: doubly ionized vacancies n = 4: singly ionized vacancies n = 2: neutral vacancies n ≤6 for isolated defects or defect complexes - log x in CeO2-x

  7. Defects and nonstoichiometry in binary oxides. Formation of shear planes Ordering of defects and formation of superstructures is observed for large deviations from stoichiometry (TiO2-δ, Nb2O5-δ, WO3-δ, ReO3-δ, etc.). Elimination of oxygen vacancies by formation of metal-rich shear planes is a common mechanisms (crystallographic shear). Formation of shear planes in ReO3 (left) and WO3 (right) by elimination of oxygen vacancies

  8. Partial Schottky disorder (TiO2–rich side) Partial Schottky disorder (BaO-rich side) Full Schottky disorder Oxygen nonstoichiometry 1eV = 96.5 kJ/mol Lattice defects and nonstoichiometry in perovskites BaTiO3

  9. Lattice defects and nonstoichiometry in perovskites BaTiO3 Ba-rich Ti-rich

  10. 1200°C Ti-rich 1320°C Ba-rich Lattice defects, nonstoichiometry and phase transitions in perovskites Cubic (paraelectric) – tetragonal (ferroelectric) phase transition in BaTiO3 Enthalpy of transition Transition temperature Ba-rich Ti-rich

  11. (1) At low p(O2) << p0(O2) (1)m = -1/6 BaTiO3 (2)m = -1/4 n-type p-type (3) At high p(O2) p(O2) > p0(O2) p0(O2) n = p Lattice defects and electrical conductivity in perovskites Z: numero di cariche; e: carica dell’elettrone; : mobilità c: concentrazione Electrical conductivity (2a) At intermediate p(O2) and R = Ba/Ti < 1 (2b) At intermediate p(O2) with R = 1 and acceptor impurities Reduction (1): H2-3 eV Oxidation (3): H1 eV

  12. Doping of perovskites: controlling defect nature and concentration Acceptor doping: the substitutional impurity has a lower charge than the regular and bring less oxygen into the lattice Donor doping: the substitutional impurity has a higher charge than the regular and bring more oxygen into the lattice

  13. La:BaTiO3 1200°C p0(O2) Doping of perovskites: influence of doping on electron conductivity • Donor doped compounds: • Black colour; • Good conductivity (>10-2 S/cm) even at RT; • Some show metallic conduction (103 S/cm, La:SrTiO3);

  14. p0(O2) Doping of perovskites: influence of doping on electron conductivity • Acceptor doped compounds • Light colour; • Good conductivity at high temperature • Many are insulators at RT; • Can be fired in reduced atmosphere retaining their dielectric properties.

  15. ABO3 A2B2O5 A4B4O11 Sr2Fe2O5 Doping of perovskites: from isolated defect to oxygen vacancy ordering and formation of layered structures Due to the high dielectric constant (20-1000) and structural stability, perovskites can accomodate a large concentration of foreign aliovalent impurities (good solvent) and related charge compensating defects (cation or oxygen vacancies). The simple model of randomly distributed isolated defects (no association) holds up to high dopant concentration (few at.% for acceptors, 10 at.% for donors). At higher dopant concentration, ordering of defects, formation of shear planes and layered structures is observed. x = 0: SrTiO3; x = 1: Sr2Fe2O5 At T < 700°C, oxygen vacancy ordering occurs in Sr2Fe2O5 (brownmillerite structure). For intermediate compositions, intergrowth of perovskite blocks and brownmillerite layer with general formula AnBnO3n-1. Perovskites doped with high concentration of acceptor impurities (10-20 at.%) shows high ionic (oxygen) and electronic (holes related to transition metals Ti, Fe, Co, Nb) conductivity. Application as mixed conductors in electrochemical devices.

  16. At low p(O2): n-type Fe2+ p-type p-type Fe4+ At high p(O2) n-type Doping of perovskites: from isolated defect to oxygen vacancy ordering and formation of layered structures La1-xSrxFeO3 with 0 < x <0.25 can be accurately described as an acceptor-doped perovskite with randomly distributed defects

  17. cubic distorted orthorhombic SrTiO3 La2Ti2O7 Doping of perovskites: excess oxygen, shear planes and layered structures Reducing atmosphere, black conducting ceramics (up to 103 Scm-1) , random distribution of defects, x up to 0.3 Oxidizing atmosphere, less conducting, O excess accomodated by formation of shear planes when x > 0.17, by small isolated defects when x<0.17 x = 0, δ = 0 : SrTiO3 x = 1, δ = 0.5 : La2Ti2O7 δ = x/2 The structure can be described as the intergrowth of perovskite layers (SrTiO3) and La2Ti2O7 layers. A layered perovskite with general formula: La4Srn-4TinO3n+2

  18. Sr3Ti2O7 P SrO P SrO SrTiO3+Sr3Ti2O7 P Sr3Ti2O7 (n = 2) Layered perovskites A typical example: Ruddlesden-Popper phases SrO(SrTiO3)n or Srn+1TinO3n+1 Aurivillius compounds: ((Bi2O2)2+(Bim-1TimO3m+1)2-) Ferroelectric & piezoelectric materials with high TC

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