Non-stoichiometry in CaCu3Ti4O12 (CCTO) ceramics Universidad Complutense de Madrid, Departamento Física Aplicada III Rainer Schmidt Non-stoichiometry in CaCu3Ti4O12 (CCTO) ceramics Rainer Schmidt, Shubhra Pandey, Derek C. Sinclair Universidad Complutense de Madrid Engineering Materials Electroceramics Research Group Grupo de Física de Materiales Complejos Dpto. Fisica Aplicada III 21st June 2013 C-32

“CaCu3Ti4O12” (CCTO) ceramics Universidad Complutense de Madrid, Departamento Física Aplicada III Rainer Schmidt Non-stoichiometry in “CaCu3Ti4O12” (CCTO) ceramics - Introduction: Giant dielectric permittivity er in CCTO Non-stoichiometry models - Phase diagram - Lattice parameters - Dielectric Spectroscopy - Combined data analysis - Conclusions

Crystal structure Giant dielectric permittivity Introduction: Giant er Rainer Schmidt Crystal structure Giant dielectric permittivity Giant dielectric permittivity er = 105 at high f T-dependent drop to er = 80 at low f Giant er has extrinsic origin!!! Single crystals: Electrode interface effect Polycrystalline ceramics: Grain boundary (GB) effect 1:3 A-site ordered perovskite (A'A''3B4O12) Strong octahedral tilting: a+a+a+ (Glazier) Cu square planar coordination Doubled simple cubic perovskite cell Space group Im-3 Homes et al. Science 293 (2001) p.673 3

Internal Barrier Layer Capacitor (IBLC) structure Introduction: Giant er Rainer Schmidt Internal Barrier Layer Capacitor (IBLC) structure Sinclair et al. Appl. Phys. Lett. 80 (2002) p.2153 The T-dependent drop to lower er is fully consistent with an IBLC model as represented by two RC element 4

Non-stoichiometry models in “CaCu3Ti4O12” Introduction: Non-stoichiometry Rainer Schmidt Non-stoichiometry models in “CaCu3Ti4O12” (A) Oxygen vacancies + Ti-reduction (B) High temperature Cu-reduction + Cu loss (C) Cu reoxidation upon cooling + internal redox Cu+ +Ti4+ → Cu2+ + Ti3+ (D) Cu-loss and Cu oxidation (E) Cu-excess and oxygen vacancies None of the models has been clealry confirmed experimentally! Li et al. Appl. Phys. Lett. 88 (2006) p.232903 Li et al. Solid State Commun. 135 (2005) 60 Fang et al. Acta Materialia 54 (2005) 2867 Schmidt et al. RSC Advances in Press (2013) 5

Internal Barrier Layer Capacitor (IBLC) structure Introduction: Giant er Rainer Schmidt Internal Barrier Layer Capacitor (IBLC) structure Fang et al., J.Am.Ceram.Soc. 87 (2004) p.2072 M. Li, PhD Thesis (2008) Sheffield Cu-rich phase bulk TEM GB Schmidt et al., J.Eur.Ceram.Soc. 87 (2004) p.2072 Cu-rich phase Grain boundaries and bulk behave very different! SEM: backscattered EDAX line-scan 6

Ternary CaO-CuO-TiO2 phase diagram Phase diagram Rainer Schmidt Ternary CaO-CuO-TiO2 phase diagram 1000 °C 1100 °C x y z D B,C E 2 7 4 6 8 5 1 3 7

Ternary CaO-CuO-TiO2 phase diagram Phase diagram Rainer Schmidt Ternary CaO-CuO-TiO2 phase diagram 1000 °C 1100 °C x y z D B,C E 2 7 4 6 8 5 1 3 8

CCTO Lattice parameters Cation ratio (fractions) Lattice parameters Rainer Schmidt CCTO Lattice parameters Compos. Number Cation ratio (fractions) Expected phases (wt.) Detected Phases (1100 °C ) a [Å] (1100 °C) a [Å] (1000 °C) 1 Ca 0.125, Cu 0.375, Ti 0.5 CCTO (1) CCTO, [CTO] 7.3925 (1) 7.3916 (4) 2 Ca 0.11, Cu 0.33, Ti 0.56 CCTO (0.88), TiO2 (0.12) CCTO, TiO2, [[CTO]] 7.3928 (1) 7.3917 (3) 3 Ca 0.14, Cu 0.3, Ti 0.56 CCTO (0.84), CTO (0.07), TiO2 (0.09) CCTO, CTO, TiO2 7.3924 (3) 7.3905 (2) 4 Ca 0.175, Cu 0.325, Ti 0.5 CCTO (0.88), CTO (0.12) CCTO, CTO 7.3931 (2) 7.3925 (3) 5 Ca 0.145, Cu 0.435, Ti 0.42 CCTO (0.74), CuO (0.17), CTO (0.09) CCTO, CTO, CuO 7.3922 (2) 7.3929 (2) 6 Ca 0.1125, Cu 0.4375, Ti 0.45 CCTO (0.9), CuO (0.1) CCTO, [CTO], CuO 7.3929 (4) 7.3911 (2) 7 Ca 0.1025, Cu 0.385, Ti 0.5125 CCTO (0.81), CuO (0.08), TiO2 (0.11) CCTO, TiO2, [CTO], CuO 7.3915 (1) 7.3903 (2) 8 Ca 0.06, Cu 0.52, Ti 0.42 CCTO (0.47), CuO (0.35), TiO2 (0.18) CCTO, TiO2, [CTO], CuO, 7.3912 (2) 7.3905 (3) 9

Dielectric permittivity vs frequency Dielectric Spectroscopy Dielectric spectroscopy Rainer Schmidt Dielectric permittivity vs frequency Dielectric Spectroscopy Ceramics sintered at 1100 °C Compositions 7 & 8 are different, 6 is intermediate. 10

Bulk dielectric permittivity vs CCTO lattice parameter Combined data analysis Rainer Schmidt Bulk dielectric permittivity vs CCTO lattice parameter In compositions 1 – 4 a solid solution is obvious, which may be responsible for increased bulk er above Clausius-Mossotti predictions 11

Bulk resistivity vs CCTO lattice parameter Combined data analysis Rainer Schmidt Bulk resistivity vs CCTO lattice parameter In compositions 5,7 & 8 a second solid solution is obvious, which may be responsible for increased GB resistivity 12

Conclusions Rainer Schmidt Two different defect mechanisms were identified in CCTO ceramics 1.) In compositions with no CuO secondary phase the detected defect mechanism can explain the large bulk dielectric permittivity. Only small changes of the bulk CCTO resistivity occur along the solid solution. It may be associated with Ca-Cu anti-site defects [PRL 99 (2007) p037602] 2.) In compositions with a CuO secondary phase the detected defect mechanism exhibits large changes in the bulk CCTO resistivity along the solid solution. It may be associated with the differences in insulating and Cu-rich grain boundaries and semiconducting Cu-deficient bulk areas. The separate analysis of the CCTO ternary phase diagram, lattice parameters of several compositions and impedance spectroscopy of sintered ceramics only gives some hints on the influence of Cu on the IBLC structure and non-stoichiometry in CCTO Only the combined analysis of all 3 experimental results allows the clear identification of two distinct defect mechanisms