Calcium copper titanate

History

CCTO was first synthesized in 1967 by Alfred Deschanvres and his coworkers. While the structural features were known, no physical properties had been measured. In 2000, Mas Subramanian and his colleagues at DuPont Central R&D discovered that CCTO displayed a dielectric constant greater than 10,000, compared to the normal dielectric SrTiO₃, which has a constant of 300 at room temperature. Since then, it has found widespread usage in capacitor applications.

Synthesis and Structure

Most compounds which form this crystal structure are made under high pressure conditions. Pure CCTO, however, can be easily synthesized by standard solid state methods via intimate mixtures of the metal carbonate and oxide precursors at temperatures between 1000 and 1200 °C.

The CaCu₃Ti₄O₁₂ structure type is derived from the cubic perovskite structure, by an octahedral tilting distortion as is GdFeO₃. In both cases the distortion is driven by a mismatch between the size of the A-cations and the cubic ReO₃ network. However, CaCu₃Ti₄O₁₂ and GdFeO₃ adopt different patterns of octahedral tilting (a⁻b⁺a⁻ and a⁺a⁺a⁺ in Glazer notation). The octahedral tilting distortion associated with the GdFeO₃ structure leads to a structure where all of the A-cation environments are identical. In contrast, the octahedral tilting distortion associated with the CaCu₃Ti₄O₁₂ structure produces a structure where 75% of the A-cation sites (A" sites) have square planar coordination, while 25% of the A-cation sites remain 12 coordinate. The square planar sites are almost always filled by Jahn-Teller ion such as Cu²⁺ or Mn³⁺, while the A' site is always occupied by a larger ion.

Dielectric Properties

Using the Clausius-Mossotti relation, the calculated intrinsic dielectric constant should be 49. However, CCTO exhibits a dielectric constant upwards of 10,200 at 1 MHz, with a low loss tangent until approximately 300 °C. In addition, the relative dielectric constant increases with decreasing frequency (in the range of 1 MHz to 1 kHz).

The colossal-dielectric phenomenon is attributed to a grain boundary (internal) barrier layer capacitance (IBLC) instead of an intrinsic property associated with the crystal structure. This barrier layer electrical microstructure with effective permittivity values in excess of 10, 000 can be fabricated by single-step processing in air at ∼1100 °C. CCTO is therefore an attractive option to the currently used BaTiO₃-based materials which require complex, multistage processing routes to produce IBLCs of similar capacity.

Because there is a large discrepancy between the observed dielectric constant and the calculated intrinsic constant, the true origin of this phenomenon is still under debate.