Crystalline ceramics: Spotting impostors

Electron microscopy and spectroscopy reveal dopant atoms in high permittivity perovskites.

Perovskites are ceramics with a structure that gives them remarkable abilities to store electric charge. The crystalline structure consists of a network of titanium and oxygen octahedra, in between which large metal ions such as calcium (Ca) and copper (Cu) are located. However, the metals are too large to fit comfortably, causing the octahedra to shift and twist. The result is an unstable internal structure that can be oriented with an electric field, making perovskites excellent materials for energy-storing capacitors.

Fig. 1: The crystal structure of CaCu₃Ti₄O₁₂ , an ordered perovskite.

A new type of perovskite, CaCu₃Ti₄O₁₂ (Fig. 1) is of interest because of its intriguing electrical properties. Current passing through this material behaves nonlinearly—characteristics which could allow the ceramic to be developed as a long-life sensor.

Dopants such as the rare earth atom lanthanum (La) are normally added to CaCu₃Ti₄O₁₂ to improve device performance. However, because the perovskite crystal is unstable, the dopant substitution position can dramatically alter the electronic behaviour. Now, Sung-Yoon Chung and colleagues¹ have used electron microscopy and spectroscopy to precisely pinpoint the location of dopant atoms in the CaCu₃Ti₄O₁₂ lattice.

The researchers prepared pure CaCu₃Ti₄O₁₂ crystals into thin sections so that the beam of a scanning transmission electron microscope (STEM) could pass through them. The resulting images showed an array of round dots arranged into a checkerboard pattern, with each dot corresponding to a column of atoms. Because each atom has a different size, the dot’s brightness reveals its identity.

Half of the rows in the CaCu₃Ti₄O₁₂ crystal were alternately light and dark. These were identified as columns of pure Cu and mixed Ca-Cu atoms. And, since Ca is a lighter atom with a lower atomic number, the mixed columns appear darker in the images.

Doping the CaCu₃Ti₄O₁₂with 5% La caused a dramatic change in the atomic images. Suddenly, metal rows that were formerly light and dark now appeared universally bright. Image simulations and spectroscopic measurements proved that La exclusively replaced Ca in the perovskite.

“Our study shows that La is selectively substituted for Ca rather than Cu,” says Chung. “Progress in STEM allows not only better image resolution, but also sensitive detection of the atomic columns due to the enhanced beam current.”