Perovskite – Crystal Structure
Shown in the image is a cubic crystal structure called the Perovskite crystal structure
All solids are aggregates of atoms. Based on the way the atoms are arranged, most solids can be classified as crystalline or amorphous (glass). In crystalline materials, the atoms constituting the solid are arranged in a periodic manner, whereas in glass they are in “arranged” in a random manner such as in liquids. The structure shown in the figure is that of a mineral called Perovskite. A typical example of the Perovskite structure pictured is barium titanate (BaTiO3). 8 Ba atoms (orange ones) “sit” at the cube corners, 1 Titanium atom (the black one) “sits” in the centre of the cube and 6 oxygen atoms (the rest) “sit” at the centre of the faces. Note that an actual crystal of bigger dimensions is built up by stacking up such cubes of much smaller dimension of the order of 5Å. Each atom at the cube corner is shared by eight other cubes, while the atoms at the center of the cube face are shared by two cubes. Hence, only 1/8 of each Ba atom and ½ of each oxygen atom actually belong to the cube shown, resulting in the formula above. There are 230 possible periodic arrangements that can fill space and these were deduced by mathematicians even before it was actually proven to be so experimentally by x-ray diffraction in the early part of the twentieth century.
Barium titanate, strontium titanate and their mixtures belong to a class of materials called ferroelectrics that exhibit a spontaneous electrical dipole whose direction can be switched by the application of an electric field. In particular, thin films and superlattices based on these materials find applications as tunable (tunable implies that the dielectric constant can be changed by the application of an electric field) capacitors for microwave frequency applications. A superlattice is made by alternately depositing very thin layers of barium titanate and strontium titanate. In Prof. Krupanidhi’s lab this is done by a thin film deposition technique called pulsed laser deposition. Such superlattices have shown better tunability (55%) than thin films of a single composition (30%). An example of the composition variation, across a superlattice determined by Secondary Ion Mass Spectroscopy fabricated in Prof. Krupanidhi’s lab by his student Asis Sarkar is shown in the lower image below. Note that the thickness of each layer of the superlattice is of the order of 10 nm. For more information on Prof. Krupanidhi’s lab visit his faculty page