barium titanate
/ BA-ree-um ty-TAY-nate /
If electroceramics had a mascot it would be barium titanate, chemical formula BaTiO3 — the white powder inside the overwhelming majority of the world's ceramic capacitors. It was discovered independently during the Second World War, when researchers hunting for a high-permittivity replacement for scarce natural mica stumbled on a titanate with a dielectric constant not of tens but of thousands, and soon realised they had found the first widely useful ferroelectric.
Its structure is the perovskite, ABO3: large Ba2+ ions at the corners of a cube, oxygens at the face centres forming an octahedron, and a small Ti4+ ion in the middle of that oxygen cage. Above about 130 degrees C the Ti4+ sits dead-centre, the cell is cubic, and the crystal is a plain (paraelectric) insulator. Cool below 130 degrees C and the cell stretches slightly along one axis into a tetragonal shape: the Ti4+ and the oxygens shift in opposite directions, giving a spontaneous polarization along that axis — this is the ferroelectric state. Two further transitions follow on cooling, tetragonal to orthorhombic near 5 degrees C and orthorhombic to rhombohedral near minus 90 degrees C. The dielectric constant is not flat: it rises toward each transition and spikes to around 10000 right at the 130-degree Curie point.
That spike is both the gift and the problem. Pure barium titanate makes a dreadful capacitor precisely because its permittivity peaks so sharply at one temperature and is far lower elsewhere; so real capacitor dielectrics are barium titanate chemically 'smeared' — doped and substituted with calcium, zirconium, and rare earths, and engineered into core-shell grains — to broaden and flatten the peak into the stable X7R or Y5V response. Barium titanate is also the base of the PTC thermistor: made semiconducting by donor doping, it switches from conducting to resistive right at its Curie point. An honest note: grain size matters enormously — fine-grained barium titanate has a higher room-temperature permittivity than coarse, one of many reasons processing, not just chemistry, sets the final device.
To make an X7R capacitor stable from minus 55 to plus 125 degrees C, engineers do not use pure BaTiO3 — its permittivity would swing wildly around 130 degrees C. Instead each grain is grown with a doped shell around a purer core, so the material is really a blend of slightly different Curie points that together give a broad, gentle plateau.
Barium titanate is the base material, but a usable capacitor is what chemistry and microstructure make of it — pure BaTiO3 is almost never used raw.
The permittivity peak at the Curie point is a liability, not a feature, for a capacitor: you want a value that does not move, so device design is largely about suppressing and broadening the very peak that makes pure barium titanate famous.