A new tricritical point in Co doped barium titanate

By means of X-ray diffraction of a polydomain single crystals of Co doped BaTiO3 we have measured the c/a ra io as a function of temperature. We found that the first order character of the tetragonal-cubic transition disappears by increasing the Co amount. We conclude that samples with 2.5% atom of Co exhibit a tricritical point. This point seems to be different than the tricritical point which is observed in pure BaTiO3 under hydrostatic pressure.