Investigation of structural phase transitions in perovskites using high energy synchrotron radiation

Superstructure and temperature dependence of the critical scattering and the spontaneous strain in SrTiO3 and YBa2Cu306+x have been studied using a new 3-crystal diffractometer of high-momentum space resolution for synchrotron radiation with energies >80 keV. The temperature dependence of the bulk critical scattering from a highly perfect SrTiO₃ crystal above the critical temperature of the cubic to tetragonal phase transition at Tc~100 K is in excellent agreement with neutron data taken at Brookhaven with the same sample as before, although the q-space resolution was 27 times better. The sharp quasi-Bragg peak discovered by means of high resolution X-ray diffraction was found only in a near surface volume, indicating that the two-length scales behaviour postulated for this phase transition is related to a skin layer at the sample surface. A low temperature study of the spontaneous strain in pure and Ca doped SrTiO₃ provided further indications for the existence of a novel phase in the quantum paraelectric regime at temperatures below 37 K. The Ortho-II superstructure of YBa2Cu₃O_6.5as well as the Ortho-II superstructure of YBa₂Cu3O_6.77were studied with the best possible resolution available today.