Depth dependence of strain, mosaicity and sharp component in the critical scattering of SrTiO₃

¹ Hamburger Synchrotronstrahlungslabor HASYLAB at Deutsches Elektronen-Synchrotron DESY - Notkestr. 85, D-22603 Hamburg, Germany
² Department of Physics, Brookhaven National Laboratory, Upton, New York 11973, USA

Received: 10 April 1997
Accepted: 9 July 1997

Abstract

The sharp component observed in the critical scattering above the cubic-to-tetragonal phase transition in is now generally considered as a surface-related phenomenon. In the present paper, for the first time, the structural difference between the bulk and a skin layer of a highly perfect crystal has been shown quantitatively by diffraction methods. High-resolution measurements have been performed with highly penetrating 121 keV synchrotron radiation X-rays. The lattice parameter d, its fluctuations , the crystal mosaicity and the critical scattering were found to be depth dependent. The two-length-scale behaviour in the critical fluctuations is confined to a skin layer approximately thick and related to an increase in strain. Approaching the crystal surface the lattice parameter decreases by , increases from the bulk value of to , the mosaicity increases from 1.0 to 8.2 seconds of arc. In addition the critical temperature determined from the temperature dependence of the 1/2(511) reflection at the surface and below, is smaller than the bulk value by 1.7 and 2.4 K, respectively.