DEPTH DEPENDENCE OF STRAIN, MOSAICITY AND SHARP COMPONENT IN THE CRITICAL SCATTERING OF SRTIO3

The sharp component observed in the critical scattering above the cubi c-to-tetragonal phase transition in SrTiO3 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 m ethods. High-resolution measurements have been performed with highly p enetrating 121 keV synchrotron radiation X-rays. The lattice parameter d, its fluctuations Delta d/d, the crystal mosaicity and the critical scattering were found to be depth dependent. The two-length-scale beh aviour in the critical fluctuations is confined to a skin layer approx imately 100 mu m thick and related to an increase in strain. Approachi ng the crystal surface the lattice parameter decreases by 6.2.10(-5) A ngstrom, Delta d/d increases from the bulk value of 5.2.10(-5) to 13.1 0(-5), the mosaicity increases from 1.0 to 8.2 seconds of are. In addi tion the critical temperature determined from the temperature dependen ce of the 1/2(511) reflection at the surface and 150 mu m below, is sm aller than the bulk value by 1.7 and 2.4 K, respectively.