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Abstract

The temperature dependence of the sharp and the broad component
of the critical scattering above the cubic-to-tetragonal phase
transition of SrTiO3 has been studied by means of high-
resolution triple-crystal diffractometry using 100-200 keV
synchrotron radiation in five samples differing with respect to
growth technique and oxygen vacancy concentrations. Emphasis is
on changes in the critical behavior, the critical temperature,
and the strain fields at the transition from bulk to surface.
The sharp component was observed only in surface near regions
of highly perfect crystals and is coupled to the occurrence of
a long-range strain gradient that was identified by an
exponential increase of mosaicity, lattice parameter
fluctuations, and Bragg-peak intensity when approaching the
surface from the bulk of the sample. Vanishing of the sharp
component was observed at the polished/etched surface of a
platelet cut off the large perfect crystal after release of
strain due to free bending of the platelet. The values of the
critical temperature observed in the bulk of the different
samples vary between 98.7less than or equal toT(c)less than or
equal to105.8 K. In the surface near regions of a highly
perfect float-zone grown crystal a variation of T-c of about
0.5 K has been found. Concerning the broad component the
critical exponent describing the temperature dependence of the
inverse correlation length kappa(b) varies between 0.73less
than or equal tonu(b)less than or equal to1.19, the exponent
for the susceptibility between 1.49less than or equal
tochi(b)less than or equal to2.9, however, the ratio of the two
exponents is almost sample independent and equal to
chi(b)/nu(b)=2.1 with a variance of 0.2, in good agreement with
the theoretical value of 1.97 obtained by LeGouillou and Zinn-
Justin [Phys. Rev. B 21, 3976 (1980)]. The occurrence of the
sharp component did not affect significantly the critical
exponents for the underlying broad component of the critical
scattering. The exponents for the sharp component observed in
surface near layers of about 100 mum thickness at the highly
perfect float zone and flux grown crystals varied between
0.58less than or equal tonu(s)less than or equal to1.30, the
values for the ratio varied between 3.3less than or equal
tochi(s)/nu(s)less than or equal to4.6. The average value of
chi(s)/nu(s) is 3.9 with a variance of 0.5, and is about twice
the ratio chi(b)/nu(b) for the broad component as suggested by
Harris.