The formation of so-called Ruddlesden-Popper planar faults was studied in S
rO-doped SrTiO3 for different quantities of SrO additions and sintering con
ditions. For small SrO additions we observed a microstructure with a unifor
m grain size distribution and the enrichment of SrO at the grain boundaries
. Larger additions of SrO produced a microstructure of elongated grains con
taining random planar faults, polytypic lamellae of more or less ordered fa
ults, and polytype loops within SrTiO3 grains. We showed that these SrTiO3
grains were elongated as a result of preferential growth of the polytypic l
amellae. In addition, we discuss a correlation between the formation of pla
nar faults embedded in the perovskite matrix at low firing temperatures and
Ruddlesden-Popper phases that are stable at higher temperatures.