Abstract
The lattice parameters of tetragonal strontium titanate are determined to about 1 part in 106 with penetrating X-rays. While no peculiarity is detected in a bulk sample, a distinct anomaly at 37 K is observed in an optically polished thin platelet derived from the same single crystal. This suggests that the anomaly is related to a high density of dislocations. A possible mechanism is that dislocations stabilize transverse antiphase boundaries which undergo a ferroelectric transition near 37 K. If so, this transition stresses the boundaries which then strain the bulk leading to the observed anomaly.