Low-temperature structural phase transition inLa2CuO4

Simultaneous electrical resistance and x-ray diffraction measurements have been made on samples of ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$. The resistance data show a local maximum near 36 K; the x-ray data reveal a low-temperature structural transition which is correlated with the electrical anomaly. In addition to the new diffraction pattern, remnants of the orthorhombically distorted ${\mathrm{K}}_2$Ni${\mathrm{F}}_4$ structure of the parent phase are seen in varying amounts in all samples at low temperatures. These observations are in agreement with recent theoretical predictions.