Structure of Thallium and Gadolinium at Low Temperatures

X-ray diffraction at temperatures from 300^∘{}K to 5^∘{}K has been employed to search for a low-temperature transformation in thallium that might account for the anomaly in the pressure coefficient of the superconducting transition temperature of this element. No evidence of instability of the normal close-packed hexagonal structure was obtained either after cooling or after cold work at various low temperatures. Similarly, the close-packed hexagonal structure of gadolinium was found to be stable to 5^∘{}K.