Gadolinium crystallizes in hexagonal, close-packed α- form at room temperature, but, when heated to temperatures above 1235 °C, it transforms into its β- form, which has a body-centered cubic structure. Gadolinium-157 has the highest thermal neutron capture cross-section among any stable nuclides at 259,000 barns. Only xenon-135 has a higher cross section, 2 million barns, but that isotope is unstable.