Abstract
The coherent αα phase transition of H in Nb has been studied in several sample shapes. The samples were loaded in situ with the critical hydrogen concentration of 0.31 H/Nb above Tc=171°C and slowly cooled. Below Tc a coherent macroscopic spinodal decomposition takes place. By x-ray scattering and examination of the relaxed crystals, the spatial arrangement of the phases was determined. The macroscopic modes so revealed depend sensitively upon sample shape through the fulfillment of the elastic boundary condition.