Two types of multistack structures in MgB2-type superconductor CaAlSi

Crystal structure analysis of an AlB2-type superconductor, CaAlSi, has been carried out using synchrotron X-rays and neutron diffraction measurements. Two different stackings along the c-axis-5-fold and 6-fold structures-with different values of T-c 6.0 and 8.2 K, respectively, are confirmed. Our results show that AI and Si atoms are ordered alternately in honeycomb layers, and Ca atoms are intercalated at regular intervals between the (AI,Si) layers in both the structures. The 5-fold and 6-fold structures are determined by the arrangement of Al and Si along the c-axis. The relation between T-c and the structure is discussed based on our structural analysis.