The heat capacity anomaly at the transition to superconductivity of the layered superconductor MgB 2 is compared to first-principles calculations with the Coulomb repulsion, µ * , as the only parameter which is fixed to give the measured T c . We solve the Eliashberg equations for both an isotropic one-band and a two-band model with different superconducting gaps on the π-band and σ-band Fermi surfaces. The agreement with experiments is considerably better for the two-band model than for the one-band model.

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The title compounds were prepared from the elements by reactions in sealed tantalum tubes in a water-cooled sample chamber of a high-frequency furnace. They crystallize with the ZrNiAl-type structure, space group P6̄2m. The structures of the cerium compounds were refined from single-crystal x-ray diffraction data: a = 767.3(1) pm, c = 410.37(4) pm, wR2 = 0.0324, 521 F2-values for CePdMg; a = 755.02(7) pm, c = 413.82(4) pm, wR2 = 0.0393, 514 F2-values for CePtMg; and a = 774.1(3) pm, c = 421.6(1) pm, wR2 = 0.0355, 395 F2-values for CeAuMg, with 14 variables for each refinement. The palladium compound shows a small homogeneity range: CePd1+xMg1-x. The structures contain two crystallographically different transition metal sites T1 and T2 which are located in tri-capped trigonal prisms: [T1 Mg6Ce3] and [T2 Ce6Mg3]. Magnetic susceptibility and heat capacity measurements reveal long-range magnetic ordering at 2.1(2) K for CePdMg, 3.6(2) K for CePtMg, and 2.0(2) K for CeAuMg. Curie-Weiss behaviour at higher temperatures shows that the cerium ions are in the 3+ oxidation state. The isotypic LaTMg compounds are Pauli paramagnetic down to lowest temperatures (T = 0.3 K). All the compounds, RETMg (RE = La, Ce; T = Pd, Pt, Au) show metallic behaviour.