Trends in electronic structures and structural properties of MAX phases: a first-principles study on M2AlC (M = Sc, Ti, Cr, Zr, Nb, Mo, Hf, or Ta), M2AlN, and hypothetical M2AlB phases (Trends in electronic structures and structural properties of MAX phases: a first-principles study on M2AlC (M = Sc, Ti, Cr, Zr, Nb, Mo, Hf, or Ta), M2AlN, and hypothetical M2AlB phases)