Consideration of the validity of the 14 valence electron rule for semiconducting chimney-ladder phase compounds

Electronic structure calculations for compounds known as Nowotny chimney-ladder (CL) phases have been performed to ascertain an empirical rule that CL compounds with a valence electron concentration (VEC) of 14 are a semiconductors. RuGa2 and RuAl2 which have a TiSi2-type structure, a prototype of the CL phase, with a VEC value of 14 are indirect-gap semiconductors with estimated band gaps of 0.235 and 0.20 eV, respectively. Ru2Si3 and Ru2Ge3 with VEC=14 are predicted to be direct-gap semiconductors but the band gap decrease in the heavier elements results in closure of the gap in Ru2Sn3. Ir3Ga5 and Ir4Ge5 will be metallic or semimetallic though their VEC values are 14. The Fermi level of Mn11Si19, whose VEC is slightly smaller than 14, is located just before the gap and seems not to be inconsistent with p-type semiconducting behavior. The Fermi levels of Rh10Ga17 and Rh17Ge22 whose VECs exceed 14 are located past the gap. Cr11Gei19 and Mo13Ge23, whose VECs are smaller than 14 would be metallic. These results show that the above rule is a rather good criterion for exploration of semiconducting chimney-ladder phase compounds but compounds with VEC=14 are not always semiconductors.