Effect of metal vacancies on the electronic band structure of hexagonal   Nb, Zr and Y diborides
  Energy band structures of metal-deficient hexagonal diborides M$_{0.75}$B$_2$ (M = Nb, Zr and Y) were calculated using the full-potential LMTO method. The metal vacancies change the density of states near the Fermi level and this effect is quite different for III-V group transition metal diborides. Contradictory data on superconductivity in diborides may be supposed to be connected with nonstoichiometry of samples. Vacancy formation energies are estimated and analyzed. 
I.R. Shein, K.I. Shein, N.I. Medvedeva, A.L. Ivanovskii