Structure and properties of (AlB2)n and (MgB2)n (n=1, ..., 10) clusters

A systematic search for energetically lowest lying structures of neutral(AlB2)(n) and(MgB2)(n) clusters with n = 1, ..., 10 is performed using density functional theory within a multistep hierarchical algorithm specially adapted for the global optimization of relatively large structures. For obtained clusters, different physical properties (energetic, electrostatic, electronic, and thermodynamic) are determined. The variation of these properties with increasing cluster size is discussed in detail. The bulk values of binding energy, specific zero point energy, ionization potential, electron affinity, collision diameter and formation enthalpy for aluminum and magnesium diborides have been obtained by means of physically sound extrapolation of the calculated data to the particles of infinite size. The temperature-dependent thermodynamic functions of(AlB2)(n) and(MgB2)(n) clusters, such as enthalpy, entropy, specific heat capacity, and reduced Gibbs energy, are evaluated with allowance for vibrational anharmonicity and for the existence of excited electronic states. The appropriate data are fitted to seven-parameter NASA (Chemkin) polynomials. The approximations of the reduced Gibbs energy applicable for extrapolation towards large clusters and even small nanoparticles are also elaborated.