The influence of transition metals (TMs) on the hydriding/dedydriding critical point of NaAlH4 has been studied using the chemical potential method. The theoretical results predict that, if the reaction system reaches equilibrium, most of the TM additives significantly increase its hydriding/dedydriding critical point in the sequence Pd>Co>Zr>Ni>Nb>Hf>Ti>Mn>Fe>V>Cu>Cr and therefore have destabilizing effects on NaAlH4 in the same sequence. Experimentally, the isothermal dehydriding kinetics of NaAlH4 is studied only with Fe, Ti, and FeTi additives. The experimental results show that the destabilizing effect of Fe and Ti additives with coarse particles is very low while more effective destabilizing ability is achieved with fine FeTi particles. These experimental results suggest that the predicted destabilizing effect of TM additive is hindered kinetically due to the coarse particles of additives, and therefore experimental verification of the destabilizing effect of TMs should be performed with nanosized particles.

Zr mono-doped and (Zr,N) co-doped ZnO are investigated by the first-principles calculations. It is found that Zr prefers to substitute Zn site under most growth conditions. The passive (N-Zr-N) complexes create a fully occupied impurity band above the valence-band maximum (VBM) of ZnO, which helps p-type conductivity by reducing the ionization energy, consistent with a new approach to overcome the doping asymmetry [Y.F.Yan, J.B.Li, S.H.Wei, and M.M. Al-Jassim, Phys. Rev. Lett. 98 (2007) 135506]. In comparison with (Ga,N) co-doping, (Zr,N) is found to be probably better dopants to push p-type conductivity in ZnO through the new approach with easier formation of the passive impurity band.

The influence of defect charge state on the magnetism of Cu doped ZnO as well as the Cu defects clustering have been investigated by the first-principles calculations. We demonstrate that p-type ZnO:Cu could have ferromagnetic (FM) property, but n-type ZnO:Cu would not have local magnetic moment. Furthermore, the neutral substitutive Cu defects are found to be favorable in clustering, which maintains the FM ordering.

We distill from first-principles spin-polarized total-energy calculations some practical rules for predicting the magnetic state (ferromagnetic/antiferromagnetic/paramagnetic) of substitutional transition-metal impurity with different charge state in various host crystal groups Ⅳ, Ⅲ-Ⅴ, Ⅱ-Ⅵ, Ⅰ-Ⅲ-Ⅵ2, and Ⅱ-Ⅳ-Ⅴ2 semiconductors. The basic mechanism is the stabilization of a ferromagnetic bond between two transition metals if the interacting orbitals are partially-occupied. These rules are then subjected to quantitative tests, which substantiate the mechanism of ferromagnetism in these systems.We discuss cases where current electronic structure calculations agree with these rules, and identify a few cases where conflicts exist. The effect of doping on transition-metal magnetic properties is also covered by these rules by considering the oxidation state changes due to doping. In addition, we systematically apply these rules to ideal substitutional impurities, contrasting our predictions with experiment. Discrepancies may be used to assess the role of various nonidealities such as presence of additional dopants, precipitates, clusters, or interstitial sites.

The efficiency of CuInSe2 based solar cell devices could improve significantly if CuGaSe2, a wider band gap chalcopyrite semiconductor, could be added to the CuInSe2 absorber layer. This is, however, limited by the difficulty of doping n-type CuGaSe2 and, hence, in its alloys with CuInSe2. Indeed, wider-gap members of semiconductor series are often more difficult to dope than lower-gap members of the same series. We find that in chalcopyrites, there are three critical values of the Fermi energy EF that control n-type doping: (i) EnF,pin is the value of EF where the energy to form Cu vacancies is zero. At this point, the spontaneously formed vacancies (=acceptors) kill all electrons. (ii) EnF,comp is the value of EF where the energy to form a Cu vacancy equals the energy to form an n-type dopant, e.g., CdCu. (iii) EnF,site is the value of EF where the formation of Cd-on-In is equal to the formation of Cd-on-Cu. For good n-type doping, EnF,pin, EnF,comp, and EnF,site need to be as high as possible in the gap. We find that these quantities are higher in the gap in CuInSe2 than in CuGaSe2, so the latter is difficult to dope n-type. In this work, we calculate all three critical Fermi energies and study theoretically the best growth condition for n-type CuInSe2 and CuGaSe2 with possible cation and anion doping. We find that the intrinsic defects such as VCu and InCu or GaCu play significant roles in doping in both chalcopyrites. For group-II cation (Cd, Zn, or Mg) doping, the best n-type growth condition is In/ Ga-rich, and maximal Se-poor, which is also the optimal condition for stabilizing the intrinsic InCu/GaCu donors. Bulk CuInSe2 can be doped at equilibrium n-type, but bulk CuGaSe2 cannot be due to the low formation energy of intrinsic Cu-vacancy. For halogen anion doping, the best n-type materials growth is still under In/ Ga-rich, and maximal Se-poor conditions. These conditions are not best for halogen substitutional defects, but are optimal for intrinsic InCu/GaCu donors. Again, CuGaSe2 cannot be doped n-type by halogen doping, while CuInSe2 can.