Hydrides metal-insulator transition

The metal-insulator transition induced by hydrogen causes also changes of the electric resistance of the sample. The physical relation is rather complicated as it depends on the electronic changes as well as hydride growth mechanism and morphology. Therefore, in most cases, the change in resistance behavior is used to determine the onset of hydride formation/ decomposition. Repeating the measurements at various applied pressures/temperatures reveals a van t Hoff plot, similar to DSC-measurements (2.3). 

In Section 2.3 the structural and optical properties of neutral and cationic Na clusters at r = 0 K as functions of size are presented and compared with experimental data recorded at low temperature. The temperature-dependent line-broadening will be illustrated by the example of Na9, since in this case a comparison with experimental data at different temperatures is particularly instructive. In Section 2.4 the results of ab initio molecular dynamics (AIMD) studies on Li9 will serve to show different temperature behavior of distinct types of structures as well as their isomerization mechanisms. The study of possible metal-insulator transitions and segregation into metallic and ionic parts in finite systems carried out on prototypes of nonstoichiometric alkali halide and alkali hydride clusters with single and multiple excess electrons is presented in Section 2.5. A comparison of structural and optical characteristics of Na F and lAnUm (n > m) series allows us to illustrate the influence of different bonding properties. 

The ground-state properties of nonstoichiometrie X Y clusters (X = Na, Li, K and Y = Cl, F) with single and multiple excess electrons have been extensively studied experimentally [41-43] and theoretically [44-46] since they are good candidates for possible metal-insulator transitions and metallic-ionic segregation in finite systems. Hydrogenation of lithium clusters has been also investigated [47, 48]. It is of interest to establish similarities and differences among properties of alkali halide and alkali hydride clusters, since both bulk materials have a common structure but the electron affinities of F and H atoms are very different (3.4 versus 0.75 eV). The question can be raised to what extent these differences are reflected in properties of small finite systems. 

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