Structural and electronic properties of germanium clathratesGe46andK8

Abstract

The structural and electronic properties of germanium clathrates Ge46 and K8Ge46 are studied by first principles calculations within the local density approximation. The equilibrium structures are obtained by ab initio pseudopotential calculation combined with dynamic minimizations. The clathrate structure is found as a low energy phase for germanium. The electronic band structures for Ge46 clathrates are calculated and the band gap is found to be considerably larger than that of the diamond phase. Due to the effect of pentagonal rings, strong similarity in electronic properties between clathrate and Ge24 fullerene structure are found. The effect of doping clathrate cages with metal atoms are examined. The K8Ge46 clathrate is found to be metallic with the conduction bands only slightly modified by K dopants. 71.20.Tx, 61.48.+c

Band-structure calculations forBa6Ge25andZerec1, Yaresko2, Thalmeier3 et al. 2002Phys. Rev. B221372View full textAdd to dashboardCite

“…This is in contrast to the situation reported for K 8 Ge 46 in Ref. 22 where the authors conclude complete charge transfer from potassium atom to the frame. Both of these cited results were critically reconsidered and charge transfer redefined in a recent paper.…”

Transport Properties of the Binary Type I Clathrate K8Ge44□2

“…There are three crystallographically distinct framework sites in this cubic structure (space group Pm 3n); 6c, 16i, and 24k. 38,56,64,65 As the sp 3 bonding in the clathrate results in an intrinsic semiconductor in the absence of guests, these donated electrons will, in binary clathrates, occupy the framework conduction bands, or remain localized at framework vacancies if the formation of such vacancies is energetically favorable. The partial structure of the type I clathrate is illustrated in Fig.…”

“…[61][62][63] In this way, electropositive guest atoms (e.g., the alkali metals) are treated as electron donors, which donate their valence electrons to the host framework, within a rigid band approximation. 38,56,64,65 As the sp 3 bonding in the clathrate results in an intrinsic semiconductor in the absence of guests, these donated electrons will, in binary clathrates, occupy the framework conduction bands, or remain localized at framework vacancies if the formation of such vacancies is energetically favorable. 39,52,56 Thus, electron-rich materials such as Na 8 Si 46 and K 8 Si 46 exhibit metallic properties.…”

Sur les phases de type clathrate du silicium et des éléments apparentés (C, Ge, Sn) : Une approche historique

“…Comme Si 46 , le clathtrate vide Si 34 correspond à un semi conducteur, mais à transition directe dont la largeur de bande interdite est là encore proche de 1,9 eV (0,7 eV plus élevé que pour Si d ). Selon Demkov et al, les résultats obtenus pour le clathrate Na x Si 136 ne confirment pas entièrement l'hypothèse selon laquelle les états du sodium constitueraient une bande d'impuretés dans la bande interdite du silicium, comme envisagée anté-rieurement, mais les états du sodium formeraient au contraire une bande étroite au bas de la bande de conduction de Si 136 46 qui a conduit à des résultats très proches des précédents [106]. La présence d'atomes de potassium dans les cages ne modifie pas de manière significative la bande de valence de Ge 46 , mais induit quelques différences au bas de la bande de conduction.…”