High-Pressure Synthesis of a New Silicon Clathrate Superconductor, Ba<sub>8</sub>Si<sub>46</sub>

Yamanaka, Shōji; Enishi, Eiji; Fukuoka, Hiroshi; Yasukawa, Masahiro

doi:10.1021/ic990778p

Cited by 349 publications

(334 citation statements)

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“…Powder x-ray diffraction ͑XRD͒ measurements showed the Al 13 and Al 16 samples to be pure type-I clathrate with no additional phases within experimental resolution, while Al 12 and Al 10 also exhibited a small amount of diamond-phase Ge. Rietveld analysis yielded best fits of Ba 8.0 Al 15.1 Ge 27.8 for the nominal Al 16 sample, and Ba 7.0 Al 12.6 Ge 33.5 for Al 12 . For Al 16 these yielded 2.0, 5.2, and 7.9 Al atoms on the 6c, 16i, and 24k framework sites respectively in the type-I structure ͑space group Pm3n, #223͒, and for Al 12 , 1.5, 2.0, and 9.1 Al atoms.…”

Section: Sample Preparation and Characterizationmentioning

confidence: 90%

“…Samples are denoted by Al x , for example Al 16 representing Ba 8 Al 16 Ge 30 . Powder x-ray diffraction ͑XRD͒ measurements showed the Al 13 and Al 16 samples to be pure type-I clathrate with no additional phases within experimental resolution, while Al 12 and Al 10 also exhibited a small amount of diamond-phase Ge. Rietveld analysis yielded best fits of Ba 8.0 Al 15.1 Ge 27.8 for the nominal Al 16 sample, and Ba 7.0 Al 12.6 Ge 33.5 for Al 12 .…”

Section: Sample Preparation and Characterizationmentioning

confidence: 99%

“…Each unit cell contains two 20-atom cages, which form pentagonal dodecahedra, and six 24-atom cages, which form tetrakaidecahedra. Recently, much work has been devoted to Ba 8 Ga 16 Ge 30 , which exhibits a particularly large thermoelectric efficiency at high temperatures. 6 An important issue has been to understand the displacement of Ba ions inside the larger cages, and the strong resulting effect on transport behavior.…”

Section: Introductionmentioning

confidence: 99%

“…It was recently shown that Al alloying in Ba 8 Ga 16−x Al x Ge 30 can act to tune the thermal conductivity, 7 however it was also shown by high-resolution crystallography that in Ba 8 Al 16 Ge 30 the interactions between Ba and group-III Al ions strongly affect the vibrational behavior of cage-center ions. 8 An additional feature which strongly affects the electronic behavior is the relative ease of breaking the Ge framework, with vacancies spontaneously formed to maintain electron balance in off-stoichiometry materials.…”

Section: Introductionmentioning

confidence: 99%

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NMR experiments and electronic structure calculations in type-I BaAlGe clathrates

Gou

Rodriguez

et al. 2009

Phys. Rev. B

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We describe 27 Al NMR experiments on Ba 8 Al x Ge 46−x type-I clathrates coupled with ab initio computational studies. For x = 16, calculated spectra determined by the ab initio results gave good agreement with the measurements, with best-fitting configurations also corresponding to the computed lowest-energy atomic arrangements. Analysis of the NMR results showed that a distribution of Knight shifts dominates the central portion of the line. Computational results demonstrate that this stems from the large variation of carrier density on different sites. Al-deficient samples with x = 12 and 13 exhibited a split central NMR peak, signaling two main local environments for Al ions, which we connected to the presence of vacancies. Modeling of the wide-line spectrum for x = 12 indicates a configuration with more Al on the 24k site than for x = 16. The results indicate the importance of nonbonding hybrids adjacent to the vacancies in the electronic structure near E F . We also address the static distortions from Pm3n symmetry in these structures.

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Section: Sample Preparation and Characterizationmentioning

confidence: 90%

Section: Sample Preparation and Characterizationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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NMR experiments and electronic structure calculations in type-I BaAlGe clathrates

Gou

Rodriguez

et al. 2009

Phys. Rev. B

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“…Sharp peaks near the Fermi level in the electronic density of states result in large Seebeck coefficients and also superconductivity for Ba-containing members. 3,4,[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Two main structural types were described during the initial studies of Si and Ge clathrate materials. [1][2][3][4][5][6][7][8][9] The type I clathrate has a primitive cubic structure (Pm3n: a o ~ 10.2 Å) based on dodecahedral [5 12 ] and tetrakaidecahedral [5 12 6 2 ] cages that are occupied by metal guest atoms to produce metallic Na 8 Si 46 whose formation is studied here.…”

Section: Introductionmentioning

confidence: 99%

Time-Resolved in Situ Synchrotron X-ray Diffraction Studies of Type 1 Silicon Clathrate Formation

et al. 2011

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Silicon clathrates are unusual open-framework solids formed by tetrahedrally-bonded silicon that show remarkable electronic and thermal properties. The type I structure has a primitive cubic unit cell containing cages occupied by metal atoms to give compositions such as Na 8 Si 46 or Na 2 Ba 6 Si 46 . Although their structure and properties are well described there is little understanding of the formation mechanism. Na 8 Si 46 is typically produced by metastable thermal decomposition under vacuum conditions from NaSi, itself an unusual structure containing Si 4 4-poly-anions. In this study we used in situ synchrotron X-ray diffraction combined with rapid X-ray detection on samples taken through a controlled temperature ramp (25-500 o C at 8 o /min) under vacuum conditions (10 -4 bar), to study the clathrate formation reaction. We also carried out complementary in situ high temperature solid state 23 Na NMR experiments using a sealed tube loaded under inert gas atmosphere conditions. We find no evidence for an intermediate amorphous phase during clathrate formation. Instead we observe an unexpectedly high degree of structural coherency between the Na 8 Si 46 clathrate and its NaSi precursor, evidenced by a smooth passage of several X-ray reflections from one structure into the other. The results indicate the possibility of an unusual, epitaxial-like, growth of the clathrate phase as Na atoms are removed from the NaSi precursor into the vacuum.

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Existierende und hypothetische intermediärvalente AgII/AgIII- und AgII/AgI-Fluoride als potentielle Supraleiter

Grochala

Hoffmann

2001

Angew. Chem.

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Um ihr Potential als leitende oder supraleitende Materialien abzuschätzen, untersuchen wir die Kristallstrukturen und die magnetischen Eigenschaften der heute bekannten etwa 100 binären, ternären und quaternären AgII‐ und AgIII‐Fluoride im festen Zustand. Das AgII‐Kation befindet sich in diesen Verbindungen im Allgemeinen in einer verzerrt‐oktaedrischen Umgebung, entweder in einer unendlichen [AgF]+‐Kette oder in [AgF2]‐Schichten. Manchmal findet man auch diskrete quadratisch‐planare [AgF4]2−‐ Ionen. Das AgIII‐Kation tritt meist in der Form isolierter quadratisch‐planarer [AgF4]−‐Ionen auf. AgIII(d8)‐Systeme sind typischerweise diamagnetisch. Demgegenüber führt das reichhaltige Spektrum an koordinativen Umgebungen des AgII (d9) in binären und ternären Fluoriden zu sehr vielfältigen magnetischen Eigenschaften, die von Paramagnetismus über temperaturunabhängigen Paramagnetismus (charakteristisch für halbgefüllte Bänder und metallisches Verhalten) und Antiferromagnetismus bis hin zu schwachem Ferromagnetismus reichen. AgII und AgIII weisen dieselben d‐Elektronenzahlen auf wie CuII (d9) bzw. CuIII (d8). F− und O2− sind isoelektronische Ionen mit abgeschlossener Schale (s2p6). Sowohl F− als auch O2− sind Liganden mit schwachem Feld. Im Lichte dieser Ähnlichkeiten und auf der Basis einiger experimenteller Hinweise untersuchen wir Analogien zwischen den supraleitenden Cupraten (CuII/CuIII‐O2−‐ und CuII/CuI‐O2−‐Systemen) und den formal gemischtvalenten AgII/AgIII‐F−‐ und AgII/AgI‐F−‐Phasen. Zu diesem Zweck berechneten wir die elektronischen Strukturen einiger strukturell charakterisierter binärer und ternärer Fluoride von AgI, AgII und AgIII und vergleichen die Ergebnisse mit ähnlichen Rechnungen für Oxocuprat‐Supraleiter. Elektronische Niveaus in der Nachbarschaft des Fermi‐Niveaus (x2−y2 oder z2) haben üblicherweise stark gemischten Ag(d)/F(p)‐Charakter und sind Ag‐F‐antibindend, sodass sie die Möglichkeit zur effizienten Schwingungskopplung bieten (wie dies für d9‐Systeme mit bedeutendem kovalenten Bindungsanteil typisch ist). Gemäß unseren Rechnungen liegt dies nicht nur an einer zufälligen Koinzidenz der Orbitalenergien; überraschenderweise weist die Ag‐F‐Bindung in AgII‐ und AgIII‐Fluoriden tatsächlich bedeutende kovalente Anteile auf. Die elektronische Zustandsdichte am Fermi‐Niveau (DOSF) von Silberfluoriden wie auch die Frequenzen der Metall‐Ligand‐Streckschwingungsmoden sind denen von Kupferoxiden ähnlich. Die oben geschilderten Eigenschaften deuten darauf hin, dass geeignet löcher‐ oder elektronendotierte AgII‐Fluoride gute BCS‐Supraleiter sein könnten. Wir analysieren ein Komproportionierungs‐/Disproportionierungs‐Gleichgewicht in löcherdotierten AgII‐Fluoriden und das mögliche Auftreten von Löchern im F(p)‐Band. Möglicherweise lässt sich bei diesen Fluoriden eine Überschneidung der Kurven der ionischen und der kovalenten Bindung (entsprechend der Formulierung AgIII‐F−/AgII‐F0) erreichen, was zu einem erheblichen Anstieg der Schwingungskopplung führen würde.

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High-Pressure Synthesis of a New Silicon Clathrate Superconductor, Ba₈Si₄₆

Cited by 349 publications

References 18 publications

NMR experiments and electronic structure calculations in type-I BaAlGe clathrates

NMR experiments and electronic structure calculations in type-I BaAlGe clathrates

Time-Resolved in Situ Synchrotron X-ray Diffraction Studies of Type 1 Silicon Clathrate Formation

Existierende und hypothetische intermediärvalente AgII/AgIII- und AgII/AgI-Fluoride als potentielle Supraleiter

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High-Pressure Synthesis of a New Silicon Clathrate Superconductor, Ba8Si46

Cited by 349 publications

References 18 publications

NMR experiments and electronic structure calculations in type-I BaAlGe clathrates

NMR experiments and electronic structure calculations in type-I BaAlGe clathrates

Time-Resolved in Situ Synchrotron X-ray Diffraction Studies of Type 1 Silicon Clathrate Formation

Existierende und hypothetische intermediärvalente AgII/AgIII- und AgII/AgI-Fluoride als potentielle Supraleiter

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High-Pressure Synthesis of a New Silicon Clathrate Superconductor, Ba₈Si₄₆