Pseudogap tuning

Shortly after, we recognized that ScCu4Ga2 (Im3) [70] might also be tuned to a QC, but the correct stoichiometry and reaction conditions were not achieved in our limited experiments. Recently, Honma and Ishimasa [71] have reported that i-QC phase forms almost exclusively from a rapidly quenched ScisCu48Ga34 composition, emphasizing a very narrow phase width and its thermodynamic metastability at room temperature. However, the failure turned us to other Ga intermetallics, which led to the pseudogap tuning concepts that follow. 

In Sect. 4.2 we expand on pseudogap tuning concepts and illustrate these ideas and applications to the isotypic Mg2Cu6Ga5, Mg2Znn, and Na2Au6ln5. Because all the ACs we have obtained have very similar structural motifs, their structural regularities will be discussed together later in Sect. 5. 

LMTO calculations on a hypothetical ScZn6 1/1 AC [82] revealed that Sc plays the same role here as in Sc-Mg-Cu-Ga 1/1 AC (Fig. 13). The Sc not only provides valence electrons to push into the pseudogap, but its d orbitals also afford mixing with Zn s, p orbitals to enhance the depth of the pseudogap. This may explain why no Mg-Zn binary or Mg-Cu-Ga ternary Tsai-type QCs exist, but the Sc-Cu-Zn i-QC [24,68] forms, although its discovery was not directed by the pseudogap tuning concept. 

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