Abstract
Although compound formation between two elements is well studied, thorough investigations make it possible to uncover new binary compounds. A re-examination of the La–Au system revealed three new phases, which were characterized with respect to their structural and electronic properties as well as thermal stability: La7Au3 (Th7Fe3 type, space group P63mc, Pearson code hP20) appears to be metastable. It can be obtained by slow crystallization from a stoichiometric melt. La3Au2 (U3Si2 type, space group P4/mbm, Pearson code tP10) is stable up to 1013 K, where it decomposes peritectically. La3Au4 (Pu3Pd4 type, space group R3̅, Pearson code hR14) is thermally stable up to at least 1273 K. In addition, the crystal structures of La2Au (anti-PbCl2 type, space group Pnma, Pearson code oP12) and α-LaAu (FeB type, space group Pnma, Pearson code oP8) could be determined by single-crystal X-ray diffraction. The electronic structures and chemical bonding have been evaluated from first principles calculations. They show that all compounds can be viewed as electron-rich, polar intermetallics.
Highlights
This opens up possibilities for crystal structure design, which likely have not been brought to full potential yet
Numerous compounds containing group 10 elements were reported to crystallize in the Pu3Pd4 structure type.[80−85] Among the aurides, structurally well-characterized representatives of this Nd.[76−78,86] family are limited to M3Au4 with M = Ca, Y, In addition, compositions with M = Ce, Pr, and Gd, Sm, Tb, and Th were assigned to this structure based on the respective powder X-ray diffraction patterns without further refinements.[78,87−91] The M3Au4 phases with rare earth metals Pr, Nd, Gd, and Tb were found to decompose peritectically at 1523−1613 K.86,88−90 In light of our experimental data, it is conceivable that La3Au4 has a similar decomposition point
We described three new binary phases in the La−Au system La7Au3, La3Au2, and La3Au4 crystallizing in known structure types, which are rather uncommon for binary gold-containing intermetallic compounds
Summary
The location of gold in the sixth period of the periodic table lends this element physical and chemical characteristics that are often strikingly different from those of the lighter congeners in group 11.1 To a large extent, these differences are associated with strong relativistic effects impacting the electronic properties of the gold valence states.[1,2] In particular, relativistic stabilization of the 6s2 electronic configuration results in a high electronegativity of gold, comparable to that of iodine on the Pauling scale,[3] some recently developed electronegativity scales suggest a somewhat lower value for Au in comparison to I.4,5 For this reason, compounds of Au with other metals frequently reveal a significant transfer of the electron density to the Au atoms, which allows their description as aurides, i.e., phases with anionic Au species. Numerous compounds containing group 10 elements were reported to crystallize in the Pu3Pd4 structure type.[80−85] Among the aurides, structurally well-characterized representatives of this Nd.[76−78,86] family are limited to M3Au4 with M = Ca, Y, In addition, compositions with M = Ce, Pr, and Gd, Sm, Tb, and Th were assigned to this structure based on the respective powder X-ray diffraction patterns without further refinements.[78,87−91] The M3Au4 phases with rare earth metals Pr, Nd, Gd, and Tb were found to decompose peritectically at 1523−1613 K.86,88−90 In light of our experimental data, it is conceivable that La3Au4 has a similar decomposition point. We found that in all cases the introduction of H into the structure has a destabilizing effect and the formation of the binary hydride LaH2 is favorable: La7Au3H(tetr)
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