On the Functionality of Complex Intermetallics: Frustration, Chemical Pressure Relief, and Potential Rattling Atoms in Y₁₁Ni₆₀C₆

Abstract

Intermetallic carbides provide excellent model systems for exploring how frustration can shape the structures and properties of inorganic materials. Combinations of several metals with carbon can be designed in which the formation of tetrahedrally close-packed (TCP) intermetallics conflicts with the C atoms' requirement of trigonal prismatic or octahedral coordination environments, as offered by the simple close-packings (SCP) of equally sized spheres. In this Article, we explore the driving forces that lead to the coexistence of these incompatible arrangements in the YbNiC-type compound YNiC (cI154), as well as potential consequences of this intergrowth for the phase's physical properties. Our focus begins on the structure's SCP regions, which appear as C-stuffed versions of a AuCu-type YNi phase that is not observed on its own in the Y-Ni system. DFT-Chemical Pressure (DFT-CP) calculations on this hypothetical YNi phase reveal large negative pressures within the Ni sublattice, as it is stretched to accommodate the size requirements of the Y atoms. In the YNiC structure, two structural mechanisms for addressing these CP issues appear: the incorporation of interstitial C atoms, and the presence of interfaces with CaCu-type domains. The relative roles of these two mechanisms are investigated with the CP analysis on a hypothetical YNiC series of C-stuffed AuCu-type phases, the Y-Ni sublattice of YNiC, and finally the full YNiC structure. Through these calculations, the C atoms appear to play the roles of relieving positive Y CPs and supporting relaxation at the AuCu-type/CaCu-type interfaces, where the cancellation occurs between opposite CPs experienced by the Y atoms in the two parent structures (following the epitaxial stabilization mechanism). The CP analysis of YNiC also highlights a sublattice of Y and Ni atoms with large negative CPs (and thus the potential for soft vibrational modes), illustrating how frustrated structures could lead to the full realization of the phonon glass-electron crystal concept.

Tutorial on Chemical Pressure Analysis: How Atomic Packing Drives Laves/Zintl Intergrowth in K3Au5Tl

“…In the actual intergrowth structure, the CP relief provided by these interfaces is supported by structural relaxation, which features the twisting of the network of vertex-sharing Au tetrahedra, the contraction of the remaining Tl-Tl contacts, and the stronger interactions at some of the K-Au contacts. The detailed matching of the atomic positions and complementary CPs at the KAu 2 /KTl interface illustrates the more general phenomenon of epitaxial stabilization in intermetallic intergrowth structures [35,[37][38][39].…”

“…This method is based on the recognition that non-optimal interatomic distances are detectable in the local pressures that surround the atoms of a solid-state lattice, as constructed from the output of DFT calculations. In the study of a variety of intermetallic phases, CP analysis has provided explanations for diverse structural phenomena, such as the insertion of interfaces into simple structures [19,20,24,25], the adoption of local icosahedral symmetry in quasicrystal approximants [26][27][28], the emergence in incommensurability [29][30][31][32][33][34], the stabilizing effects of various kinds of single point substitutions [23][24][25]35,36], and the formation of intergrowth structures [27,35,[37][38][39]. The CP maps generated in the process can also be used to analyze the forces involved in chemical bonding and molecular structure [40][41][42][43].…”

Rational Design of Superconducting Metal Hydrides via Chemical Pressure Tuning

“…Negative CP has previously been correlated with soft atomic motions, [39] with cases where an atom bears negative CP in all directions, such as those seen here, as a possible indicator of a rattling atom. [40] In fact, it has been pointed out that such a dispersionless rattling mode of the boron atoms within their H 8 cubes is evident in the phonon band structure of LaBH 8 , as described in Ref. [10] Thus, the hydrogenic network of vertex-sharing cubes and tetrahedra can be stabilized to incorporate La atoms by stuffing either set of holes.…”

Inducing Complexity in Intermetallics through Electron–Hole Matching: The Structure of Fe14Pd17Al69

“…Examples include the interpenetration of polar and non‐polar regions in NaCd 2 , the coexistence of radially and periodically packed domains in Ta 39.1 Cu 5.4 Al 55.4 , the micelle‐like segregation of Ca–Cd and Cu–Cd interactions in the quasicrystal approximant Ca 10 Cu 2 Cd 27 , and the intergrowth of intermetallic and carbide units in Mn 16 SiC 4 . Such segregated structures have the added advantage that rattling atoms may emerge at the domain interfaces for enhanced phonon scattering …”

“…[7] Such segregated structures have the added advantage that rattling atoms may emerge at the domain interfaces for enhanced phonon scattering. [8] Any attempt to induce this type of frustration, of course, needs ameans of preventing simple phase segregation of the incompatible domains (analogous to the covalent bond linking the head group and tail in as urfactant molecule). Thed esire for closed-shell electron counts can provide one such attractive force.I ntermetallic structures are becoming increasingly understandable in terms of bonding schemes, such as the Zintl concept, [9] Mott-Jones model, [10] or the 18Àn rule.…”