Toward Predicting the Assembly of Modular Intermetallics from Chemical Pressure Analysis: The Interface Nucleus Approach

Complex intermetallic phases are often constructed from domains derived from simpler structures arranged into hierarchical assemblies. These modular arrangements offer intriguing prospects, such as the integration of the properties of distinct compounds into a single material or for the emergence of new properties from the interactions among different domains. In this article, we develop a strategy for the design of such complex structures, which we term the interface nucleus approach. Within this framework, the assembly of complex structures is facilitated by interface nuclei: geometrical motifs shared by two parent structures that serve as a region of overlap to nucleate or seed the formation of a combined structure. Our central hypothesis is that the formation of an interface between structures at these motifs creates opportunities for the relief of atomic packing stresses, as revealed by Density Functional Theory-Chemical Pressure (DFT-CP) analysis: when corresponding interatomic contacts in two structures exhibit complementarity─negative CP with positive CP or intense CP with mild CP─the intergrowth allows for a more balanced packing arrangement. To illustrate the application of the interface nucleus concept, we analyze three modular intermetallic structures, the σ-phase (FeCr), PuNi3, and Ca6Cu6Al5 types. In each case, the assembly of the structure can be connected to complementary CP features in an interface nucleus shared by its parent structures, while the distribution of the interface nuclei in the parents serves to template the geometry of the overall framework. In this way, the interface nucleus approach points toward avenues for the design of modular intermetallics from the CP schemes of potential partner structures.

Role of Partial Vacancy and Structural Distortion in the Stability of Nonstoichiometric Phases Ni7-δInSe2-xSx (1.26 ≥ δ ≥ 0.94; 0 ≤ x ≤ 1.33)

This study explores the structure and stability of partly disordered sulfur-substituted Ni5.74InSe2 (I4/mmm, a = 3.6766(1) Å, c = 18.8178(10) Å, Z = 2). The structure of Ni7-δInSe2-xSx (x = 0.2, 0.36, 0.66, 0.80, 0.94) compounds is isotypic to their parent Ni5.74InSe2 and can be viewed as alternating heterometallic Cu3Au-type ∞2[Ni3In] slabs and defective Cu2Sb-type ∞2[Ni4-δ(Se/S)2] slabs along the [001]-axis. Similar to the parent Se-compound, the Ni-Ch (Ch = chalcogen) fragment is non-stoichiometric and possesses a partially occupied Ni-site. It was observed that with sulfur insertion at the selenium site of Ni5.74InSe2, the interatomic distance between the partially occupied nickel and mixed (S/Se) sites decreases from ∼2.24 to ∼1.95 Å, and the occupancy of the disordered nickel site simultaneously increases. The limiting composition Ni6.06InSe0.67S1.33 (x = 1.33, δ = 0.94) is formed in the sulfur-rich region. Its average structure resembles the Ni6SnS2-type and has a similar motif to Ni5.74InSe2; the only difference is that Cu3Au-type ∞2[Ni3In] alternates with two types of Ni-Ch fragments (Cu2Sb or Li2O type units). By using first-principles electronic structure calculations, we explained the presence of partially disordered nickel sites in the Ni-Ch fragment and rationalized why the nickel site occupancy increases with sulfur insertion.

Merging the AuCu3- and BaAl4-based structure motifs: flux-assisted synthesis, crystal, and electronic structure of Ca2Pt7XP4-δ phosphide platinides (X = Al, Ti, and Zn)

Three quaternary phosphide platinides, Ca₂Pt₇AlP_3.00(4), Ca₂Pt₇TiP_3.24(4), and Ca₂Pt₇ZnP_2.78(2), were synthesized by a high-temperature technique using lead as a flux. According to the single-crystal diffraction data, they are isotypic and crystallize in the tetragonal space group I4/mmm with Z = 2 (Ca₂Pt₇AlP_3.00(4): a = 3.9893(6) Å, c = 26.832(5) Å; Ca₂Pt₇TiP_3.24(4): a = 3.99610(10) Å, c = 26.9074(17) Å; Ca₂Pt₇ZnP_2.78(2): a = 4.0020(2) Å, c = 26.5549(17) Å) and thus represent first europium-free compounds of the Eu₂Pt₇AlP_2.95 structure type. Their structures can be described as an intergrowth of the AuCu₃- and CaBe₂Ge₂-type blocks. DFT calculations predict metallic conductivity and non-magnetic state for all three compounds. Bonding analysis based on the Bader charge distribution and ELF topology reveals a combination of localized covalent and ionic interactions in the CaBe₂Ge₂-type fragments and complex pattern of pairwise, multi-center, and ionic interactions in the AuCu₃-type fragments that closely reproduces bonding in the parent Pt₃X (X = Al, Ti, Zn) binary intermetallics.

Nickel - p-block metal mixed chalcogenides based on AuCu3-type fragments: iodine-assisted synthesis as a way of obtaining new structures

Two new mixed nickel-gallium chalcogenides, Ni9.39Ga2S2 and Ni5.80GaTe2, and a new mixed nickel-indium telluride, Ni5.78InTe2, have been synthesized by a high-temperature ampoule route with the addition of iodine, and characterized from single-crystal or powder diffraction data. They belong to the relatively uncommon Ni7-xMQ2/Ni10-xM2Q2 type of structures (M = Ge, Sn, Sb, In), and are built from p-block metal-centered nickel cuboctahedra, alternating along the c axis with defective Cu2Sb-type nickel-chalcogen ones. Both tellurium-containing compounds show a small degree of orthorhombic distortion with respect to the idealized tetragonal structure, only detectable in the powder diffraction data. No phase transition to the tetragonal structure was detected for Ni5.80GaTe2 by the in situ powder diffraction measurements from room temperature to 550 °C. DFT calculations show close relationships of electronic structures of these ternary compounds to their parent intermetallics, Ni3M (M = Ga, In). Metallic conductivity and paramagnetic properties are predicted for all three with the latter confirmed by magnetic measurements. The bonding patterns, investigated via the ELF topological analysis, show multi-centered nickel - p-block metal bonds in the AuCu3-type fragments and pairwise covalent interactions in the nickel-chalcogen fragments. Both Ni7-xMTe2 compounds showed no structural or compositional changes upon high-temperature mid-pressure hydrogenation.