New insights into the application of the valence rules in Zintl phases—Crystal and electronic structures of Ba7Ga4P9, Ba7Ga4As9, Ba7Al4Sb9, Ba6CaAl4Sb9, and Ba6CaGa4Sb9

Metal-Rich Phosphides Obtained from the Lead Flux: Synthesis, Crystal, and Electronic Structure of Sr5Pt12P9 and BaPt3P2

Using a high-temperature ampoule technique and lead metal as a flux, we have grown single crystals and determined crystal structures from single-crystal X-ray diffraction data of two metal-rich phosphides, Sr₅Pt₁₂P₉ (P 2₁/m, a = 6.1472(3) Å, b = 25.1713(13) Å, c = 6.4635(3) Å, β = 99.604(2)°, Z = 2, R₁ = 0.0326, wR₂ = 0.0786) and BaPt₃P₂ (P 2₁2₁2₁, a = 6.3605(6) Å, b = 6.8541(7) Å, c = 11.3493(12) Å, Z = 4, R₁ = 0.0231, wR₂ = 0.0501). Both compounds belong to their own structure types and feature 3D networks of Pt and P atoms, with the channels occupied by alkaline earth metal cations. Density functional theory calculations reveal Sr₅Pt₁₂P₉ to be a metal, while BaPt₃P₂ is a narrow-gap semiconductor with a band gap of 0.24 eV. Bonding analysis shows that both compounds feature networks of prominent covalent localized Pt-P bonds, responsible for their structural stability, as well as additional weaker and, likely, less localized Pt-Pt interactions.

Flux Growth of Phosphide and Arsenide Crystals

Flux crystal growth has been widely applied to explore new phases and grow crystals of emerging materials. To accommodate the needs of high-quality single crystals, the flux crystal growth should be reliable, controllable, and predictable. The selections of suitable flux and growth conditions remain empirical due to the lack of systematic investigation especially for reactions, which involve highly volatile components, such as P and As. Considering the flux elements, often the system in question is a quaternary or a higher multinary system, which drastically increases complexity. In this manuscript, on the examples of flux growth of phosphides and arsenides, guidelines of flux selections, existing challenges, and future directions are discussed. We expect that the field will be further developed by applying in situ techniques and computational modeling of the nucleation and growth kinetics. Additionally, leveraging variables other than temperature, such as applied pressure, will make flux growth a more powerful tool in the future.