Quasi-one-dimensional tetracyanoplatinates: first-principles computational study of the electronic structure, optical properties, and polarons

Tetracyanoplatinate chains have highly anisotropic electronic and optical properties that are readily tuned by their chemical environment. In Krogmann's salt K₂Pt(CN)₄X_0.3 (X = Br, Cl), known as KCP(X), the partial oxidation of Pt leads to metallic conductivity at room temperature. KCP(X) was heavily studied in the previous century due to its charge density wave physics and similarity to proposed excitonic superconductors, but modern first-principles electronic structure theory is lacking. We compare the electronic structure, phonon spectra, and optical spectra computed from density functional theory methods for KCP(X) and related materials. In particular, conduction band anti-crossings with flat bands derived from K ions are observed in KCP(Br). Semiconducting Cs₄[Pt(CN)₄](CF₃SO₃)₂ (TCP) is shown to have increased electronic and optical anisotropy relative to KCP(X) due to large triflate spacer molecules between platinate chains. Finally, the structure of polarons in TCP are investigated from density functional perturbation theory approaches.