BaCuTP2 (T = Al, Ga, In): a semiconducting black sheep in the ThCr2Si2 intermetallic family

Abstract

The ThCr₂Si₂ structure type has been well explored for decades with diverse magnetic, superconducting, and heavy-fermion behavior. For transition metal-containing ThCr₂Si₂-type compounds, a metallic band structure and properties are typical. In this work, a rare example of semiconducting BaCuTP₂ (T = Al, Ga, In) materials is reported. BaCuTP₂ materials retain the tetragonal I4/mmm ThCr₂Si₂-type crystal structure with a large c/a ratio of ∼3.3, where Cu and T metals jointly occupy the Cr-site. Edge-sharing (Cu/T)P₄ tetrahedra form [CuTP₂]²⁻ layers stacked along the crystallographic [001] direction, with the Ba²⁺ cations located in the interlayer spaces. Solid state NMR revealed partial short-range ordering in the Cu/T sublattice. The composition of the produced phases is electron balanced, Ba²⁺Cu¹⁺T³⁺(P³⁻)₂. High values of Seebeck coefficients were experimentally observed due to the high valley degeneracy in the band structure. Heat capacity and structural studies show that Ba exhibits anisotropic ‘rattling-like’ behavior along the [001] direction in Ga- and In-containing compounds. A combination of Ba rattling, short range Cu/T ordering, and a recently discovered coupling between acoustic and optical phonons for ThCr₂Si₂-type phosphides, resulted in ultralow thermal conductivity (<0.50 W m⁻¹ K⁻¹) for the title compounds.