BaCu_2-xT_xP₂ (T = Al, Ga, In, Si, or {Si+Zn}): A New Group Semiconducting 'Golden Geese' in The ThCr₂Si₂ Family with Ultra-Low Thermal Conductivity and High Seebeck Coefficien

ThCr₂Si₂-type layered materials are a large family of compounds with applications ranging from thermoelectricity to magnetism, with most of them showing metallic behavior. In this study, we synthesized a variety of new ThCr₂Si₂-type materials with the general formula BaCu_2-<i>x</i><i>T_x</i>P₂ (<i>T</i> = Al, Ga, In, Si, or {Si+Zn}), most being charge balanced semiconductors, a rarity in this family. They all crystallize in the ThCr₂Si₂-type tetragonal <i>I</i>4/<i>mmm</i> space group, with Cu/<i>T</i> jointly occupying the same 4<i>d</i> crystallographic site. In the case of BaCuAlP₂ and BaCu_1.33Si_0.67P₂, Ba atom occupies a single crystallographic site. However, the introduction of Zn in the BaCu_1.33Si_0.67P₂system results in the expansion of the unit cell by 4%, splitting the Ba atom along the crystallographic <i>c</i>-direction. Similar structural distortions are observed for BaCuGaP₂ and BaCuInP₂ as well. Such structural disorder of the Ba atoms leads to the occurrence of clathrate-like rattling behavior along the <i>c</i>-direction, as observed from heat capacity measurements. This in turn leads to ultra-low thermal conductivity (as low as ~0.4 W/m-K at 550 °C) at high temperatures. All BaCu_2-<i>x</i><i>T_x</i>P₂ materials show semiconducting behavior, making them potential solar absorbers. The composition with the lowest Zn-content, BaCu_1.3Zn_0.2Si_0.5P₂, exhibits a clear semiconductor-to-metal transition upon heating above 155 K. The materials show high Seebeck coefficients, such as ~300 <i>μ</i>V/K at 550 °C for BaCuAlP₂, making them promising candidates for thermoelectric applications. Band structure and density-of-states calculations on ordered hypothetical structures reveal clear semiconducting nature for the triel-based materials and semi-metallic nature for the Si-based ones.