New layered quaternary BaCu6Sn2As4-x and BaCu6Sn2P4-x phases: crystal growth and physical properties

Abstract We report two new quaternary BaCu6Sn2As4-x and BaCu6Sn2P4-x phases with a new structure type, which are fully characterized by single crystal X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). These two phases crystallize in tetragonal cell with space group I4/mmm (#139) and Pearson symbol tI26. The refined lattice parameters are a = 4.164(1) A, c = 24.088(3) A for BaCu6Sn2As4-x, and a = 4.053(2) A, c = 24.08(1) A for BaCu6Sn2P4-x, respectively. They possess a distinct layered feature composed of Cu6Sn2Pnx (Pn = P, As) layers sandwiched by the Ba atoms. The Cu-Sn framework of the Cu6Sn2Pnx layer is closely related to the well-known Cu2Sb-type structure, and Pn atoms are found occupying two interstitial sites caused by close packing of Cu and Sn atoms or capped on the top of square nets formed by Cu atoms. The compounds are also characterized by electrical transport measurements down to 2 K.