DOE OSTI2020
Ca3Si2As4 crystallizes in the monoclinic P2_1/c space group. The structure is three-dimensional. there are three inequivalent Ca2+ sites. In the first Ca2+ site, Ca2+ is bonded to six As+1.50- atoms to form CaAs6 octahedra that share corners with four CaAs6 octahedra, corners with four SiSiAs3 tetrahedra, edges with nine CaAs6 octahedra, and edges with two equivalent SiSiAs3 tetrahedra. The corner-sharing octahedra tilt angles range from 2–6°. There are a spread of Ca–As bond distances ranging from 2.99–3.18 Å. In the second Ca2+ site, Ca2+ is bonded to six As+1.50- atoms to form CaAs6 octahedra that share corners with four CaAs6 octahedra, corners with four SiSiAs3 tetrahedra, edges with nine CaAs6 octahedra, and edges with two SiSiAs3 tetrahedra. The corner-sharing octahedra tilt angles range from 2–9°. There are a spread of Ca–As bond distances ranging from 2.99–3.10 Å. In the third Ca2+ site, Ca2+ is bonded to six As+1.50- atoms to form CaAs6 octahedra that share corners with four CaAs6 octahedra, corners with eight SiSiAs3 tetrahedra, edges with eight CaAs6 octahedra, and an edgeedge with one SiSiAs3 tetrahedra. The corner-sharing octahedra tilt angles range from 3–9°. There are a spread of Ca–As bond distances ranging from 2.96–3.13 Å. There are two inequivalent Si sites. In the first Si site, Si is bonded to one Si and three As+1.50- atoms to form SiSiAs3 tetrahedra that share corners with three AsCa5Si octahedra, corners with eight CaAs6 octahedra, corners with three SiSiAs3 tetrahedra, and edges with two CaAs6 octahedra. The corner-sharing octahedra tilt angles range from 14–82°. The Si–Si bond length is 2.35 Å. There are a spread of Si–As bond distances ranging from 2.39–2.42 Å. In the second Si site, Si is bonded to one Si and three As+1.50- atoms to form distorted SiSiAs3 tetrahedra that share corners with three AsCa4Si2 octahedra, corners with eight CaAs6 octahedra, a cornercorner with one SiSiAs3 tetrahedra, and edges with three CaAs6 octahedra. The corner-sharing octahedra tilt angles range from 19–80°. There are a spread of Si–As bond distances ranging from 2.35–2.43 Å. There are four inequivalent As+1.50- sites. In the first As+1.50- site, As+1.50- is bonded to five Ca2+ and one Si atom to form AsCa5Si octahedra that share corners with seven AsCa4Si2 octahedra, a cornercorner with one SiSiAs3 tetrahedra, and edges with ten AsCa4Si2 octahedra. The corner-sharing octahedra tilt angles range from 4–92°. In the second As+1.50- site, As+1.50- is bonded to four Ca2+ and two equivalent Si atoms to form AsCa4Si2 octahedra that share corners with eight AsCa4Si2 octahedra, corners with two equivalent SiSiAs3 tetrahedra, and edges with eight AsCa4Si2 octahedra. The corner-sharing octahedra tilt angles range from 4–92°. In the third As+1.50- site, As+1.50- is bonded to four Ca2+ and two Si atoms to form AsCa4Si2 octahedra that share corners with eight AsCa5Si octahedra, corners with two SiSiAs3 tetrahedra, and edges with eight AsCa5Si octahedra. The corner-sharing octahedra tilt angles range from 5–93°. In the fourth As+1.50- site, As+1.50- is bonded to five Ca2+ and one Si atom to form AsCa5Si octahedra that share corners with seven AsCa4Si2 octahedra, a cornercorner with one SiSiAs3 tetrahedra, and edges with ten AsCa5Si octahedra. The corner-sharing octahedra tilt angles range from 4–87°.