DOE OSTI2020
K3Er4(CuTe2)5 crystallizes in the orthorhombic Pmn2_1 space group. The structure is three-dimensional. there are three inequivalent K1+ sites. In the first K1+ site, K1+ is bonded in a 7-coordinate geometry to seven Te2- atoms. There are a spread of K–Te bond distances ranging from 3.47–3.71 Å. In the second K1+ site, K1+ is bonded to seven Te2- atoms to form distorted KTe7 pentagonal bipyramids that share corners with six ErTe6 octahedra, corners with eight CuTe4 tetrahedra, edges with three ErTe6 octahedra, edges with three CuTe4 tetrahedra, a faceface with one ErTe6 octahedra, and faces with two equivalent KTe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 36–62°. There are a spread of K–Te bond distances ranging from 3.45–3.66 Å. In the third K1+ site, K1+ is bonded in a body-centered cubic geometry to eight Te2- atoms. There are a spread of K–Te bond distances ranging from 3.58–3.95 Å. There are four inequivalent Er3+ sites. In the first Er3+ site, Er3+ is bonded to six Te2- atoms to form ErTe6 octahedra that share corners with five ErTe6 octahedra, corners with four equivalent KTe7 pentagonal bipyramids, corners with four CuTe4 tetrahedra, edges with two equivalent ErTe6 octahedra, edges with four CuTe4 tetrahedra, a faceface with one KTe7 pentagonal bipyramid, and a faceface with one CuTe4 tetrahedra. The corner-sharing octahedra tilt angles range from 44–48°. There are a spread of Er–Te bond distances ranging from 2.99–3.19 Å. In the second Er3+ site, Er3+ is bonded to six Te2- atoms to form ErTe6 octahedra that share corners with five ErTe6 octahedra, corners with two equivalent CuTe4 tetrahedra, edges with two equivalent ErTe6 octahedra, and edges with four CuTe4 tetrahedra. The corner-sharing octahedra tilt angles range from 44–46°. There are a spread of Er–Te bond distances ranging from 3.00–3.10 Å. In the third Er3+ site, Er3+ is bonded to six Te2- atoms to form ErTe6 octahedra that share corners with five ErTe6 octahedra, corners with four CuTe4 tetrahedra, edges with two equivalent ErTe6 octahedra, edges with two equivalent KTe7 pentagonal bipyramids, edges with two equivalent CuTe4 tetrahedra, and a faceface with one CuTe4 tetrahedra. The corner-sharing octahedra tilt angles range from 44–48°. There are a spread of Er–Te bond distances ranging from 2.99–3.16 Å. In the fourth Er3+ site, Er3+ is bonded to six Te2- atoms to form ErTe6 octahedra that share corners with five ErTe6 octahedra, corners with two equivalent KTe7 pentagonal bipyramids, corners with three CuTe4 tetrahedra, edges with two equivalent ErTe6 octahedra, an edgeedge with one KTe7 pentagonal bipyramid, edges with four CuTe4 tetrahedra, and a faceface with one CuTe4 tetrahedra. The corner-sharing octahedral tilt angles are 44°. There are a spread of Er–Te bond distances ranging from 2.95–3.16 Å. There are five inequivalent Cu1+ sites. In the first Cu1+ site, Cu1+ is bonded to four Te2- atoms to form CuTe4 tetrahedra that share corners with four equivalent KTe7 pentagonal bipyramids, corners with four CuTe4 tetrahedra, edges with four ErTe6 octahedra, and an edgeedge with one KTe7 pentagonal bipyramid. There are two shorter (2.65 Å) and two longer (2.70 Å) Cu–Te bond lengths. In the second Cu1+ site, Cu1+ is bonded to four Te2- atoms to form CuTe4 tetrahedra that share corners with two equivalent KTe7 pentagonal bipyramids, corners with three CuTe4 tetrahedra, and edges with four ErTe6 octahedra. There are a spread of Cu–Te bond distances ranging from 2.63–2.78 Å. In the third Cu1+ site, Cu1+ is bonded to four Te2- atoms to form distorted CuTe4 tetrahedra that share corners with five ErTe6 octahedra, corners with two equivalent KTe7 pentagonal bipyramids, corners with four CuTe4 tetrahedra, edges with two equivalent ErTe6 octahedra, an edgeedge with one CuTe4 tetrahedra, and a faceface with one ErTe6 octahedra. The corner-sharing octahedra tilt angles range from 49–61°. There are a spread of Cu–Te bond distances ranging from 2.64–2.70 Å. In the fourth Cu1+ site, Cu1+ is bonded to four Te2- atoms to form distorted CuTe4 tetrahedra that share corners with five ErTe6 octahedra, corners with four CuTe4 tetrahedra, edges with two equivalent ErTe6 octahedra, an edgeedge with one CuTe4 tetrahedra, and a faceface with one ErTe6 octahedra. The corner-sharing octahedra tilt angles range from 46–61°. There are a spread of Cu–Te bond distances ranging from 2.61–2.72 Å. In the fifth Cu1+ site, Cu1+ is bonded to four Te2- atoms to form CuTe4 tetrahedra that share corners with three ErTe6 octahedra, corners with nine CuTe4 tetrahedra, edges with two equivalent ErTe6 octahedra, edges with two equivalent KTe7 pentagonal bipyramids, and a faceface with one ErTe6 octahedra. The corner-sharing octahedra tilt angles range from 17–47°. There are three shorter (2.64 Å) and one longer (2.91 Å) Cu–Te bond lengths. There are ten inequivalent Te2- sites. In the first Te2- site, Te2- is bonded in a 7-coordinate geometry to four K1+, two equivalent Er3+, and one Cu1+ atom. In the second Te2- site, Te2- is bonded in a 7-coordinate geometry to four K1+, two equivalent Er3+, and one Cu1+ atom. In the third Te2- site, Te2- is bonded to one K1+, three Er3+, and two equivalent Cu1+ atoms to form distorted edge-sharing TeKEr3Cu2 pentagonal pyramids. In the fourth Te2- site, Te2- is bonded to one K1+, three Er3+, and two equivalent Cu1+ atoms to form distorted edge-sharing TeKEr3Cu2 pentagonal pyramids. In the fifth Te2- site, Te2- is bonded in a 7-coordinate geometry to two equivalent K1+, two Er3+, and three Cu1+ atoms. In the sixth Te2- site, Te2- is bonded in a 6-coordinate geometry to two equivalent K1+, two Er3+, and two equivalent Cu1+ atoms. In the seventh Te2- site, Te2- is bonded in a 7-coordinate geometry to three Er3+ and four Cu1+ atoms. In the eighth Te2- site, Te2- is bonded in a 5-coordinate geometry to three Er3+ and two Cu1+ atoms. In the ninth Te2- site, Te2- is bonded in a 8-coordinate geometry to four K1+, two equivalent Er3+, and two Cu1+ atoms. In the tenth Te2- site, Te2- is bonded in a 7-coordinate geometry to four K1+, two equivalent Er3+, and one Cu1+ atom.