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K4NaNb5O15 crystallizes in the monoclinic Pm space group. The structure is three-dimensional. there are eight inequivalent K1+ sites. In the first K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share corners with eleven KO12 cuboctahedra, faces with six KO12 cuboctahedra, and faces with eight NbO6 octahedra. There are a spread of K–O bond distances ranging from 2.77–3.13 Å. In the second K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share corners with twelve KO12 cuboctahedra, faces with six KO12 cuboctahedra, and faces with eight NbO6 octahedra. There are a spread of K–O bond distances ranging from 2.76–3.07 Å. In the third K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share corners with eleven KO12 cuboctahedra, faces with six KO12 cuboctahedra, and faces with eight NbO6 octahedra. There are a spread of K–O bond distances ranging from 2.77–3.08 Å. In the fourth K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share corners with four KO12 cuboctahedra, faces with two equivalent KO12 cuboctahedra, and faces with eight NbO6 octahedra. There are a spread of K–O bond distances ranging from 2.73–3.07 Å. In the fifth K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share corners with eight KO12 cuboctahedra, faces with four KO12 cuboctahedra, and faces with eight NbO6 octahedra. There are a spread of K–O bond distances ranging from 2.76–3.07 Å. In the sixth K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share corners with twelve KO12 cuboctahedra, faces with six KO12 cuboctahedra, and faces with eight NbO6 octahedra. There are a spread of K–O bond distances ranging from 2.76–3.07 Å. In the seventh K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share corners with twelve KO12 cuboctahedra, faces with six KO12 cuboctahedra, and faces with eight NbO6 octahedra. There are a spread of K–O bond distances ranging from 2.77–3.07 Å. In the eighth K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share corners with eight KO12 cuboctahedra, faces with four KO12 cuboctahedra, and faces with eight NbO6 octahedra. There are a spread of K–O bond distances ranging from 2.74–3.07 Å. There are two inequivalent Na1+ sites. In the first Na1+ site, Na1+ is bonded in a 12-coordinate geometry to nine O2- atoms. There are a spread of Na–O bond distances ranging from 2.59–3.02 Å. In the second Na1+ site, Na1+ is bonded in a 9-coordinate geometry to nine O2- atoms. There are a spread of Na–O bond distances ranging from 2.57–3.02 Å. There are ten inequivalent Nb5+ sites. In the first Nb5+ site, Nb5+ is bonded to six O2- atoms to form NbO6 octahedra that share corners with six NbO6 octahedra and faces with four KO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 1–18°. There are a spread of Nb–O bond distances ranging from 1.89–2.27 Å. In the second Nb5+ site, Nb5+ is bonded to six O2- atoms to form NbO6 octahedra that share corners with six NbO6 octahedra and faces with four KO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 2–18°. There are a spread of Nb–O bond distances ranging from 1.89–2.27 Å. In the third Nb5+ site, Nb5+ is bonded to six O2- atoms to form NbO6 octahedra that share corners with six NbO6 octahedra and faces with eight KO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 1–14°. There are a spread of Nb–O bond distances ranging from 1.88–2.29 Å. In the fourth Nb5+ site, Nb5+ is bonded to six O2- atoms to form NbO6 octahedra that share corners with six NbO6 octahedra and faces with eight KO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 2–14°. There are a spread of Nb–O bond distances ranging from 1.88–2.28 Å. In the fifth Nb5+ site, Nb5+ is bonded to six O2- atoms to form NbO6 octahedra that share corners with six NbO6 octahedra and faces with eight KO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 2–14°. There are a spread of Nb–O bond distances ranging from 1.88–2.28 Å. In the sixth Nb5+ site, Nb5+ is bonded to six O2- atoms to form NbO6 octahedra that share corners with six NbO6 octahedra and faces with four equivalent KO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 1–18°. There are a spread of Nb–O bond distances ranging from 1.88–2.25 Å. In the seventh Nb5+ site, Nb5+ is bonded to six O2- atoms to form NbO6 octahedra that share corners with six NbO6 octahedra and faces with six KO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 1–16°. There are a spread of Nb–O bond distances ranging from 1.89–2.25 Å. In the eighth Nb5+ site, Nb5+ is bonded to six O2- atoms to form NbO6 octahedra that share corners with six NbO6 octahedra and faces with eight KO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 2–14°. There are a spread of Nb–O bond distances ranging from 1.88–2.26 Å. In the ninth Nb5+ site, Nb5+ is bonded to six O2- atoms to form NbO6 octahedra that share corners with six NbO6 octahedra and faces with eight KO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 2–14°. There are a spread of Nb–O bond distances ranging from 1.88–2.25 Å. In the tenth Nb5+ site, Nb5+ is bonded to six O2- atoms to form NbO6 octahedra that share corners with six NbO6 octahedra and faces with six KO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 3–18°. There are a spread of Nb–O bond distances ranging from 1.88–2.27 Å. There are thirty inequivalent O2- sites. In the first O2- site, O2- is bonded in a 5-coordinate geometry to two equivalent K1+, one Na1+, and two equivalent Nb5+ atoms. In the second O2- site, O2- is bonded in a 2-coordinate geometry to three K1+, one Na1+, and two equivalent Nb5+ atoms. In the third O2- site, O2- is bonded in a 2-coordinate geometry to four K1+ and two equivalent Nb5+ atoms. In the fourth O2- site, O2- is bonded in a 2-coordinate geometry to four K1+ and two equivalent Nb5+ atoms. In the fifth O2- site, O2- is bonded in a 2-coordinate geometry to three K1+ and two equivalent Nb5+ atoms. In the sixth O2- site, O2- is bonded in a 2-coordinate geometry to two K1+, two equivalent Na1+, and two equivalent Nb5+ atoms. In the seventh O2- site, O2- is bonded in a 2-coordinate geometry to two K1+, two equivalent Na1+, and two equivalent Nb5+ atoms. In the eighth O2- site, O2- is bonded in a 2-coordinate geometry to four K1+ and two equivalent Nb5+ atoms. In the ninth O2- site, O2- is bonded in a 2-coordinate geometry to four K1+ and two equivalent Nb5+ atoms. In the tenth O2- site, O2- is bonded in a 2-coordinate geometry to four K1+ and two equivalent Nb5+ atoms. In the eleventh O2- site, O2- is bonded in a 1-coordinate geometry to two equivalent K1+, two equivalent Na1+, and two Nb5+ atoms. In the twelfth O2- site, O2- is bonded in a 1-coordinate geometry to two equivalent K1+, two equivalent Na1+, and two Nb5+ atoms. In the thirteenth O2- site, O2- is bonded in a 6-coordinate geometry to four K1+ and two Nb5+ atoms. In the fourteenth O2- site, O2- is bonded in a 6-coordinate geometry to four K1+ and two Nb5+ atoms. In the fifteenth O2- site, O2- is bonded in a 1-coordinate geometry to four K1+ and two Nb5+ atoms. In the sixteenth O2- site, O2- is bonded in a 4-coordinate geometry to two equivalent K1+ and two Nb5+ atoms. In the seventeenth O2- site, O2- is bonded in a 6-coordinate geometry to four K1+ and two Nb5+ atoms. In the eighteenth O2- site, O2- is bonded in a 2-coordinate geometry to four K1+ and two Nb5+ atoms. In the nineteenth O2- site, O2- is bonded in a 2-coordinate geometry to four K1+ and two Nb5+ atoms. In the twentieth O2- site, O2- is bonded in a 4-coordinate geometry to two equivalent K1+ and two Nb5+ atoms. In the twenty-first O2- site, O2- is bonded in a 6-coordinate geometry to two equivalent K1+, two equivalent Na1+, and two Nb5+ atoms. In the twenty-second O2- site, O2- is bonded in a 6-coordinate geometry to two equivalent K1+, two equivalent Na1+, and two Nb5+ atoms. In the twenty-third O2- site, O2- is bonded in a 6-coordinate geometry to four K1+ and two Nb5+ atoms. In the twenty-fourth O2- site, O2- is bonded in a 6-coordinate geometry to four K1+ and two Nb5+ atoms. In the twenty-fifth O2- site, O2- is bonded in a 6-coordinate geometry to four K1+ and two Nb5+ atoms. In the twenty-sixth O2- site, O2- is bonded in a 6-coordinate geometry to two equivalent K1+, two equivalent Na1+, and two Nb5+ atoms. In the twenty-seventh O2- site, O2- is bonded in a 6-coordinate geometry to four K1+ and two Nb5+ atoms. In the twenty-eighth O2- site, O2- is bonded in a 6-coordinate geometry to four K1+ and two Nb5+ atoms. In the twenty-ninth O2- site, O2- is bonded in a 6-coordinate geometry to four K1+ and two Nb5+ atoms. In the thirtieth O2- site, O2- is bonded in a 6-coordinate geometry to two equivalent K1+, two equivalent Na1+, and two Nb5+ atoms.

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