KErF4 crystallizes in the trigonal P3_1 space group. The structure is three-dimensional. there are six inequivalent K1+ sites. In the first K1+ site, K1+ is bonded to eight F1- atoms to form distorted KF8 hexagonal bipyramids that share a cornercorner with one ErF7 pentagonal bipyramid, edges with two KF8 hexagonal bipyramids, and edges with six ErF7 pentagonal bipyramids. There are a spread of K–F bond distances ranging from 2.56–3.05 Å. In the second K1+ site, K1+ is bonded in a 8-coordinate geometry to eight F1- atoms. There are a spread of K–F bond distances ranging from 2.63–3.06 Å. In the third K1+ site, K1+ is bonded to eight F1- atoms to form distorted KF8 hexagonal bipyramids that share a cornercorner with one ErF7 pentagonal bipyramid, edges with two KF8 hexagonal bipyramids, and edges with six ErF7 pentagonal bipyramids. There are a spread of K–F bond distances ranging from 2.62–3.02 Å. In the fourth K1+ site, K1+ is bonded in a 8-coordinate geometry to eight F1- atoms. There are a spread of K–F bond distances ranging from 2.59–3.07 Å. In the fifth K1+ site, K1+ is bonded to eight F1- atoms to form distorted KF8 hexagonal bipyramids that share a cornercorner with one ErF7 pentagonal bipyramid, edges with two KF8 hexagonal bipyramids, and edges with six ErF7 pentagonal bipyramids. There are a spread of K–F bond distances ranging from 2.62–3.08 Å. In the sixth K1+ site, K1+ is bonded in a 8-coordinate geometry to eight F1- atoms. There are a spread of K–F bond distances ranging from 2.61–2.90 Å. There are six inequivalent Er3+ sites. In the first Er3+ site, Er3+ is bonded to seven F1- atoms to form ErF7 pentagonal bipyramids that share a cornercorner with one KF8 hexagonal bipyramid, corners with three equivalent ErF7 pentagonal bipyramids, edges with three KF8 hexagonal bipyramids, and an edgeedge with one ErF7 pentagonal bipyramid. There are a spread of Er–F bond distances ranging from 2.15–2.31 Å. In the second Er3+ site, Er3+ is bonded to seven F1- atoms to form ErF7 pentagonal bipyramids that share corners with five ErF7 pentagonal bipyramids, edges with three KF8 hexagonal bipyramids, and an edgeedge with one ErF7 pentagonal bipyramid. There are a spread of Er–F bond distances ranging from 2.21–2.28 Å. In the third Er3+ site, Er3+ is bonded to seven F1- atoms to form ErF7 pentagonal bipyramids that share a cornercorner with one KF8 hexagonal bipyramid, corners with three equivalent ErF7 pentagonal bipyramids, edges with three KF8 hexagonal bipyramids, and an edgeedge with one ErF7 pentagonal bipyramid. There are a spread of Er–F bond distances ranging from 2.13–2.31 Å. In the fourth Er3+ site, Er3+ is bonded to seven F1- atoms to form ErF7 pentagonal bipyramids that share corners with five ErF7 pentagonal bipyramids, edges with three KF8 hexagonal bipyramids, and an edgeedge with one ErF7 pentagonal bipyramid. There are a spread of Er–F bond distances ranging from 2.21–2.28 Å. In the fifth Er3+ site, Er3+ is bonded to seven F1- atoms to form ErF7 pentagonal bipyramids that share a cornercorner with one KF8 hexagonal bipyramid, corners with three equivalent ErF7 pentagonal bipyramids, edges with three KF8 hexagonal bipyramids, and an edgeedge with one ErF7 pentagonal bipyramid. There are a spread of Er–F bond distances ranging from 2.15–2.31 Å. In the sixth Er3+ site, Er3+ is bonded to seven F1- atoms to form ErF7 pentagonal bipyramids that share corners with five ErF7 pentagonal bipyramids, edges with three KF8 hexagonal bipyramids, and an edgeedge with one ErF7 pentagonal bipyramid. There are a spread of Er–F bond distances ranging from 2.21–2.28 Å. There are twenty-four inequivalent F1- sites. In the first F1- site, F1- is bonded in a 4-coordinate geometry to two K1+ and two Er3+ atoms. In the second F1- site, F1- is bonded in a 2-coordinate geometry to two K1+ and two Er3+ atoms. In the third F1- site, F1- is bonded in a 3-coordinate geometry to one K1+ and two equivalent Er3+ atoms. In the fourth F1- site, F1- is bonded in a 2-coordinate geometry to one K1+ and two Er3+ atoms. In the fifth F1- site, F1- is bonded in a 4-coordinate geometry to two K1+ and two Er3+ atoms. In the sixth F1- site, F1- is bonded in a 2-coordinate geometry to two K1+ and two Er3+ atoms. In the seventh F1- site, F1- is bonded in a 2-coordinate geometry to one K1+ and two Er3+ atoms. In the eighth F1- site, F1- is bonded in a 2-coordinate geometry to one K1+ and two equivalent Er3+ atoms. In the ninth F1- site, F1- is bonded in a 2-coordinate geometry to two K1+ and two Er3+ atoms. In the tenth F1- site, F1- is bonded in a 4-coordinate geometry to two K1+ and two Er3+ atoms. In the eleventh F1- site, F1- is bonded in a 3-coordinate geometry to one K1+ and two equivalent Er3+ atoms. In the twelfth F1- site, F1- is bonded in a 2-coordinate geometry to one K1+ and two Er3+ atoms. In the thirteenth F1- site, F1- is bonded to three K1+ and one Er3+ atom to form a mixture of edge and corner-sharing FK3Er tetrahedra. In the fourteenth F1- site, F1- is bonded in a 4-coordinate geometry to two K1+ and two Er3+ atoms. In the fifteenth F1- site, F1- is bonded to three K1+ and one Er3+ atom to form a mixture of distorted edge and corner-sharing FK3Er tetrahedra. In the sixteenth F1- site, F1- is bonded in a 4-coordinate geometry to two K1+ and two Er3+ atoms. In the seventeenth F1- site, F1- is bonded in a 4-coordinate geometry to two K1+ and two Er3+ atoms. In the eighteenth F1- site, F1- is bonded to three K1+ and one Er3+ atom to form a mixture of distorted edge and corner-sharing FK3Er tetrahedra. In the nineteenth F1- site, F1- is bonded in a 4-coordinate geometry to two K1+ and two Er3+ atoms. In the twentieth F1- site, F1- is bonded to three K1+ and one Er3+ atom to form a mixture of distorted edge and corner-sharing FK3Er tetrahedra. In the twenty-first F1- site, F1- is bonded to three K1+ and one Er3+ atom to form a mixture of distorted edge and corner-sharing FK3Er tetrahedra. In the twenty-second F1- site, F1- is bonded in a 4-coordinate geometry to two K1+ and two Er3+ atoms. In the twenty-third F1- site, F1- is bonded in a 4-coordinate geometry to two K1+ and two Er3+ atoms. In the twenty-fourth F1- site, F1- is bonded to three K1+ and one Er3+ atom to form a mixture of distorted edge and corner-sharing FK3Er tetrahedra.