BaCaO2 crystallizes in the orthorhombic Pnma space group. Ba(1) is bonded to three equivalent O(1) and four equivalent O(2) atoms to form distorted BaO7 pentagonal bipyramids that share corners with four equivalent Ba(1)O7 pentagonal bipyramids, corners with three equivalent Ca(1)O5 trigonal bipyramids, edges with four equivalent Ba(1)O7 pentagonal bipyramids, edges with seven equivalent Ca(1)O5 trigonal bipyramids, and faces with two equivalent Ba(1)O7 pentagonal bipyramids. Ca(1) is bonded to two equivalent O(2) and three equivalent O(1) atoms to form CaO5 trigonal bipyramids that share corners with three equivalent Ba(1)O7 pentagonal bipyramids, corners with four equivalent Ca(1)O5 trigonal bipyramids, edges with seven equivalent Ba(1)O7 pentagonal bipyramids, and edges with two equivalent Ca(1)O5 trigonal bipyramids. There are two inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Ba(1) and three equivalent Ca(1) atoms to form distorted OBa3Ca3 octahedra that share corners with nine equivalent O(2)Ba4Ca2 octahedra, edges with three equivalent O(2)Ba4Ca2 octahedra, edges with six equivalent O(1)Ba3Ca3 octahedra, and  a faceface with one O(2)Ba4Ca2 octahedra. The corner-sharing octahedral tilt angles range from 8-60°. In the second O site, O(2) is bonded to four equivalent Ba(1) and two equivalent Ca(1) atoms to form OBa4Ca2 octahedra that share corners with four equivalent O(2)Ba4Ca2 octahedra, corners with nine equivalent O(1)Ba3Ca3 octahedra, edges with two equivalent O(2)Ba4Ca2 octahedra, edges with three equivalent O(1)Ba3Ca3 octahedra,  a faceface with one O(1)Ba3Ca3 octahedra, and faces with two equivalent O(2)Ba4Ca2 octahedra. The corner-sharing octahedral tilt angles range from 8-60°.
BaCaO2 crystallizes in the orthorhombic Pnma space group. Ba(1) is bonded to three equivalent O(1) and four equivalent O(2) atoms to form distorted BaO7 pentagonal bipyramids that share corners with four equivalent Ba(1)O7 pentagonal bipyramids, corners with three equivalent Ca(1)O5 trigonal bipyramids, edges with four equivalent Ba(1)O7 pentagonal bipyramids, edges with seven equivalent Ca(1)O5 trigonal bipyramids, and faces with two equivalent Ba(1)O7 pentagonal bipyramids. There is one shorter (2.83 Å) and two longer (2.98 Å) Ba(1)-O(1) bond lengths. There are two shorter (2.76 Å) and two longer (2.82 Å) Ba(1)-O(2) bond lengths. Ca(1) is bonded to two equivalent O(2) and three equivalent O(1) atoms to form CaO5 trigonal bipyramids that share corners with three equivalent Ba(1)O7 pentagonal bipyramids, corners with four equivalent Ca(1)O5 trigonal bipyramids, edges with seven equivalent Ba(1)O7 pentagonal bipyramids, and edges with two equivalent Ca(1)O5 trigonal bipyramids. There is one shorter (2.35 Å) and one longer (2.37 Å) Ca(1)-O(2) bond length. There are two shorter (2.31 Å) and one longer (2.48 Å) Ca(1)-O(1) bond length. There are two inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Ba(1) and three equivalent Ca(1) atoms to form distorted OBa3Ca3 octahedra that share corners with nine equivalent O(2)Ba4Ca2 octahedra, edges with three equivalent O(2)Ba4Ca2 octahedra, edges with six equivalent O(1)Ba3Ca3 octahedra, and  a faceface with one O(2)Ba4Ca2 octahedra. The corner-sharing octahedral tilt angles range from 8-60°. In the second O site, O(2) is bonded to four equivalent Ba(1) and two equivalent Ca(1) atoms to form OBa4Ca2 octahedra that share corners with four equivalent O(2)Ba4Ca2 octahedra, corners with nine equivalent O(1)Ba3Ca3 octahedra, edges with two equivalent O(2)Ba4Ca2 octahedra, edges with three equivalent O(1)Ba3Ca3 octahedra,  a faceface with one O(1)Ba3Ca3 octahedra, and faces with two equivalent O(2)Ba4Ca2 octahedra. The corner-sharing octahedral tilt angles range from 8-60°.