Mg14YSi crystallizes in the hexagonal P-6m2 space group. There are five inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Mg(4), four equivalent Mg(1), four equivalent Mg(3), and two equivalent Y(1) atoms to form MgY2Mg10 cuboctahedra that share corners with four equivalent Si(1)Mg12 cuboctahedra, corners with six equivalent Mg(1)Y2Mg10 cuboctahedra, corners with eight equivalent Mg(2)Mg10Si2 cuboctahedra, edges with two equivalent Y(1)Mg12 cuboctahedra, edges with four equivalent Mg(4)Mg12 cuboctahedra, edges with four equivalent Mg(1)Y2Mg10 cuboctahedra, edges with eight equivalent Mg(3)YMg10Si cuboctahedra, faces with two equivalent Mg(2)Mg10Si2 cuboctahedra, faces with two equivalent Mg(4)Mg12 cuboctahedra, faces with two equivalent Y(1)Mg12 cuboctahedra, faces with four equivalent Mg(1)Y2Mg10 cuboctahedra, and faces with ten equivalent Mg(3)YMg10Si cuboctahedra. In the second Mg site, Mg(2) is bonded to two equivalent Mg(4), four equivalent Mg(2), four equivalent Mg(3), and two equivalent Si(1) atoms to form distorted MgMg10Si2 cuboctahedra that share corners with four equivalent Y(1)Mg12 cuboctahedra, corners with six equivalent Mg(2)Mg10Si2 cuboctahedra, corners with eight equivalent Mg(1)Y2Mg10 cuboctahedra, edges with two equivalent Si(1)Mg12 cuboctahedra, edges with four equivalent Mg(2)Mg10Si2 cuboctahedra, edges with four equivalent Mg(4)Mg12 cuboctahedra, edges with eight equivalent Mg(3)YMg10Si cuboctahedra, faces with two equivalent Mg(4)Mg12 cuboctahedra, faces with two equivalent Mg(1)Y2Mg10 cuboctahedra, faces with two equivalent Si(1)Mg12 cuboctahedra, faces with four equivalent Mg(2)Mg10Si2 cuboctahedra, and faces with ten equivalent Mg(3)YMg10Si cuboctahedra. In the third Mg site, Mg(1) is bonded to two equivalent Mg(4); four Mg(1,1); four equivalent Mg(3); and two equivalent Y(1) atoms to form MgY2Mg10 cuboctahedra that share corners with four equivalent Si(1)Mg12 cuboctahedra; corners with six equivalent Mg(1)Y2Mg10 cuboctahedra; corners with eight equivalent Mg(2)Mg10Si2 cuboctahedra; edges with two equivalent Y(1)Mg12 cuboctahedra; edges with four equivalent Mg(4)Mg12 cuboctahedra; edges with four Mg(1,1)Y2Mg10 cuboctahedra; edges with eight equivalent Mg(3)YMg10Si cuboctahedra; faces with two equivalent Mg(2)Mg10Si2 cuboctahedra; faces with two equivalent Mg(4)Mg12 cuboctahedra; faces with two equivalent Y(1)Mg12 cuboctahedra; faces with four Mg(1,1)Y2Mg10 cuboctahedra; and faces with ten equivalent Mg(3)YMg10Si cuboctahedra. In the fourth Mg site, Mg(3) is bonded to two Mg(1,1); two equivalent Mg(2); two equivalent Mg(4); four equivalent Mg(3); one Y(1); and one Si(1) atom to form distorted MgYMg10Si cuboctahedra that share corners with four equivalent Mg(4)Mg12 cuboctahedra; corners with fourteen equivalent Mg(3)YMg10Si cuboctahedra; edges with two equivalent Mg(4)Mg12 cuboctahedra; edges with two equivalent Y(1)Mg12 cuboctahedra; edges with two equivalent Si(1)Mg12 cuboctahedra; edges with four equivalent Mg(2)Mg10Si2 cuboctahedra; edges with four Mg(1,1)Y2Mg10 cuboctahedra; edges with four equivalent Mg(3)YMg10Si cuboctahedra;  a faceface with one Y(1)Mg12 cuboctahedra;  a faceface with one Si(1)Mg12 cuboctahedra; faces with two equivalent Mg(4)Mg12 cuboctahedra; faces with five equivalent Mg(2)Mg10Si2 cuboctahedra; faces with five Mg(1,1)Y2Mg10 cuboctahedra; and faces with six equivalent Mg(3)YMg10Si cuboctahedra. In the fifth Mg site, Mg(4) is bonded to three Mg(1,1); three equivalent Mg(2); and six equivalent Mg(3) atoms to form MgMg12 cuboctahedra that share corners with six equivalent Mg(4)Mg12 cuboctahedra; corners with twelve equivalent Mg(3)YMg10Si cuboctahedra; edges with six equivalent Mg(2)Mg10Si2 cuboctahedra; edges with six Mg(1,1)Y2Mg10 cuboctahedra; edges with six equivalent Mg(3)YMg10Si cuboctahedra; faces with two equivalent Mg(4)Mg12 cuboctahedra; faces with three equivalent Mg(2)Mg10Si2 cuboctahedra; faces with three Mg(1,1)Y2Mg10 cuboctahedra; faces with three equivalent Y(1)Mg12 cuboctahedra; faces with three equivalent Si(1)Mg12 cuboctahedra; and faces with six equivalent Mg(3)YMg10Si cuboctahedra. Y(1) is bonded to six Mg(1,1) and six equivalent Mg(3) atoms to form YMg12 cuboctahedra that share corners with six equivalent Y(1)Mg12 cuboctahedra; corners with twelve equivalent Mg(2)Mg10Si2 cuboctahedra; edges with six Mg(1,1)Y2Mg10 cuboctahedra; edges with twelve equivalent Mg(3)YMg10Si cuboctahedra; faces with two equivalent Si(1)Mg12 cuboctahedra; faces with six equivalent Mg(4)Mg12 cuboctahedra; faces with six Mg(1,1)Y2Mg10 cuboctahedra; and faces with six equivalent Mg(3)YMg10Si cuboctahedra. Si(1) is bonded to six equivalent Mg(2) and six equivalent Mg(3) atoms to form SiMg12 cuboctahedra that share corners with six equivalent Si(1)Mg12 cuboctahedra; corners with twelve Mg(1,1)Y2Mg10 cuboctahedra; edges with six equivalent Mg(2)Mg10Si2 cuboctahedra; edges with twelve equivalent Mg(3)YMg10Si cuboctahedra; faces with two equivalent Y(1)Mg12 cuboctahedra; faces with six equivalent Mg(2)Mg10Si2 cuboctahedra; faces with six equivalent Mg(4)Mg12 cuboctahedra; and faces with six equivalent Mg(3)YMg10Si cuboctahedra.
Mg14YSi crystallizes in the hexagonal P-6m2 space group. There are five inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Mg(4), four equivalent Mg(1), four equivalent Mg(3), and two equivalent Y(1) atoms to form MgY2Mg10 cuboctahedra that share corners with four equivalent Si(1)Mg12 cuboctahedra, corners with six equivalent Mg(1)Y2Mg10 cuboctahedra, corners with eight equivalent Mg(2)Mg10Si2 cuboctahedra, edges with two equivalent Y(1)Mg12 cuboctahedra, edges with four equivalent Mg(4)Mg12 cuboctahedra, edges with four equivalent Mg(1)Y2Mg10 cuboctahedra, edges with eight equivalent Mg(3)YMg10Si cuboctahedra, faces with two equivalent Mg(2)Mg10Si2 cuboctahedra, faces with two equivalent Mg(4)Mg12 cuboctahedra, faces with two equivalent Y(1)Mg12 cuboctahedra, faces with four equivalent Mg(1)Y2Mg10 cuboctahedra, and faces with ten equivalent Mg(3)YMg10Si cuboctahedra. Both Mg(1)-Mg(4) bond lengths are 3.13 Å. There are two shorter (3.09 Å) and two longer (3.31 Å) Mg(1)-Mg(1) bond lengths. All Mg(1)-Mg(3) bond lengths are 3.25 Å. Both Mg(1)-Y(1) bond lengths are 3.20 Å. In the second Mg site, Mg(2) is bonded to two equivalent Mg(4), four equivalent Mg(2), four equivalent Mg(3), and two equivalent Si(1) atoms to form distorted MgMg10Si2 cuboctahedra that share corners with four equivalent Y(1)Mg12 cuboctahedra, corners with six equivalent Mg(2)Mg10Si2 cuboctahedra, corners with eight equivalent Mg(1)Y2Mg10 cuboctahedra, edges with two equivalent Si(1)Mg12 cuboctahedra, edges with four equivalent Mg(2)Mg10Si2 cuboctahedra, edges with four equivalent Mg(4)Mg12 cuboctahedra, edges with eight equivalent Mg(3)YMg10Si cuboctahedra, faces with two equivalent Mg(4)Mg12 cuboctahedra, faces with two equivalent Mg(1)Y2Mg10 cuboctahedra, faces with two equivalent Si(1)Mg12 cuboctahedra, faces with four equivalent Mg(2)Mg10Si2 cuboctahedra, and faces with ten equivalent Mg(3)YMg10Si cuboctahedra. Both Mg(2)-Mg(4) bond lengths are 3.23 Å. There are two shorter (3.14 Å) and two longer (3.25 Å) Mg(2)-Mg(2) bond lengths. All Mg(2)-Mg(3) bond lengths are 3.12 Å. Both Mg(2)-Si(1) bond lengths are 3.20 Å. In the third Mg site, Mg(1) is bonded to two equivalent Mg(4); four Mg(1,1); four equivalent Mg(3); and two equivalent Y(1) atoms to form MgY2Mg10 cuboctahedra that share corners with four equivalent Si(1)Mg12 cuboctahedra; corners with six equivalent Mg(1)Y2Mg10 cuboctahedra; corners with eight equivalent Mg(2)Mg10Si2 cuboctahedra; edges with two equivalent Y(1)Mg12 cuboctahedra; edges with four equivalent Mg(4)Mg12 cuboctahedra; edges with four Mg(1,1)Y2Mg10 cuboctahedra; edges with eight equivalent Mg(3)YMg10Si cuboctahedra; faces with two equivalent Mg(2)Mg10Si2 cuboctahedra; faces with two equivalent Mg(4)Mg12 cuboctahedra; faces with two equivalent Y(1)Mg12 cuboctahedra; faces with four Mg(1,1)Y2Mg10 cuboctahedra; and faces with ten equivalent Mg(3)YMg10Si cuboctahedra. Both Mg(1)-Mg(4) bond lengths are 3.13 Å. There is one shorter (3.09 Å) and one longer (3.31 Å) Mg(1)-Mg(1) bond length. All Mg(1)-Mg(3) bond lengths are 3.25 Å. Both Mg(1)-Y(1) bond lengths are 3.20 Å. In the fourth Mg site, Mg(3) is bonded to two Mg(1,1); two equivalent Mg(2); two equivalent Mg(4); four equivalent Mg(3); one Y(1); and one Si(1) atom to form distorted MgYMg10Si cuboctahedra that share corners with four equivalent Mg(4)Mg12 cuboctahedra; corners with fourteen equivalent Mg(3)YMg10Si cuboctahedra; edges with two equivalent Mg(4)Mg12 cuboctahedra; edges with two equivalent Y(1)Mg12 cuboctahedra; edges with two equivalent Si(1)Mg12 cuboctahedra; edges with four equivalent Mg(2)Mg10Si2 cuboctahedra; edges with four Mg(1,1)Y2Mg10 cuboctahedra; edges with four equivalent Mg(3)YMg10Si cuboctahedra;  a faceface with one Y(1)Mg12 cuboctahedra;  a faceface with one Si(1)Mg12 cuboctahedra; faces with two equivalent Mg(4)Mg12 cuboctahedra; faces with five equivalent Mg(2)Mg10Si2 cuboctahedra; faces with five Mg(1,1)Y2Mg10 cuboctahedra; and faces with six equivalent Mg(3)YMg10Si cuboctahedra. Both Mg(3)-Mg(4) bond lengths are 3.21 Å. There are two shorter (3.13 Å) and two longer (3.26 Å) Mg(3)-Mg(3) bond lengths. The Mg(3)-Y(1) bond length is 3.23 Å. The Mg(3)-Si(1) bond length is 3.07 Å. In the fifth Mg site, Mg(4) is bonded to three Mg(1,1); three equivalent Mg(2); and six equivalent Mg(3) atoms to form MgMg12 cuboctahedra that share corners with six equivalent Mg(4)Mg12 cuboctahedra; corners with twelve equivalent Mg(3)YMg10Si cuboctahedra; edges with six equivalent Mg(2)Mg10Si2 cuboctahedra; edges with six Mg(1,1)Y2Mg10 cuboctahedra; edges with six equivalent Mg(3)YMg10Si cuboctahedra; faces with two equivalent Mg(4)Mg12 cuboctahedra; faces with three equivalent Mg(2)Mg10Si2 cuboctahedra; faces with three Mg(1,1)Y2Mg10 cuboctahedra; faces with three equivalent Y(1)Mg12 cuboctahedra; faces with three equivalent Si(1)Mg12 cuboctahedra; and faces with six equivalent Mg(3)YMg10Si cuboctahedra. Y(1) is bonded to six Mg(1,1) and six equivalent Mg(3) atoms to form YMg12 cuboctahedra that share corners with six equivalent Y(1)Mg12 cuboctahedra; corners with twelve equivalent Mg(2)Mg10Si2 cuboctahedra; edges with six Mg(1,1)Y2Mg10 cuboctahedra; edges with twelve equivalent Mg(3)YMg10Si cuboctahedra; faces with two equivalent Si(1)Mg12 cuboctahedra; faces with six equivalent Mg(4)Mg12 cuboctahedra; faces with six Mg(1,1)Y2Mg10 cuboctahedra; and faces with six equivalent Mg(3)YMg10Si cuboctahedra. Si(1) is bonded to six equivalent Mg(2) and six equivalent Mg(3) atoms to form SiMg12 cuboctahedra that share corners with six equivalent Si(1)Mg12 cuboctahedra; corners with twelve Mg(1,1)Y2Mg10 cuboctahedra; edges with six equivalent Mg(2)Mg10Si2 cuboctahedra; edges with twelve equivalent Mg(3)YMg10Si cuboctahedra; faces with two equivalent Y(1)Mg12 cuboctahedra; faces with six equivalent Mg(2)Mg10Si2 cuboctahedra; faces with six equivalent Mg(4)Mg12 cuboctahedra; and faces with six equivalent Mg(3)YMg10Si cuboctahedra.