Ca3Si2As4 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to one As(1), one As(2), two equivalent As(3), and two equivalent As(4) atoms to form distorted CaAs6 octahedra that share corners with two equivalent Ca(2)As6 octahedra, corners with two equivalent Ca(3)As6 octahedra, corners with three equivalent Si(1)SiAs3 tetrahedra, edges with two equivalent Ca(1)As6 octahedra, edges with three equivalent Ca(3)As6 octahedra, and edges with four equivalent Ca(2)As6 octahedra. The corner-sharing octahedral tilt angles range from 2-7°. In the second Ca site, Ca(2) is bonded to one As(1), one As(2), two equivalent As(3), and two equivalent As(4) atoms to form CaAs6 octahedra that share corners with two equivalent Ca(1)As6 octahedra, corners with two equivalent Ca(3)As6 octahedra,  a cornercorner with one Si(1)SiAs3 tetrahedra, edges with two equivalent Ca(2)As6 octahedra, edges with three equivalent Ca(3)As6 octahedra, edges with four equivalent Ca(1)As6 octahedra, and  an edgeedge with one Si(1)SiAs3 tetrahedra. The corner-sharing octahedral tilt angles range from 2-9°. In the third Ca site, Ca(3) is bonded to one As(3), one As(4), two equivalent As(1), and two equivalent As(2) atoms to form CaAs6 octahedra that share corners with two equivalent Ca(1)As6 octahedra, corners with two equivalent Ca(2)As6 octahedra, corners with four equivalent Si(1)SiAs3 tetrahedra, edges with two equivalent Ca(3)As6 octahedra, edges with three equivalent Ca(1)As6 octahedra, edges with three equivalent Ca(2)As6 octahedra, and  an edgeedge with one Si(1)SiAs3 tetrahedra. The corner-sharing octahedral tilt angles range from 3-9°. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one Si(2), one As(2), and two equivalent As(1) atoms to form distorted SiSiAs3 tetrahedra that share  a cornercorner with one Ca(2)As6 octahedra,  a cornercorner with one As(2)Ca4Si2 octahedra,  a cornercorner with one As(3)Ca5Si octahedra,  a cornercorner with one As(4)Ca5Si octahedra, corners with three equivalent Ca(1)As6 octahedra, corners with four equivalent Ca(3)As6 octahedra, corners with two equivalent Si(1)SiAs3 tetrahedra,  an edgeedge with one Ca(2)As6 octahedra, and  an edgeedge with one Ca(3)As6 octahedra. The corner-sharing octahedral tilt angles range from 13-82°. In the second Si site, Si(2) is bonded in a 4-coordinate geometry to one Si(1), one As(2), one As(3), and one As(4) atom. There are four inequivalent As sites. In the first As site, As(4) is bonded to one Ca(3), two equivalent Ca(1), two equivalent Ca(2), and one Si(2) atom to form AsCa5Si octahedra that share corners with two equivalent As(1)Ca4Si2 octahedra, corners with two equivalent As(3)Ca5Si octahedra, corners with three equivalent As(2)Ca4Si2 octahedra,  a cornercorner with one Si(1)SiAs3 tetrahedra, edges with two equivalent As(1)Ca4Si2 octahedra, edges with two equivalent As(2)Ca4Si2 octahedra, edges with two equivalent As(4)Ca5Si octahedra, and edges with four equivalent As(3)Ca5Si octahedra. The corner-sharing octahedral tilt angles range from 4-92°. In the second As site, As(1) is bonded to one Ca(1), one Ca(2), two equivalent Ca(3), and two equivalent Si(1) atoms to form AsCa4Si2 octahedra that share corners with two equivalent As(2)Ca4Si2 octahedra, corners with two equivalent As(4)Ca5Si octahedra, corners with four equivalent As(3)Ca5Si octahedra,  an edgeedge with one As(3)Ca5Si octahedra, edges with two equivalent As(1)Ca4Si2 octahedra, edges with two equivalent As(4)Ca5Si octahedra, and edges with three equivalent As(2)Ca4Si2 octahedra. The corner-sharing octahedral tilt angles range from 4-92°. In the third As site, As(2) is bonded to one Ca(1), one Ca(2), two equivalent Ca(3), one Si(1), and one Si(2) atom to form AsCa4Si2 octahedra that share  a cornercorner with one As(3)Ca5Si octahedra, corners with two equivalent As(1)Ca4Si2 octahedra, corners with two equivalent As(2)Ca4Si2 octahedra, corners with three equivalent As(4)Ca5Si octahedra,  a cornercorner with one Si(1)SiAs3 tetrahedra, edges with two equivalent As(4)Ca5Si octahedra, edges with three equivalent As(1)Ca4Si2 octahedra, and edges with three equivalent As(3)Ca5Si octahedra. The corner-sharing octahedral tilt angles range from 5-94°. In the fourth As site, As(3) is bonded to one Ca(3), two equivalent Ca(1), two equivalent Ca(2), and one Si(2) atom to form AsCa5Si octahedra that share  a cornercorner with one As(2)Ca4Si2 octahedra, corners with two equivalent As(4)Ca5Si octahedra, corners with four equivalent As(1)Ca4Si2 octahedra,  a cornercorner with one Si(1)SiAs3 tetrahedra,  an edgeedge with one As(1)Ca4Si2 octahedra, edges with two equivalent As(3)Ca5Si octahedra, edges with three equivalent As(2)Ca4Si2 octahedra, and edges with four equivalent As(4)Ca5Si octahedra. The corner-sharing octahedral tilt angles range from 4-88°.
Ca3Si2As4 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to one As(1), one As(2), two equivalent As(3), and two equivalent As(4) atoms to form distorted CaAs6 octahedra that share corners with two equivalent Ca(2)As6 octahedra, corners with two equivalent Ca(3)As6 octahedra, corners with three equivalent Si(1)SiAs3 tetrahedra, edges with two equivalent Ca(1)As6 octahedra, edges with three equivalent Ca(3)As6 octahedra, and edges with four equivalent Ca(2)As6 octahedra. The corner-sharing octahedral tilt angles range from 2-7°. The Ca(1)-As(1) bond length is 3.17 Å. The Ca(1)-As(2) bond length is 3.13 Å. There is one shorter (2.98 Å) and one longer (2.99 Å) Ca(1)-As(3) bond length. There is one shorter (2.97 Å) and one longer (3.00 Å) Ca(1)-As(4) bond length. In the second Ca site, Ca(2) is bonded to one As(1), one As(2), two equivalent As(3), and two equivalent As(4) atoms to form CaAs6 octahedra that share corners with two equivalent Ca(1)As6 octahedra, corners with two equivalent Ca(3)As6 octahedra,  a cornercorner with one Si(1)SiAs3 tetrahedra, edges with two equivalent Ca(2)As6 octahedra, edges with three equivalent Ca(3)As6 octahedra, edges with four equivalent Ca(1)As6 octahedra, and  an edgeedge with one Si(1)SiAs3 tetrahedra. The corner-sharing octahedral tilt angles range from 2-9°. The Ca(2)-As(1) bond length is 3.09 Å. The Ca(2)-As(2) bond length is 3.06 Å. There is one shorter (3.01 Å) and one longer (3.03 Å) Ca(2)-As(3) bond length. There is one shorter (2.98 Å) and one longer (3.03 Å) Ca(2)-As(4) bond length. In the third Ca site, Ca(3) is bonded to one As(3), one As(4), two equivalent As(1), and two equivalent As(2) atoms to form CaAs6 octahedra that share corners with two equivalent Ca(1)As6 octahedra, corners with two equivalent Ca(2)As6 octahedra, corners with four equivalent Si(1)SiAs3 tetrahedra, edges with two equivalent Ca(3)As6 octahedra, edges with three equivalent Ca(1)As6 octahedra, edges with three equivalent Ca(2)As6 octahedra, and  an edgeedge with one Si(1)SiAs3 tetrahedra. The corner-sharing octahedral tilt angles range from 3-9°. The Ca(3)-As(3) bond length is 2.95 Å. The Ca(3)-As(4) bond length is 2.98 Å. There is one shorter (3.03 Å) and one longer (3.12 Å) Ca(3)-As(1) bond length. There is one shorter (3.04 Å) and one longer (3.12 Å) Ca(3)-As(2) bond length. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one Si(2), one As(2), and two equivalent As(1) atoms to form distorted SiSiAs3 tetrahedra that share  a cornercorner with one Ca(2)As6 octahedra,  a cornercorner with one As(2)Ca4Si2 octahedra,  a cornercorner with one As(3)Ca5Si octahedra,  a cornercorner with one As(4)Ca5Si octahedra, corners with three equivalent Ca(1)As6 octahedra, corners with four equivalent Ca(3)As6 octahedra, corners with two equivalent Si(1)SiAs3 tetrahedra,  an edgeedge with one Ca(2)As6 octahedra, and  an edgeedge with one Ca(3)As6 octahedra. The corner-sharing octahedral tilt angles range from 13-82°. The Si(1)-Si(2) bond length is 2.33 Å. The Si(1)-As(2) bond length is 2.37 Å. There is one shorter (2.39 Å) and one longer (2.40 Å) Si(1)-As(1) bond length. In the second Si site, Si(2) is bonded in a 4-coordinate geometry to one Si(1), one As(2), one As(3), and one As(4) atom. The Si(2)-As(2) bond length is 2.41 Å. The Si(2)-As(3) bond length is 2.33 Å. The Si(2)-As(4) bond length is 2.34 Å. There are four inequivalent As sites. In the first As site, As(4) is bonded to one Ca(3), two equivalent Ca(1), two equivalent Ca(2), and one Si(2) atom to form AsCa5Si octahedra that share corners with two equivalent As(1)Ca4Si2 octahedra, corners with two equivalent As(3)Ca5Si octahedra, corners with three equivalent As(2)Ca4Si2 octahedra,  a cornercorner with one Si(1)SiAs3 tetrahedra, edges with two equivalent As(1)Ca4Si2 octahedra, edges with two equivalent As(2)Ca4Si2 octahedra, edges with two equivalent As(4)Ca5Si octahedra, and edges with four equivalent As(3)Ca5Si octahedra. The corner-sharing octahedral tilt angles range from 4-92°. In the second As site, As(1) is bonded to one Ca(1), one Ca(2), two equivalent Ca(3), and two equivalent Si(1) atoms to form AsCa4Si2 octahedra that share corners with two equivalent As(2)Ca4Si2 octahedra, corners with two equivalent As(4)Ca5Si octahedra, corners with four equivalent As(3)Ca5Si octahedra,  an edgeedge with one As(3)Ca5Si octahedra, edges with two equivalent As(1)Ca4Si2 octahedra, edges with two equivalent As(4)Ca5Si octahedra, and edges with three equivalent As(2)Ca4Si2 octahedra. The corner-sharing octahedral tilt angles range from 4-92°. In the third As site, As(2) is bonded to one Ca(1), one Ca(2), two equivalent Ca(3), one Si(1), and one Si(2) atom to form AsCa4Si2 octahedra that share  a cornercorner with one As(3)Ca5Si octahedra, corners with two equivalent As(1)Ca4Si2 octahedra, corners with two equivalent As(2)Ca4Si2 octahedra, corners with three equivalent As(4)Ca5Si octahedra,  a cornercorner with one Si(1)SiAs3 tetrahedra, edges with two equivalent As(4)Ca5Si octahedra, edges with three equivalent As(1)Ca4Si2 octahedra, and edges with three equivalent As(3)Ca5Si octahedra. The corner-sharing octahedral tilt angles range from 5-94°. In the fourth As site, As(3) is bonded to one Ca(3), two equivalent Ca(1), two equivalent Ca(2), and one Si(2) atom to form AsCa5Si octahedra that share  a cornercorner with one As(2)Ca4Si2 octahedra, corners with two equivalent As(4)Ca5Si octahedra, corners with four equivalent As(1)Ca4Si2 octahedra,  a cornercorner with one Si(1)SiAs3 tetrahedra,  an edgeedge with one As(1)Ca4Si2 octahedra, edges with two equivalent As(3)Ca5Si octahedra, edges with three equivalent As(2)Ca4Si2 octahedra, and edges with four equivalent As(4)Ca5Si octahedra. The corner-sharing octahedral tilt angles range from 4-88°.