NaNb2PO8 crystallizes in the monoclinic P2_1 space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded in a 6-coordinate geometry to one O(1), one O(10), one O(12), one O(4), one O(5), and one O(9) atom. In the second Na site, Na(2) is bonded in a 7-coordinate geometry to one O(11), one O(12), one O(13), one O(14), one O(15), one O(7), and one O(8) atom. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(10), one O(15), one O(3), one O(5), one O(7), and one O(9) atom to form NbO6 octahedra that share corners with three equivalent Nb(3)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra, and corners with two equivalent P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 7-20°. In the second Nb site, Nb(2) is bonded to one O(11), one O(13), one O(14), one O(2), and two equivalent O(6) atoms to form NbO6 octahedra that share corners with two equivalent Nb(2)O6 octahedra, corners with three equivalent Nb(4)O6 octahedra, and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 5-23°. In the third Nb site, Nb(3) is bonded to one O(1), one O(3), one O(5), one O(9), and two equivalent O(16) atoms to form NbO6 octahedra that share corners with two equivalent Nb(3)O6 octahedra, corners with three equivalent Nb(1)O6 octahedra, and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 7-20°. In the fourth Nb site, Nb(4) is bonded to one O(11), one O(12), one O(13), one O(2), one O(4), and one O(8) atom to form NbO6 octahedra that share corners with three equivalent Nb(2)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra, and corners with two equivalent P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 5-23°. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(10), one O(12), one O(14), and one O(4) atom to form PO4 tetrahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra, and corners with two equivalent Nb(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 12-34°. In the second P site, P(2) is bonded to one O(1), one O(15), one O(7), and one O(8) atom to form PO4 tetrahedra that share  a cornercorner with one Nb(3)O6 octahedra,  a cornercorner with one Nb(4)O6 octahedra, and corners with two equivalent Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 8-25°. There are sixteen inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Na(1), one Nb(3), and one P(2) atom. In the second O site, O(2) is bonded in a linear geometry to one Nb(2) and one Nb(4) atom. In the third O site, O(3) is bonded in a linear geometry to one Nb(1) and one Nb(3) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Na(1), one Nb(4), and one P(1) atom. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one Na(1), one Nb(1), and one Nb(3) atom. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to two equivalent Nb(2) atoms. In the seventh O site, O(7) is bonded in a 3-coordinate geometry to one Na(2), one Nb(1), and one P(2) atom. In the eighth O site, O(8) is bonded in a 2-coordinate geometry to one Na(2), one Nb(4), and one P(2) atom. In the ninth O site, O(9) is bonded in a distorted T-shaped geometry to one Na(1), one Nb(1), and one Nb(3) atom. In the tenth O site, O(10) is bonded in a 3-coordinate geometry to one Na(1), one Nb(1), and one P(1) atom. In the eleventh O site, O(11) is bonded in a distorted T-shaped geometry to one Na(2), one Nb(2), and one Nb(4) atom. In the twelfth O site, O(12) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Nb(4), and one P(1) atom. In the thirteenth O site, O(13) is bonded in a distorted T-shaped geometry to one Na(2), one Nb(2), and one Nb(4) atom. In the fourteenth O site, O(14) is bonded in a 3-coordinate geometry to one Na(2), one Nb(2), and one P(1) atom. In the fifteenth O site, O(15) is bonded in a 3-coordinate geometry to one Na(2), one Nb(1), and one P(2) atom. In the sixteenth O site, O(16) is bonded in a linear geometry to two equivalent Nb(3) atoms.
NaNb2PO8 crystallizes in the monoclinic P2_1 space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded in a 6-coordinate geometry to one O(1), one O(10), one O(12), one O(4), one O(5), and one O(9) atom. The Na(1)-O(1) bond length is 2.49 Å. The Na(1)-O(10) bond length is 2.61 Å. The Na(1)-O(12) bond length is 2.76 Å. The Na(1)-O(4) bond length is 2.39 Å. The Na(1)-O(5) bond length is 2.53 Å. The Na(1)-O(9) bond length is 2.55 Å. In the second Na site, Na(2) is bonded in a 7-coordinate geometry to one O(11), one O(12), one O(13), one O(14), one O(15), one O(7), and one O(8) atom. The Na(2)-O(11) bond length is 2.68 Å. The Na(2)-O(12) bond length is 2.87 Å. The Na(2)-O(13) bond length is 2.47 Å. The Na(2)-O(14) bond length is 2.49 Å. The Na(2)-O(15) bond length is 2.52 Å. The Na(2)-O(7) bond length is 2.53 Å. The Na(2)-O(8) bond length is 2.66 Å. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(10), one O(15), one O(3), one O(5), one O(7), and one O(9) atom to form NbO6 octahedra that share corners with three equivalent Nb(3)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra, and corners with two equivalent P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 7-20°. The Nb(1)-O(10) bond length is 2.08 Å. The Nb(1)-O(15) bond length is 2.18 Å. The Nb(1)-O(3) bond length is 1.95 Å. The Nb(1)-O(5) bond length is 1.87 Å. The Nb(1)-O(7) bond length is 2.23 Å. The Nb(1)-O(9) bond length is 1.85 Å. In the second Nb site, Nb(2) is bonded to one O(11), one O(13), one O(14), one O(2), and two equivalent O(6) atoms to form NbO6 octahedra that share corners with two equivalent Nb(2)O6 octahedra, corners with three equivalent Nb(4)O6 octahedra, and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 5-23°. The Nb(2)-O(11) bond length is 2.00 Å. The Nb(2)-O(13) bond length is 2.17 Å. The Nb(2)-O(14) bond length is 2.14 Å. The Nb(2)-O(2) bond length is 1.91 Å. There is one shorter (1.87 Å) and one longer (1.99 Å) Nb(2)-O(6) bond length. In the third Nb site, Nb(3) is bonded to one O(1), one O(3), one O(5), one O(9), and two equivalent O(16) atoms to form NbO6 octahedra that share corners with two equivalent Nb(3)O6 octahedra, corners with three equivalent Nb(1)O6 octahedra, and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 7-20°. The Nb(3)-O(1) bond length is 2.15 Å. The Nb(3)-O(3) bond length is 1.90 Å. The Nb(3)-O(5) bond length is 2.05 Å. The Nb(3)-O(9) bond length is 2.11 Å. There is one shorter (1.90 Å) and one longer (1.94 Å) Nb(3)-O(16) bond length. In the fourth Nb site, Nb(4) is bonded to one O(11), one O(12), one O(13), one O(2), one O(4), and one O(8) atom to form NbO6 octahedra that share corners with three equivalent Nb(2)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra, and corners with two equivalent P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 5-23°. The Nb(4)-O(11) bond length is 1.90 Å. The Nb(4)-O(12) bond length is 2.24 Å. The Nb(4)-O(13) bond length is 1.84 Å. The Nb(4)-O(2) bond length is 1.94 Å. The Nb(4)-O(4) bond length is 2.20 Å. The Nb(4)-O(8) bond length is 2.08 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(10), one O(12), one O(14), and one O(4) atom to form PO4 tetrahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra, and corners with two equivalent Nb(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 12-34°. The P(1)-O(10) bond length is 1.57 Å. The P(1)-O(12) bond length is 1.54 Å. The P(1)-O(14) bond length is 1.54 Å. The P(1)-O(4) bond length is 1.55 Å. In the second P site, P(2) is bonded to one O(1), one O(15), one O(7), and one O(8) atom to form PO4 tetrahedra that share  a cornercorner with one Nb(3)O6 octahedra,  a cornercorner with one Nb(4)O6 octahedra, and corners with two equivalent Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 8-25°. The P(2)-O(1) bond length is 1.54 Å. The P(2)-O(15) bond length is 1.54 Å. The P(2)-O(7) bond length is 1.54 Å. The P(2)-O(8) bond length is 1.56 Å. There are sixteen inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Na(1), one Nb(3), and one P(2) atom. In the second O site, O(2) is bonded in a linear geometry to one Nb(2) and one Nb(4) atom. In the third O site, O(3) is bonded in a linear geometry to one Nb(1) and one Nb(3) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Na(1), one Nb(4), and one P(1) atom. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one Na(1), one Nb(1), and one Nb(3) atom. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to two equivalent Nb(2) atoms. In the seventh O site, O(7) is bonded in a 3-coordinate geometry to one Na(2), one Nb(1), and one P(2) atom. In the eighth O site, O(8) is bonded in a 2-coordinate geometry to one Na(2), one Nb(4), and one P(2) atom. In the ninth O site, O(9) is bonded in a distorted T-shaped geometry to one Na(1), one Nb(1), and one Nb(3) atom. In the tenth O site, O(10) is bonded in a 3-coordinate geometry to one Na(1), one Nb(1), and one P(1) atom. In the eleventh O site, O(11) is bonded in a distorted T-shaped geometry to one Na(2), one Nb(2), and one Nb(4) atom. In the twelfth O site, O(12) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Nb(4), and one P(1) atom. In the thirteenth O site, O(13) is bonded in a distorted T-shaped geometry to one Na(2), one Nb(2), and one Nb(4) atom. In the fourteenth O site, O(14) is bonded in a 3-coordinate geometry to one Na(2), one Nb(2), and one P(1) atom. In the fifteenth O site, O(15) is bonded in a 3-coordinate geometry to one Na(2), one Nb(1), and one P(2) atom. In the sixteenth O site, O(16) is bonded in a linear geometry to two equivalent Nb(3) atoms.