Al_0.5Nb_1.5(PO₄)₃

Single crystals of the title compound, aluminium niobium triphosphate, Al_0.5Nb_1.5(PO₄)₃, have been synthesized by a high-temperature reaction in a platinium crucible. The Al^III and Nb^V atoms occupy the same site on the axis, with disorder in the ratio of 1:3. The fundamental building units of the title structure are isolated Al/NbO₆ octa­hedra and PO₄ tetra­hedra (. 2 symmetry), which are further inter­locked by corner-sharing O atoms, leading to a three-dimensional framework structure with infinite channels along the a axis.

Related literature

For related structures, see: Aatiq & Bakri, (2007); Boilot et al. (1987); Chakir et al. (2006); Hong (1976); Masquelier et al. (2000); Trubach et al. (2004); Rodrigo et al. (1989); Zatovskii et al. (2006); Zhao et al. (2009). For compounds with the same structure type, see: Benmokhtar et al. (2007); Leclaire et al. (1989). For related structures, see: Brochu et al. (1997).

Experimental

Crystal data

• Al_0.5Nb_1.5(PO₄)₃

• M_r = 437.76

• Trigonal,

• a = 8.5679 (6) Å

• c = 21.898 (2) Å

• V = 1392.14 (19) ų

• Z = 6

• Mo Kα radiation

• μ = 2.51 mm⁻¹

• T = 293 K

• 0.15 × 0.05 × 0.05 mm

Data collection

• Bruker SMART 1K CCD area-detector diffractometer

• Absorption correction: multi-scan (SADABS; Bruker, 1997) T_min = 0.704, T_max = 0.885

• 2295 measured reflections

• 302 independent reflections

• 298 reflections with I > 2σ(I)

• R_int = 0.029

Refinement

• R[F² > 2σ(F²)] = 0.027

• wR(F²) = 0.064

• S = 1.39

• 302 reflections

• 27 parameters

• Δρ_max = 0.45 e Å⁻³

• Δρ_min = −0.39 e Å⁻³

Data collection: SMART (Bruker, 1997); cell refinement: SAINT (Bruker, 1997); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2004); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Supporting information

S1. Comment

The mixed phosphates AM₂(PO₄)₃ family (A = alkali metals; M = Ti, Zr, Ge, Sn) which usually belong to the NASICON (Na₃Zr₂Si₂PO₁₂: Boilot, et al., 1987) or the NZP (NaZr₂(PO₄)₃: Hong, 1976) structure-type have been extensively investigated for the low thermal expansion behavior of some members. The crystal structure that features a flexible three-dimensional framework of PO₄ tetrahedra sharing comers with MO₆ octahedra, is amenable to a wide variety of chemical substitutions at the various crystallographic positions, thus yielding a large number of closely related compounds, such as Na₃MgZr(PO₄)₃ (Chakir, et al., 2006), Na₃Fe₂(PO₄)₃ (Masquelier, et al., 2000), NaFeNb(PO₄)₃ (Zatovskii, et al., 2006), NaTi₂(PO₄)₃ (Rodrigo, et al., 1989) and NaGe₂P₃O12 (Zhao et al., 2009). The three-dimensional network consisting of PO₄ and MO₆ octahedra delimit two different types of channels in which the A atoms are usually located to compensate the negative charges. It is reported that the A atoms can completely empty in some areas, such as Fe_0.5Nb_1.5(PO₄)₃ (Trubach, et al., 2004) and Fe_0.5Sb_1.5(PO₄)₃ (Aatiq & Bakri, 2007), Nb₂(PO₄)₃(Leclaire, et al.,1989) and Fe_0.5Ti₂(PO₄)₃(Benmokhtar, et al., 2007), etc. In order to inrich this type of compounds, we synthesis the compound Al_0.5Nb_1.5(PO₄)₃ by a high-temperature reaction and determine the crystal structure from single-crystal X-ray diffraction analysis.

As shown in Fig. 1, the asymmetric unit of Al_0.5Nb_1.5(PO₄)₃ contains a single P and Al/Nb atoms. The P atom is four coordinated by four oxygen atoms, forming isolated PO₄ tetrahedron. Al and Nb atoms are in mixed occupancy disorder locating at the 3 axes with the moral ratio of 1: 3, being coordinated by six oxygen atoms to form Al/NbO₆ octahedra. Al/NbO₆ octahedra and PO₄ tetrahedra are further interconnected via corner-sharing O atoms to form the three-dimensional framework of Al_0.5Nb_1.5(PO₄)₃, as shown in Fig. 2. The Al/Nb—O bonds have two groups of different distances, that is, 1.913 (3) and 1.949 (3) Å. The PO₄ tetrahedra are regular with two groups of P–O bond distances of 1.521 (3) and 1.529 (3) Å, and O–P–O bond angles weak dispersion from 107.91 (16) to 111.3 (2)^o, which is about the ideal value of 109.48°. On the other hand, this structure can be viewed as a NZP structure, in which the Na atom sites empty and the Zr atoms site are replaced by Al and Nb atoms in disordered manner on the principle of aliovalent pair combination Zr⁴⁺ → 0.25 A l³⁺ + 0.73 N b⁵⁺.

Related literature

For related structures, see: Aatiq & Bakri, (2007); Boilot et al. (1987); Chakir et al. (2006); Hong (1976); Masquelier et al. (2000); Trubach et al. (2004); Rodrigo et al. (1989); Zatovskii et al. (2006); Zhao et al. (2009). For compounds with the same structure type, see: Benmokhtar et al. (2007); Leclaire et al. (1989). For related structures, see: Brochu et al. (1997).

S2. Experimental

The finely ground reagents K₂CO₃, Al₂O₃, Nb₂O₅ and NH₄H₂PO₄ were mixed in the molar ratio K: Al: Nb: P = 1: 3: 10: 20, were placed in a Pt crucible, and heated at 573 K for 4 h. The mixture was then re-ground and heated at 1473 K for 20 h, then cooled to 973 K at a rate of 3 K h^-1, and finally quenched to room temperature. A few colorless crystals of the title compound with prismatic shape were obtained.

S3. Refinement

The structure contains substitutional disorder in which Al1 and Nb1 occupy the same position. The atomic positional and anisotropic displacement parameters of Al1 and Nb1 atoms were constrained to be identical by using EADP and EXYZ constraint instructions (SHELXL97; Sheldrick, 2008). The ratio of Al1 and Nb1 was fixed to 1: 3 to achieve charge balance.

Computing details

Data collection: SMART (Bruker, 1997); cell refinement: SAINT (Bruker, 1997); data reduction: SAINT (Bruker, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2004); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures

Fig. 1. The expanded asymmetric unit of Al0.5Nb1.5(PO4)3 showing the coordination environments of the P and Al/Nb atoms. The displacement ellipsoids are drawn at the 50% probability level.[Symmetry codes: (i) x, y, z; (ii) -x + y, -x, z; (iii) -y, x-y, z; (iv) 0.66667 - x, 0.33333 - x + y, 0.83333 - z; (v) 0.66667 - y, 0.33333 - x, -0.16667 + z; (vi) -1/3 + x, 1/3 + x-y, -0.16667 + z; (vii) -0.33333 - x + y, -2/3 + y, -0.16667 + z.]

Fig. 2. View of the crystal structure of Al0.5Nb1.5(PO4)3 along [010]. PO4 and Al/NbO6 units are given in the polyhedral representation.

aluminium(III) triniobium(V) phosphate(V)

Crystal data

Al0.5Nb1.5(PO4)3	Dx = 3.133 Mg m^−3
Mr = 437.76	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3c	Cell parameters from 247 reflections
Hall symbol: -R 3 2"c	θ = 2.6–25.0°
a = 8.5679 (6) Å	µ = 2.51 mm^−1
c = 21.898 (2) Å	T = 293 K
V = 1392.14 (19) Å^3	Prism, colourless
Z = 6	0.15 × 0.05 × 0.05 mm
F(000) = 1254

Data collection

Bruker SMART 1K CCD area-detector diffractometer	302 independent reflections
Radiation source: fine-focus sealed tube	298 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.029
ω scans	θmax = 25.7°, θmin = 3.3°
Absorption correction: multi-scan (SADABS; Bruker, 1997)	h = −7→10
Tmin = 0.704, Tmax = 0.885	k = −10→8
2295 measured reflections	l = −26→21

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.027	Secondary atom site location: difference Fourier map
wR(F^2) = 0.064	w = 1/[σ^2(Fo^2) + (0.0163P)^2 + 17.3988P] where P = (Fo^2 + 2Fc^2)/3
S = 1.39	(Δ/σ)max < 0.001
302 reflections	Δρmax = 0.45 e Å^−3
27 parameters	Δρmin = −0.39 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Nb1	0.0000	0.0000	0.35896 (3)	0.0091 (2)	0.75
Al1	0.0000	0.0000	0.35896 (3)	0.0091 (2)	0.25
P1	0.3333	0.38482 (17)	0.4167	0.0143 (4)
O1	0.1675 (4)	0.1984 (4)	0.40796 (12)	0.0173 (6)
O2	0.3025 (4)	0.4696 (4)	0.47305 (12)	0.0194 (7)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Nb1	0.0092 (3)	0.0092 (3)	0.0090 (4)	0.00460 (14)	0.000	0.000
Al1	0.0092 (3)	0.0092 (3)	0.0090 (4)	0.00460 (14)	0.000	0.000
P1	0.0179 (8)	0.0126 (5)	0.0141 (7)	0.0089 (4)	−0.0043 (6)	−0.0022 (3)
O1	0.0172 (15)	0.0132 (14)	0.0183 (14)	0.0053 (13)	−0.0039 (12)	−0.0051 (11)
O2	0.0253 (16)	0.0164 (15)	0.0162 (14)	0.0102 (14)	−0.0008 (12)	−0.0052 (11)

Geometric parameters (Å, º)

Nb1—O1	1.913 (3)	P1—O2	1.521 (3)
Nb1—O1^i	1.913 (3)	P1—O2^vi	1.521 (3)
Nb1—O1^ii	1.913 (3)	P1—O1^vi	1.529 (3)
Nb1—O2^iii	1.949 (3)	P1—O1	1.529 (3)
Nb1—O2^iv	1.949 (3)	O2—Al1^vii	1.949 (3)
Nb1—O2^v	1.949 (3)	O2—Nb1^vii	1.949 (3)
O1—Nb1—O1^i	91.63 (12)	O1^ii—Nb1—O2^v	89.81 (12)
O1—Nb1—O1^ii	91.63 (12)	O2^iii—Nb1—O2^v	88.66 (12)
O1^i—Nb1—O1^ii	91.63 (12)	O2^iv—Nb1—O2^v	88.66 (12)
O1—Nb1—O2^iii	89.81 (12)	O2—P1—O2^vi	111.3 (2)
O1^i—Nb1—O2^iii	89.86 (12)	O2—P1—O1^vi	110.32 (15)
O1^ii—Nb1—O2^iii	177.90 (12)	O2^vi—P1—O1^vi	107.91 (16)
O1—Nb1—O2^iv	177.90 (12)	O2—P1—O1	107.91 (16)
O1^i—Nb1—O2^iv	89.81 (12)	O2^vi—P1—O1	110.32 (15)
O1^ii—Nb1—O2^iv	89.86 (12)	O1^vi—P1—O1	109.1 (2)
O2^iii—Nb1—O2^iv	88.66 (12)	P1—O1—Nb1	152.96 (18)
O1—Nb1—O2^v	89.86 (12)	P1—O2—Al1^vii	155.8 (2)
O1^i—Nb1—O2^v	177.90 (12)	P1—O2—Nb1^vii	155.8 (2)

Symmetry codes: (i) −x+y, −x, z; (ii) −y, x−y, z; (iii) −y+2/3, −x+1/3, z−1/6; (iv) −x+y−1/3, y−2/3, z−1/6; (v) x−1/3, x−y+1/3, z−1/6; (vi) −x+2/3, −x+y+1/3, −z+5/6; (vii) −x+y+1/3, y+2/3, z+1/6.