LiHo(PO₃)₄

Lithium holmium(III) polyphosphate(V), LiHo(PO₃)₄, belongs to the type I of polyphosphates with general formula ALn(PO₃)₄, where A is a monovalent cation and Ln is a trivalent rare earth cation. In the crystal structure, the polyphosphate chains spread along the b-axis direction, with a repeat period of four tetra­hedra and 2₁ inter­nal symmetry. The Li and Ho atoms are both located on twofold rotation axes and are surrounded by four and eight O atoms, leading to a distorted tetra­hedral and dodeca­hedral coordination, respectively. The HoO₈ polyhedra are isolated from each other, the closest Ho⋯Ho distance being 5.570 (1) Å.

Related literature

For isotypic LiLn(PO₃)₄ structures, see: Ben Zarkouna & Driss (2004) for Ln = Yb; Ben Zarkouna et al. (2005) for Ln = Er; Ben Zarkouna et al. (2007) for Ln = Tb. For related structures, see: Amami et al. (2004) (NaHo(PO₃)₄); Palkina et al. (1976) (β-KHo(PO₃)₄); Tranqui et al. (1972) (HoP₅O₁₄). For general background, see: Brown & Altermatt (1985); Durif (1995); Liu et al. (1999); Palkina et al. (1981); Inter­national Tables for X-Ray Crystallography (1968).

Experimental

Crystal data

• LiHo(PO₃)₄

• M_r = 487.75

• Monoclinic, C 2/c

• a = 16.280 (3) Å

• b = 7.039 (2) Å

• c = 9.561 (2) Å

• β = 125.99 (2)°

• V = 886.5 (4) ų

• Z = 4

• Mo Kα radiation

• μ = 9.72 mm⁻¹

• T = 293 (2) K

• 0.31 × 0.14 × 0.07 mm

Data collection

• Enraf–Nonius CAD-4 diffractometer

• Absorption correction: ψ scan (North et al., 1968) T_min = 0.095, T_max = 0.378 (expected range = 0.128–0.507)

• 1500 measured reflections

• 1078 independent reflections

• 1076 reflections with I > 2σ(I)

• R_int = 0.016

• 2 standard reflections frequency: 120 min intensity decay: none

Refinement

• R[F² > 2σ(F²)] = 0.025

• wR(F²) = 0.071

• S = 1.12

• 1078 reflections

• 84 parameters

• Δρ_max = 2.75 e Å⁻³

• Δρ_min = −3.18 e Å⁻³

Table 1
Selected bond lengths (Å)

Li—O2^i	1.962 (9)
Li—O6	1.999 (9)
Ho—O3	2.287 (3)
Ho—O1^i	2.348 (4)
Ho—O2^ii	2.411 (3)
Ho—O6	2.516 (3)
P1—O1	1.488 (4)
P1—O6	1.499 (3)
P1—O4	1.589 (4)
P1—O5^iii	1.594 (3)
P2—O2	1.486 (3)
P2—O3	1.488 (3)
P2—O5	1.588 (3)
P2—O4	1.601 (4)
Symmetry codes: (i) ; (ii) ; (iii) .

Data collection: CAD-4 EXPRESS (Duisenberg, 1992; Macíček & Yordanov, 1992); cell refinement: CAD-4 EXPRESS; Macíček & Yordanov, 1992); data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1998); software used to prepare material for publication: SHELXL97.

Supporting information

S1. Comment

The present paper is an extension of our earlier work on polyphosphates of general formula LiLn(PO₃)₄ [Ln = Yb (Ben Zarkouna & Driss, 2004), Er (Ben Zarkouna et al., 2005) and Tb (Ben Zarkouna et al., 2007)]. This family is now expanded to include the lithium holmium polyphosphate, LiHo(PO₃)₄, which might be of particular interest in the area of luminescent materials, because an avalanche up-conversion emission was observed previously for Ho³⁺-containing compounds (Liu et al., 1999).

The LiLn(PO₃)₄ polyphosphates are isostructural and belong to form I according to the classification of Palkina et al. (1981). In this type of arrangement, helical ribbons, _∞(PO₃)_n, formed by corner-sharing of PO₄ tetrahedra, spread parallel to the b axis. The period of the chains corresponds to four tetrahedra but, owing to the presence of 2₁ screw axes, only two of them are crystallographically independent. The P—O bonds involving terminal O atoms are the shortest within a PO₄ tetrahedron, because of the dπ-pπ orbital overlap (Durif, 1995). In LiHo(PO₃)₄, the mean P—O bond lengths for terminal and bridging O atoms, are of 1.490 and 1.593 Å, respectively. The O—P—O angles are within the range 101.2 (2)–119.6 (2) °, the smallest and the largest ones are those involving the longest and the shortest P—O bonds, respectively. The P—O—P bridges exhibit angles of 131.6 (2) and 134.8 (2)°, larger than the O—P—O angles. The Li and Ho atoms are arranged alternately on twofold rotation axes at roughly similar spacings [3.486 (1) and 3.553 (1) Å] in comparison with the quite different spacings between K and Ho atoms [3.721 (2) and 8.589 (2) Å] in β-KHo(PO₃)₄ (Palkina et al., 1976). The Ho atom is surrounded by eight terminal oxygen atoms forming a distorted dodecahedron, with an average Ho—O distance of 2.390 Å, while the Li atom is located inside an irregular tetrahedron with a mean Li—O distance of 1.980 Å. All these bond lengths are conform with those mentioned in the literature (International Tables for X-Ray Crystallography, 1968; Durif, 1995; Amami et al., 2004).

By sharing edges, the HoO₈ and LiO₄ polyhedra are joined to produce infinite linear chains running along the b axis (Fig. 1), in contrast to crystal structure of NaHo(PO₃)₄ (form II) (Amami et al., 2004) where zigzag chains of face-sharing HoO₈ and NaO₈ polyhedra are observed. As shown in Fig. 2, each chain shares corners of its polyhedra with four adjacent polyphosphate chains.

It is noteworthy that no O atom is common to two Ho atoms, and the closest Ho···Ho distance of 5.570 (1) Å in LiHo(PO₃)₄ is comparable with that found in HoP₅O₁₄ (Tranqui et al., 1972). Bond-valence-sum values (Brown & Altermatt, 1985) are 0.998, 3.054, 4.936 and 4.967 valence units for Li, Ho, P1 and P2, respectively, in good agreement with the expected formal charges.

Related literature

For isotypic LiLn(PO₃)₄ structures, see: Ben Zarkouna & Driss (2004) for Ln = Yb; Ben Zarkouna et al. (2005) for Ln = Er; Ben Zarkouna et al. (2007) for Ln = Tb. For related structures, see: Amami et al. (2004) (NaHo(PO₃)₄); Palkina et al. (1976) (β-KHo(PO₃)₄); Tranqui et al. (1972) (HoP₅O₁₄). For general background, see: Brown & Altermatt (1985); Durif (1995); Liu et al. (1999); Palkina et al. (1981); International Tables for X-Ray Crystallography (1968).

S2. Experimental

The title compound was prepared in single crystalline form using the flux method. At room temperature, 3 g of Li₂CO₃ and 0.5 g of Ho₂O₃ were slowly added to 10 ml of H₃PO₄ (85%_wt) in a glassy carbon crucible. The resulting mixture was then progressively heated to 573 K and kept at this temperature for 8 days. After cooling to room temperature and removal of the excess phosphoric flux with boiling water, pale yellow crystals of LiHo(PO₃)₄ were separated.

S3. Refinement

The highest peak is located 0.87 Å from Ho and the deepest hole is located 1.00 Å from the same atom.

Computing details

Data collection: CAD-4 EXPRESS (Duisenberg, 1992; Macíček & Yordanov, 1992); cell refinement: CAD-4 EXPRESS (Duisenberg, 1992; Macíček & Yordanov, 1992); data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1998); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. : Alternating arrangement of HoO8 and LiO4 polyhedra projected along the a axis. PO4 tetrahedra are omitted for clarity.
	Fig. 2. : Projection of the LiHo(PO3)4 structure along the b axis.

Lithium holmium(III) polyphosphate(V)

Crystal data

LiHo(PO3)4	F(000) = 904
Mr = 487.75	Dx = 3.654 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -C 2yc	Cell parameters from 25 reflections
a = 16.280 (3) Å	θ = 10.1–14.7°
b = 7.039 (2) Å	µ = 9.72 mm^−1
c = 9.561 (2) Å	T = 293 K
β = 125.99 (2)°	Plate, pale yellow
V = 886.5 (4) Å^3	0.31 × 0.14 × 0.07 mm
Z = 4

Data collection

Enraf–Nonius CAD-4 diffractometer	1076 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.016
Graphite monochromator	θmax = 28.0°, θmin = 3.1°
ω/2θ scans	h = −21→17
Absorption correction: ψ scan (North et al., 1968)	k = −2→9
Tmin = 0.095, Tmax = 0.378	l = 0→12
1500 measured reflections	2 standard reflections every 120 min
1078 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: heavy method
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.025	w = 1/[σ^2(Fo^2) + (0.0459P)^2 + 12.7265P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.071	(Δ/σ)max < 0.001
S = 1.12	Δρmax = 2.75 e Å^−3
1078 reflections	Δρmin = −3.18 e Å^−3
84 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0014 (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
Li	0.0000	0.708 (2)	0.2500	0.015 (3)
Ho	0.0000	0.20312 (4)	0.2500	0.0073 (1)
P1	0.13774 (8)	0.5517 (2)	0.6157 (1)	0.0067 (2)
P2	0.14654 (8)	0.1504 (2)	0.6961 (1)	0.0068 (2)
O1	0.1137 (3)	0.7154 (5)	0.6843 (5)	0.0118 (7)
O2	0.0711 (3)	0.0845 (5)	0.7255 (5)	0.0116 (6)
O3	0.1280 (3)	0.1150 (5)	0.5262 (4)	0.0121 (7)
O4	0.1568 (3)	0.3740 (5)	0.7337 (4)	0.0107 (6)
O5	0.2552 (2)	0.0776 (5)	0.8526 (4)	0.0105 (6)
O6	0.0652 (2)	0.5022 (5)	0.4280 (4)	0.0104 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Li	0.018 (6)	0.011 (6)	0.020 (7)	0.000	0.012 (6)	0.000
Ho	0.0063 (2)	0.0076 (2)	0.0071 (2)	0.000	0.0034 (1)	0.000
P1	0.0052 (5)	0.0069 (5)	0.0072 (5)	−0.0002 (4)	0.0031 (4)	−0.0001 (4)
P2	0.0051 (5)	0.0072 (5)	0.0073 (5)	−0.0001 (4)	0.0032 (4)	0.0009 (4)
O1	0.012 (2)	0.009 (2)	0.016 (2)	0.001 (1)	0.009 (1)	−0.002 (1)
O2	0.008 (1)	0.012 (2)	0.016 (2)	0.001 (1)	0.008 (1)	0.002 (1)
O3	0.012 (2)	0.014 (2)	0.008 (1)	0.002 (1)	0.004 (1)	−0.000 (1)
O4	0.015 (2)	0.007 (1)	0.009 (1)	0.001 (1)	0.007 (1)	0.002 (1)
O5	0.005 (1)	0.016 (2)	0.010 (1)	0.002 (1)	0.005 (1)	0.002 (1)
O6	0.007 (1)	0.014 (2)	0.006 (1)	−0.001 (1)	0.001 (1)	−0.000 (1)

Geometric parameters (Å, º)

Li—O2^i	1.962 (9)	Ho—O6	2.516 (3)
Li—O2^ii	1.962 (9)	Ho—O6^iii	2.516 (3)
Li—O6^iii	1.999 (9)	P1—O1	1.488 (4)
Li—O6	1.999 (9)	P1—O6	1.499 (3)
Ho—O3^iii	2.287 (3)	P1—O4	1.589 (4)
Ho—O3	2.287 (3)	P1—O5^vi	1.594 (3)
Ho—O1^ii	2.348 (4)	P2—O2	1.486 (3)
Ho—O1^i	2.348 (4)	P2—O3	1.488 (3)
Ho—O2^iv	2.411 (3)	P2—O5	1.588 (3)
Ho—O2^v	2.411 (3)	P2—O4	1.601 (4)
O2^i—Li—O2^ii	83.7 (5)	O6—P1—O5^vi	105.0 (2)
O2^i—Li—O6^iii	119.57 (14)	O4—P1—O5^vi	102.7 (2)
O2^ii—Li—O6^iii	125.85 (14)	O2—P2—O3	119.6 (2)
O2^i—Li—O6	125.85 (14)	O2—P2—O5	107.9 (2)
O2^ii—Li—O6	119.57 (14)	O3—P2—O5	111.9 (2)
O6^iii—Li—O6	87.2 (5)	O2—P2—O4	104.6 (2)
O1—P1—O6	118.8 (2)	O3—P2—O4	109.7 (2)
O1—P1—O4	106.8 (2)	O5—P2—O4	101.2 (2)
O6—P1—O4	110.8 (2)	P1—O4—P2	131.6 (2)
O1—P1—O5^vi	111.7 (2)	P2—O5—P1^vii	134.8 (2)

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