LiHo(PO(3))(4).

Lithium holmium(III) polyphosphate(V), LiHo(PO(3))(4), belongs to the type I of polyphosphates with general formula ALn(PO(3))(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 tetra-hedra and 2(1) inter-nal 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 tetra-hedral and dodeca-hedral coordination, respectively. The HoO(8) polyhedra are isolated from each other, the closest Ho⋯Ho distance being 5.570 (1) Å.

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2216).  (Ben Zarkouna et al., 2005) and Tb (Ben Zarkouna et al., 2007)]. This family is now expanded to include the lithium holmium polyphosphate, LiHo(PO 3 ) 4 , which might be of particular interest in the area of luminescent materials, because an avalanche up-conversion emission was observed previously for Ho 3+ -containing compounds (Liu et al., 1999).
The LiLn(PO 3 ) 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, ∞ (PO 3 ) n , formed by corner-sharing of PO 4 tetrahedra, spread parallel to the b axis. The period of the chains corresponds to four tetrahedra but, owing to the presence of 2 1 screw axes, only two of them are crystallographically independent. The P-O bonds involving terminal O atoms are the shortest within a PO 4 tetrahedron, because of the dπ-pπ orbital overlap (Durif, 1995). In LiHo ( (International Tables for X-Ray Crystallography Durif, 1995;Amami et al., 2004).
By sharing edges, the HoO 8 and LiO 4 polyhedra are joined to produce infinite linear chains running along the b axis ( Fig. 1), in contrast to crystal structure of NaHo(PO 3 ) 4 (form II) (Amami et al., 2004) where zigzag chains of face-sharing HoO 8 and NaO 8 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(PO 3 ) 4 is comparable with that found in HoP 5 O 14 (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.

supplementary materials sup-2 Experimental
The title compound was prepared in single crystalline form using the flux method. At room temperature, 3 g of Li 2 CO 3 and 0.5 g of Ho 2 O 3 were slowly added to 10 ml of H 3 PO 4 (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(PO 3 ) 4 were separated.

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

Data collection
Enraf-Nonius CAD-4 diffractometer R int = 0.016 Radiation source: fine-focus sealed tube θ max = 28.0º Monochromator: graphite θ min = 3.1º T = 293(2) K h = −21→17 ω/2θ scans k = −2→9 Absorption correction: ψ scan (North et al., 1968) l = 0→12 Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.