The type IV polymorph of KEu(PO3)4

Single crystals of KEu(PO3)4, potassium europium(III) polyphosphate, were obtained by solid-state reactions. This monoclinic form is the second polymorph described for this composition and belongs to type IV of long-chain polyphosphates with general formula A I B III(PO3)4. It is isotypic with its KEr(PO3)4 and KDy(PO3)4 homologues. The crystal structure is built of infinite helical chains of corner-sharing PO4 tetrahedra with a repeating unit of eight tetrahedra. These chains are further linked by isolated EuO8 square antiprisms, forming a three-dimensional framework. The K+ ions are located in pseudo-hexagonal channels running along [01] and are surrounded by nine O atoms in a distorted environment.

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2282).

Comment
Rare earth polyphosphates are interesting materials and bear potential applications (Rashchi & Finch, 2000;Barsukov et al., 2004). The title compound is a member of a large family of polyphosphates with general formula A I B III (PO 3 ) 4 (where A I is a monovalent cation: Li, Na, K, Rb, Cs, Tl, NH 4 , Ag and B III is a trivalent cation: Ln,Y, Bi). It is now well known that these compounds are classified into seven structural types usually labelled by roman numerals from I to VII. A short recapitulation of the main crystal chemical characteristics of these seven structural types has recently been given by Jaouadi et al. (2003). The KEu(PO 3 ) 4 polymorph described in this article belongs to the IV structural type.
The structure of this type IV polymorph is built of infinite helical chains of corner-sharing PO 4 tetrahedra further linked by isolated EuO 8 square antiprisms. The (PO 3 ) ∞ chains exhibit a repeating unit of eight PO 4 tetrahedra ( Fig. 1) and are running along the [101] direction. The three-dimensional framework resulting from the edge-sharing between the PO 4 tetrahedra and the EuO 8 square antiprisms exhibits pseudo hexagonal channels where the K + ions reside. The K + ion is 9-coordinated by oxygen atoms with distances ranging from 2.789 (2) Å to 3.370 (3) Å. By sharing corners, the KO 9 coordination polyhedra form corrugated chains running along the [010] direction (Fig. 2). Whereas the K atoms are separated by 6.599 (2) Å within the chain, the shortest K-K distance in the structure, 4.770 (2) Å, occurs between two adjacent (KO 9 ) ∞ chains. This shortest distance corresponds to the separation between two K + ions within the channels of the structure running along the [201] direction. This separation distance A I -A I is strongly dependent on the nature of the A I element and decreases as the size of the A I element increases. For instance, in the A I Gd(PO 3 ) 4 homologue series, where A I = K, Rb, Cs, this A I -A I shortest distance varies from 4.801 Å for K to 4.211 Å for Cs (4.524 Å for Rb). For CsEu(PO 3 ) 4 the shortest Cs-Cs distance is equal to 4.237Å (Zhu et al. 2009).
For isotypic AB(PO 3 ) 4 structures, where A is an alkali metal, Tl or NH 4 + , and B is a rare earth element, see: Palkina et al. (1977) for TlNd, Maksimova et al. (1978) for RbNd, Dago et al. (1980) for KEr, Maksimova et al. (1981) for CsNd, Maksimova et al. (1982) for RbHo, Horchani et al. (2004) for RbEr, Rekik et al. (2004) for KGd, Naïli & Mhiri (2005) for CsGd, Ben Zarkouna et al. (2006) for (NH 4 )Gd, Khlissa & Férid (2006) for RbTb, Ettis et al. (2006)  A mixture of these reagents in the molar ratio 34:57:9 was used for the synthesis. The mixture was heated at 473 K for 6 h, then at 573 K for 6 h and finally at 873 K for 24 h. The furnace was then cooled down first to 773 K at the rate of 2 K . h -1 and then to room temperature at the rate of K . h -1 . Single crystals were extracted from the batch by washing with hot water.
Besides crystals of the title compound, crystals of the type III polymorph (Hu et al., 1984)) have also been obtained.