KPr(PO3)4

Single crystals of the title compound, potassium praseodymium(III) polyphosphate, were obtained by solid-state reaction. The monoclinic non-centrosymmetric structure is isotypic with all other KLn(PO3)4 analogues from Ln = La to Er, inclusive. The crystal structure of these long-chain polyphosphates is built up from infinite crenelated polyphosphate chains of corner-sharing PO4 tetrahedra with a repeating unit of four tetrahedra. These chains, running along [100], are arranged in a pseudo-tetragonal rod packing and are further linked by isolated PrO8 square antiprisms [Pr—O = 2.3787 (9)–2.5091 (8) Å], forming a three-dimensional framework. The K+ ions reside in channels parallel to [010] and exhibit a highly distorted coordination sphere by eight O atoms at distances ranging from 2.7908 (9) to 3.1924 (11) Å.


Comment
Long chains polyphosphates with general formula A I B III (PO 3 ) 4 have been classified into seven structural types, labelled from I to VII (Durif, 1995;Jaoudi et al. 2003). All long-chains polyphosphates of formula KLn(PO 3 ) 4 (Ln = rare earth elements) reported up to now (Zhu et al., 2009;Horchani-Naifer et al., 2008;Parreu et al., 2006;Xing et al., 1987;Ninghai et al., 1984;Lin et al., 1983;Krutik et al., 1980;Hong et al., 1975) adopt type III except for KYb(PO 3 ) 4 (Palkina et al. 1981) which is the only presently known member of type V. Most of these potassium polyphosphates are dimorphic and crystallize with both the type III and the type IV polymorphs. KCe(PO 3 ) 4 which has been shown to crystallize with either the type II and the type III is the first exception. The second exception is concerned with the Er member of this series presenting the type VII polymorph besides both type III and type IV polymorphs. Moreover, type III long-chain polyphosphates do not exist for monovalent cations other than K + . The structure of the title compound also fits in this type III isotypic series.
The crystal structure of the title compound is built from crenelated chains with a repeating unit of four corner-sharing tetrahedra, as displayed in Fig. 1. The chains are further linked by isolated PrO 8 square antiprisms to form the three-dimensional framework. Each PrO 8 polyhedron (Pr-O distances range from 2.3787 (9) to 2.5091 (8) Å) is connected through vertices to four (PO 3 ) ∞ chains stacked in a pseudo-tetragonal rod packing as shown in Fig. 2. Figure 2 also shows that within this pseudo-tetragonal rod packing, two adjacent chains are twisted by ca. 90 ° whereas two opposite chains are parallel.
The relative disposition of the chains running along the [100] direction accounts for the strong non-centrosymmetric character of the structure. Figure 3 displays details of the connections between the PrO 8 square antiprisms and the four chains surrounding each antiprism. One of the four chains (labelled C 1 ) is attached in a tridentate fashion on a triangular face of the square antiprism whereas the opposite and parallel chain (labelled C 2 ) is connected only through a vertex (Fig. 3a). The two other chains which are adjacent to the first one are attached in a bidentate fashion. The first of these two chains (labelled C 3 ) is linked through a bidentate diphosphate group attached on one side of one square face of the square antiprism (Fig.   3b). The second chain (labelled C 4 ) is connected at the ends of one diagonal of the second square face of the antiprism (Fig.   3c) through corners of the terminal PO 4 groups of the crenel-shaped tetraphosphate group corresponding to the repeating unit of the chain. This polyhedral linkage delimits channels running along [010] where the K + ions lie in a highly distorted environment defined by eight oxygen atoms at distances ranging from 2.7908 (9) to 3.1924 (11) Å.
For the cyclophosphate structure with the same composition KPr(PO 3 ) 4 , see: .

Experimental
Crystals of the title compound were synthesized by reacting Pr 6 O 11 with (NH 4 )H 2 PO 4 and K 2 CO 3 in a platinum crucible.
A mixture of these reagents in the molar ratio 14: 66: 20 was used for the synthesis. The mixture has first been heated at 473 K for 12 h and then the temperature has been increased up to 573 K and maintained for 12 h before to be raised at 853 supplementary materials sup-2 K and kept for 24 additional hours. At the end of this heating step, the muffle furnace was cooled down first to 673 K at the rate of 2 K h -1 and subsequently to room temperature by switching the power off. Single crystals were extracted from the batch by leaching with hot water.

Refinement
The highest residual peak in the final difference Fourier map was located 0.46 Å from atom Pr and the deepest hole was located 0.47 Å from atom K. Fig. 1. View of the repeating unit of the (PO 3 ) ∞ chains. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (vi) -x + 1, y + 1/2, -z + 1; (vii) x, y, z -1; (viii) -x, y + 1/2, -z + 1; (ix) x -1, y, z.]    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.