α-Ba2P2O7

Single crystals of α-Ba2P2O7, dibarium diphosphate, were obtained by solid-state reaction. The orthorhombic structure is isotypic with α-Sr2P2O7 and is the second polymorph obtained for this composition. The structure is built from two different BaO9 polyhedra (both with m symmetry), with Ba—O distances in the ranges 2.7585 (10)–3.0850 (6) and 2.5794 (13)–2.9313 (4) Å. These polyhedra are further linked by sharing corners along [010] and either edges or triangular faces perpendicularly to [010] to form the three-dimensional framework. This polyhedral linkage delimits large channels parallel to [010] where the P2O7 diphosphate anions are located. These groups (symmetry m) are characterized by a P—O—P angle of 131.52 (9)° and an eclipsed conformation. They are connected to the BaO9 polyhedra through edges and corners.


Comment
Due to their potential applications as optical materials (Pang et al., 2009, Peng et al., 2010, alkaline earth diphosphates exhibit at the present time a growing interest. Since the optical properties are strongly related to the crystal structure, the study of polymorphism in these materials is worth investigating. Besides the hexagonal form σ-Ba 2 P 2 O 7 described by ElBelghitti et al. (1995), the title compound is the second polymorph for this composition obtained in the form of single-crystals. The orthorhombic title compound α-Ba 2 P 2 O 7 is isotypic with α-Sr 2 P 2 O 7 (Hagman et al. 1968;Grenier & Masse, 1977;Barbier & Echard, 1998) and closely related to BaPbP 2 O 7 . The existence of this polymorph has previously been mentioned by Durif (1995) and several other authors (McCauley & Hummel, 1968;Mehdi et al., 1977;Bian et al., 2004;Kokhanovskii, 2004).
The structure of the α-polymorph is built up from two different BaO 9 polyhedra with Ba-O distances ranging from 2.7585 (10) Å to 3.0850 (6) Å and from 2.5794 (13) Å to 2.9313 (4) Å, respectively. Figure 1 displays details of these coordination polyhedra as well as their linkage by corners, edges or triangular faces to form the three-dimensional framework.
This polyhedral linkage delimits large channels parallel to [010] where the P 2 O 7 diphosphate groups are located (Fig. 2).
These groups (symmetry m) are characterized by a P-O-P angle of 131.52 (9)° and an eclipsed conformation. They are connected to the BaO 9 polyhedra through edges and corners. Each oxygen atom of the P 2 O 7 groups, apart from the O1 bridging oxygen, is bonded to three Ba atoms and one phosphorus atom. During the anisotropic refinement of the displacement parameters of the isotypic α-Sr 2 P 2 O 7 structure (Barbier & Echard, 1998), the authors observed a very strong anisotropy along the b direction for both O5 and O1 atoms located in the (010) mirror plane and interpreted this phenomenon as a possible atomic-scale disorder associated with the local loss of mirror symmetry resulting in a non-centrosymmetric structural arrangement. Such an outstanding feature is still present in our anisotropic structure refinement, although the amplitudes of the atomic displacement parameters are significantly lower, especially for the U 22 component of the O5 atom. If the strong anisotropy of the O1 atom is perfectly understandable because of its bridging role, that of the O5 atom strongly bonded to two Ba1 atoms at 3.0850 (6) Å, one Ba2 atom at 2.5794 (13) Å and one P1 atom at 1.5067 (13) Å is more surprising.
However, due to the relative homogeneity of the values of the isotropic displacement parameters of all oxygen atoms, it does not seem necessary to envisage any atomic-scale disorder for the O5 atom in the present case.
Simultaneous with our refinement, an independent study of the α-Ba 2 P 2 O 7 structure from a hydrothermally grown crystal was reported by Heyward et al. (2010). The results of both refinements in terms of geometric parameters are the same within the threefold standard deviation.

Experimental
Single crystals of the title compound have been obtained during the study of the phase relationships in the ternary system Na 2 O-BaO-P 2 O 5 . They were synthesized in the solid state by reacting Na 2 CO 3 , BaCO 3 and (NH 4 ) 2 HPO 4 in a platinum crucible. A mixture of these reagents taken in the molar ratio 1: 1: 2 was carefully ground in an agate mortar and successively supplementary materials sup-2 heated at 373 K, 573 K and 773 K for 24 h at each temperature. After a new grinding, the reacting mixture was submitted to a final heat treatment at 973 K for 2 days followed by a slow cooling to room temperature at the rate of 5 K h -1 . After an abundant washing of the batch with hot water, single crystals of the title compound could have been extracted.