Redetermination of β-Ba(PO3)2

In comparison with the previous structure determination of the β-modification of barium catena-polyphosphate that was based on Weissenberg film data [Grenier et al. (1967 ▶). Bull. Soc. Fr. Minéral. Cristallogr. 90, 24–31], the current CCD-data-based redetermination reveals all atoms with anisotropic displacement parameters, standard uncertainties for the atomic coordinates, and the determination of the absolute structure. Moreover, a much higher accuracy in terms of the bond-length distribution for the polyphosphate chain, with two shorter and two longer P—O distances, was achieved. The structure consists of polyphosphate chains extending parallel to [100] with a periodicity of two PO4 tetrahedra. The Ba2+ cations are located between the chains and are surrounded by ten O atoms in the form of a distorted coordination polyhedron, with Ba—O distances ranging from 2.765 (3) to 3.143 (3) Å, also reflecting the higher precision of the current redetermination.

In comparison with the previous structure determination of the -modification of barium catena-polyphosphate that was based on Weissenberg film data . Bull. Soc. Fr. Minéral. Cristallogr. 90,[24][25][26][27][28][29][30][31], the current CCDdata-based redetermination reveals all atoms with anisotropic displacement parameters, standard uncertainties for the atomic coordinates, and the determination of the absolute structure. Moreover, a much higher accuracy in terms of the bond-length distribution for the polyphosphate chain, with two shorter and two longer P-O distances, was achieved. The structure consists of polyphosphate chains extending parallel to [100] with a periodicity of two PO 4 tetrahedra. The Ba 2+ cations are located between the chains and are surrounded by ten O atoms in the form of a distorted coordination polyhedron, with Ba-O distances ranging from 2.765 (3) to 3.143 (3) Å , also reflecting the higher precision of the current redetermination.

Comment
Polymorphism of Ba(PO 3 ) 2 with three modifications has been reported by Grenier & Martin (1975): The stable β-form transforms to the high-temperature α-form at 1058 K, and the γ-form transforms at 978 K to the β-form. Structure determinations were carried out for the γ-form (Coing-Boyat et al., 1978) and for the β-form . The crystal structure of α-Ba(PO 3 ) 2 is yet unknown. Comparative discussions of the structural set-up of the β-and γ-form of Ba(PO 3 ) 2 and of other divalent long-chain polyphosphates were given by Durif (1995).
During experiments intended to isolate crystals of α-Ba(PO 3 ) 2 by quenching the reaction product from the recrystallized melt at temperatures above the indicated transition point, high-quality crystals of β-Ba(PO 3 ) 2 were obtained instead. Since the first structure refinement of this modification was based on Weissenberg film data and converged with a relatively high residual R = 0.1, with atoms refined only with isotropic displacement factors and without indication of standard uncertainties for the fractional atomic coordinates, a re-refinement of the structure with modern CCD-based data seemed appropriate. The results of this re-refinement are reported here, confirming in principle the results of Grenier et al. (1967), however, achieving bond lengths and angles with much higher accuracy and precision, as exemplified by a comparison of the P-O bond length (Table 1).
The catena-polyphosphate chain has a periodicity of two PO 4 tetrahedra and extends parallel to [100] (Fig. 1). In comparison with the previous structure refinement , the determined bond lengths of the present refinement are in much better agreement with the usually observed bond length distribution in such long-chain polyphosphates (Durif, 1995), with two shorter and two longer P-O distances, each with similar values (Table 1).
The Ba 2+ cation is located between the chains and is surrounded by ten oxygen atoms in an irregular coordination sphere with Ba-O distances in the range from 2.765 (3) to 3.143 (3) Å (Fig. 2).

Experimental
Stoichiometric amounts of BaCO 3 and (NH 4 ) 2 HPO 4 (molar ratio 1:2) with a 3% excess of the phosphate precursor were finely ground, heated in a platinum crucible to 1173 K and slowly cooled to 1073 K at a rate of 2 K h -1 . Then the crucible was quenched in a cold water bath. Colourless fragments of the title compound were cut from the clear, transparent reaction product.

Refinement
In contrast to the previous structure refinement  with a = 4.510 (2), b = 13.44 (2) c = 8.36 (5) Å, the reduced cell setting was chosen for the current refinement. Structure data were finally standardized with STRUCTURE-TIDY (Gelato & Parthé, 1987