Apatite-type SrPr(4)(SiO(4))(3)O.

Single crystals of the title compound, strontium tetra-praseo-dymium tris-(silicate) oxide, SrPr(4)(SiO(4))(3)O, have been grown by the self-flux method using SrCl(2). The structure is isotypic with the apatite supergroup family having the generic formula (IX)M1(2) (VII)M2(3)((IV)TO(4))(3)X, where M = alkaline earth and rare earth metals, T = Si and X = O. The M1 site (3.. symmetry) is occupied by Pr and Sr atoms with almost even proportions and is surrounded by nine O atoms forming a tricapped trigonal prism. The M2 site (m.. symmetry) is almost exclusively occupied by Pr and surrounded by seven O atoms, forming a distorted penta-gonal bipyramid. The Si atom (m.. symmetry) is surrounded by two O (m.. symmetry) and two O atoms in general positions, forming an isolated SiO(4) tetra-hedron. Another O atom at the inversion centre (.. symmetry) is surrounded by three M2 sites, forming an equilateral triangle perpendicular to the c axis.


Apatite-type
In the title compound, the M1 site (site symmetry 3..) is occupied by Pr and Sr with almost even proportions, and surrounded by three O1, three O2 and three O3 atoms, forming a tri-capped trigonal prism. The M site (m.. symmetry) is almost exclusively occupied by Pr, and surrounded by one O1, one O2, four O3 and one O4 forming a distorted pentagonal bipyramid. The isolated SiO 4 tetrahedra are composed of one O1, one O2 and two O3 atoms. They show a slight angular distortion and exhibit m.. symmetry. The O atom at the X site with 6.. symmetry is located at the centre of the M2 triangle and its anisotropic displacement ellipsoid is more than two times prolate along the c axis, a feature also observed for other members of the rare-earth orthosilicate oxyapatite family. The present compound is isostructural with CaLa 4 (SiO 4 ) 3 O (Schroeder & Mathew, 1978).

Experimental
All chemicals used were of analytical grade (3 N) from Kojundo Chemical Laboratory Co. Ltd., Japan. Powders of Pr 2 O 3 (1.643 g), SiO 2 (0.359 g) and SrCl 2 (3.006 g) were mixed together and put into a platinum crucible (30 ml). The crucible capped with a platinum lid was then placed on alumina powder in an alumina crucible. The double crucible was heated in air to 1373 K at the rate of 100 K/h, held for 6 h at 1373 K, cooled at the rate of 20 K/h to 923 K, and then furnace-cooled by turning off the power. The flux components were washed away by distilled water in an oven at 373 K. Crystals were hexagonal prismatic with the longest dimension of 150 µm. Energy dispersive spectroscopy indicated that the crystals contained no elements other than Sr, Pr, Si and O. The Sr:Pr ratio was also estimated from eight samples to be 1.04:3.96 with an estimated standard uncertainty of ±0.06. Therefore, the stoichiometric composition of SrPr 4 Si 3 O 13 was assumed for the refinement of X-ray data.

Refinement
Possibility of lower symmetries like P6 3 or P3 for the crystal was discarded in the course of refinements because no significant improvement was obtained. Defects at the O sites were not observed from the preliminary population analysis.
Presence of interstitial O atoms were not detected in the difference Fourier maps. Populations of Sr and Pr at the M1 and M2 sites were refined with a restraint to satisfy the charge neutrality for the stoichiometric composition SrPr 4 Si 3 O 13 . The supplementary materials sup-2 positional and atomic displacement parameters of Sr and Pr at both sites were constrained to unity, taking into account the respective site symmetries. The highest remaining peak is 0.45 Å from M2 and the deepest hole is 0.04 Å from M1. Figures   Fig. 1. The crystal structure of SrPr 4 (SiO 4 ) 3 O in projection along [001]. Pr and Sr atoms are given as red, Si as dark blue, and O atoms as light blue ellipsoids at the 50% probability level.

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes.
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 > 2σ(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.