Pr 5 Si 3 N 9

Single crystals of Pr 5 Si 3 N 9 , pentapraseodymium trisilicon nonanitride, were obtained by the reaction of elemental praseodymium with silicon diimide in a radio-frequency furnace at 1873 K. The crystal structure consists of a chain-like Si—N substructure of corner-sharing SiN 4 tetrahedra. An additional Q 1-type [SiN 4 ] unit is attached to every second tetrahedron directed alternately in opposite directions. The resulting branched chains interlock with each other, building up a three-dimensional structure. The central atoms of the Q 1-type [SiN 4 ] unit and of its attached tetrahedron are situated on a mirror plane, as are two of the four crystallographically unique Pr 3+ ions. The latter are coordinated by six to ten N atoms, with Pr—N distances similar to those of other rare earth nitridosilicates.

Single crystals of Pr 5 Si 3 N 9 , pentapraseodymium trisilicon nonanitride, were obtained by the reaction of elemental praseodymium with silicon diimide in a radio-frequency furnace at 1873 K. The crystal structure consists of a chainlike Si-N substructure of corner-sharing SiN 4 tetrahedra. An additional Q 1 -type [SiN 4 ] unit is attached to every second tetrahedron directed alternately in opposite directions. The resulting branched chains interlock with each other, building up a three-dimensional structure. The central atoms of the Q 1type [SiN 4 ] unit and of its attached tetrahedron are situated on a mirror plane, as are two of the four crystallographically unique Pr 3+ ions. The latter are coordinated by six to ten N atoms, with Pr-N distances similar to those of other rare earth nitridosilicates.
Si3 atoms are located on a mirrow plane which is co-planar to [100]. Due to the constitution of the terminal tetrahedra the chains interlock zipper-like with each other (Fig. 2). The Pr 3+ ions (yellow) are located between the chains. The coordination numbers of the Pr 3+ ions range between six (for Pr4) and ten (for Pr 3) with Pr-N distances varying from 2.310 (11) to 3.053 (2) Å. The geometric parameters of Pr 5 Si 3 N 9 are in the usual ranges and correspond with those of the isotypic compounds Ln 5 Si 3 N 9 (Ln = La, Ce) and other nitridosilicates (Schnick, 2001;Lissner & Schleid, 2004).

Experimental
Pr 5 Si 3 N 9 was synthesized by the reaction of Pr (swarf, 99.9%, Chempur, Karlsruhe) and silicon diimide (Schnick & Huppertz, 1997) which were thoroughly mixed in a glove box (Unilab, MBraun). The mixture was heated in a tungsten crucible in a radio-frequency furnace (Schnick et al., 1999) under purified N 2 up to 1873 K within 1 h. This temperature was retained for 5 h, and the crucible thereafter cooled down to 1073 K in 35 h before quenching to room temperature within 1 h. Pr 5 Si 3 N 9 could be obtained as air-sensitive dark-yellow crystals with PrN as by-product.

Refinement
In the final Fourier map the highest peak is 0.19 Å from atom Si1 and the deepest hole is 0.64 Å from atom Pr4. Fig. 1. Presentation of the Si-N substructure, with anisotropic displacement parameters drawn at the 50% probability level. SiO 4 tetrahedra are depicted purple for Si1, light blue for Si2 and dark blue for Si3.

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