Crystal structure of Ag3Dy2(NO3)9 and quantitative comparison to isotypic compounds

The crystal structure of the title compound and its particular relation to isotypic compounds is considered.

In this work a new member of this group of compounds, Ag 3 Dy 2 (NO 3 ) 9 , is presented, the first one containing Ag and Dy, which has been found to crystallize in the abovementioned structure type.

Structural commentary
Similar to many related compounds, the title compound was obtained from a melt of nitrates, in this case silver nitrate and dysprosium nitrate pentahydrate. However, while for synthesis of related compounds, oxides are often used as lanthanide sources and the respective alkali metal nitrate or a eutectic combination of nitrates act as solvent as well as nitrate donor, in the present experimental setting the nitrates can be deployed in stoichiometric amounts. The crystals, which were found to be suitable for structure determination were obtained from a 2:1 mixture of Ag and Dy nitrates, i.e. a slight excess of AgNO 3 , as described in the experimental section. The surplus Ag is present as remaining AgNO 3 as well as elemental silver after partial thermal or light-induced decomposition. So far, no hint of another compound with a 2:1 composition of metals in the Ag/Dy system, as could be expected for smaller lanthanides similar to the alkali metal or ammonium systems (Manek & Meyer, 1992, 1993a, has been observed. Ag 3 Dy 2 (NO 3 ) 9 ( Fig. 1) crystallizes in space group P4 1 32 with most atoms at general positions except for Ag, N1 and O1 at 12d and Dy at 8c Wyckoff positions. The asymmetric unit comprises one Ag, one Dy, two N, and five O atoms. The Dy atom, being located on a threefold axis, is coordinated by six bidentate nitrate anions with Dy-O distances of 2.557 (11)-2.732 (11) Å (see Fig. 2a), the surrounding oxygen atoms form a distorted icosahedron (Fig. 2b). The polyhedra are connected to neighbouring icosahedra via common vertices, and inside this polyhedron the Dy atom is slightly off-centre, shown by formation of the shortest Dy-O distances to O3 and O4 as part of the same NO 3 À anion (the lower one in Fig. 2b), most probably driven by repulsion of next-neighbour Dy atoms. The silver atom is also coordinated by five nitrate ions in exclusively bidentate manner (Fig. 3). The Ag-O distances span quite a large range, so besides eight distances between 2.741 (11) and 3.004 (11) Å two relatively short distances of 2.383 (15) Å are found. These short bonds include oxygen atoms in almost opposite positions, which form an O-Ag-O angle of 154.7 (6) , indicating the preferred formation of AgO 2 dumbbells even in an environment of quite rigid complex anions, for instance observed in Ag 4 SiO 4 (Klein & Jansen, 2008), in contrast to a more spherical 'alkali metallike' coordination as in Ag 3 SbO 4 (distorted rock salt structure; Klein & Jansen, 2010 Twelvefold coordination of the Dy 3+ ion by six bidentate nitrate ions in Ag 3 Dy 2 (NO 3 ) 9 : (a) view along the threefold symmetry axis; (b) distorted icosahedron around Dy. Atoms are drawn at the 60% probability level.

Figure 1
Unit cell of Ag 3 Dy 2 (NO 3 ) 9 , view along the c axis, atomic displacement ellipsoids are drawn with a probability of 60%.
largest axis of the displacement ellipsoid perpendicular to the AgO 2 dumbbell direction (see Fig. 3), which also represents the largest extension of an anisotropic parameter of all atoms in this structure (see supporting information, U 22 ). The two independent nitrate ions are perfectly planar, with O-N-O angle sums of 360.00 and 359.79 around N1 and N2, respectively. Both the nitrate ions are situated between three bidentately coordinated metal atoms forming almost planar AgDy 2 (NO 3 ) and Ag 2 Dy(NO 3 ) units, respectively, as illustrated in Fig. 4. The longest N-O distances and the smallest O-N-O angles are found in the direction of coordinated Dy atoms, and in addition the Ag atom coordination, including a short Ag-O distance shows an O-N-O angle slightly below the mean value. The appearance of this structure type for the combination Ag-Dy is somewhat remarkable. While silver as an atypical single-charged cation deforms its direct environment slightly to achieve a more convenient coordinative situation as explained above, dysprosium represents the heaviest lanthanide and, thus, the one with the smallest ionic radius observed in this structure type so far (Shannon, 1976), and a twelvecoordinate site seems to be unusual for this small lanthanide. This view is supported by the finding that compounds that include smaller lanthanide cations avoid to adopt this structure type in favour of another structure with a smaller coordination number and even a slightly different composition (A/ Ln = 2:1; Manek & Meyer, 1992, 1993a. Additionally, this might be confirmed by the 'underbonding' of the Dy cation, as the bond-valence sums (Brown & Altermatt, 1985) are calculated to be 2.51 valence units for the threefold positively charged ion, according to the parameters of Brese & O'Keeffe (1991).
The crystal structure has been quantitatively compared to isotypic structures by applying the program compstru (de la Flor et al., 2016), available at the Bilbao Crystallographic Server (Aroyo et al., 2006). With Ag 3 Dy 2 (NO 3 ) 9 as the reference structure, Table 1 lists the absolute distances between paired atoms as well as the arithmetic mean of the distance (d av ) between paired atoms, the degree of lattice deviation (S) and the measure of similarity (Á). Generally, the low values for S and Á indicate a close relationship between all phases, including the trend to increasing numbers at larger differences of lattice parameters from Na to Rb compounds. The differences of d av , S, and Á are of course determined in a higher degree by the more differing radii of the (more frequent) alkali metal cations than by those of the more similar lanthanide ions. Significantly, in all cases the largest displacements between atom pairs are observed for O5, i.e. the closest Ag-coordinating O atom, confirming the special bonding situation for Ag including the above-mentioned AgO 2 dumbbells. Consequently, the whole NO 3 anion, of which O5 is a part, is shifted slightly more than the atoms of the other anion. The Ag atom is also affected, as indicated by Planar surrounding of the two independent nitrate anions: NO 3 (1) (upper) coordinating two Dy and one Ag, view perpendicular to the twofold symmetry axis through Ag, N1, and O1; NO 3 (2) (lower) coordinating one Dy and two Ag, the short Ag-O5 bond is drawn thicker than other Ag-O bonds. All atoms are shown at the 60% probability level. [Symmetry codes:

Figure 3
Coordination of the Ag + cation by five bidentate nitrate anions. The shorter Ag-O bonds, which define the AgO 2 dumbbell, are emphasized, displacement ellipsoids are drawn at the 60% probability level.
[Symmetry codes: (ii) y, z, x; (iii) x + 1 4 , Àz + 1 4 , y À 1 4 .] higher Ag-A displacements than those of the lanthanide cation pairs, while the coordination of the Ln cations remains similar (distortedly icosahedral, slightly off-centered), just accompanied by decreasing Ln-O distances with decreasing cation radii. An exception represents the, so far, only known Na structure, where the similarity as well as the relative displacements are about one order lower than for all other examples, indicating that the packing is distorted to a similar degree by the small Na cation as in the title compound by the Ag cation. However, the closest Ag-O distance is shorter than all Na-O distances in the related Na 3 Nd 2 (NO 3 ) 9 .

Synthesis and crystallization
An alumina crucible was charged with 359 mg AgNO 3 (2.1 mmol; Merck; p.A.) and 495 mg Dy(NO 3 ) 3 Á5H 2 O (1.1 mmol; Alfa Aesar; 99.99%). The mixture was melted together at 573 K for 72 h in an Ar atmosphere, and was cooled down to 453 K at a rate of 0.1 K min À1 . Within an amorphous yellow-grey matrix, pale-yellow plates were found that were hygroscopic. EDX measurements on several crystals confirm the presence of Ag and Dy as the only elements heavier than oxygen. For the X-ray data collection, crystals were immersed into perfluoroalkyl ether, which covers and acts as glue on a glass tip during the data collection at low temperatures.

Trisilver Didysprosium nonanitrate
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refined as a 2-component inversion twin.