Crystal structures of Ca4+xY3–xSi7O15+xN5–x (0 ≤ x ≤ 1) comprising of an isolated [Si7(O,N)19] unit

The solid solution series Ca4+xY3–xSi7O15+xN5–x with x = 0, 0.5 and 1, crystallizes isotypically with a [Si7(O,N)19] unit as a characteristic building unit.


Chemical context
Silicon oxynitrides (or oxynitridosilicates) containing an alkaline-earth or a rare-earth metal cation have been extensively studied due to their potential applications as phosphors for white-light-emitting diodes (Takeda et al., 2018). Recently, the exploration range for new silicon oxynitrides has been expanded to compounds with alkaline-earth and rare-earth metal cations. In this regard,  (Park et al., 2012) or BaYSi 2 O 5 N (Kobayashi et al., 2017) were synthesized and their crystal structures determined. The corresponding oxide or nitride forms are unknown for these materials. At the same time, the introduction of multiple anions contributes to the formation of otherwise unattainable silicate units in single anion compounds. In addition to the compounds mentioned above, for example, Ce 4 [Si 4 O 4 N 6 ]O has a hyperbolic layer structure, which is composed of an [SiO 3 N] unit connected by three cyclic [Si 3 O 3 N 6 ] units through corner-sharing (Irran et al., 2000).
While exploring new oxynitrides, we obtained SrSiO 2.64 N 0.24 with a single-chain inosilicate structure, which has not been realized for Sr-or Sr-rich metasilicate oxides and nitrides (Kobayashi et al., 2018). In the present work, the synthesis and Crystal structure of Ca 4.5 Y 2.5 Si 7 O 15.5 N 4.5 (2) drawn with cation-centered polyhedra. Colour code as in Fig. 1.

Figure 3
Crystal structure of Ca 5 Y 2 Si 7 O 16 N 4 (3) drawn with cation-centered polyhedra. Colour code as in Fig. 1.

Figure 4
Representative for all structures, the atomic arrangement around Si atoms in the structure of Ca 4 Y 3 Si 7 O 15 N 5 (1). Displacement ellipsoids are drawn at the 90% probability level. [Symmetry codes: (i) 1 À y, 1 + x À y, z; (ii) Àx + y, 1 À x, z; (iii) x, y, 1 2 À z; (iv) 1 À y, 1 + x À y, 1 2 À z; (v) Àx + y, 1 À x, 1 2 À z.]  Table 1. In agreement with the higher electronegativity of oxygen when compared to nitrogen, the Si-N bonds are systematically longer than Si-O bonds. (2 mol% Ce to Y) for 1 and 3 were ground in the presence of 20 wt% CaF 2 (Wako Chemical, 99.9%) as a flux. The mixtures were pelletized at 20 MPa, put on an alumina boat with a carbon sheet dish (Toyo Tanso, 0.1 mm of thickness) and calcined at 1733 K for 4 h under 100 ml min À1 of flowing nitrogen. The reaction mixtures were slowly cooled under different conditions: to 1373 K at a rate of 30 K h À1 , to 1173 K at a rate of 100 K h À1 and to RT by turning off the power for 1, and to 1373 K at a rate of 60 K h À1 , to 1173 K at a rate of 100 K h À1 and to RT by turning off the power for 2 and 3.

Synthesis and crystallization
After roughly grinding the recrystallized fused pellets, the powders obtained were washed with 5 M HCl(aq.) and distilled water, followed by drying at 353 K. Colourless platelet-like single crystals were selected from the reaction products. Each crystal was cut into two portions. One was affixed to a Mitegen (R) micro-mount device with a drop of Paratone N oil for single-crystal X-ray analysis. The other part was used for elemental analysis by energy dispersive X-ray (EDX) spectrometry using a scanning electron microscope (Hitachi, SU1510) equipped with an EDX detector (Horiba, X-act). EDX analysis indicated a Ca:Y:Si ratio of 0.266 (9):0.237 (4):0.497 (9) for 1, of 0.325 (9):0.183 (5)

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. In the initial stages of the refinements, all Si3 positions were treated as being located at the the 2d site, corresponding to a triangular environment of three N5 atoms. As a result of the strong anisotropy of the displacement ellipsoids along the c axis, the Si3 sites were subsequently refined as being split with half occupancy and a mirror symmetry element at z = AE0.25. When the occupancies of the disordered Ca 2+ and Y 3+ sites were refined freely, the ratios of Ca:Y were 7:86:6.14 for 1, 10.07:3.93 for 2, and 9:88:4.12 for 3. Reliability factors for these refinements are summarized in Table 3. All values are almost the same, and the differences between the refined structures are within standard uncertainties. For the final steps of refinements, values obtained by EDX spectrometry were idealized under consideration of charge neutrality. Incorporation of Ce in single crystals of 1 and 3 was confirmed by their photoluminescence (the data are not shown). However, the contamination was ignored because of the small amount (2 mol% relative to Y, that is, Ca 4 Y 2.93 Ce 0.06 Si 7 O 15 N 5 for 1 and Ca 5 Y 1.96 Ce 0.04 Si 7 O 15 N 5 for 3). Actually, consideration of the presence of Ce had only a marginal effect on refinement parameters and refined structures.
Five sites around the silicon atoms were detected. Although it is difficult to distinguish between oxygen and nitrogen atoms by XRD analysis alone, site occupancies of oxygen and nitrogen sites were determined from coordination environments, bond lengths, and bond-valence sums (Morgan, 1986;Fuertes, 2006;Braun et al., 2010;Maak et al., 2017). Following Pauling's second crystal rule, the site at Wyckoff position 6h is coordinated by three Si atoms and thus should be occupied by nitrogen (N5) alone. The relatively long bond length of Si3-(O,N)4, 1.804 (6), 1.769 (8), and 1.765 (7) Å for 1, 2, and 3, respectively, indicate that the (O,N)4 site at the 4f position also might be occupied by nitrogen. Under consideration of charge neutrality for the different compositions in 1-3, this site was refined as being occupationally disordered by oxygen and nitrogen for 2 and 3.
Δρ max = 1.25 e Å −3 Δρ min = −0.56 e Å −3 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x (14)     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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (<1)  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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )