Sr3(Al3+xSi13−x)(N21−xO2+x):Eu2+ (x ∼ 0): a monoclinic modification of Sr-sialon

The structure of the title compound, Sr-bearing oxonitridoaluminosilicate (Sr-sialon), contains two types of channels running along the a axis, with the three unique Sr atoms (coordinatioon number seven) residing in the larger one. The channels cross a three-dimensional Si–Al–O–N network, in which the Si and Al atoms are in a tetrahedral coordination with N and O atoms. The chemical composition of the crystal is close to Sr3Al3Si13N21O2 (tristrontium trialuminium tridecasilicon henicosanitride dioxide), which can be expressed as a mixture of SrSiN2, Si3N4, AlN, and SiO2 components in the molar ratio 3:3:3:1. The crystal studied was metrically orthorhombic, consisting of four twin components related by metric merohedry.

The structure of the title compound, Sr-bearing oxonitridoaluminosilicate (Sr-sialon), contains two types of channels running along the a axis, with the three unique Sr atoms (coordinatioon number seven) residing in the larger one. The channels cross a three-dimensional Si-Al-O-N network, in which the Si and Al atoms are in a tetrahedral coordination with N and O atoms. The chemical composition of the crystal is close to Sr 3 Al 3 Si 13 N 21 O 2 (tristrontium trialuminium tridecasilicon henicosanitride dioxide), which can be expressed as a mixture of SrSiN 2 , Si 3 N 4 , AlN, and SiO 2 components in the molar ratio 3:3:3:1. The crystal studied was metrically orthorhombic, consisting of four twin components related by metric merohedry.
The Si(Al)1-10 sites and N(O)1-16 sites in the dreier ring layer are almost exclusively occupied by Si and N, respectively, within the experimental error. The mean bond distance of the Si tetrahedra in the dreier ring layer is 1.76 (2) Å which is close to 1.72 Å as expected from the sum of ionic radii of Si 4+ adn N 3- (Shannon, 1976). The N(O)17-20 sites are located at the boundary of the dreier and sechser ring layers and occupied equally by N and O atoms. The Si(Al)11-14 sites in the sechser ring layers contain Si and Al atoms. The Si(Al)15 -16 and Si(Al)17 -18 split pair sites in the sechser ring layers are half filled with Si or Al. The N(O)21-23 sites are located in the sechser ring layer and occupied by N almost exclusively within the experimental error. All the Si(Al) tetrahedra in sechser ring layer have at least one long bond of circa 1.80 Å, suggesting a statistical presence of Al-N bonds of ca. 1.85 Å (Shannon, 1976). This geometrical feature supports the result of population analysis for the structure containing two different atoms with similar scattering powers for X-rays at the same site.
The geometry around the Si(Al)15-16 and Si(Al)17-18 split pairs in the sechser ring layer is shown in Fig. 3. The configuration varies depending upon the real location of Si(Al) atom at either site of the split pair. The N23 site bridges two Si(Al) atoms in tetrahedral coordination.
The Sr atoms reside in the larger channel coordinating to 7 N(O) atoms at distances less than 3.1 Å. The Eu atoms displace 4-6% Sr atoms at Sr(Eu)2-3 sites, whereas none at Sr(Eu)1 within the experimental error. Since Sr(Eu)2-3 sites enriches O around themselves, a possible coupling of the Eu and O distributions in the structure can be pointed out. Oeckler et al. (2009) reported that the composition of their Sr-bearing sialon Sr 5 Al 5+x Si 21-x N 35-x O 2+x :Eu 2+ (x~0) can be expressed as '5551' , i.e., 5SrSiN 2 +5AlN+5Si 3 N 4 +1SiO 2 , when x=0. In this context, the composition of the present crystal, The present analysis on the 3331 Sr-sialon was carried out independently and in a different approach against the study on the 3331 Eu-sialon by Michiue et al., (2009). Although these 3331 sialons were finally found to have similar structures, a complicated microstructure like twins in crystals of Sr-sialon should be noted, as given in Refinement. This may suggest a presence of microstress in channels, which can be introduced by the replacement of Eu with Sr having a slightly larger ionic radius.

Experimental
The powder sample with a starting composition of Sr 6 Si 27 Al 6 O 6 N 42 was prepared from α-Si 3 N 4 (SN-E10, Ube Industries Ltd.), SrO (Kojyundo Chemical Laboratory Co., Ltd.), and AlN (Type F, Tokuyama Co., Ltd.). The mixture was ground in the Si 3 N 4 mortar and pestle. The powder sample was filled in h-BN crucible and fired in a graphite resistance furnace at 2173 K for 24 hours under 1 MPa nitrogen atmosphere. The chemical composition was not analyzed on this batch, but done on another Eu-doped (3.0 at%) batch using EPMA, resulting in Sr 5.82 Eu 0.18 Al 6.67 Si 26.77 N 41.50 O 4.11 . This composition can be rewritten as (Sr,Eu) 3 (Al 3+x Si 13-x )(N 21-x O 2+x ) (x~0.06±0.32). A small amount of Eu was also detected from the X-ray diffraction analysis for the crystals sampled from the batch in which no Eu was included as starting components. This probably came from a contamination from the furnace or the crucible.

Refinement
The reciprocal sections synthesized from the frame data indicated no significant diffuse scattering for any reflections. Several very weak reflections were found not coincident at the lattice points expected for the present structure. They were neglected because their locations differed from sample to sample. No significant departure from the metrical orthrhombicity was detected from the laboratory source and the synchrotron X-ray experiments. The Laue symmetry was well approximated by mmm. However, all trials of structure solution assuming the orthorhombic space groups were unsuccessful or only revealed that almost all the atom sites except for Sr should assume the split atom model. The best R1 factor was 0.11 for P2 1 22 1 .
The possibility of orthorhombic-mimicking monoclinic structure was then examined. From the systematic absence of The point group of the crystal is 112 which is a merohedry in monoclinic crystal system. The symmetry element for the inversion twin is m' of the monoclinic holohedry, 112'/m'. In addition, since the crystal lattice is metrically orthorhombic, 'double twinning' can take place in which 'twin by merohedry' and 'twin by metric merohedry' occur at the same time (Coch, 2004;Nespolo, 2004). In the twin by metric merohedry, the twin element can be chosen from the symmetry elements of the orthorhombic holohedry, 2"/m"2"/m"2"/m". Possible combinations are categorized into five groups, (1) 112 (no twin), (2) 112/m' (inversion twin by merohedry), (3) m"12, 1m"2, and m"m"2 (reflection twin by metric merohedry), (4) 2"12, 12"2, and 2"2"2 (rotation twin by metric merohedry), and (5) m"2"2, 2"/m"12, 2"/m"2"/m"2, 2"12/m', 1m"2/m', and 2"/m"2"/m"2/m' (twin by double merohedry), each of which contains the same set of equivalent points. All these groups were examined through the least-squares procedure. The R1 factors are 0.0635 for group 1, 0.595 for group 2, 0.0481 for group 3, 0.475 for group 4, and 0.0447 for group 5. The double merohedry model (group 5) was finally adopted from comparing R1 factors, residual electrons, and K factors. The K factor improved significantly to 0.98 which is close to the ideal value of 1. The refined volume fractions of the individual twin components assuming 2"12/m' in the crystal are 28 (1)%, 22 (1)%, 21 (1)%, and 29 (1)%, where the second individual is related to the first by the two-fold rotation about a, and the third and forth are the inversions of the first and second, respectively.
Anisotropic ADPs for Sr(Eu) and isotropic ones for the other atoms were employed in the refinement. It was difficult to refine ADPs of all crystallographically independent Al(Si) and N(O) atom sites simultaneously, mainly due to insufficient high-angle diffraction data. As mentioned in Comment, the structure is composed of the dreier and sechser ring layers.  c) and (e) were supposed to be almost equally occupied by the two atoms. To reduce ambiguities, fixed population numbers were assumed in the final stage of refinements except for Sr(Eu)2 and Sr(Eu)3. These constraints led the composition of the crystal to (Sr,Eu) 3 (Al 3+x Si 13-x )(N 21-x O 2+x ) with x=0, which agreed with the EPMA analysis on a compound synthesized in similar conditions as mentioned in Experimental. Fig. 1. The asymmetric unit of the structure ; the displacement ellipsoids are plotted at the 80% probability level.     (14) Si8-N3 iv 1.789 (11)