Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270104026666/bc1056sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270104026666/bc1056Isup2.hkl |
Ca metal and CaF2 (3:1 molar ratio) were mixed in a glove-bag under Ar and placed in an Ni crucible. The crucible was then inserted into a silica tube that was sealed from air while allowing dynamic flow of inert gas. The reaction mixture was heated to 1273 K under Ar for 1 h and then cooled to 473 K, at which point the gas flow was switched from Ar to N2. The reactants were heated to 1273 K for 12 h, and were subsequently cooled at a rate of 40 K h−1 to 473 K and then to room temperature at a non-controlled rate.
Data collection: SMART (Bruker, 1997–2002); cell refinement: SAINT-Plus (Bruker, 2003); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXS97 in SHELXTL (Bruker, 2000); program(s) used to refine structure: SHELXL97 in SHELXTL; molecular graphics: SHELXP in SHELXTL; software used to prepare material for publication: SHELXP in SHELXTL.
Ca2NF | Dx = 2.987 Mg m−3 |
Mr = 113.17 | Mo Kα radiation, λ = 0.71073 Å |
Cubic, Fd3m | Cell parameters from 2393 reflections |
Hall symbol: -F 4vw 2vw | θ = 3.5–28.3° |
a = 10.0215 (8) Å | µ = 4.22 mm−1 |
V = 1006.46 (14) Å3 | T = 100 K |
Z = 16 | Prism, light yellow |
F(000) = 896 | 0.11 × 0.09 × 0.08 mm |
Bruker SMART APEX CCD diffractometer | 83 independent reflections |
Radiation source: fine-focus sealed tube | 81 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.031 |
ω scans | θmax = 28.3°, θmin = 3.5° |
Absorption correction: empirical (using intensity measurements) (SADABS in SAINT-Plus; Bruker, 2003) | h = −13→13 |
Tmin = 0.540, Tmax = 0.717 | k = −13→13 |
2393 measured reflections | l = −13→13 |
Refinement on F2 | 0 restraints |
Least-squares matrix: full | Primary atom site location: structure-invariant direct methods |
R[F2 > 2σ(F2)] = 0.028 | Secondary atom site location: difference Fourier map |
wR(F2) = 0.064 | w = 1/[σ2(Fo2) + (0.0109P)2 + 23.8669P] where P = (Fo2 + 2Fc2)/3 |
S = 1.39 | (Δ/σ)max < 0.001 |
83 reflections | Δρmax = 0.40 e Å−3 |
10 parameters | Δρmin = −0.41 e Å−3 |
Ca2NF | Z = 16 |
Mr = 113.17 | Mo Kα radiation |
Cubic, Fd3m | µ = 4.22 mm−1 |
a = 10.0215 (8) Å | T = 100 K |
V = 1006.46 (14) Å3 | 0.11 × 0.09 × 0.08 mm |
Bruker SMART APEX CCD diffractometer | 83 independent reflections |
Absorption correction: empirical (using intensity measurements) (SADABS in SAINT-Plus; Bruker, 2003) | 81 reflections with I > 2σ(I) |
Tmin = 0.540, Tmax = 0.717 | Rint = 0.031 |
2393 measured reflections |
R[F2 > 2σ(F2)] = 0.028 | 0 restraints |
wR(F2) = 0.064 | w = 1/[σ2(Fo2) + (0.0109P)2 + 23.8669P] where P = (Fo2 + 2Fc2)/3 |
S = 1.39 | Δρmax = 0.40 e Å−3 |
83 reflections | Δρmin = −0.41 e Å−3 |
10 parameters |
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 F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
Ca | 0.50988 (7) | 0.74012 (7) | 0.50988 (7) | 0.0068 (4) | |
N | 0.5000 | 0.5000 | 0.5000 | 0.0051 (13) | |
F1 | 0.5000 | 0.0000 | 0.5000 | 0.050 (4) | 0.913 (14) |
F2 | 0.6250 | 0.1250 | 0.6250 | 0.005 (12) | 0.17 (3) |
U11 | U22 | U33 | U12 | U13 | U23 | |
Ca | 0.0068 (4) | 0.0068 (4) | 0.0068 (4) | 0.0001 (3) | −0.0001 (3) | 0.0001 (3) |
N | 0.0051 (13) | 0.0051 (13) | 0.0051 (13) | −0.0008 (15) | −0.0008 (15) | −0.0008 (15) |
F1 | 0.050 (4) | 0.050 (4) | 0.050 (4) | 0.043 (4) | 0.043 (4) | 0.043 (4) |
F2 | 0.005 (12) | 0.005 (12) | 0.005 (12) | 0.000 | 0.000 | 0.000 |
Ca—F2i | 2.3412 (12) | N—Caix | 2.4105 (7) |
Ca—N | 2.4105 (7) | N—Cai | 2.4105 (7) |
Ca—Nii | 2.4105 (7) | N—Caii | 2.4105 (7) |
Ca—Niii | 2.4105 (7) | F1—Cax | 2.6081 (8) |
Ca—F1iv | 2.6081 (8) | F1—Caxi | 2.6081 (8) |
Ca—F1v | 2.6081 (8) | F1—Cai | 2.6081 (8) |
Ca—F1vi | 2.6081 (8) | F1—Caiv | 2.6081 (8) |
Ca—Cavii | 3.263 (2) | F1—Cav | 2.6081 (8) |
Ca—Caiii | 3.263 (2) | F1—Caxii | 2.6081 (8) |
Ca—Caii | 3.263 (2) | F2—Caxiii | 2.3412 (12) |
Ca—Caviii | 3.5487 (3) | F2—Cax | 2.3412 (12) |
Ca—Caix | 3.5487 (3) | F2—Caxii | 2.3412 (12) |
N—Caviii | 2.4105 (7) | F2—Cai | 2.3412 (12) |
N—Caiii | 2.4105 (7) | ||
F2i—Ca—N | 121.94 (2) | F1vi—Ca—Caviii | 132.538 (18) |
F2i—Ca—Nii | 121.94 (2) | Cavii—Ca—Caviii | 122.593 (18) |
N—Ca—Nii | 94.61 (3) | Caiii—Ca—Caviii | 90.0 |
F2i—Ca—Niii | 121.94 (2) | Caii—Ca—Caviii | 62.627 (18) |
N—Ca—Niii | 94.61 (3) | F2i—Ca—Caix | 88.154 (13) |
Nii—Ca—Niii | 94.61 (3) | N—Ca—Caix | 42.600 (18) |
F2i—Ca—F1iv | 51.66 (2) | Nii—Ca—Caix | 137.013 (12) |
N—Ca—F1iv | 89.732 (4) | Niii—Ca—Caix | 93.19 (2) |
Nii—Ca—F1iv | 173.59 (4) | F1iv—Ca—Caix | 47.132 (14) |
Niii—Ca—F1iv | 89.732 (4) | F1v—Ca—Caix | 86.81 (2) |
F2i—Ca—F1v | 51.66 (2) | F1vi—Ca—Caix | 132.538 (18) |
N—Ca—F1v | 89.732 (4) | Cavii—Ca—Caix | 122.593 (18) |
Nii—Ca—F1v | 89.732 (4) | Caiii—Ca—Caix | 62.627 (18) |
Niii—Ca—F1v | 173.59 (4) | Caii—Ca—Caix | 90.0 |
F1iv—Ca—F1v | 85.57 (3) | Caviii—Ca—Caix | 54.75 (4) |
F2i—Ca—F1vi | 51.66 (2) | Caviii—N—Caiii | 180.00 (3) |
N—Ca—F1vi | 173.59 (4) | Caviii—N—Caix | 85.20 (4) |
Nii—Ca—F1vi | 89.732 (4) | Caiii—N—Caix | 94.80 (4) |
Niii—Ca—F1vi | 89.732 (4) | Caviii—N—Cai | 85.20 (4) |
F1iv—Ca—F1vi | 85.57 (3) | Caiii—N—Cai | 94.80 (4) |
F1v—Ca—F1vi | 85.57 (3) | Caix—N—Cai | 85.20 (4) |
F2i—Ca—Cavii | 144.7 | Caviii—N—Ca | 94.80 (4) |
N—Ca—Cavii | 93.33 (2) | Caiii—N—Ca | 85.20 (4) |
Nii—Ca—Cavii | 47.400 (18) | Caix—N—Ca | 94.80 (4) |
Niii—Ca—Cavii | 47.400 (18) | Cai—N—Ca | 180.00 (3) |
F1iv—Ca—Cavii | 137.132 (14) | Caviii—N—Caii | 94.80 (4) |
F1v—Ca—Cavii | 137.132 (14) | Caiii—N—Caii | 85.20 (4) |
F1vi—Ca—Cavii | 93.08 (2) | Caix—N—Caii | 180.0 |
F2i—Ca—Caiii | 144.7 | Cai—N—Caii | 94.80 (4) |
N—Ca—Caiii | 47.400 (18) | Ca—N—Caii | 85.20 (4) |
Nii—Ca—Caiii | 93.33 (2) | Cax—F1—Caxi | 85.74 (3) |
Niii—Ca—Caiii | 47.400 (18) | Cax—F1—Cai | 94.26 (3) |
F1iv—Ca—Caiii | 93.08 (2) | Caxi—F1—Cai | 180.0 |
F1v—Ca—Caiii | 137.132 (14) | Cax—F1—Caiv | 180.0 |
F1vi—Ca—Caiii | 137.132 (14) | Caxi—F1—Caiv | 94.26 (3) |
Cavii—Ca—Caiii | 60.0 | Cai—F1—Caiv | 85.74 (3) |
F2i—Ca—Caii | 144.7 | Cax—F1—Cav | 85.74 (3) |
N—Ca—Caii | 47.400 (18) | Caxi—F1—Cav | 94.26 (3) |
Nii—Ca—Caii | 47.400 (18) | Cai—F1—Cav | 85.74 (3) |
Niii—Ca—Caii | 93.33 (2) | Caiv—F1—Cav | 94.26 (3) |
F1iv—Ca—Caii | 137.132 (14) | Cax—F1—Caxii | 94.26 (3) |
F1v—Ca—Caii | 93.08 (2) | Caxi—F1—Caxii | 85.74 (3) |
F1vi—Ca—Caii | 137.132 (14) | Cai—F1—Caxii | 94.26 (3) |
Cavii—Ca—Caii | 60.0 | Caiv—F1—Caxii | 85.74 (3) |
Caiii—Ca—Caii | 60.0 | Cav—F1—Caxii | 180.0 |
F2i—Ca—Caviii | 88.154 (13) | Caxiii—F2—Cax | 109.5 |
N—Ca—Caviii | 42.600 (18) | Caxiii—F2—Caxii | 109.5 |
Nii—Ca—Caviii | 93.19 (2) | Cax—F2—Caxii | 109.5 |
Niii—Ca—Caviii | 137.013 (12) | Caxiii—F2—Cai | 109.5 |
F1iv—Ca—Caviii | 86.81 (2) | Cax—F2—Cai | 109.5 |
F1v—Ca—Caviii | 47.132 (14) | Caxii—F2—Cai | 109.5 |
Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) x, −y+5/4, −z+5/4; (iii) −x+5/4, −y+5/4, z; (iv) x, −y+3/4, −z+3/4; (v) −x+3/4, −y+3/4, z; (vi) x, y+1, z; (vii) −x+5/4, y, −z+5/4; (viii) x−1/4, y−1/4, −z+1; (ix) −x+1, y−1/4, z−1/4; (x) −x+1, y−3/4, z+1/4; (xi) x, y−1, z; (xii) x+1/4, y−3/4, −z+1; (xiii) x+1/4, −y+1, z+1/4. |
Experimental details
Crystal data | |
Chemical formula | Ca2NF |
Mr | 113.17 |
Crystal system, space group | Cubic, Fd3m |
Temperature (K) | 100 |
a (Å) | 10.0215 (8) |
V (Å3) | 1006.46 (14) |
Z | 16 |
Radiation type | Mo Kα |
µ (mm−1) | 4.22 |
Crystal size (mm) | 0.11 × 0.09 × 0.08 |
Data collection | |
Diffractometer | Bruker SMART APEX CCD diffractometer |
Absorption correction | Empirical (using intensity measurements) (SADABS in SAINT-Plus; Bruker, 2003) |
Tmin, Tmax | 0.540, 0.717 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 2393, 83, 81 |
Rint | 0.031 |
(sin θ/λ)max (Å−1) | 0.667 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.028, 0.064, 1.39 |
No. of reflections | 83 |
No. of parameters | 10 |
w = 1/[σ2(Fo2) + (0.0109P)2 + 23.8669P] where P = (Fo2 + 2Fc2)/3 | |
Δρmax, Δρmin (e Å−3) | 0.40, −0.41 |
Computer programs: SMART (Bruker, 1997–2002), SAINT-Plus (Bruker, 2003), SAINT-Plus, SHELXS97 in SHELXTL (Bruker, 2000), SHELXL97 in SHELXTL, SHELXP in SHELXTL.
Ca—F2i | 2.3412 (12) | Ca—Caiii | 3.263 (2) |
Ca—N | 2.4105 (7) | Ca—Caiv | 3.5487 (3) |
Ca—F1ii | 2.6081 (8) | ||
F2i—Ca—Nv | 121.94 (2) | Nvi—Ca—F1ii | 89.732 (4) |
N—Ca—Nv | 94.61 (3) | F1ii—Ca—F1vii | 85.57 (3) |
Nv—Ca—F1ii | 173.59 (4) |
Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) x, −y+3/4, −z+3/4; (iii) −x+5/4, y, −z+5/4; (iv) x−1/4, y−1/4, −z+1; (v) x, −y+5/4, −z+5/4; (vi) −x+5/4, −y+5/4, z; (vii) −x+3/4, −y+3/4, z. |
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The compound Ca2NF belongs to a largely unexplored class of extended inorganic materials first referred to as pseudo-oxides by Andersson (1967), as they have compositions that can be derived from oxides by replacement of O2− with N3− and F− ions. Andersson (1970) published the first quantitative structural study on nitride fluorides, in which three phases in the Mg/N/F system were prepared and characterized by powder X-ray diffraction methods. The three phases, L—Mg2NF, Mg3NF3 and H—Mg2NF, all have structures related to rocksalt to various extents, depending on anion ordering. In the H—Mg2NF phase, for example, no ordering of N and F atoms occurs and the rocksalt-type structure of MgO is observed. The L—Mg2NF phase, on the other hand, is tetragonal (i.e. anti-LiFeO2-type structure) due to ordering of N and F atoms along the c axis, and has a nearly doubled c axis relative to the a and b axes. Ehrlich et al. (1971) subsequently completed a study of M2NF (M = Ca, Sr and Ba), in which they reported the rocksalt structure for all three phases (i.e. simialr to the analogous oxides) using the Guinier powder diffraction technique. Galy et al. (1971) also reported a rocksalt-type structure for Ca2NF on the basis of powder X-ray diffraction results. We previously reported (Nicklow et al., 2001) the preparation and single-crystal X-ray structure of a Ca2NF phase with the L—Mg2NF-type structure. The present study concerns a newly observed phase in the Ca/N/F system, with doubling of the cell axes along all three directions relative to rocksalt-type Ca2NF. This phase is isostructural with doubled-cubic Sr2NF reported previously (Wagner, 2002).
Crystals for the present study were grown from the melt of a mixture of Ca metal and CaF2 reacting in a dynamic flow of N2 gas. The yellow samples, including that selected for analysis, were typically small and highly air (i.e. moisture) sensitive. They were also invariably multicrystalline, usually consisting of a predominant crystallite with one or more other components at symmetrically unrelated orientations. GEMINI (Bruker, 2000) was used to find the orientation matrix of the predominant Ca2NF crystallite in the selected sample, and the final refined lattice parameter was ultimately determined to be 10.0215 (8) Å. It was immediately recognized that this parameter is approximately double that of the rocksalt-type Ca2NF reported by Ehrlich et al. (1971), with a cell parameter of 4.937 Å. On the basis of the overall analysis of systematic absences, the space group was optimally assigned as Fd3 m (No. 227). Included in the observed data were 29 low-intensity forbidden reflections, all of type (0kl) with k+l≠4n, which are forbidden by the presence of the diamond glide planes. These are accounted for by the refined model described below.
Initial stages of the structure refinement yielded three crystallographically distinct octahedral positions, which were assigned as Ca (32 e), N (16 d) and F1 (16c), respectively. Following these assignments, appreciable electron density remained at one of the tetrahedral sites (8a), and this was optimally refined as an interstitial F atom. The occupancy of this site was constrained to be equal to vacancies at the nearby F1 octahedral position; thus a Frenkel defect was modeled. Also, the sum of the occupancies of these two partially filled F sites was restrained to keep the total F content fixed at 16 atoms per unit cell. The resulting final empirical composition was Ca2N(F0.913)oct(F0.087)tet, corresponding to an average of 1.4 interstitial F atoms per unit cell.
Fig. 1 shows the coordination sphere for one Ca atom, with atoms plotted as 50% probability displacement ellipsoids. The severely misshapen ellipsoids at the octahedral F1 sites reflect the partial occupancy and disorder at this site, as expected from the nearby Frenkel defect. Any one of three adjacent F1 atoms can be positioned 2.170 Å away into the nearby F2 (i.e. tetrahedral) site in this Frenkel defect model, with the remaining two F1 atoms expected to relax away from F2. Thus the ellipsoids are directed away from (and towards) the nearest partially occupied F2 tetrahedral sites, and the 16c special position at which F1 is located is really an average position with partial occupancy. Note that attempts to refine a split-atom model for the F1 position failed to yield a stable refinement, as might be expected given that the largest principal axis of the displacement ellipsoid is 0.136 Å2.
Fig. 2 shows the unit-cell plot, and selected bond angles and lengths are given in Table 1. From the figure, ordering of N and F atoms along all three cell axes is clearly seen, accounting for the doubling of the cell edges relative to rocksalt as well as the presence of the diamond glide planes. An interstitial F2 atom is also shown in the figure, with a nearby F1 atom missing, which results in local loss of the diamond glide plane. Therefore, considering the average vacancy of 1.4 F1 sites per unit cell, the model accounts for the presence of weak reflections in the data mentioned previously that are forbidden for the diamond glide planes. It is also evident in Fig. 1 and from Table 1 that the CaF3N3 octahedra are distorted, with N—Ca—N angles greater than 90° and F1—Ca—F1 angles smaller. This distortion is merely a consequence of the fact that the Ca atom is closer to the N/N/N face of the octahedron than to the F/F/F face, since the Ca—N bonds are shorter than the Ca—F1 bonds, as also indicated in Table 1. Thus the Ca atom is positioned off the octahedral center (i.e. on a lower-symmetry site) relative to Ca in rocksalt-type CaO, whereas the N and F atoms have essentially rocksalt-type positions. In addition, as expected, the N—Ca—N angle is larger than the corresponding angle in doubled-cubic Sr2NF (93.89°; Wagner, 2002).
Bond valence sums were calculated using the empirical parameters of Brese & O'Keeffe (1991), which were reported as 2.14 for Ca—N bonds and 1.842 for Ca—F bonds. The resulting values are 1.86 for Ca, 2.89 for N, 0.76 for F1 and 1.04 for F2 atoms. These results indicate that the atoms on the doubled-cubic lattice positions are somewhat underbonded. Initially, one might consider that this result can be explained by the relatively expanded lattice caused by the presence of the distorted octahedra. For example, note that doubling of Ehrlich's reported ideal rocksalt-type cell would give a lattice dimension of 4.936 Å × 2 = 9.872 Å, which is significantly smaller than the value of 10.0215 (8) Å for the present structure. However, the bond valence sums for both the rocksalt-type Ca2NF phase of Ehrlich et al. (1971) and our previously reported L—Ca2NF phase (Nicklow et al., 2001) indicate underbonding as well (e.g. the values for Ca in the two phases are 1.79 and 1.77, respectively). Therefore, further explanation for underbonding in the present structure must be provided by crystal chemical factors other than lattice expansion. O'Keefe & Hyde (1984) have previously noted that atoms in oxides with large cation-to-anion ratios (i.e. one or greater) tend to be underbonded, and attribute this phenomenon to bond elongation from relatively strong cation–cation interactions in such compounds. We have consistently observed the same phenomenon in other M2NF (M = Ca, Sr and Ba) compounds, and have discussed it in detail elsewhere (Nicklow et al., 2001; Wagner, 2002; Seibel & Wagner, 2004).