Comment

The title compound, NH₄ScF₄, (I), was produced during our exploratory studies in the hydrothermal chemistry of organically templated scandium fluorides (Stephens et al., 2004; Stephens & Lightfoot, 2006). This compound arises from an in situ breakdown of the organic template (see Experimental). It is related to several other layered fluorides of stoichiometry AMF₄ and may also be regarded as an n = 1 Dion-Jacobson phase (Dion et al., 1981).

The asymmetric unit (Fig. 1) contains one unique Sc site on a general position, having quite regular octahedral symmetry (Table 1). There are five crystallographically independent F sites and two N sites, both on mirror planes.

Fig. 2 shows that (I) is built from puckered layers of vertex-sharing octahedra, typical of layered perovskites. The layers are eclipsed relative to each other with respect to the a axis, but staggered by b/4 along b. Extensive hydrogen bonding occurs between adjacent layers, mediated by the ammonium cations (Table 2). The only N-HF hydrogen bonds are those between ammonium cations and the `apical' F atoms that project into the interlayer space. This hydrogen bonding fulfills bond-valence requirements around the apical F atoms F2 and F3, as shown in Table 3.

Both the raw diffraction data and the derived model display a significant degree of pseudosymmetry (e.g. the bond lengths involving F1/F4 and F2/F3). An alternative model in space group Amma, with b' = b/2, was also considered. However, our refinements confirm that the chosen model is correct; refinements in the higher-symmetry model lead to anomalously elongated ellipsoids for the `in-plane' F atoms, transverse to the Sc-F-Sc linkages. In fact, the pseudosymmetry is due to `octahedral tilting', and the difference between the two models is clarified in Fig. 3. In addition to the tilting relative to the b axis, shown in Fig. 2(b), there is a second tilt mode around the c axis, which is allowed in the correct Pmcn model but forbidden in the approximate Amma model. The two models are analogous to those in KFeF₄ (Lapasset et al., 1986), which undergoes a structural phase transition from KFeF₄(III) to KFeF₄(II) at 368 K, corresponding to Pmcn to Amma. We have not explored the possibility of such a phase transition in the present case.

In comparison with other compositionally related AMF₄ compounds, (I) is isotypic with KGaF₄ (Courbion et al., 1989). Interestingly, however, more precisely similar compositions have different structure types; for example, NH₄FeF₄ (Leblanc et al., 1985) has the same type of layer as (I), but the [ScF₄]^- sheets are eclipsed along both a and b, whereas KScF₄, has a unique `corrugated sheet' structure containing both cis- and trans-vertex-sharing octahedra (Champarnaud-Mesjard & Frit, 1992).

Figure 1 The asymmetric unit of (I), with displacement ellipsoids drawn at the 50% probability level [symmetry code: (i) -x, y + , -z + ].

Figure 2 (a) A projection along [100], showing staggered octahedra. (b) A projection along [010], showing eclipsed octahedra.

Figure 3 A view of a single layer along [001], showing (a) the Pmcn model with the additional c axis tilt mode and (b) the Amma model, with b' = b/2.

Experimental

Scandium oxide (0.138 g), water (5 ml) and a 40% aqueous solution of HF (0.5 ml) were heated in a Teflon-lined steel autoclave for 1 h at 463 K. To this, ethylene glycol (5 ml) and 1,3-diaminopropane (0.4 ml) were added, and the resulting mixture was heated at the same temperature for four days. The product was filtered off, washed with water and allowed to dry at room temperature overnight. Powder X-ray diffraction revealed predominantly (I) as the product, together with a small amount of ScF₃. Compound (I) was heated to 1073 K at a rate of 5 K min^-1 under N₂ gas. Thermogravimmetric analysis shows a single-step weight loss of 19.6% from 573 to 698 K (20% calculated). Powder X-ray diffraction of the residue shows that this decomposition product is ScF₃.

Crystal data

NH4ScF4 Mr = 139.00 Orthorhombic, P m c n a = 7.862 (2) Å b = 8.088 (2) Å c = 13.503 (4) Å V = 858.6 (4) Å3 Z = 8 Dx = 2.151 Mg m-3 Mo K radiation = 1.67 mm-1 T = 93 (2) K Prism, colourless 0.10 × 0.03 × 0.02 mm

Data collection

Rigaku/MSC CCD area-detector diffractometer scans Absorption correction: multi-scan (SADABS; Bruker, 1999) Tmin = 0.825, Tmax = 0.970 5191 measured reflections 847 independent reflections 587 reflections with I > 2(I) Rint = 0.026 max = 25.3°

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.028 wR(F2) = 0.102 S = 1.11 847 reflections 75 parameters Only H-atom coordinates refined w = 1/[2(Fo2) + (0.0519P)2 + 0.1485P] where P = (Fo2 + 2Fc2)/3 (/)max = 0.005 max = 0.40 e Å-3 min = -0.40 e Å-3

Table 1 Selected geometric parameters (Å, °) Sc-F2 1.9904 (12)  Sc-F3 1.9919 (12) Sc-F1 2.0272 (7) Sc-F4 2.0273 (7) Sc-F5i 2.0320 (19) Sc-F5 2.0325 (19) F2-Sc-F3 179.61 (9) F1-Sc-F4 179.82 (7) F5i-Sc-F5 179.713 (18) Scii-F1-Sc 151.46 (10) Sciii-F4-Sc 151.82 (10) Sciv-F5-Sc 168.48 (8) Symmetry codes: (i) ; (ii) ; (iii) ; (iv) -x, .

Table 2 Hydrogen-bond geometry (Å, °) D-HA D-H HA DA D-HA N1-H1F2 0.91 (3) 1.97 (3) 2.836 (2) 159 (2) N1-H2F3v 0.97 (4) 2.11 (3) 2.858 (3) 132.0 (9) N1-H3F2vi 0.86 (4) 2.21 (3) 2.853 (3) 131.3 (14) N2-H4F3vii 0.84 (4) 2.24 (3) 2.867 (3) 132.1 (14) N2-H5F3 0.93 (3) 1.95 (3) 2.837 (2) 158 (2) N2-H6F2viii 0.99 (4) 2.11 (3) 2.872 (4) 132.1 (9) Symmetry codes: (v) ; (vi) ; (vii) , -y+2, -z+1; (viii) .

Table 3 Bond valence parameters Bond sij sij Sc F2 0.54 - Sc F3 0.53 - Sc F1 0.49 - Sc F4 0.49 - Sc F5 0.48 - Sc F5 0.48 -   - 3.00 F1 Sc 0.49 × 2 0.97 F2 Sc 0.54 0.54 F3 Sc 0.53 0.53 F4 Sc 0.49 × 2 0.97 F5 Sc 0.48 - F5 Sc 0.48 0.96 Note: sij values calculated for B = 0.37; Brese & O'Keeffe (1991) extrapolated.

Systematic absences were consistent with space groups Pmcn (62) or P2₁cn (33). Successful refinement of the structure in centrosymmetric Pmcn, together with the lack of any contradictory physical property measurements, meant that this was preferred over P2₁cn. Pmcn [a non-standard setting of Pnma (62)] was chosen as this defines the perovskite-like layers to lie perpendicular to c, which is the convention in perovskite chemistry. H atoms were located from a difference Fourier map and were refined freely; the U_iso(H) values were fixed at 0.018 Å².

Data collection: CrystalClear (Rigaku/MSC, 2005); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: SHELXL97.

Supplementary data for this paper are available from the IUCr electronic archives (Reference: BC3020 ). Services for accessing these data are described at the back of the journal.