The crystal structure of KScP2O7

The crystal structure of the pyrophosphate KScP2O7 crystallizes in the KAlP2O7 structure type and is compared with other structures of the KM IIIP2O7 series (M = Al–Y).


Chemical context
Metal-phosphates with open framework structures raise large interest due to a rich crystal chemistry (Clearfield, 1988) and possible interesting applications, e.g. as non-linear optical materials, solid-state electrolytes, ionic conductors, battery materials or sensors (Hagerman & Poeppelmeier, 1995;Vı "tiņ š et al., 2000). In the context of solid-state electrolytes, we recently investigated the Na-super ionic conducting NaSICON-type compounds Na 3 Sc 2 (PO 4 ) 3 (NSP) and Ag 3 Sc 2 (PO 4 ) 3 (ASP) in terms of their structural phase-transition sequences and ionic conductivities (Rettenwander et al., 2018;Ladenstein et al., 2020;Redhammer et al., 2020). To elucidate the role of the alkali metals on symmetry, we intended to synthesize the potassium analogue of NSP with flux-growth techniques. Using a method applied by Sljukic et al. (1967) for the synthesis of large crystals of NaSICON-type KZr 2 (PO 4 ) 3 , however, did not yield the intended compound K 3 Sc 2 (PO 4 ) 3 , but the title diphosphate KScP 2 O 7 instead.
In this contribution, we present the determination of the crystal structure of KScP 2 O 7 , not reported so far, and compare it with the series of other K-containing diphosphates.

Structural commentary
The title compound crystallizes in space group P2 1 /c and is isostructural with KAlP 2 O 7 (Ng & Calvo, 1973). It contains one distinct K and Sc atom site, two distinct P atom and seven different oxygen-atom positions, all of them on general position 4 e. The basic building unit is a pyrophosphate group, which is formed by two distinct PO 4 tetrahedra ( Fig. 1). They share the O4 oxygen atom, and the bridging P1-O4 and P2-O4 bond lengths are distinctly longer [1.6128 (6) and 1.6076 (6) Å , respectively] than the three shorter terminal P-O bonds [ranging between 1.4944 (7) and 1.5207 (6) Å ]. These latter distances are those to the oxygen atoms which are shared with the ScO 6 octahedra. The tetrahedral O-P-O angles involving the bridging oxygen atom O4 are generally smaller, those involving the terminal oxygen atoms distinctly larger than the ideal O-T-O angle of 109.5 . This -together with the difference in bond lengths between bridging and nonbridging T-O bonds -induces polyhedral distortion (especially for the tetrahedral angle variance, TAV), which is distinctly larger for the P1 tetrahedron. Likewise, the average bond length is slightly larger for the P1O 4 tetrahedron than for the P2O 4 tetrahedron (Table 1). When comparing average bond lengths and polyhedral distortion parameters of the series of KM III P 2 O 7 structures (M = Al to Y), no clear variations with the ionic radius of the M cations can be found from the available data for tetrahedral structure units and distortion parameters, and they remain almost constant. The parameters for KScP 2 O 7 fit well into the data of the other KM III P 2 O 7 structures. The tetrahedral bridging angle P1-O4-P2 amounts to 125.80 (5) and is distinctly larger than that of KAlP 2 O 7 [123.2 (11) ]. On the other hand, here a clear trend of increasing bridging angle with increasing size of the M cation is evident, i.e. the pyrophosphate group is stretched to account for the increase in size of the M cations ( Table 1).
The terminal oxygen atoms of the pyrophosphate group share their corners with five neighbouring ScO 6 octahedra. Following Leclaire et al. (1989), two phosphate tetrahedra and one octahedron form the basic {ScP 2 O 11 } 9units (cf. Fig. 1), which are connected with units of the same kind via cornersharing to make up a sheet parallel to (001), as depicted in The principal building unit of KScP 2 O 7 shown with displacement ellipsoids at the 95% probability level. [Symmetry codes: (i) x, 1 2 À y,  ScO 6 octahedron of one layer shares its O3 iii and O6 i corners (symmetry codes refer to Fig. 1) with one PO 4 tetrahedron each of the layer below and above. Generally, all corners of the octahedron are shared with neighbouring PO 4 tetrahedra, whereby all oxygen atoms except O4 directly connect the octahedron with a pyrophosphate P 2 O 7 group, and the O2 ii and O5 iv oxygen atoms join the octahedron with two PO 4 tetrahedra within the abovementioned layer parallel to (001). Additionally, the O1, O5 iv and O7 oxygen atoms ( Fig. 1) are also bonded to one K I cation each. The average Sc-O bond length is 2.085 Å while individual bond lengths range between 2.0736 (7) and 2.1122 (6) Å with one shorter bond (Sc-O6 i ) of 2.0346 (7) Å . A similar behaviour with one significantly shorter M-O bond is also observed in other KM III P 2 O 7 compounds and seems to be a more general feature. The Sc-O6 i and the Sc-O3 iii bonds, which point towards [001] and connect different (001) layers, both are the shortest within the ScO 6 octahedron. Assuming that these two bonds are those to the axial oxygen atoms of the octahedron, the coordination polyhedron appears to be slightly compressed. Also, Ng & Calvo (1973) noted for KAlP 2 O 7 that the axial bonds are considerably shorter that the equatorial ones within the (001) layer and -more generally speaking -this is also found in other KM III P 2 O 7 compounds. The ScO 6 octahedron in the title compound is only slightly distorted in terms of bond lengths and bond-angle variance (Table 1). It is worth noting that KAlP 2 O 7 shows the most regular octahedral coordination of all KM III P 2 O 7 structures compared here, and the distortion increases with increasing size of the octahedral cation as depicted in Fig. 3a. The average <M-O> bond lengths also scale well with the ionic radius of the M site cation and are positively correlated (Fig. 3b).
Large heptagonal cavities are formed in the skeleton of octahedral and tetrahedral units that are made up from four tetrahedrally and two octahedrally coordinated sites within the (001) layer. The stacking of the layers leads to channels running parallel to [001] where the potassium cations are hosted. They are tenfold coordinated with K-O bond lengths ranging between 2.7837 (7) Å and 3.3265 (9) Å , the average K-O bond length being 3.072 Å . As for <M-O>, the average K-O bond length also increases with increasing size of the M site cation, i.e. the channel size increases also.
Using bond-valence energy landscape map (BVEL) calculations, an estimation of possible diffusion pathways of alkali ions in a compound can be facilitated. Using the program SoftBV (Chen & Adams, 2017;Chen et al., 2019) such calculations were performed on KScP 2 O 7 and reveal two energy minima. The lowest lying minimum is indeed occupied by the K I cation, a second one is present at x, y, z = 0.271, 0.317, 0.438 (interstitial i1) and is unoccupied. A one-dimensional diffusion pathway is evident (Fig. 4) Table 1 Selected structural and distortional parameters (Å , ) of KM III P 2 O 7 compounds.
Vol. = polyhedral volume (Å 3 ), DI = distortion index, ECoN = effective coordination number, OQE = octahedral quadratic elongation, OAV = octahedral angle variance ( 2 ), TQE = tetrahedral quadratic elongation, TAV = tetrahedral angle variance ( 2 ). All calculations were performed using VESTA (Momma & Izumi 2011; for the mathematical meaning see the VESTA Handbook); ionic radii were taken from Shannon (1976).  (7) 127.4 (6) Notes: (a) data from Riou et al. (1988); (b) data from Genkin & Timofeeva (1989). regular K site to the interstitial i1 site; to move it from i1 to the next K1 site needs $1.3 eV. Interestingly, the percolation energy in e.g. Fe III , Mo III and In III compounds of the KM III P 2 O 7 series is distinctly higher with around 1.8 eV as estimated from BVEL maps. Generally, a partial substitution of trivalent cations by divalent ones might be of interest to increase the content of alkaline ions (here K I ), which most probably could be found on the interstitial i1 site.

Synthesis and crystallization
The title compound was grown during attempts to synthesize NaSICON-type K 3 Sc 2 (PO 4 ) 3 adopting a flux growth protocol set up by Sljukic et al. (1967). Sc 2 O 3 and KH 2 PO 4 were mixed in stoichiometric quantities (molar ratio 2:3) and B 2 O 3 was added as a flux with a sixfold quantity of that of Sc 2 O 3 . The complete mixture was transferred to a platinum crucible, covered with a lid, and heated in a chamber furnace to 1473 K, held at this temperature for 24 h and then slowly cooled down to 1073 K at a rate of 3 K h À1 . Between 1073 K and room temperature the cooling rate was 50 K h À1 . The synthesis batch was immersed in hot water to dissolve the B 2 O 3 and remaining K-phosphates. The residual contained single-phase KScP 2 O 7 as checked by powder X-ray diffraction and showed Bond-valence energy landscape map at levels of 1.5 eV above the minimum (yellow) viewed along [100]; the crystal structure of KScP 2 O 7 is overlayed, the K sites lie within channels.

Potassium scandium diphosphate
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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )