Crystal structure of KNaCuP2O7, a new member of the diphosphate family

KNaCuP2O7 is a member of the diphosphate family and crystallizes isotypically with α-Na2CuP2O7. The structure exhibits nearly eclipsed diphosphate units, distorted CuO5 square-pyramids, and distorted NaO7 and KO9 polyhedra as the main building units.


Chemical context
In order to improve the ionic conductivity in copper diphosphates with general formula MM'CuP 2 O 7 (M, M' = monovalent cation), we attempted to partially replace the potassium cations in K 2 CuP 2 O 7 by smaller sodium cations. In the K 2 CuP 2 O 7 structure, the alkali cations are located in the interlayer space between corrugated [CuP 2 O 7 ] 2À anionic layers. Reducing the size of the cation can increase its mobility, and therefore might improve the material's charge-transport behaviour. The diphosphate group directly connected to three CuO 5 polyhedra in the structure of KNaCuP 2 O 7 . Displacement ellipsoids are drawn at the 50% probability level.
Several attempts were made to prepare crystals of the hypothetical solid solution K 2-x Na x CuP 2 O 7 , with x in the range 0 to 2. All of the attempts led to a compound with x = 1, i.e. KNaCuP 2 O 7 , the crystal structure of which is reported in this communication.

Figure 3
Projection of the KNaCuP 2 O 7 structure along [100], showing the corrugated interlayer space housing the cations K + in the 'L' sites and Na + in the 'S' sites. Displacement ellipsoids are drawn at the 99% probability level. Notes: qi = formal oxydation number; sof(i) = site occupation factor; d ar (i) = average distance; d med (i) = weighted average distance; CN(i) = coordination number; ECoN(i)= effective coordination number; cat = dispersion factor on cationic charges measuring the deviation of the computed charges; cat =[AE i (q i À Q i ) 2 /N À 1] 1/2 = 0.019.

Figure 2
The CuO 5 square-pyramid with neighbouring diphosphate groups in the structure of KNaCuP 2 O 7 . Displacement ellipsoids are drawn at the 50% probability level. 5 , as defined by Addison et al. (1984), has a value of 0.26, indicating a distorted square-pyramidal coordination environment (the value for an ideal square-pyramid is 0 while that of an ideal trigonal bipyramid is 1). Each CuO 5 polyhedron shares its vertices with two P 2 O 7 4À anions, one of which is chelating and the other belongs to two different P 2 O 7 groups ( Fig. 2). This linkage leads to layers extending parallel to (010) (Fig. 3).
As in the crystal structures of K 2 CuP 2 O 7 and -Na 2 CuP 2 O 7 , the crystal structure of KNaCuP 2 O 7 exhibits two independent sites hosting the K + and Na + cations. The first site is larger ('L') and is occupied by K + cations. It is limited by two neighbouring anionic layers (Fig. 3). The resulting KO 9 coordination polyhedron is considerably distorted (Fig. 4, Table 1). Its bond-valence sum is 0.98 valence units ( Table 2).
The second site is smaller ('S') and is occupied by Na + cations. It is surrounded by three CuO 5 and five PO 4 polyhedra, delimiting an ellipsoidal cavity as shown in Figs. 3 and 5. The irregular coordination sphere of Na + is made up of seven oxygen atoms and shows an effective coordination number (ECoN; Nespolo et al., 2001)

Figure 4
The nine-coordinated K + cation in the large 'L' site within the interlayer space in the structure of KNaCuP 2 O 7 . Displacement ellipsoids are drawn at 50% probability level. [Symmetry codes:
compiled in the ICSD (ICSD, 2017). It is apparent that the size of the cation(s) defines the structure type.

Synthesis and crystallization
Crystals of KNaCuP 2 O 7 were obtained from a mixture of KH 2 PO 4 , NaH 2 PO 4 and CuO in the molar ratio K:Na:Cu:P = 1:1:2:2. The stoichiometric mixture was finely ground and heated in a porcelain crucible at 623 K for 12 h to eliminate volatile products. The temperature was then increased to 873 K and held for 15 d until fusion was reached. The sample was slowly cooled (5 K d À1 ) to 500 K and finally allowed to cool radiatively to room temperature. The product was washed with water and rinsed with an aqueous solution of HCl (low concentration). Only one type of regular light-blue prismatic crystals was observed. The obtained crystals were ground and checked by powder X-ray diffraction. Rietveld analysis with the program TOPAS 4.2 (Coelho, 2009) revealed a single-phase product of KNaCuP 2 O 7 .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The occupancy of the Na + and K + sites was checked by independent refinement of the site occupation factors. In each case, full occupancy was observed without the contribution of the other cation. The maximum and minimum electron densities remaining in the difference-  program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

Crystal data
KNaCuP 2  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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Cu 0.76039 (5)