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

Fitouri, I.; Boughzala, H.

doi:10.1107/S2056989018000130

research communications

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ISSN: 2056-9890

Volume 74| Part 2| February 2018| Pages 109-112

https://doi.org/10.1107/S2056989018000130

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

Ines Fitouri ^a and Habib Boughzala ^a ^*

^aLaboratoire de Materiaux et Cristallochimie, Faculté des Sciences de Tunis, Université de Tunis El Manar, 2092 Manar II Tunis, Tunisia
^*Correspondence e-mail: habib.boughzala@ipein.rnu.tn

Edited by M. Weil, Vienna University of Technology, Austria (Received 6 December 2017; accepted 3 January 2018; online 9 January 2018)

Potassium sodium copper(II) diphosphate(V), KNaCuP₂O₇, was synthesized by solid-state reactions. It crystallizes in the α-Na₂CuP₂O₇ structure type in space group P2₁/n. In the crystal, CuO₅ square-pyramids are linked to nearly eclipsed P₂O₇ groups by sharing corners to build up corrugated layers with composition [CuP₂O₇]²⁻ that extend parallel to (010). The K⁺ and Na⁺ cations reside in the interlayer space and are connected to nine and seven O atoms, respectively. The structural model was validated by bond-valence-sum (BVS) and charge-distribution (CHARDI) analysis.

Keywords: crystal structure; diphosphate; eclipsed conformation; isotypism.

CCDC reference: 1814470

1. Chemical context

In order to improve the ionic conductivity in copper diphosphates with general formula MM'CuP₂O₇ (M, M' = monovalent cation), we attempted to partially replace the potassium cations in K₂CuP₂O₇ by smaller sodium cations. In the K₂CuP₂O₇ structure, the alkali cations are located in the interlayer space between corrugated [CuP₂O₇]²⁻ anionic layers. Reducing the size of the cation can increase its mobility, and therefore might improve the material's charge-transport behaviour.

Several attempts were made to prepare crystals of the hypothetical solid solution K_2–xNa_xCuP₂O₇, with x in the range 0 to 2. All of the attempts led to a compound with x = 1, i.e. KNaCuP₂O₇, the crystal structure of which is reported in this communication.

2. Structural commentary

The title compound KNaCuP₂O₇ crystallizes isotypically with α-Na₂CuP₂O₇ (Erragh et al., 1995 ) and also shows resemblance with one form of K₂CuP₂O₇ (ElMaadi et al., 1995 ). It is built up by corrugated [CuP₂O₇]²⁻ layers with the alkali cations situated in the interlayer space. The anionic layers consist of a nearly eclipsed diphosphate group [O2—P2—P1—O6 torsion angle = 15.90 (1)°] linked to CuO₅ square-pyramids by sharing five of the terminal oxygen atoms (O2, O3, O5, O6, O7). The bridging atom O4 of the diphosphate unit is not involved in metal coordination, and atom O1 coordinates to the alkali cations in the interlayer space (Fig. 1).

Figure 1
The diphosphate group directly connected to three CuO₅ polyhedra in the structure of KNaCuP₂O₇. Displacement ellipsoids are drawn at the 50% probability level.

The P2—O4—P1 bridging angle [119.01 (11)°] of the diphosphate anion is close to those observed in other similar diarsenate and diphosphate structures, such as KCr_1/4Al_3/4As₂O₇ [118.50 (14)°; Bouhassine & Boughzala, 2017 ], CsCrAs₂O₇ [118.7 (2)°; Bouhassine & Boughzala, 2015 ], KAlAs₂O₇ [118.3 (2)°; Boughzala & Jouini, 1995 ], α-Na₂CuP₂O₇ [118.65 (1)°; Erragh et al., 1995] and K₂CuP₂O₇ [120.41 (3)°; ElMaadi et al., 1995]. As expected, the Cu—O bond length to the apical oxygen atom O5 is significantly longer than the Cu—O distances to the basal oxygen atoms of the square-pyramid (Table 1). The calculated bond-valence sum (Brown, 2002 ; Adams, 2003 ) of 1.94 valence units for the Cu site is in good agreement with the expected value of 2 for divalent copper. The geometry index of the CuO₅ polyhedron τ₅, 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 CuO₅ polyhedron shares its vertices with two P₂O₇⁴⁻ anions, one of which is chelating and the other belongs to two different P₂O₇ groups (Fig. 2). This linkage leads to layers extending parallel to (010) (Fig. 3).

Table 1
Selected bond lengths (Å)

Cu—O2	1.9328 (18)	Na—O5	2.398 (2)
Cu—O3ⁱ	1.9427 (18)	Na—O7ⁱ	2.4442 (19)
Cu—O6	1.9743 (17)	Na—O3^iv	2.772 (2)
Cu—O7ⁱ	1.9872 (17)	Na—O5ⁱ	2.815 (2)
Cu—O5ⁱⁱ	2.3225 (19)	Na—O7^iv	2.878 (2)
P1—O1	1.482 (2)	K—O2	2.721 (2)
P1—O2	1.5246 (19)	K—O5ⁱⁱⁱ	2.7245 (18)
P1—O3	1.5313 (19)	K—O1ⁱ	2.764 (2)
P1—O4	1.6272 (17)	K—O6^v	2.7969 (19)
P2—O5	1.4958 (17)	K—O7^vi	2.8450 (19)
P2—O6	1.5252 (17)	K—O3^vii	2.8630 (18)
P2—O7	1.5277 (16)	K—O1	3.036 (2)
P2—O4	1.6148 (18)	K—O2^vii	3.1973 (19)
Na—O1ⁱⁱⁱ	2.249 (2)	K—O3ⁱ	3.257 (2)
Na—O6^iv	2.397 (2)
Symmetry codes: (i) x+1, y, z; (ii) $[x+{\script{1\over 2}}, -y+{\script{3\over 2}}, z+{\script{1\over 2}}]$ ; (iii) -x+1, -y+1, -z; (iv) $[x+{\script{1\over 2}}, -y+{\script{3\over 2}}, z-{\script{1\over 2}}]$ ; (v) $[-x+{\script{3\over 2}}, y-{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (vi) $[-x+{\script{1\over 2}}, y-{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (vii) -x+1, -y+1, -z+1.

Figure 2
The CuO₅ square-pyramid with neighbouring diphosphate groups in the structure of KNaCuP₂O₇. Displacement ellipsoids are drawn at the 50% probability level.

Figure 3
Projection of the KNaCuP₂O₇ 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.

As in the crystal structures of K₂CuP₂O₇ and α-Na₂CuP₂O₇, the crystal structure of KNaCuP₂O₇ 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 KO₉ 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 CuO₅ and five PO₄ 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 ) of 5.2 (for other ECoN values, see Table 2). The Na—O bonds lengths can be divided in groups of four short [2.249 (2)–2.4442 (19) Å] and three long [2.772 (2)–2.878 (2) Å] bonds (Table 1). Its bond-valence sum is 0.98 valence units (Table 2).

Table 2
CHARDI and BVS analysis of cation polyhedra in KNaCuP₂O₇

Cation	qi()·sof(i)	Q(i)	V(i)·sof(i)	CN(i)	ECoN(i)	d_ar(i)	d_med(i)
Cu	2.000	1.94	1.995	5	4.35	2.031	2.031
P1	5.000	5.07	4.921	4	3.84	1.540	1.541
P2	5.000	5.03	4.938	4	3.88	1.540	1.540
K	1.000	0.98	1.103	9	5.20	2.622	2.564
Na	1.000	0.98	1.106	7	7.80	2.912	2.912
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; σ_cat = dispersion factor on cationic charges measuring the deviation of the computed charges; σ_cat=[Σ_i(q_i − Q_i)²/N − 1]^1/2 = 0.019.

Figure 4
The nine-coordinated K⁺ cation in the large `L' site within the interlayer space in the structure of KNaCuP₂O₇. Displacement ellipsoids are drawn at 50% probability level. [Symmetry codes: (i) 1 + x, y, z; (ii) 1 − x, 1 − y, −z; (iii) $[{3\over 2}]$ − x, − $[{1\over 2}]$ + y, $[{1\over 2}]$ − z; (iv) $[{1\over 2}]$ − x, − $[{1\over 2}]$ + y, $[{1\over 2}]$ − z; (v) 1 − x, 1 − y, 1 − z.]

Figure 5
The surrounding of the seven-coordinated Na⁺ cation in the `S' site in the structure of KNaCuP₂O₇. Displacement ellipsoids are drawn at 50% probability level. [Symmetry codes: (i) 1 + x, y, z; (ii) 1 − x, 1 − y, −z; (iii) $[{1\over 2}]$ + x, $[{3\over 2}]$ − y, − $[{1\over 2}]$ + z].

3. Database survey

Table 3 summarizes lattice parameters and the symmetry of related MM'CuP₂O₇ (M, M' = monovalent cation) compounds compiled in the ICSD (ICSD, 2017 ). It is apparent that the size of the cation(s) defines the structure type.

Table 3
Structural data (Å, °) for the M,M′₂CuP₂O₇ family of compounds

Compound	Space group	a	b	c	β	Z	Reference
Li₂CuP₂O₇	I2/a	14.068 (2)	4.8600 (8)	8.604 (1)	98.97 (1)	4	Spirlet et al. (1993 )
Li₂CuP₂O₇	C2/c	15.3360 (14)	4.8733 (13)	8.6259 (16)	114.795 (10)	4	Gopalakrishna et al. (2008 )
α-Na₂CuP₂O₇	P2₁/n	8.823 (3)	13.494 (3)	5.108 (2)	92.77 (3)	4	Erragh et al. (1995)
β-Na₂CuP₂O₇	C2/c	14.715 (1)	5.704 (2)	8.066 (1)	115.14 (1)	4	Etheredge et al. (1995 )
K₂CuP₂O₇	Pbnm	9.509 (4)	14.389 (6)	5.276 (2)		4	ElMaadi et al. (1995)
K₂CuP₂O₇	P $[\overline{4}]$ 2₁m	8.056 (2)		5.460 (11)		2	Keates et al. (2014 )
α-Rb₂CuP₂O₇	Pmcn	5.183 (1)	10.096 (1)	15.146 (2)		4	Chernyatieva et al. (2008 )
β-Rb₂CuP₂O₇	Cc	7,002 (1)	12.751 (3)	9.773 (2)	110.93 (3)	4	Shvanskaya et al. (2012 )
Cs₂CuP₂O₇	Cc	7.460 (6)	12.973 (10)	9.980 (8)	111.95	4	Mannasova et al. (2016 )
NaCsCuP₂O₇	Pmn2₁	5.147 (2)	15.126 (3)	9.717 (5)		4	Chernyatieva et al. (2009 )
Na_1.12Ag_0.88CuP₂O₇	C2/c	15.088 (2)	5.641 (1)	8.171 (1)	116.11 (1)	4	Bennazha et al. (2002 )

4. Synthesis and crystallization

Crystals of KNaCuP₂O₇ were obtained from a mixture of KH₂PO₄, NaH₂PO₄ 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⁻¹) 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 KNaCuP₂O₇.

5. 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-Fourier map are located 0.67 Å from O7 and 0.48 Å from Cu, respectively.

Table 4
Experimental details

Crystal data
Chemical formula	KNaCuP₂O₇
M_r	299.57
Crystal system, space group	Monoclinic, P2₁/n
Temperature (K)	298
a, b, c (Å)	5.176 (3), 13.972 (5), 9.067 (3)
β (°)	91.34 (2)
V (Å³)	655.6 (5)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	4.51
Crystal size (mm)	0.18 × 0.13 × 0.09

Data collection
Diffractometer	Enraf–Nonius CAD-4
Absorption correction	ψ scan (North et al., 1968 )
T_min, T_max	0.868, 0.997
No. of measured, independent and observed [I > 2σ(I)] reflections	3300, 1425, 1291
R_int	0.036
(sin θ/λ)_max (Å⁻¹)	0.638

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.020, 0.059, 1.01
No. of reflections	1425
No. of parameters	110
Δρ_max, Δρ_min (e Å⁻³)	0.48, −0.42
Computer programs: CAD-4 EXPRESS (Enraf–Nonius, 1994 ), XCAD4 (Harms & Wocadlo, 1995 ), SHELXS97 (Sheldrick, 2008 ), SHELXL2014/7 (Sheldrick, 2015 ), DIAMOND (Brandenburg, 2008 ) and publCIF (Westrip, 2010 ).

Supporting information

CCDC reference: 1814470

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2056989018000130/wm5429sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989018000130/wm5429Isup2.hkl

checkCIF report

Computing details top

Data collection: CAD-4 EXPRESS (Enraf–Nonius, 1994); cell refinement: CAD-4 EXPRESS (Enraf–Nonius, 1994); data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); 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).

Potassium sodium copper(II) diphosphate(V) top

Crystal data top

KNaCuP₂O₇	F(000) = 580
M_r = 299.57	D_x = 3.035 Mg m⁻³
Monoclinic, P2₁/n	Mo Kα radiation, λ = 0.71073 Å
a = 5.176 (3) Å	Cell parameters from 25 reflections
b = 13.972 (5) Å	θ = 10–15°
c = 9.067 (3) Å	µ = 4.51 mm⁻¹
β = 91.34 (2)°	T = 298 K
V = 655.6 (5) Å³	Prism, blue
Z = 4	0.18 × 0.13 × 0.09 mm

Data collection top

Enraf–Nonius CAD-4 diffractometer	R_int = 0.036
Radiation source: fine-focus sealed tube	θ_max = 27.0°, θ_min = 2.7°
ω/2θ scans	h = −6→6
Absorption correction: ψ scan (North et al., 1968)	k = −2→17
T_min = 0.868, T_max = 0.997	l = −11→11
3300 measured reflections	2 standard reflections every 120 min
1425 independent reflections	intensity decay: 1.1%
1291 reflections with I > 2σ(I)

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.020	w = 1/[σ²(F_o²) + (0.0261P)² + 0.7316P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.059	(Δ/σ)_max = 0.001
S = 1.01	Δρ_max = 0.48 e Å⁻³
1425 reflections	Δρ_min = −0.42 e Å⁻³
110 parameters	Extinction correction: SHELXL-2014/7 (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0120 (9)

Special details top

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 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 > 2sigma(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Cu	0.76039 (5)	0.66590 (2)	0.21888 (3)	0.01093 (12)
P1	0.26527 (12)	0.54589 (5)	0.24314 (7)	0.01115 (15)
P2	0.25245 (11)	0.68972 (4)	0.02910 (6)	0.00898 (15)
Na	0.7521 (2)	0.70912 (8)	−0.16272 (11)	0.0192 (2)
K	0.75717 (11)	0.40697 (4)	0.38439 (6)	0.01850 (15)
O1	0.2223 (4)	0.44115 (15)	0.2502 (2)	0.0223 (4)
O2	0.5278 (3)	0.57557 (14)	0.3077 (2)	0.0176 (4)
O3	0.0576 (3)	0.60643 (15)	0.31633 (19)	0.0179 (4)
O4	0.2588 (3)	0.57729 (12)	0.07029 (18)	0.0126 (4)
O5	0.2938 (3)	0.69884 (14)	−0.13300 (18)	0.0160 (4)
O6	0.4685 (3)	0.73465 (13)	0.12308 (19)	0.0138 (4)
O7	−0.0125 (3)	0.72712 (13)	0.07246 (19)	0.0139 (4)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cu	0.00808 (16)	0.01226 (18)	0.01244 (17)	0.00043 (11)	0.00010 (11)	0.00296 (11)
P1	0.0103 (3)	0.0102 (3)	0.0130 (3)	0.0000 (2)	0.0007 (2)	0.0029 (2)
P2	0.0088 (3)	0.0099 (3)	0.0082 (3)	0.0002 (2)	0.0008 (2)	0.0009 (2)
Na	0.0190 (5)	0.0213 (6)	0.0172 (5)	−0.0021 (4)	−0.0001 (4)	0.0020 (5)
K	0.0220 (3)	0.0153 (3)	0.0182 (3)	0.0022 (2)	−0.0008 (2)	0.0000 (2)
O1	0.0272 (10)	0.0115 (10)	0.0280 (10)	−0.0046 (8)	0.0001 (8)	0.0049 (8)
O2	0.0123 (8)	0.0198 (10)	0.0206 (9)	−0.0026 (8)	−0.0022 (7)	0.0093 (8)
O3	0.0151 (9)	0.0261 (10)	0.0126 (8)	0.0075 (8)	0.0008 (7)	0.0036 (8)
O4	0.0178 (8)	0.0093 (8)	0.0107 (8)	−0.0006 (7)	0.0012 (6)	−0.0003 (7)
O5	0.0213 (9)	0.0179 (9)	0.0088 (8)	0.0015 (8)	0.0025 (7)	0.0013 (7)
O6	0.0115 (8)	0.0115 (8)	0.0181 (8)	−0.0002 (7)	−0.0041 (6)	0.0011 (7)
O7	0.0091 (8)	0.0154 (9)	0.0173 (8)	0.0016 (7)	0.0030 (6)	0.0044 (8)

Geometric parameters (Å, º) top

Cu—O2	1.9328 (18)	Na—P2ⁱ	3.0977 (12)
Cu—O3ⁱ	1.9427 (18)	Na—P2^viii	3.1316 (12)
Cu—O6	1.9743 (17)	Na—Cu^ix	3.2481 (11)
Cu—O7ⁱ	1.9872 (17)	Na—Cu^viii	3.3550 (11)
Cu—O5ⁱⁱ	2.3225 (19)	K—O2	2.721 (2)
Cu—Naⁱⁱ	3.2481 (11)	K—O5^vii	2.7245 (18)
Cu—Naⁱⁱⁱ	3.3550 (11)	K—O1ⁱ	2.764 (2)
Cu—K^iv	3.4966 (7)	K—O6^x	2.7969 (19)
Cu—Na	3.5117 (10)	K—O7^xi	2.8450 (19)
Cu—K	3.9169 (7)	K—O3^v	2.8630 (18)
P1—O1	1.482 (2)	K—O1	3.036 (2)
P1—O2	1.5246 (19)	K—O2^v	3.1973 (19)
P1—O3	1.5313 (19)	K—O3ⁱ	3.257 (2)
P1—O4	1.6272 (17)	K—P1^v	3.4455 (9)
P1—K	3.4260 (9)	K—Cu^x	3.4965 (7)
P1—K^v	3.4455 (9)	O1—Na^vii	2.249 (2)
P1—K^vi	3.5329 (9)	O1—K^vi	2.764 (2)
P2—O5	1.4958 (17)	O2—K^v	3.1973 (19)
P2—O6	1.5252 (17)	O3—Cu^vi	1.9427 (18)
P2—O7	1.5277 (16)	O3—Naⁱⁱⁱ	2.772 (2)
P2—O4	1.6148 (18)	O3—K^v	2.8630 (18)
P2—Na^vi	3.0977 (12)	O3—K^vi	3.257 (2)
P2—Naⁱⁱⁱ	3.1316 (12)	O5—Cu^ix	2.3225 (19)
P2—Na	3.1617 (12)	O5—K^vii	2.7245 (18)
Na—O1^vii	2.249 (2)	O5—Na^vi	2.815 (2)
Na—O6^viii	2.397 (2)	O6—Naⁱⁱⁱ	2.397 (2)
Na—O5	2.398 (2)	O6—K^iv	2.7968 (19)
Na—O7ⁱ	2.4442 (19)	O7—Cu^vi	1.9872 (17)
Na—O3^viii	2.772 (2)	O7—Na^vi	2.4441 (19)
Na—O5ⁱ	2.815 (2)	O7—K^xii	2.8451 (19)
Na—O7^viii	2.878 (2)	O7—Naⁱⁱⁱ	2.878 (2)

O2—Cu—O3ⁱ	91.46 (8)	O5—Na—Cu^ix	45.56 (5)
O2—Cu—O6	91.34 (7)	O7ⁱ—Na—Cu^ix	127.14 (6)
O3ⁱ—Cu—O6	176.22 (8)	O3^viii—Na—Cu^ix	36.58 (4)
O2—Cu—O7ⁱ	160.62 (8)	O5ⁱ—Na—Cu^ix	146.13 (6)
O3ⁱ—Cu—O7ⁱ	90.77 (7)	O7^viii—Na—Cu^ix	37.24 (4)
O6—Cu—O7ⁱ	87.43 (7)	P2ⁱ—Na—Cu^ix	150.57 (4)
O2—Cu—O5ⁱⁱ	109.22 (7)	P2^viii—Na—Cu^ix	58.62 (2)
O3ⁱ—Cu—O5ⁱⁱ	92.15 (8)	P2—Na—Cu^ix	65.37 (2)
O6—Cu—O5ⁱⁱ	84.52 (7)	O1^vii—Na—Cu^viii	108.65 (6)
O7ⁱ—Cu—O5ⁱⁱ	89.93 (7)	O6^viii—Na—Cu^viii	35.44 (4)
O2—Cu—Naⁱⁱ	134.76 (6)	O5—Na—Cu^viii	148.47 (6)
O3ⁱ—Cu—Naⁱⁱ	58.24 (7)	O7ⁱ—Na—Cu^viii	81.19 (5)
O6—Cu—Naⁱⁱ	118.00 (6)	O3^viii—Na—Cu^viii	77.32 (5)
O7ⁱ—Cu—Naⁱⁱ	61.20 (5)	O5ⁱ—Na—Cu^viii	43.13 (4)
O5ⁱⁱ—Cu—Naⁱⁱ	47.48 (5)	O7^viii—Na—Cu^viii	86.18 (4)
O2—Cu—Naⁱⁱⁱ	72.89 (6)	P2ⁱ—Na—Cu^viii	64.76 (2)
O3ⁱ—Cu—Naⁱⁱⁱ	134.08 (6)	P2^viii—Na—Cu^viii	57.50 (2)
O6—Cu—Naⁱⁱⁱ	44.76 (5)	P2—Na—Cu^viii	151.44 (4)
O7ⁱ—Cu—Naⁱⁱⁱ	118.06 (6)	Cu^ix—Na—Cu^viii	103.22 (3)
O5ⁱⁱ—Cu—Naⁱⁱⁱ	55.94 (5)	O2—K—O5^vii	102.87 (6)
Naⁱⁱ—Cu—Naⁱⁱⁱ	103.22 (3)	O2—K—O1ⁱ	96.73 (6)
O2—Cu—K^iv	136.72 (6)	O5^vii—K—O1ⁱ	78.10 (6)
O3ⁱ—Cu—K^iv	123.31 (6)	O2—K—O6^x	163.30 (6)
O6—Cu—K^iv	53.04 (5)	O5^vii—K—O6^x	63.38 (5)
O7ⁱ—Cu—K^iv	54.45 (5)	O1ⁱ—K—O6^x	71.95 (6)
O5ⁱⁱ—Cu—K^iv	51.11 (5)	O2—K—O7^xi	127.36 (6)
Naⁱⁱ—Cu—K^iv	65.54 (2)	O5^vii—K—O7^xi	66.49 (5)
Naⁱⁱⁱ—Cu—K^iv	64.45 (2)	O1ⁱ—K—O7^xi	127.46 (6)
O2—Cu—Na	121.97 (6)	O6^x—K—O7^xi	58.05 (5)
O3ⁱ—Cu—Na	120.87 (5)	O2—K—O3^v	115.68 (6)
O6—Cu—Na	59.42 (5)	O5^vii—K—O3^v	141.38 (6)
O7ⁱ—Cu—Na	42.40 (5)	O1ⁱ—K—O3^v	98.78 (6)
O5ⁱⁱ—Cu—Na	115.40 (5)	O6^x—K—O3^v	78.91 (6)
Naⁱⁱ—Cu—Na	102.98 (2)	O7^xi—K—O3^v	87.26 (6)
Naⁱⁱⁱ—Cu—Na	103.54 (2)	O2—K—O1	51.17 (5)
K^iv—Cu—Na	64.59 (2)	O5^vii—K—O1	71.36 (6)
O2—Cu—K	39.55 (6)	O1ⁱ—K—O1	126.29 (7)
O3ⁱ—Cu—K	56.02 (6)	O6^x—K—O1	125.76 (6)
O6—Cu—K	127.43 (5)	O7^xi—K—O1	77.84 (5)
O7ⁱ—Cu—K	131.41 (5)	O3^v—K—O1	132.24 (6)
O5ⁱⁱ—Cu—K	122.07 (5)	O2—K—O2^v	87.12 (5)
Naⁱⁱ—Cu—K	112.35 (2)	O5^vii—K—O2^v	137.25 (6)
Naⁱⁱⁱ—Cu—K	110.35 (2)	O1ⁱ—K—O2^v	142.70 (6)
K^iv—Cu—K	172.759 (11)	O6^x—K—O2^v	109.37 (5)
Na—Cu—K	122.43 (2)	O7^xi—K—O2^v	74.42 (5)
O1—P1—O2	112.67 (12)	O3^v—K—O2^v	47.81 (5)
O1—P1—O3	114.78 (12)	O1—K—O2^v	84.43 (5)
O2—P1—O3	108.17 (11)	O2—K—O3ⁱ	54.42 (5)
O1—P1—O4	107.95 (11)	O5^vii—K—O3ⁱ	110.06 (5)
O2—P1—O4	107.13 (10)	O1ⁱ—K—O3ⁱ	49.03 (5)
O3—P1—O4	105.64 (10)	O6^x—K—O3ⁱ	119.15 (5)
O1—P1—K	62.31 (8)	O7^xi—K—O3ⁱ	176.12 (5)
O2—P1—K	50.40 (8)	O3^v—K—O3ⁱ	94.86 (5)
O3—P1—K	132.38 (7)	O1—K—O3ⁱ	102.97 (5)
O4—P1—K	120.75 (7)	O2^v—K—O3ⁱ	109.40 (5)
O1—P1—K^v	97.92 (9)	O2—K—P1	25.58 (4)
O2—P1—K^v	67.78 (7)	O5^vii—K—P1	86.44 (4)
O3—P1—K^v	55.22 (7)	O1ⁱ—K—P1	112.63 (5)
O4—P1—K^v	153.15 (7)	O6^x—K—P1	148.42 (4)
K—P1—K^v	77.51 (2)	O7^xi—K—P1	102.86 (4)
O1—P1—K^vi	47.78 (8)	O3^v—K—P1	128.38 (4)
O2—P1—K^vi	132.43 (7)	O1—K—P1	25.61 (4)
O3—P1—K^vi	67.06 (8)	O2^v—K—P1	85.94 (4)
O4—P1—K^vi	119.91 (7)	O3ⁱ—K—P1	78.37 (3)
K—P1—K^vi	96.10 (2)	O2—K—P1^v	93.47 (4)
K^v—P1—K^vi	72.974 (19)	O5^vii—K—P1^v	156.78 (4)
O5—P2—O6	113.20 (10)	O1ⁱ—K—P1^v	116.62 (5)
O5—P2—O7	111.96 (10)	O6^x—K—P1^v	102.54 (4)
O6—P2—O7	111.49 (10)	O7^xi—K—P1^v	90.50 (4)
O5—P2—O4	107.91 (10)	O3^v—K—P1^v	26.06 (4)
O6—P2—O4	105.10 (10)	O1—K—P1^v	108.15 (4)
O7—P2—O4	106.66 (10)	O2^v—K—P1^v	26.19 (3)
O5—P2—Na^vi	65.03 (7)	O3ⁱ—K—P1^v	92.83 (4)
O6—P2—Na^vi	150.48 (8)	P1—K—P1^v	102.49 (2)
O7—P2—Na^vi	51.00 (7)	O2—K—Cu^x	139.21 (4)
O4—P2—Na^vi	103.13 (7)	O5^vii—K—Cu^x	41.57 (4)
O5—P2—Naⁱⁱⁱ	147.30 (9)	O1ⁱ—K—Cu^x	93.78 (5)
O6—P2—Naⁱⁱⁱ	48.05 (7)	O6^x—K—Cu^x	34.34 (3)
O7—P2—Naⁱⁱⁱ	66.21 (7)	O7^xi—K—Cu^x	34.63 (3)
O4—P2—Naⁱⁱⁱ	103.48 (7)	O3^v—K—Cu^x	101.36 (5)
Na^vi—P2—Naⁱⁱⁱ	116.33 (3)	O1—K—Cu^x	91.51 (4)
O5—P2—Na	46.71 (7)	O2^v—K—Cu^x	107.32 (4)
O6—P2—Na	70.95 (7)	O3ⁱ—K—Cu^x	141.53 (4)
O7—P2—Na	149.66 (7)	P1—K—Cu^x	115.54 (2)
O4—P2—Na	101.43 (7)	P1^v—K—Cu^x	116.45 (2)
Na^vi—P2—Na	111.56 (3)	P1—O1—Na^vii	153.68 (14)
Naⁱⁱⁱ—P2—Na	118.02 (3)	P1—O1—K^vi	108.83 (11)
O1^vii—Na—O6^viii	89.29 (8)	Na^vii—O1—K^vi	93.05 (7)
O1^vii—Na—O5	92.89 (8)	P1—O1—K	92.08 (9)
O6^viii—Na—O5	126.37 (7)	Na^vii—O1—K	86.20 (7)
O1^vii—Na—O7ⁱ	111.80 (8)	K^vi—O1—K	126.29 (7)
O6^viii—Na—O7ⁱ	116.12 (7)	P1—O2—Cu	125.16 (11)
O5—Na—O7ⁱ	112.48 (7)	P1—O2—K	104.02 (10)
O1^vii—Na—O3^viii	150.23 (8)	Cu—O2—K	113.55 (8)
O6^viii—Na—O3^viii	79.37 (6)	P1—O2—K^v	86.03 (8)
O5—Na—O3^viii	72.83 (6)	Cu—O2—K^v	128.21 (9)
O7ⁱ—Na—O3^viii	97.87 (7)	K—O2—K^v	92.88 (5)
O1^vii—Na—O5ⁱ	85.37 (7)	P1—O3—Cu^vi	126.56 (11)
O6^viii—Na—O5ⁱ	67.10 (6)	P1—O3—Naⁱⁱⁱ	106.61 (10)
O5—Na—O5ⁱ	166.46 (9)	Cu^vi—O3—Naⁱⁱⁱ	85.17 (8)
O7ⁱ—Na—O5ⁱ	56.39 (5)	P1—O3—K^v	98.72 (8)
O3^viii—Na—O5ⁱ	114.42 (7)	Cu^vi—O3—K^v	134.68 (8)
O1^vii—Na—O7^viii	91.46 (7)	Naⁱⁱⁱ—O3—K^v	83.27 (6)
O6^viii—Na—O7^viii	56.27 (5)	P1—O3—K^vi	87.28 (9)
O5—Na—O7^viii	70.10 (6)	Cu^vi—O3—K^vi	94.34 (7)
O7ⁱ—Na—O7^viii	156.06 (5)	Naⁱⁱⁱ—O3—K^vi	163.09 (7)
O3^viii—Na—O7^viii	59.34 (5)	K^v—O3—K^vi	85.14 (5)
O5ⁱ—Na—O7^viii	123.32 (6)	P2—O4—P1	119.01 (11)
O1^vii—Na—P2ⁱ	93.49 (6)	P2—O5—Cu^ix	128.83 (11)
O6^viii—Na—P2ⁱ	94.77 (5)	P2—O5—Na	106.28 (10)
O5—Na—P2ⁱ	138.41 (6)	Cu^ix—O5—Na	86.95 (7)
O7ⁱ—Na—P2ⁱ	29.06 (4)	P2—O5—K^vii	139.70 (11)
O3^viii—Na—P2ⁱ	114.67 (5)	Cu^ix—O5—K^vii	87.32 (5)
O5ⁱ—Na—P2ⁱ	28.80 (4)	Na—O5—K^vii	90.88 (6)
O7^viii—Na—P2ⁱ	150.56 (5)	P2—O5—Na^vi	86.16 (8)
O1^vii—Na—P2^viii	96.04 (6)	Cu^ix—O5—Na^vi	80.94 (6)
O6^viii—Na—P2^viii	28.24 (4)	Na—O5—Na^vi	166.46 (9)
O5—Na—P2^viii	98.55 (5)	K^vii—O5—Na^vi	82.58 (6)
O7ⁱ—Na—P2^viii	136.17 (6)	P2—O6—Cu	126.10 (11)
O3^viii—Na—P2^viii	61.77 (4)	P2—O6—Naⁱⁱⁱ	103.71 (9)
O5ⁱ—Na—P2^viii	94.99 (5)	Cu—O6—Naⁱⁱⁱ	99.80 (7)
O7^viii—Na—P2^viii	29.06 (3)	P2—O6—K^iv	134.97 (10)
P2ⁱ—Na—P2^viii	121.50 (4)	Cu—O6—K^iv	92.63 (6)
O1^vii—Na—P2	99.77 (6)	Naⁱⁱⁱ—O6—K^iv	89.12 (6)
O6^viii—Na—P2	151.35 (6)	P2—O7—Cu^vi	124.98 (11)
O5—Na—P2	27.01 (4)	P2—O7—Na^vi	99.93 (9)
O7ⁱ—Na—P2	85.78 (5)	Cu^vi—O7—Na^vi	104.36 (7)
O3^viii—Na—P2	79.46 (5)	P2—O7—K^xii	137.96 (10)
O5ⁱ—Na—P2	140.25 (5)	Cu^vi—O7—K^xii	90.92 (6)
O7^viii—Na—P2	96.10 (4)	Na^vi—O7—K^xii	89.80 (6)
P2ⁱ—Na—P2	111.56 (3)	P2—O7—Naⁱⁱⁱ	84.73 (7)
P2^viii—Na—P2	123.17 (4)	Cu^vi—O7—Naⁱⁱⁱ	81.55 (6)
O1^vii—Na—Cu^ix	115.94 (6)	Na^vi—O7—Naⁱⁱⁱ	167.87 (8)
O6^viii—Na—Cu^ix	86.18 (5)	K^xii—O7—Naⁱⁱⁱ	79.43 (5)

Symmetry codes: (i) x+1, y, z; (ii) x+1/2, −y+3/2, z+1/2; (iii) x−1/2, −y+3/2, z+1/2; (iv) −x+3/2, y+1/2, −z+1/2; (v) −x+1, −y+1, −z+1; (vi) x−1, y, z; (vii) −x+1, −y+1, −z; (viii) x+1/2, −y+3/2, z−1/2; (ix) x−1/2, −y+3/2, z−1/2; (x) −x+3/2, y−1/2, −z+1/2; (xi) −x+1/2, y−1/2, −z+1/2; (xii) −x+1/2, y+1/2, −z+1/2.

CHARDI and BVS analysis of cation polyhedra in KNaCuP₂O₇ top

Cation	qi()·sof(i)	Q(i)	V(i)·sof(i)	CN(i)	ECoN(i)	d_ar(i)	d_med(i)
Cu	2.000	1.94	1.995	5	4.35	2.031	2.031
P1	5.000	5.07	4.921	4	3.84	1.540	1.541
P2	5.000	5.03	4.938	4	3.88	1.540	1.540
K	1.000	0.98	1.103	9	5.20	2.622	2.564
Na	1.000	0.98	1.106	7	7.80	2.912	2.912

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; σ_cat = dispersion factor on cationic charges measuring the deviation of the computed charges; σ_cat=[Σ_i(q_i-Q_i)²/N-1]^1/2 = 0.019.

Structural data (Å, °) for the M,M'₂CuP₂O₇ family of compounds top

Compound	Space group	a	b	c	β	Z	Reference
Li₂CuP₂O₇	I2/a	14.068 (2)	4.8600 (8)	8.604 (1)	98.97 (1)	4	Spirlet et al. (1993)
Li₂CuP₂O₇	C2/c	15.3360 (14)	4.8733 (13)	8.6259 (16)	114.795 (10)	4	Gopalakrishna et al. (2008)
α-Na₂CuP₂O₇	P2₁/n	8.823 (3)	13.494 (3)	5.108 (2)	92.77 (3)	4	Erragh et al. (1995)
β-Na₂CuP₂O₇	C2/c	14.715 (1)	5.704 (2)	8.066 (1)	115.14 (1)	4	Etheredge et al. (1995)
K₂CuP₂O₇	Pbnm	9.509 (4)	14.389 (6)	5.276 (2)		4	ElMaadi et al. (1995)
K₂CuP₂O₇	P42₁m	8.056 (2)		5.460 (11)		2	Keates et al. (2014)
α-Rb₂CuP₂O₇	Pmcn	5.183 (1)	10.096 (1)	15.146 (2)		4	Chernyatieva et al. (2008)
β-Rb₂CuP₂O₇	Cc	7,002 (1)	12.751 (3)	9.773 (2)	110.93 (3)	4	Shvanskaya et al. (2012)
Cs₂CuP₂O₇	Cc	7.460 (6)	12.973 (10)	9.980 (8)	111.95	4	Mannasova et al. (2016)
NaCsCuP₂O₇	Pmn2₁	5.147 (2)	15.126 (3)	9.717 (5)		4	Chernyatieva et al. (2009)
Na_1.12Ag_0.88CuP₂O₇	C2/c	15.088 (2)	5.641 (1)	8.171 (1)	116.11 (1)	4	Bennazha et al. (2002)

Acknowledgements

We acknowledge the assistance of the staff of the Tunisian Laboratory of Materials and Crystallography during the data collection.

Funding information

Funding for this research was provided by: Ministère de l'enseignement supérieur et de la recherche scientifique (Tunisie).

References

Adams, S. (2003). softBV. University of Göttingen, Germany. Google Scholar
Addison, A. W., Rao, T. N., Reedijk, J., van Rijn, J. & Verschoor, G. C. (1984). J. Chem. Soc. Dalton Trans. pp. 1349–1356. CSD CrossRef Web of Science Google Scholar
Bennazha, J., Boukhari, A. & Holt, E. M. (2002). Acta Cryst. C58, i87–i89. Web of Science CrossRef CAS IUCr Journals Google Scholar
Boughzala, H. & Jouini, T. (1995). Acta Cryst. C51, 179–181. CrossRef CAS Web of Science IUCr Journals Google Scholar
Bouhassine, M. A. & Boughzala, H. (2015). Acta Cryst. E71, 636–639. Web of Science CSD CrossRef IUCr Journals Google Scholar
Bouhassine, M. A. & Boughzala, H. (2017). Acta Cryst. E73, 345–348. Web of Science CSD CrossRef IUCr Journals Google Scholar
Brandenburg, K. (2008). DIAMOND. Crystal Impact GbR, Bonn, Germany. Google Scholar
Brown, I. D. (2002). The Chemical Bond in Inorganic Chemistry: The Bond Valence Model. Oxford University Press. Google Scholar
Chernyatieva, A. P., Krivovichev, S. V. & Spiridonova, D. V. (2008). Book of Abstracts, VI International Conference on Inorganic Materials, pp. 3–143. Dresden: Elsevier. Google Scholar
Chernyatieva, A. P., Krivovichev, S. V. & Spiridonova, D. V. (2009). Proceedings of the XX Russian Conference of Young Scientists in Memory of the Corresponding Member of the Academy of Sciences of the USSR, K. O. Kratts. Google Scholar
Coelho, A. A. (2009). TOPAS 4.2. www. topas-academic. net Google Scholar
ElMaadi, A., Boukhari, A. & Holt, E. M. (1995). J. Alloys Compd. 223, 13–17. CrossRef CAS Web of Science Google Scholar
Enraf–Nonius (1994). CAD–4 EXPRESS. Enraf–Nonius, Delft, The Netherlands. Google Scholar
Erragh, F., Boukhari, A., Abraham, F. & Elouadi, B. (1995). J. Solid State Chem. 120, 23–31. CrossRef CAS Web of Science Google Scholar
Etheredge, K. M. S. & Hwu, S. J. (1995). Inorg. Chem. 34, 1495–1499. CrossRef CAS Web of Science Google Scholar
Gopalakrishna, G. S., Mahesh, M. J., Ashamanjari, K. G. & Prasad, J. S. (2008). Mater. Res. Bull. 43, 1171–1178. Web of Science CrossRef CAS Google Scholar
Harms, K. & Wocadlo, S. (1995). XCAD4. University of Marburg, Germany. Google Scholar
ICSD (2017). Inorganic Crystal Structure Database. FIZ-Karlsruhe, Germany. https://www.fiz-karlsruhe. de/fiz/products/icsd/welcome. html Google Scholar
Keates, A. C., Wang, Q. & Weller, M. T. (2014). J. Solid State Chem. 210, 10–14. Web of Science CrossRef CAS Google Scholar
Mannasova, A. A., Chernyatieva, A. P. & Krivovichev, S. V. (2016). Z. Kristallogr. 231, 65–69. CAS Google Scholar
Nespolo, M., Ferraris, G., Ivaldi, G. & Hoppe, R. (2001). Acta Cryst. B57, 652–664. Web of Science CrossRef CAS IUCr Journals Google Scholar
North, A. C. T., Phillips, D. C. & Mathews, F. S. (1968). Acta Cryst. A24, 351–359. CrossRef IUCr Journals Web of Science Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Sheldrick, G. M. (2015). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Shvanskaya, L. V., Yakubovich, O. V. & Urusov, V. S. (2012). Dokl. Phys. Chem. 442, 19–26. Web of Science CrossRef CAS Google Scholar
Spirlet, M. R., Rebizant, J. & Liegeois-Duyckaerts, M. (1993). Acta Cryst. C49, 209–211. CrossRef CAS Web of Science IUCr Journals Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar