The crystal structure of KScP2O7

Redhammer, G.J.; Tippelt, G.

doi:10.1107/S2056989020010427

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Volume 76| Part 9| September 2020| Pages 1412-1416

https://doi.org/10.1107/S2056989020010427

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The crystal structure of KScP₂O₇

Günther J. Redhammer ^a ^* and Gerold Tippelt ^a

^aChemistry and Physics of Materials, University of Salzburg, Jakob-Haringerstr. 2A, 5020 Salzburg, Austria
^*Correspondence e-mail: guenther.redhammer@sbg.ac.at

Edited by M. Weil, Vienna University of Technology, Austria (Received 24 July 2020; accepted 28 July 2020; online 4 August 2020)

Single crystals of KScP₂O₇, potassium scandium diphosphate, were grown in a borate flux. The title compound crystallizes isotypically with KAlP₂O₇ in space-group type P2₁/c, Z = 4. The main building block is an {ScP₂O₁₁}^9– unit, forming layers parallel to (001). These layers are stacked along [001] via common corners of octahedral and tetrahedral units to span up large heptagonal cavities that host the potassium cations with a coordination number of 10. The P—O—P bridging angle increases with increasing size of the octahedrally coordinated M^III cation, as do the K—O distances within a series of KM^IIIP₂O₇ compounds (M^III = Al to Y with ionic radii r = 0.538 to 0.90 Å).

Keywords: KAlP₂O₇ structure type; pyrophosphate; scandium; isotypism; structure determination; crystal structure.

CCDC reference: 2019936

1. 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 ; Vītiņš et al., 2000 ). In the context of solid-state electrolytes, we recently investigated the Na-super ionic conducting NaSICON-type compounds Na₃Sc₂(PO₄)₃ (NSP) and Ag₃Sc₂(PO₄)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 KZr₂(PO₄)₃, however, did not yield the intended compound K₃Sc₂(PO₄)₃, but the title diphosphate KScP₂O₇ instead.

Vītiņš et al. (2000) reviewed that for such A^IM^IIIP₂O₇ (A = Li, Na, K, Rb, Cs, Tl; M = Al, Ga, Fe, In, Sc, …) compounds six different structure types can be distinguished. They confirm that structure type I, involving compounds with large A site cations, is the largest group, showing P2₁/c space-group symmetry. KAlP₂O₇ (Ng & Calvo, 1973 ) is regarded as the aristo-structure of group I with around 45 different compositions as compiled in the Inorganic Crystal Structure Database (ICSD; Zagorac et al., 2019 ). For K-containing compounds, further materials are KCrP₂O₇ (Gentil et al., 1997 ), KGaP₂O₇ (Genkin & Timofeeva, 1989 ), KFeP₂O₇ (Riou et al., 1988 ; Genkin & Timofeeva, 1989), KVP₂O₇ (Benhamada et al., 1991 ), KTiP₂O₇ (Zatovsky et al., 2000 ), KMoP₂O₇ (Leclaire et al., 1989 ; Chen et al., 1989 ), KInP₂O₇ (Zhang et al., 2004 ), KLuP₂O₇ (Yuan et al., 2007 ), KYbP₂O₇ (Horchani-Naifer & Férid, 2007 ), KErP₂O₇ (Chaker et al., 2016 ) and KYP₂O₇ (Yuan et al., 2007). Synthesis and lattice parameters of Ce^III-doped polycrystalline KScP₂O₇ as well as the luminescence properties were reported recently by Zhang et al. (2016 ); however, no atomic coordinates were given.

In this contribution, we present the determination of the crystal structure of KScP₂O₇, not reported so far, and compare it with the series of other K-containing diphosphates.

2. Structural commentary

The title compound crystallizes in space group P2₁/c and is isostructural with KAlP₂O₇ (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 PO₄ 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 ScO₆ 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 non-bridging 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 P1O₄ tetrahedron than for the P2O₄ tetrahedron (Table 1). When comparing average bond lengths and polyhedral distortion parameters of the series of KM^IIIP₂O₇ 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 KScP₂O₇ fit well into the data of the other KM^IIIP₂O₇ structures. The tetrahedral bridging angle P1—O4—P2 amounts to 125.80 (5)° and is distinctly larger than that of KAlP₂O₇ [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).

Table 1
Selected structural and distortional parameters (Å, °) of KM^IIIP₂O₇ compounds

Vol. = polyhedral volume (Å³), 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 ).

M	Al	Cr	Ga	Fe^a	Fe^b	V	Ti	Mo	Sc	In	Lu	Yb	Er	Y
ionic radius	0.536	0.615	0.62	0.645	0.645	0.64	0.67	0.69	0.745	0.8	0.861	0.868	0.89	0.9
<K—O>	2.949	2.975	2.982	3.017	2.993	2.998	3.029	3.029	3.072	3.056	3.118	3.130	3.119	3.150
Vol.	44.03	45.21	45.23	50.08	45.73	46.03	47.21	47.63	49.40	48.45	52.04	52.11	52.09	53.25
DI	0.0442	0.0519	0.0484	0.0484	0.0535	0.0560	0.0596	0.0636	0.0620	0.0641	0.0695	0.0735	0.0813	0.0774
ECoN	8.84	8.38	8.54	8.58	8.29	8.22	7.89	7.51	7.66	7.29	7.09	6.63	6.80	6.40

<M—O>	1.889	1.973	1.964	2.021	1.991	2.000	2.036	2.091	2.085	2.124	2.199	2.207	2.301	2.266
Vol.	8.95	10.20	10.06	10.97	10.46	10.63	11.19	12.10	12.02	12.68	14.05	14.19	15.86	15.38
DI	0.0102	0.0091	0.0076	0.0313	0.0106	0.0154	0.0220	0.0086	0.0100	0.0073	0.0139	0.0116	0.0266	0.0074
OQE	1.0025	1.0031	1.0033	1.0032	1.0036	1.0036	1.0048	1.0045	1.0040	1.0047	1.0068	1.0070	1.0170	1.0060
OAV	7.97	10.31	11.42	6.47	11.79	10.66	12.98	15.40	13.52	16.38	22.62	23.79	54.07	20.97
ECoN	5.96	5.97	5.98	5.73	5.96	5.92	5.85	5.97	5.96	5.99	5.95	5.95	5.75	5.98

<P1—O>	1.536	1.542	1.540	1.537	1.537	1.540	1.540	1.536	1.537	1.538	1.545	1.531	1.494	1.518
Vol.	1.85	1.87	1.86	1.85	1.85	1.86	1.87	1.85	1.86	1.85	1.87	1.83	1.68	1.78
DI	0.0240	0.0241	0.0240	0.0124	0.0237	0.0245	0.0234	0.0256	0.0245	0.0281	0.0329	0.0264	0.0584	0.0326
TQE	1.0049	1.0053	1.0049	1.0042	1.0048	1.0047	1.0037	1.0052	1.0043	1.0059	1.0080	1.0043	1.0179	1.0074
TAV	19.55	21.29	19.79	17.39	18.95	18.34	14.44	20.41	16.80	22.82	27.35	16.12	37.83	24.70
ECoN	3.89	3.89	3.88	3.97	3.89	3.89	3.90	3.88	3.89	3.85	3.77	3.87	3.14	3.77

<P2—O>	1.531	1.535	1.535	1.530	1.531	1.535	1.536	1.534	1.535	1.530	1.546	1.535	1.509	1.529
Vol.	1.83	1.85	1.85	1.84	1.84	1.85	1.85	1.85	1.85	1.83	1.88	1.85	1.72	1.83
DI	0.0241	0.0242	0.0260	0.0088	0.0241	0.0227	0.0247	0.0231	0.0236	0.0272	0.0470	0.0237	0.0400	0.0351
TQE	1.0034	1.0032	1.0035	1.0011	1.0030	1.0026	1.0027	1.0027	1.0027	1.0034	1.0058	1.0024	1.0206	1.0038
TAV	12.54	11.68	12.48	4.49	10.67	9.18	9.12	9.60	9.48	10.99	9.51	7.94	71.43	4.61
ECoN	3.87	3.87	3.86	3.98	3.88	3.89	3.88	3.88	3.89	3.80	3.61	3.89	3.57	3.71

P1—O4—P2	123.18 (11)	123.68 (10)	123.8 (2)	n.d.	124.32 (10)	124.24	125.0 (2)	124.97 (15)	125.80 (5)	125.6 (5)	123.8 (9)	127.5 (3)	123.72 (7)	127.4 (6)
Notes: (a) data from Riou et al. (1988); (b) data from Genkin & Timofeeva (1989).

Figure 1
The principal building unit of KScP₂O₇ shown with displacement ellipsoids at the 95% probability level. [Symmetry codes: (i) x, $[{1\over 2}]$ − y, $[{1\over 2}]$ + z; (ii) 1 − x, − $[{1\over 2}]$ + y, $[{3\over 2}]$ − z; (iii) x, $[{1\over 2}]$ − y, − $[{1\over 2}]$ + z; (iv) 2 − x, − $[{1\over 2}]$ + y, $[{3\over 2}]$ − z].

The terminal oxygen atoms of the pyrophosphate group share their corners with five neighbouring ScO₆ octahedra. Following Leclaire et al. (1989), two phosphate tetrahedra and one octahedron form the basic {ScP₂O₁₁}^9– units (cf. Fig. 1), which are connected with units of the same kind via corner-sharing to make up a sheet parallel to (001), as depicted in Fig. 2. These layers are stacked along [001] in such a way that a ScO₆ octahedron of one layer shares its O3ⁱⁱⁱ and O6ⁱ corners (symmetry codes refer to Fig. 1) with one PO₄ tetrahedron each of the layer below and above.

Figure 2
(a) Layer of laterally interconnected {ScP₂O₁₁}⁹⁻ units, forming a layer parallel to (001), the K^I cations are hosted in the channels extending along [001]; (b) the three-dimensional framework structure of KScP₂O₇, viewed along [010].

Generally, all corners of the octahedron are shared with neighbouring PO₄ tetrahedra, whereby all oxygen atoms except O4 directly connect the octahedron with a pyrophosphate P₂O₇ group, and the O2ⁱⁱ and O5^iv oxygen atoms join the octahedron with two PO₄ tetrahedra within the above-mentioned 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—O6ⁱ) of 2.0346 (7) Å. A similar behaviour with one significantly shorter M—O bond is also observed in other KM^IIIP₂O₇ compounds and seems to be a more general feature. The Sc—O6ⁱ and the Sc—O3ⁱⁱⁱ bonds, which point towards [001] and connect different (001) layers, both are the shortest within the ScO₆ 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 KAlP₂O₇ 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^IIIP₂O₇ compounds. The ScO₆ 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 KAlP₂O₇ shows the most regular octahedral coordination of all KM^IIIP₂O₇ 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).

Figure 3
Variation of the octahedral angle variance (OAV) (a) and the average M^III—O bond lengths (b) as a function of the ionic radius of the M site cation across the series of KM^IIIP₂O₇ compounds (M = Al to Y).

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 KScP₂O₇ 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), involving the i1 position, and is oriented parallel to [001]. An estimated activation energy of ∼0.3 eV would be needed to move a potassium ion from the 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^IIIP₂O₇ 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.

Figure 4
Bond-valence energy landscape map at levels of 1.5 eV above the minimum (yellow) viewed along [100]; the crystal structure of KScP₂O₇ is overlayed, the K sites lie within channels.

3. Synthesis and crystallization

The title compound was grown during attempts to synthesize NaSICON-type K₃Sc₂(PO₄)₃ adopting a flux growth protocol set up by Sljukic et al. (1967). Sc₂O₃ and KH₂PO₄ were mixed in stoichiometric quantities (molar ratio 2:3) and B₂O₃ was added as a flux with a sixfold quantity of that of Sc₂O₃. 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⁻¹. Between 1073 K and room temperature the cooling rate was 50 K h⁻¹. The synthesis batch was immersed in hot water to dissolve the B₂O₃ and remaining K-phosphates. The residual contained single-phase KScP₂O₇ as checked by powder X-ray diffraction and showed single crystals of irregular to needle-like form with well-developed faces. The crystals are colorless and highly transparent with sizes up to 140 µm in lengths and ∼80 µm in diameter.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2.

Table 2
Experimental details

Crystal data
Chemical formula	KScP₂O₇
M_r	258
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	293
a, b, c (Å)	7.4634 (1), 10.3902 (1), 8.3747 (1)
β (°)	106.49
V (Å³)	622.72 (1)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	2.35
Crystal size (mm)	0.16 × 0.09 × 0.08

Data collection
Diffractometer	Bruker SMART APEX CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 )
T_min, T_max	0.38, 0.52
No. of measured, independent and observed [I > 2σ(I)] reflections	20981, 2985, 2850
R_int	0.021
(sin θ/λ)_max (Å⁻¹)	0.837

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.018, 0.049, 1.05
No. of reflections	2985
No. of parameters	101
Δρ_max, Δρ_min (e Å⁻³)	0.79, −0.66
Computer programs: APEX2 and SAINT (Bruker, 2012 ), SIR2014 (Burla et al., 2005 ), SHELXL2014/7 (Sheldrick, 2015 ), DIAMOND (Brandenburg, 2006 ), ORTEP for Windows and WinGX publication routines (Farrugia, 2012 ).

Supporting information

CCDC reference: 2019936

Crystal structure: contains datablocks global, I. DOI: https://doi.org/10.1107/S2056989020010427/wm5578sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989020010427/wm5578Isup2.hkl

checkCIF report

Computing details top

Data collection: APEX2 (Bruker, 2012); cell refinement: SAINT (Bruker, 2012); data reduction: SAINT (Bruker, 2012); program(s) used to solve structure: SIR2014 (Burla et al., 2005); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2006), ORTEP for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX publication routines (Farrugia, 2012).

Potassium scandium diphosphate top

Crystal data top

KScP₂O₇	F(000) = 504
M_r = 258	D_x = 2.752 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 20981 reflections
a = 7.4634 (1) Å	θ = 2.9–36.5°
b = 10.3902 (1) Å	µ = 2.35 mm⁻¹
c = 8.3747 (1) Å	T = 293 K
β = 106.49°	Prismatic, colorless
V = 622.72 (1) Å³	0.16 × 0.09 × 0.08 mm
Z = 4

Data collection top

Bruker SMART APEX CCD diffractometer	2850 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.021
rotation, ω–scans at 4 different φ positions	θ_max = 36.5°, θ_min = 2.9°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −12→12
T_min = 0.38, T_max = 0.52	k = −17→17
20981 measured reflections	l = −13→13
2985 independent reflections

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: structure-invariant direct methods
R[F² > 2σ(F²)] = 0.018	w = 1/[σ²(F_o²) + (0.0257P)² + 0.274P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.049	(Δ/σ)_max = 0.001
S = 1.05	Δρ_max = 0.79 e Å⁻³
2985 reflections	Δρ_min = −0.66 e Å⁻³
101 parameters	Extinction correction: SHELXL2014/7 (Sheldrick 2015), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0429 (15)
0 constraints

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.

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

	x	y	z	U_iso*/U_eq
K1	0.82123 (3)	0.67829 (3)	0.94124 (3)	0.02342 (6)
Sc1	0.76534 (2)	0.09952 (2)	0.74255 (2)	0.00607 (4)
P1	0.55749 (3)	0.36164 (2)	0.80923 (2)	0.00673 (5)
P2	0.86703 (3)	0.40292 (2)	0.67637 (3)	0.00681 (5)
O1	0.54575 (9)	0.22058 (6)	0.76034 (9)	0.01132 (11)
O2	0.36433 (9)	0.42041 (6)	0.76741 (9)	0.01313 (11)
O3	0.67337 (11)	0.38889 (7)	0.98492 (9)	0.01712 (13)
O4	0.66159 (9)	0.43502 (6)	0.69104 (8)	0.01167 (11)
O5	0.99440 (9)	0.50481 (6)	0.77947 (8)	0.01003 (10)
O6	0.85616 (11)	0.40831 (7)	0.49554 (8)	0.01632 (13)
O7	0.92157 (9)	0.27011 (6)	0.75056 (8)	0.01088 (10)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
K1	0.02013 (10)	0.02973 (12)	0.01843 (10)	0.00408 (8)	0.00226 (7)	−0.00089 (8)
Sc1	0.00647 (6)	0.00539 (6)	0.00639 (6)	0.00013 (4)	0.00190 (4)	−0.00015 (4)
P1	0.00662 (8)	0.00633 (8)	0.00731 (8)	0.00074 (6)	0.00208 (6)	−0.00046 (6)
P2	0.00755 (8)	0.00703 (8)	0.00616 (8)	−0.00186 (6)	0.00246 (6)	−0.00017 (5)
O1	0.0091 (2)	0.0070 (2)	0.0185 (3)	0.00026 (18)	0.0050 (2)	−0.00208 (19)
O2	0.0086 (2)	0.0100 (2)	0.0214 (3)	0.00284 (19)	0.0052 (2)	−0.0002 (2)
O3	0.0209 (3)	0.0207 (3)	0.0073 (3)	0.0010 (3)	−0.0001 (2)	−0.0025 (2)
O4	0.0086 (2)	0.0124 (2)	0.0150 (3)	0.00151 (19)	0.0050 (2)	0.0054 (2)
O5	0.0103 (2)	0.0097 (2)	0.0107 (2)	−0.00398 (18)	0.00393 (19)	−0.00307 (18)
O6	0.0199 (3)	0.0229 (3)	0.0069 (2)	−0.0082 (2)	0.0049 (2)	−0.0016 (2)
O7	0.0098 (2)	0.0069 (2)	0.0159 (3)	−0.00059 (18)	0.0035 (2)	0.00091 (19)

Geometric parameters (Å, º) top

K1—O5	2.7837 (7)	P1—O3	1.5068 (7)
K1—O7ⁱ	2.7966 (7)	P1—O2	1.5123 (7)
K1—O1ⁱⁱ	2.8141 (7)	P1—O1	1.5176 (6)
K1—O7ⁱⁱⁱ	2.9876 (7)	P1—O4	1.6128 (6)
K1—O5ⁱ	3.0259 (7)	P1—K1^vii	3.5569 (3)
K1—O2ⁱⁱ	3.1505 (7)	P2—O6	1.4944 (7)
K1—O3	3.2592 (8)	P2—O5	1.5178 (6)
K1—O4	3.2835 (7)	P2—O7	1.5207 (6)
K1—O2^iv	3.2927 (7)	P2—O4	1.6076 (6)
K1—O6ⁱⁱⁱ	3.3265 (9)	P2—K1ⁱ	3.4856 (3)
K1—P2ⁱ	3.4856 (3)	P2—K1^viii	3.6237 (3)
K1—P1ⁱⁱ	3.5570 (3)	O1—K1^vii	2.8141 (7)
Sc1—O6^v	2.0346 (7)	O2—Sc1ⁱⁱ	2.0883 (6)
Sc1—O3^vi	2.0736 (7)	O2—K1^vii	3.1505 (7)
Sc1—O2^vii	2.0884 (6)	O2—K1^iv	3.2927 (7)
Sc1—O5^viii	2.0989 (6)	O3—Sc1^v	2.0736 (7)
Sc1—O1	2.1045 (6)	O5—Sc1ⁱⁱⁱ	2.0990 (6)
Sc1—O7	2.1122 (6)	O5—K1ⁱ	3.0259 (7)
Sc1—K1^viii	3.9059 (3)	O6—Sc1^vi	2.0346 (7)
Sc1—K1^vi	3.9333 (3)	O6—K1^viii	3.3265 (9)
Sc1—K1ⁱ	4.1501 (3)	O7—K1ⁱ	2.7966 (7)
Sc1—K1^vii	4.2870 (3)	O7—K1^viii	2.9875 (7)

O5—K1—O7ⁱ	106.35 (2)	O2^vii—Sc1—K1^vi	56.82 (2)
O5—K1—O1ⁱⁱ	108.45 (2)	O5^viii—Sc1—K1^vi	49.513 (18)
O7ⁱ—K1—O1ⁱⁱ	145.07 (2)	O1—Sc1—K1^vi	135.284 (19)
O5—K1—O7ⁱⁱⁱ	59.145 (18)	O7—Sc1—K1^vi	118.557 (19)
O7ⁱ—K1—O7ⁱⁱⁱ	93.300 (18)	K1^viii—Sc1—K1^vi	70.210 (7)
O1ⁱⁱ—K1—O7ⁱⁱⁱ	106.983 (19)	O6^v—Sc1—K1ⁱ	52.43 (2)
O5—K1—O5ⁱ	78.29 (2)	O3^vi—Sc1—K1ⁱ	126.74 (2)
O7ⁱ—K1—O5ⁱ	50.565 (17)	O2^vii—Sc1—K1ⁱ	140.66 (2)
O1ⁱⁱ—K1—O5ⁱ	135.51 (2)	O5^viii—Sc1—K1ⁱ	79.508 (19)
O7ⁱⁱⁱ—K1—O5ⁱ	113.314 (18)	O1—Sc1—K1ⁱ	94.313 (18)
O5—K1—O2ⁱⁱ	116.03 (2)	O7—Sc1—K1ⁱ	37.741 (18)
O7ⁱ—K1—O2ⁱⁱ	115.93 (2)	K1^viii—Sc1—K1ⁱ	66.889 (5)
O1ⁱⁱ—K1—O2ⁱⁱ	48.919 (17)	K1^vi—Sc1—K1ⁱ	128.501 (4)
O7ⁱⁱⁱ—K1—O2ⁱⁱ	72.254 (18)	O6^v—Sc1—K1^vii	112.70 (2)
O5ⁱ—K1—O2ⁱⁱ	164.277 (19)	O3^vi—Sc1—K1^vii	67.21 (2)
O5—K1—O3	71.10 (2)	O2^vii—Sc1—K1^vii	75.012 (19)
O7ⁱ—K1—O3	103.820 (19)	O5^viii—Sc1—K1^vii	150.317 (19)
O1ⁱⁱ—K1—O3	84.749 (19)	O1—Sc1—K1^vii	34.402 (18)
O7ⁱⁱⁱ—K1—O3	130.140 (19)	O7—Sc1—K1^vii	110.455 (18)
O5ⁱ—K1—O3	55.064 (17)	K1^viii—Sc1—K1^vii	131.220 (7)
O2ⁱⁱ—K1—O3	133.582 (19)	K1^vi—Sc1—K1^vii	101.144 (6)
O5—K1—O4	47.582 (17)	K1ⁱ—Sc1—K1^vii	128.415 (5)
O7ⁱ—K1—O4	140.014 (19)	O3—P1—O2	113.29 (4)
O1ⁱⁱ—K1—O4	67.811 (18)	O3—P1—O1	114.72 (4)
O7ⁱⁱⁱ—K1—O4	94.293 (17)	O2—P1—O1	110.44 (4)
O5ⁱ—K1—O4	90.703 (17)	O3—P1—O4	105.51 (4)
O2ⁱⁱ—K1—O4	103.768 (18)	O2—P1—O4	105.11 (4)
O3—K1—O4	44.629 (17)	O1—P1—O4	106.97 (4)
O5—K1—O2^iv	120.669 (19)	O3—P1—K1^vii	144.71 (3)
O7ⁱ—K1—O2^iv	72.463 (19)	O2—P1—K1^vii	62.23 (3)
O1ⁱⁱ—K1—O2^iv	87.172 (19)	O1—P1—K1^vii	49.33 (3)
O7ⁱⁱⁱ—K1—O2^iv	165.347 (19)	O4—P1—K1^vii	109.38 (3)
O5ⁱ—K1—O2^iv	54.972 (17)	O3—P1—K1	56.74 (3)
O2ⁱⁱ—K1—O2^iv	116.65 (2)	O2—P1—K1	95.62 (3)
O3—K1—O2^iv	53.305 (18)	O1—P1—K1	153.20 (3)
O4—K1—O2^iv	94.610 (18)	O4—P1—K1	58.24 (3)
O5—K1—O6ⁱⁱⁱ	97.331 (19)	K1^vii—P1—K1	152.503 (8)
O7ⁱ—K1—O6ⁱⁱⁱ	55.891 (18)	O6—P2—O5	113.29 (4)
O1ⁱⁱ—K1—O6ⁱⁱⁱ	121.365 (19)	O6—P2—O7	112.29 (4)
O7ⁱⁱⁱ—K1—O6ⁱⁱⁱ	46.342 (17)	O5—P2—O7	110.41 (4)
O5ⁱ—K1—O6ⁱⁱⁱ	100.309 (18)	O6—P2—O4	106.93 (4)
O2ⁱⁱ—K1—O6ⁱⁱⁱ	72.478 (18)	O5—P2—O4	105.59 (4)
O3—K1—O6ⁱⁱⁱ	153.88 (2)	O7—P2—O4	107.91 (3)
O4—K1—O6ⁱⁱⁱ	140.332 (18)	O6—P2—K1ⁱ	140.37 (3)
O2^iv—K1—O6ⁱⁱⁱ	122.924 (18)	O5—P2—K1ⁱ	59.96 (3)
O5—K1—P2ⁱ	90.537 (15)	O7—P2—K1ⁱ	51.21 (3)
O7ⁱ—K1—P2ⁱ	25.078 (13)	O4—P2—K1ⁱ	112.48 (3)
O1ⁱⁱ—K1—P2ⁱ	150.579 (16)	O6—P2—K1^viii	66.61 (3)
O7ⁱⁱⁱ—K1—P2ⁱ	102.011 (14)	O5—P2—K1^viii	104.90 (3)
O5ⁱ—K1—P2ⁱ	25.736 (12)	O7—P2—K1^viii	53.74 (3)
O2ⁱⁱ—K1—P2ⁱ	140.943 (16)	O4—P2—K1^viii	148.68 (3)
O3—K1—P2ⁱ	80.358 (14)	K1ⁱ—P2—K1^viii	77.363 (6)
O4—K1—P2ⁱ	115.250 (14)	O6—P2—K1	126.27 (3)
O2^iv—K1—P2ⁱ	63.593 (13)	O5—P2—K1	42.96 (2)
O6ⁱⁱⁱ—K1—P2ⁱ	76.323 (13)	O7—P2—K1	121.13 (3)
O5—K1—P1ⁱⁱ	117.439 (16)	O4—P2—K1	62.66 (3)
O7ⁱ—K1—P1ⁱⁱ	131.596 (16)	K1ⁱ—P2—K1	77.719 (8)
O1ⁱⁱ—K1—P1ⁱⁱ	24.144 (13)	K1^viii—P2—K1	146.911 (7)
O7ⁱⁱⁱ—K1—P1ⁱⁱ	92.076 (14)	P1—O1—Sc1	127.58 (4)
O5ⁱ—K1—P1ⁱⁱ	154.610 (15)	P1—O1—K1^vii	106.53 (3)
O2ⁱⁱ—K1—P1ⁱⁱ	25.133 (12)	Sc1—O1—K1^vii	120.60 (3)
O3—K1—P1ⁱⁱ	108.821 (15)	P1—O2—Sc1ⁱⁱ	140.31 (4)
O4—K1—P1ⁱⁱ	87.268 (13)	P1—O2—K1^vii	92.64 (3)
O2^iv—K1—P1ⁱⁱ	99.950 (13)	Sc1ⁱⁱ—O2—K1^vii	124.31 (3)
O6ⁱⁱⁱ—K1—P1ⁱⁱ	97.304 (14)	P1—O2—K1^iv	106.05 (3)
P2ⁱ—K1—P1ⁱⁱ	151.985 (9)	Sc1ⁱⁱ—O2—K1^iv	91.12 (2)
O6^v—Sc1—O3^vi	178.96 (3)	K1^vii—O2—K1^iv	87.202 (17)
O6^v—Sc1—O2^vii	91.12 (3)	P1—O3—Sc1^v	162.86 (5)
O3^vi—Sc1—O2^vii	89.85 (3)	P1—O3—K1	100.52 (4)
O6^v—Sc1—O5^viii	91.86 (3)	Sc1^v—O3—K1	92.33 (3)
O3^vi—Sc1—O5^viii	88.54 (3)	P2—O4—P1	125.80 (4)
O2^vii—Sc1—O5^viii	88.65 (3)	P2—O4—K1	91.56 (3)
O6^v—Sc1—O1	89.20 (3)	P1—O4—K1	97.07 (3)
O3^vi—Sc1—O1	90.26 (3)	P2—O5—Sc1ⁱⁱⁱ	133.60 (4)
O2^vii—Sc1—O1	100.03 (3)	P2—O5—K1	115.23 (3)
O5^viii—Sc1—O1	171.24 (3)	Sc1ⁱⁱⁱ—O5—K1	105.40 (2)
O6^v—Sc1—O7	89.01 (3)	P2—O5—K1ⁱ	94.31 (3)
O3^vi—Sc1—O7	90.06 (3)	Sc1ⁱⁱⁱ—O5—K1ⁱ	98.65 (2)
O2^vii—Sc1—O7	173.98 (3)	K1—O5—K1ⁱ	101.71 (2)
O5^viii—Sc1—O7	85.32 (2)	P2—O6—Sc1^vi	163.73 (5)
O1—Sc1—O7	86.00 (2)	P2—O6—K1^viii	89.04 (3)
O6^v—Sc1—K1^viii	110.60 (2)	Sc1^vi—O6—K1^viii	98.58 (3)
O3^vi—Sc1—K1^viii	69.08 (2)	P2—O7—Sc1	131.85 (4)
O2^vii—Sc1—K1^viii	125.45 (2)	P2—O7—K1ⁱ	103.71 (3)
O5^viii—Sc1—K1^viii	43.401 (17)	Sc1—O7—K1ⁱ	114.72 (3)
O1—Sc1—K1^viii	128.378 (19)	P2—O7—K1^viii	102.03 (3)
O7—Sc1—K1^viii	49.150 (18)	Sc1—O7—K1^viii	98.52 (2)
O6^v—Sc1—K1^vi	125.03 (2)	K1ⁱ—O7—K1^viii	100.37 (2)
O3^vi—Sc1—K1^vi	55.89 (2)

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

Selected structural and distortional parameters of KM^IIIP₂O₇ compounds top

Vol. = polyhedral volume, DI = distortion index, ECoN = effective coordination number, OQE = octahedral quadratic elongation, OAV = octahedral angle variance, TQE = tetrahedral quadratic elongation, TAV = tetrahedral angle variance. All calculations were performed using VESTA (Momma & Izumi 2011; for the mathematical meaning see the VESTA Handbook); ionic radii were taken from Shannon (1976).

M	Al	Cr	Ga	Fe^a	Fe^b	V	Ti	Mo	Sc	In	Lu	Yb	Er	Y
ionic radius (Å)	0.536	0.615	0.62	0.645	0.645	0.64	0.67	0.69	0.745	0.8	0.861	0.868	0.89	0.9
<K—O> (Å)	2.949	2.975	2.982	3.017	2.993	2.998	3.029	3.029	3.072	3.056	3.118	3.130	3.119	3.150
Vol. (Å³)	44.03	45.21	45.23	50.08	45.73	46.03	47.21	47.63	49.40	48.45	52.04	52.11	52.09	53.25
DI	0.0442	0.0519	0.0484	0.0484	0.0535	0.0560	0.0596	0.0636	0.0620	0.0641	0.0695	0.0735	0.0813	0.0774
ECoN	8.84	8.38	8.54	8.58	8.29	8.22	7.89	7.51	7.66	7.29	7.09	6.63	6.80	6.40

<M—O> (Å)	1.889	1.973	1.964	2.021	1.991	2.000	2.036	2.091	2.085	2.124	2.199	2.207	2.301	2.266
Vol. (Å³)	8.95	10.20	10.06	10.97	10.46	10.63	11.19	12.10	12.02	12.68	14.05	14.19	15.86	15.38
DI	0.0102	0.0091	0.0076	0.0313	0.0106	0.0154	0.0220	0.0086	0.0100	0.0073	0.0139	0.0116	0.0266	0.0074
OQE	1.0025	1.0031	1.0033	1.0032	1.0036	1.0036	1.0048	1.0045	1.0040	1.0047	1.0068	1.0070	1.0170	1.0060
OAV (°²)	7.97	10.31	11.42	6.47	11.79	10.66	12.98	15.40	13.52	16.38	22.62	23.79	54.07	20.97
ECoN	5.96	5.97	5.98	5.73	5.96	5.92	5.85	5.97	5.96	5.99	5.95	5.95	5.75	5.98

<P1—O> (Å)	1.536	1.542	1.540	1.537	1.537	1.540	1.540	1.536	1.537	1.538	1.545	1.531	1.494	1.518
Vol. (Å³)	1.85	1.87	1.86	1.85	1.85	1.86	1.87	1.85	1.86	1.85	1.87	1.83	1.68	1.78
DI	0.0240	0.0241	0.0240	0.0124	0.0237	0.0245	0.0234	0.0256	0.0245	0.0281	0.0329	0.0264	0.0584	0.0326
TQE	1.0049	1.0053	1.0049	1.0042	1.0048	1.0047	1.0037	1.0052	1.0043	1.0059	1.0080	1.0043	1.0179	1.0074
TAV (°²)	19.55	21.29	19.79	17.39	18.95	18.34	14.44	20.41	16.80	22.82	27.35	16.12	37.83	24.70
ECoN	3.89	3.89	3.88	3.97	3.89	3.89	3.90	3.88	3.89	3.85	3.77	3.87	3.14	3.77

<P2—O> (Å)	1.531	1.535	1.535	1.530	1.531	1.535	1.536	1.534	1.535	1.530	1.546	1.535	1.509	1.529
Vol. (Å³)	1.83	1.85	1.85	1.84	1.84	1.85	1.85	1.85	1.85	1.83	1.88	1.85	1.72	1.83
DI	0.0241	0.0242	0.0260	0.0088	0.0241	0.0227	0.0247	0.0231	0.0236	0.0272	0.0470	0.0237	0.0400	0.0351
TQE	1.0034	1.0032	1.0035	1.0011	1.0030	1.0026	1.0027	1.0027	1.0027	1.0034	1.0058	1.0024	1.0206	1.0038
TAV (°²)	12.54	11.68	12.48	4.49	10.67	9.18	9.12	9.60	9.48	10.99	9.51	7.94	71.43	4.61
ECoN	3.87	3.87	3.86	3.98	3.88	3.89	3.88	3.88	3.89	3.80	3.61	3.89	3.57	3.71

P1—O4—P2 (°)	123.18 (11)	123.68 (10)	123.8 (2)	n.d.	124.32 (10)	124.24	125.0 (2)	124.97 (15)	125.80 (5)	125.6 (5)	123.8 (9)	127.5 (3)	123.72 (7)	127.4 (6)

Notes: (a )data from Riou et al. (1988); (b) data from Genkin & Timofeeva (1989).

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