Reinvestigation of KMg1/3Nb2/3OPO4

Babaryk, A.A.; Bon, V.; Zatovsky, I.V.; Slobodyanik, N.S.; Pekhnyo, V.

doi:10.1107/S1600536810004617

inorganic compounds

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Volume 66| Part 3| March 2010| Pages i15-i16

doi:10.1107/S1600536810004617

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Reinvestigation of KMg_1/3Nb_2/3OPO₄ †

Artem A. Babaryk,^a ^* Vladimir Bon,^b Igor V. Zatovsky,^a Nikolay S. Slobodyanik ^a and Vasily Pekhnyo ^b

^aInorganic Chemistry Department, Faculty of Chemistry, Taras Shevchenko Kyiv National University, Kyiv, Ukraine, and ^bInstitute of General and Inorganic Chemistry, NAS Ukraine, prosp. Palladina 32/34, 03680 Kyiv, Ukraine
^*Correspondence e-mail: babaryk@bigmir.net, babaryk@univ.kiev.ua

(Received 19 January 2010; accepted 5 February 2010; online 10 February 2010)

The crystal structure of potassium magnesium niobium oxide phosphate, KMg_1/3Nb_2/3OPO₄, which was described in the space group P4₃22 [McCarron & Calabrese, (1993 ). J. Solid State Chem. 102, 354–361], has been redetermined in the revised space group P4₁. Accordingly, the assignment of the space group P4₃22 and, therefore, localization of K at a single half-occupied position, as noted in the previous study, proved to be an artifact. As a consequence, two major and two minor positions of K are observed due to the splitting along [001], as first noted for KTiOPO₄ structure analogues. It has been shown that the geometry of the {M^II_1/3Nb_2/3O_6/2}_∞ framework is almost unaffected by the lowering of symmetry.

Related literature

For the previous study of the title compound, see: McCarron & Calabrese (1993). For K-splitting in KTiOPO₄ (KTP) isostructures, see: Belokoneva et al. (1997 ); Streltsov et al. (1998 ); Delarue et al. (1999 ); Nordborg (2000 ); Norberg & Ishizawa (2005 ) and in Nb-substituted KTP, see: Alekseeva et al. (2003 ); Dudka et al. (2005 ). For tetragonal aliovalent analogues related to KTiOPO₄, see: Peuchert et al. (1995 ); Babaryk et al. (2007b ). For the relationship between the crystal symmetry class and non-zero χ⁽²⁾ coefficients, see: Authier (2003 ); Babaryk et al. (2007a ). For method used to determine the absolute structure, see: Flack & Bernardinelli (2000 ).

Experimental

Crystal data

K_0.96(Mg_0.32Nb_0.68O)(PO₄)
M_r = 219.46
Tetragonal, P 4₁
a = 6.5261 (1) Å
c = 10.8427 (4) Å
V = 461.79 (2) Å³
Z = 4
Mo Kα radiation
μ = 3.02 mm⁻¹
T = 293 K
0.3 × 0.3 × 0.3 mm

Data collection

Bruker APEXII diffractometer
Absorption correction: multi-scan [SORTAV (Blessing, 1995 ) and SADABS (Bruker, 2007 )] T_min = 0.603, T_max = 0.746
2467 measured reflections
1036 independent reflections
1020 reflections with I > 2σ(I)
R_int = 0.017

Refinement

R[F² > 2σ(F²)] = 0.025
wR(F²) = 0.055
S = 1.20
1036 reflections
107 parameters
3 restraints
Δρ_max = 0.62 e Å⁻³
Δρ_min = −0.54 e Å⁻³
Absolute structure: Flack (1983 ), 336 Friedel pairs
Flack parameter: 0.08 (10)

Table 1
Comparison of bond lengths and angles (Å, °)

	This work	McCarron & Calabrese (1993)
P1—O1	1.531 (5)	1.533 (3)
P1—O2ⁱ	1.541 (5)	1.540 (3)
Mg(Nb)1—O1ⁱⁱⁱ	2.116 (5)	2.124 (3)
Mg(Nb)1—O3	2.059 (3)	2.069 (3)
Mg(Nb)1—O5ⁱⁱⁱ	1.888 (4)	1.890 (1)
O1—P1—O3	110.9 (2)	111.5 (1)
O3—P1—O2ⁱ	108.0 (2)	107.9 (2)
O5—Mg(Nb)1—O1ⁱⁱⁱ	173.55 (14)	173.4 (1)
O5—Mg(Nb)1—O2ⁱⁱⁱ	173.74 (14)	173.4 (1)
O4—Mg(Nb)1—O3	163.53 (10)	163.1 (2)
Symmetry codes: (i) $[-y+2, x, z+{\script{1\over 4}}]$ ; (iii) $[-y+1, x, z+{\script{1\over 4}}]$ .

Data collection: APEX2 (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT, and Blessing (1987 , 1989 ); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XP in SHELXTL (Sheldrick, 2008) and SLANT PLANE in WinGX (Farrugia, 1999 ); software used to prepare material for publication: SHELXTL.

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536810004617/br2136sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536810004617/br2136Isup2.hkl

checkCIF report

Comment top

Originally KMg_1/3Nb_2/3PO₅ was described by McCarron III & Calabrese (1993) who determined that it crystallized in non-polar space group P4₃22 (95) chosen on the assumption of the almost zero-order experimental value of second harmonic response (SHG) relatively to α-SiO₂ as standard. The asymmetric part of the unit cell consists of one Mg(Nb)(.2.), P1 (.2.) and O1 (..2) possesing the two-fold axes while K1 occupies a general position with q=0.5. O2, O3 are full-occupied. K1 is distant from its equivalent position (y, x, 1/4-z) at 1.582 (7) Å. Also the authors reported about the absolute configuration assignment based on exceptionally on ΔR_w(F)=0.15 as criterion. The title compound can be described as {Mg_1/3Nb_2/3O_6/2}_∞ mutually perpendicular octahedral chains running along the principal crystal axes. PO_4/2 tetrahedra link {Mg_1/3Nb_2/3O_6/2}₂ fragments of adjacent chains via the common O vertices. K atoms reside in the tunnel cavities of the anionic framework parallel to the [001] direction. However, indication of non-zero SHG value contradicts the assighment of 422 space groups, which have nil components of the χ⁽²⁾-tensor (see Authier et al., 2003). An attempt to revise this model was done earlier (Babaryk et al., 2007a) for Mn- and Co-containing isostructural compounds leading to lower symmetry lattices [space group P4₁ (76) or P4₃ (78), depending on the Flack parameter] where K1 (4a) and K2 (4a) positions were treated as independent.

This work aims to unify the description of all isostructural series of KM^II_1/3Nb_2/3OPO₄ (M = Mg, Mn, Co) based on a K-split structural model. Improvements brought by this model were tested on Mg-containing isostructure of the series.

Unlike the previous work, all refiments were carried out on F²_obs, the absolute structure of the title compound was estimated on the basis of the Flack parameter with the TWIN/BASF instruction (Sheldrick, 2008) by sparse-matrix least squares (Flack & Bernardinelli, 2000). The initial coordinates were taken from the Babaryk et al., (2007a). The initial ratio of Mg and Nb occupancies was fixed at the ideal value 1/2 while occupancies of K(1) and K(2) were free and refined to the following convergence parameters: R = 0.038, wR = 0.082, S = 1.113 and residual electron densities Δρ_max/Δρ_min = 1.41/-0.86 e Å³ [σ(Δρ) = 0.14 e/Å³]. The highest observed peak (1.41 e Å³) is distant from 0.92Å from K1 and the lowest one at a distance of 0.27 Å from K(2) (Fig. 1a,c) and corresponds to splitting along the c-axis that was earlier shown (Norberg et al., 2005) for KTiOPO₄ only in a case of synchrotron radiation experiments. In the modified model occupancies of K atoms were set of 0.25 equally and further refinement was performed with linear restraints on the occupancies to fit the charge balance. The refinement converged to R = 0.026, wR = 0.055, S = 1.195 and residual electron densities Δρ_max/Δρ_min = 0.62/-0.54 e Å³ [σ(Δρ) = 0.11 e Å³] (Fig. 1 b,d). Comparison of result obtained on the previously reported model tested on the same dataset lead to less satisfactoral results R = 0.043, wR = 0.095, S =1.112 and Δρ_max/Δρ_min = 1.23/-0.88 e Å³ [σ(Δρ) = 0.19 e Å³] in P4₁22 (93) [Flack parameter x = 0.8 (10)]. The highest peak (1.23 e Å³) observed on the (F_o-F_c)-map distant from K1 at 0.95 Å.

The asymmetric and cell units are shown on the Fig. 2 and 3.

The comparison of P—O and Mg/Nb—O bond length and angles (Table 1) produce rather comparable values in this study and previous ones, thus, the geometry of {M^II_1/3Nb_2/3O_6/2}_∞ is almost not affected by the lowering of symmetry. The major differences between the previously reported and this investigation are concerned with K-splittig phenomena. To the the best of our knowledge this is the first example among tetragonal KTP-analogues (Peuchert et al., 1995; Babaryk et al., 2007b) where it is observed. K-splitting phenomena for KTiOPO₄ family of compounds is intesively studied over the last decade. Examples of splitting at room temperature for KTP-isostructures become more evident, verbi causa, RbSbOGeO₄ (Belokoneva et al., 1997), RbTiOAsO₄ (Streltsov et al., 2000), CsTiOAsO₄ (Nordborg, 2000), KTiOPO₄ (Norberg & Ishizawa, 2005) using synchrotron irradiation experiments. In case X-ray study of KTP such splitting was found only upon heating at 673 and 973 K (Delarue et al., 1999). Insert of 7-11% at. of Nb into KTP allow to introduce the cation-split model for refinement and, noteworthy, occupation of minor K-position growth with increasing Nb-content in KTiOPO₄ (Alekseeva et al., 2003). Recent precision investigation (30 K) of 7% at. Nb-doped KTP crystals (Dudka et al., 2005) showed K distribution over the split position is almost indentical to results obtained at room temperature. Thus, it is likely to introduce the K-split model for refinement of Nb-containing KTP analogues as well as it was applied above.

The final distribution of K over the positions K1A/K1B is equiproportional to K1B/K2B ones that are of ca. 7/3. The arrangement of K atoms is almost symmetrical, the distance between major K1A and minor K1B positions is 0.70 (2) Å that is almost equal to 0.69 (3) Å K(2 A)—K(2B) separation. The distance between major K1A and K2A positions is something larger [1.729 (8) Å] in counterpart of 1.21 (3) Å between K1B and K2B, while corresponding cross-distances are equal to 1.58 (3) Å. The discrepancies between the coordination of major positions K1A, K2A and minor K1B and K2B is also observed in this study similar to that reported earlier (Norberg & Ishizawa, 2005). Both K1A and K2A atoms formed eight K—O contacts (Fig. 4a) according to a scheme [2+6]: two of them are in a range of 2.592 (7)-2.713 (7) Å corresponding to ionic-covalent type and six are longer ones within the limits 2.862 (1)-3.138 (1) Å demonstrating ionic bonding type. In counterpart to this each K1B and K2B are closed with seven O atoms (Fig. 4b), three of them are being short [2.572 (1)-2.784 (2) Å], other three ones are longer [2.96 (3)-3.26 (4) Å]. The seventh contact is distinguished by the value of 3.37 (3) Å so it is hardly side with others.

One can conclude that the cation subblattice arrangement is silmilar either for KTP isostructures or its aliovalent analogues, however a measure of K-splitting "strength" depends on the charge of the polyvalent metal constituent.

Related literature top

For the previous study of the title compound, see: McCarron & Calabrese (1993). For K-splitting in KTiOPO₄ (KTP) isostructures, see: Belokoneva et al. (1997); Streltsov et al. (1998); Delarue et al. (1999); Nordborg (2000); Norberg & Ishizawa (2005) and in Nb-substituted KTP, see: Alekseeva et al. (2003); Dudka et al. (2005). For tetragonal aliovalent analogues related to KTiOPO₄, see: Peuchert et al. (1995); Babaryk et al. (2007b). For the relationship between the crystal symmetry class and non-zero χ⁽²⁾ coefficients, see: Authier (2003); Babaryk et al. (2007a). For method used to determine the absolute structure, see: Flack & Bernardinelli (2000). For related literature on what subject?, see: Blessing (1987, 1989).

Experimental top

For preparation of the title compound 1.072 (1) g of KPO₃, 0.122 (1) g of MgO, 0.806 (1) g of Nb₂O₅ and 10.000 (1) g of K₂Mo₂O₇ were mixed together and well grounded in an agate mortar. All the reagents were of analytical grade purity. The mixture was melted in a 50 ml platinum crucible at 1273 K. Melted solution was cooled at a rate 20 K×h^-1 up to 1133 (5) K. Crystalline precipitate was freed from the liquid flux by decantation. The crystals were washed from the rest of the solidified component with hot 5%—aqueous (NaPO₃)_x. Crystalline phase consists of well-shaped colourless biaxial crystals (with size distribution from 0.2 to 0.4 mm of length) in major unless the rear pale-red prismatic crystals of K_5+xNb_8-xMg_xP₅O₃₄ (unpublished results). It is likely to obtaine pure compound at more slow cooling of the initial melt.

Evaluation of the elements quantities was performed on an X-ray fluorescence energy dispersive spectrometer Elvax Light and established Mo presence at a level of 0.01% at., which has not been taken into account at the structure refinements. Exact compositions of title compound was found using a Spectroflame Modula ICP instrument. Analysis found: K, 12.12; Mg, 4.11; Nb, 8.57; P, 12.60; O, 63.94; Mo, 0.06%. K_0.96Mg_0.32Nb_0.68PO₅ requires: K, 12.06; Mg, 4.02; Nb, 8.54; P, 12.56; O, 62.81%.

Computing details top

Data collection: APEX2 (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XP in SHELXTL (Sheldrick, 2008) and SLANT PLANE in WinGX (Farrugia, 1999); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top

	Fig. 1. Comparison of difference Fourier maps through the K1 and K2 sites accounting (a, c) and without (b, d) the K-splitting. Contour intervals are 0.015 e/A3. The contours are drawn solid (positive) and dashed (negative) lines, respectively. Map scope is 4×4 Å.
	Fig. 2. View of the the asymmetric unit. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) -y+2, x, z+1/4 (ii) x+1, y, z; (iii) -y+1, x, z+1/4].
	Fig. 3. View of the the cell unit. Colour codes: oxygen is red, magnesium and niobium are green, phosphorus is purple, potassium is grey.
	Fig. 4. Oxygen cages of K1A (a) and K1B (b). Short and long K–O contact are showed as solid lines and dashed lines, respecively. [Symmetry codes: (iv) y-1, -x+1, z-1/4; (v) y, -x+2, z-1/4;(vi) -x+1, -y+2, z-1/2; (vii) -x+1, -y+1, z-1/2; (viii) -y+1, x, z-3/4; (ix) y, -x+1, z-1/4

potassium magnesium niobium oxide phosphate top

Crystal data top

K_0.96(Mg_0.32Nb_0.68O)(PO₄)	D_x = 3.158 Mg m⁻³
M_r = 219.46	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P4₁	Cell parameters from 2061 reflections
Hall symbol: P 4w	θ = 3.1–30.6°
a = 6.5261 (1) Å	µ = 3.02 mm⁻¹
c = 10.8427 (4) Å	T = 293 K
V = 461.79 (2) Å³	Block, colourless
Z = 4	0.3 × 0.3 × 0.3 mm
F(000) = 420.4

Data collection top

Bruker APEXII diffractometer	1036 independent reflections
Radiation source: sealed x-ray tube	1020 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.017
ϕ or ω oscillation scans	θ_max = 30.0°, θ_min = 3.1°
Absorption correction: multi-scan [SORTAV (Blessing, 1995) and SADABS (Bruker, 2007)]	h = −8→5
T_min = 0.603, T_max = 0.746	k = −9→5
2467 measured reflections	l = −7→15

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	w = 1/[σ²(F_o²) + 1.2901P] where P = (F_o² + 2F_c²)/3
R[F² > 2σ(F²)] = 0.025	(Δ/σ)_max = 0.001
wR(F²) = 0.055	Δρ_max = 0.62 e Å⁻³
S = 1.20	Δρ_min = −0.54 e Å⁻³
1036 reflections	Extinction correction: SHELXL
107 parameters	Extinction coefficient: 0.0115 (15)
3 restraints	Absolute structure: Flack (1983), 336 Friedel pairs
Primary atom site location: isomorphous structure methods	Absolute structure parameter: 0.08 (10)

Crystal data top

K_0.96(Mg_0.32Nb_0.68O)(PO₄)	Z = 4
M_r = 219.46	Mo Kα radiation
Tetragonal, P4₁	µ = 3.02 mm⁻¹
a = 6.5261 (1) Å	T = 293 K
c = 10.8427 (4) Å	0.3 × 0.3 × 0.3 mm
V = 461.79 (2) Å³

Data collection top

Bruker APEXII diffractometer	1036 independent reflections
Absorption correction: multi-scan [SORTAV (Blessing, 1995) and SADABS (Bruker, 2007)]	1020 reflections with I > 2σ(I)
T_min = 0.603, T_max = 0.746	R_int = 0.017
2467 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.025	3 restraints
wR(F²) = 0.055	Δρ_max = 0.62 e Å⁻³
S = 1.20	Δρ_min = −0.54 e Å⁻³
1036 reflections	Absolute structure: Flack (1983), 336 Friedel pairs
107 parameters	Absolute structure parameter: 0.08 (10)

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 > σ(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	Occ. (<1)
O1	0.9717 (6)	0.5139 (5)	0.8485 (5)	0.0163 (9)
O2	0.5141 (5)	0.9718 (6)	0.8260 (5)	0.0169 (10)
O3	0.8102 (4)	0.7859 (4)	0.9827 (3)	0.0144 (7)
O4	0.1898 (4)	0.7865 (4)	0.9406 (3)	0.0133 (7)
O5	0.5433 (5)	0.5429 (5)	0.8361 (4)	0.0132 (6)
P1	1.00000 (13)	0.65145 (13)	0.96154 (16)	0.01016 (19)
K1A	0.2428 (18)	0.7800 (11)	0.5076 (6)	0.034 (3)	0.344 (17)
K1B	0.155 (4)	0.827 (2)	0.531 (3)	0.056 (7)	0.148 (16)
K2A	0.2198 (14)	0.758 (2)	0.6659 (7)	0.034 (3)	0.332 (19)
K2B	0.173 (3)	0.844 (5)	0.642 (3)	0.054 (7)	0.140 (18)
Mg1	0.49981 (6)	0.74099 (6)	0.96157 (9)	0.01355 (13)	0.3214 (12)
Nb1	0.49981 (6)	0.74099 (6)	0.96157 (9)	0.01355 (13)	0.6786 (12)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O1	0.0156 (15)	0.0163 (18)	0.017 (2)	0.0044 (12)	−0.0035 (17)	−0.0086 (13)
O2	0.0172 (19)	0.0154 (15)	0.018 (2)	0.0050 (12)	0.0065 (14)	0.0085 (17)
O3	0.0125 (12)	0.0158 (12)	0.015 (2)	0.0014 (10)	0.0002 (14)	−0.0028 (13)
O4	0.0126 (12)	0.0154 (12)	0.012 (2)	−0.0010 (10)	0.0003 (13)	0.0015 (13)
O5	0.0132 (14)	0.0124 (14)	0.0139 (13)	−0.0007 (8)	0.0017 (14)	−0.0026 (14)
P1	0.0071 (3)	0.0133 (4)	0.0101 (4)	0.0002 (3)	−0.0020 (4)	−0.0003 (5)
K1A	0.049 (5)	0.027 (2)	0.026 (3)	−0.022 (3)	−0.007 (2)	0.0018 (18)
K1B	0.044 (9)	0.019 (5)	0.10 (2)	−0.019 (5)	0.019 (10)	−0.006 (6)
K2A	0.026 (3)	0.049 (5)	0.027 (3)	−0.021 (3)	0.0017 (19)	−0.010 (2)
K2B	0.018 (6)	0.042 (10)	0.10 (2)	−0.016 (6)	−0.003 (7)	0.008 (11)
Mg1	0.00835 (18)	0.0162 (2)	0.0161 (2)	0.00006 (16)	−0.0009 (2)	0.0003 (3)
Nb1	0.00835 (18)	0.0162 (2)	0.0161 (2)	0.00006 (16)	−0.0009 (2)	0.0003 (3)

Geometric parameters (Å, º) top

P1—O1	1.531 (5)	K1A—O3^ix	2.993 (7)
P1—O2ⁱ	1.541 (5)	K1A—O5^x	3.002 (12)
P1—O3	1.535 (3)	K1A—O2^vii	3.004 (9)
P1—O4ⁱⁱ	1.537 (3)	K1A—O5^viii	3.138 (13)
Nb1—O1ⁱⁱⁱ	2.116 (5)	K2A—O2^iv	2.592 (7)
Nb1—O2	2.107 (5)	K2A—O1^vi	2.709 (8)
Nb1—O3	2.059 (3)	K2A—O4^iv	2.862 (10)
Nb1—O4	2.057 (3)	K2A—O2	2.941 (7)
Nb1—O5ⁱⁱⁱ	1.888 (4)	K2A—O4	2.990 (7)
Nb1—O5	1.898 (4)	K2A—O5^x	3.008 (14)
K1A—K1B	0.70 (2)	K2A—O1^xi	3.012 (9)
K1A—K2B	1.58 (3)	K2A—O5	3.134 (15)
K1A—K2A	1.729 (8)	K1B—O2^iv	2.572 (11)
K1B—K2B	1.21 (3)	K1B—O3^vii	2.591 (13)
K2A—K2B	0.69 (3)	K1B—O1^vi	2.784 (18)
K1B—K2A	1.58 (3)	K1B—O4^iv	2.96 (3)
K1B—K2B^iv	2.53 (2)	K1B—O1^viii	3.09 (3)
K1B—K2A^iv	2.89 (3)	K1B—O3^ix	3.26 (3)
K1A—K2B^iv	2.91 (3)	K1B—O2^vii	3.37 (3)
K2A—K1B^v	2.89 (3)	K2B—O1^vi	2.574 (13)
K2B—K1B^v	2.53 (2)	K2B—O1^xi	3.37 (3)
K2B—K1A^v	2.91 (3)	K2B—O2^iv	2.773 (19)
K1A—O1^vi	2.593 (6)	K2B—O2	3.10 (3)
K1A—O2^iv	2.713 (7)	K2B—O3^vii	2.97 (4)
K1A—O3^vii	2.867 (9)	K2B—O4^iv	2.586 (15)
K1A—O1^viii	2.935 (7)	K2B—O4	3.26 (4)

O1—P1—O3	110.9 (2)	O2^iv—K2A—O4	112.1 (3)
O1—P1—O4ⁱⁱ	108.4 (2)	O1^vi—K2A—O4	105.9 (2)
O3—P1—O4ⁱⁱ	110.17 (14)	O4^iv—K2A—O4	80.48 (18)
O1—P1—O2ⁱ	108.55 (17)	O2—K2A—O4	54.93 (16)
O3—P1—O2ⁱ	108.0 (2)	O2^iv—K2A—O5^x	83.3 (3)
O4ⁱⁱ—P1—O2ⁱ	110.8 (2)	O1^vi—K2A—O5^x	81.5 (2)
O1^vi—K1A—O1^viii	163.1 (5)	O4^iv—K2A—O5^x	142.9 (3)
O2^iv—K1A—O1^viii	52.22 (12)	O2—K2A—O5^x	91.3 (3)
O3^vii—K1A—O1^viii	122.2 (3)	O4—K2A—O5^x	111.9 (5)
O1^vi—K1A—O3^ix	112.3 (3)	O2^iv—K2A—O1^xi	63.26 (17)
O2^iv—K1A—O3^ix	106.1 (2)	O1^vi—K2A—O1^xi	154.5 (3)
O3^vii—K1A—O3^ix	80.42 (17)	O4^iv—K2A—O1^xi	58.02 (14)
O1^viii—K1A—O3^ix	55.08 (15)	O2—K2A—O1^xi	102.4 (3)
O1^vi—K1A—O5^x	83.5 (3)	O4—K2A—O1^xi	48.97 (16)
O2^iv—K1A—O5^x	81.4 (2)	O5^x—K2A—O1^xi	102.8 (5)
O3^vii—K1A—O5^x	143.1 (3)	O2^iv—K2A—O5	109.7 (5)
O1^viii—K1A—O5^x	91.1 (3)	O1^vi—K2A—O5	88.5 (3)
O3^ix—K1A—O5^x	111.7 (4)	O4^iv—K2A—O5	131.6 (3)
O1^vi—K1A—O2^vii	63.37 (16)	O2—K2A—O5	54.9 (2)
O2^iv—K1A—O2^vii	154.8 (3)	O4—K2A—O5	59.1 (2)
O3^vii—K1A—O2^vii	57.92 (13)	O5^x—K2A—O5	53.5 (3)
O1^viii—K1A—O2^vii	102.6 (3)	O1^xi—K2A—O5	74.9 (3)
O3^ix—K1A—O2^vii	49.03 (15)	O1^vi—K2B—O4^iv	156.9 (14)
O5^x—K1A—O2^vii	102.7 (4)	O1^vi—K2B—O2^iv	139.1 (10)
O1^vi—K1A—O5^viii	110.0 (4)	O4^iv—K2B—O2^iv	61.3 (3)
O2^iv—K1A—O5^viii	88.7 (2)	O1^vi—K2B—O3^vii	58.7 (6)
O3^vii—K1A—O5^viii	131.4 (3)	O4^iv—K2B—O3^vii	102.9 (12)
O1^viii—K1A—O5^viii	54.89 (19)	O2^iv—K2B—O3^vii	123.5 (13)
O3^ix—K1A—O5^viii	58.8 (2)	O1^vi—K2B—O2	51.1 (4)
O5^x—K1A—O5^viii	53.5 (2)	O4^iv—K2B—O2	126.3 (11)
O2^vii—K1A—O5^viii	74.8 (3)	O2^iv—K2B—O2	137.1 (15)
O2^iv—K1B—O3^vii	157.4 (12)	O3^vii—K2B—O2	97.3 (5)
O2^iv—K1B—O1^vi	138.6 (9)	O1^vi—K2B—O4	102.0 (9)
O3^vii—K1B—O1^vi	61.2 (3)	O4^iv—K2B—O4	79.7 (8)
O2^iv—K1B—O4^iv	58.8 (5)	O2^iv—K2B—O4	100.2 (10)
O3^vii—K1B—O4^iv	103.1 (10)	O3^vii—K2B—O4	132.0 (8)
O1^vi—K1B—O4^iv	123.3 (11)	O2—K2B—O4	50.9 (6)
O2^iv—K1B—O1^viii	51.2 (4)	O5ⁱⁱⁱ—Nb1—O5	93.97 (9)
O3^vii—K1B—O1^viii	126.4 (11)	O5ⁱⁱⁱ—Nb1—O4	91.86 (13)
O1^vi—K1B—O1^viii	137.0 (13)	O5—Nb1—O4	99.56 (15)
O4^iv—K1B—O1^viii	97.5 (5)	O5ⁱⁱⁱ—Nb1—O3	99.36 (15)
O2^iv—K1B—O3^ix	102.2 (8)	O5—Nb1—O3	91.70 (13)
O3^vii—K1B—O3^ix	79.6 (7)	O4—Nb1—O3	163.53 (10)
O1^vi—K1B—O3^ix	100.1 (8)	O5ⁱⁱⁱ—Nb1—O2	173.74 (14)
O4^iv—K1B—O3^ix	132.4 (8)	O5—Nb1—O2	88.9 (2)
O1^viii—K1B—O3^ix	51.0 (5)	O4—Nb1—O2	82.15 (13)
O2^iv—K2A—O1^vi	141.9 (3)	O3—Nb1—O2	86.13 (14)
O2^iv—K2A—O4^iv	59.99 (17)	O5ⁱⁱⁱ—Nb1—O1ⁱⁱⁱ	88.8 (2)
O1^vi—K2A—O4^iv	130.3 (6)	O5—Nb1—O1ⁱⁱⁱ	173.55 (14)
O2^iv—K2A—O2	162.7 (6)	O4—Nb1—O1ⁱⁱⁱ	86.18 (13)
O1^vi—K2A—O2	52.19 (12)	O3—Nb1—O1ⁱⁱⁱ	82.08 (13)
O4^iv—K2A—O2	122.2 (3)	O2—Nb1—O1ⁱⁱⁱ	89.02 (10)

Symmetry codes: (i) −y+2, x, z+1/4; (ii) x+1, y, z; (iii) −y+1, x, z+1/4; (iv) y−1, −x+1, z−1/4; (v) −y+1, x+1, z+1/4; (vi) y, −x+2, z−1/4; (vii) −x+1, −y+2, z−1/2; (viii) −x+1, −y+1, z−1/2; (ix) −y+1, x, z−3/4; (x) y, −x+1, z−1/4; (xi) x−1, y, z.

Experimental details

Crystal data
Chemical formula	K_0.96(Mg_0.32Nb_0.68O)(PO₄)
M_r	219.46
Crystal system, space group	Tetragonal, P4₁
Temperature (K)	293
a, c (Å)	6.5261 (1), 10.8427 (4)
V (Å³)	461.79 (2)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	3.02
Crystal size (mm)	0.3 × 0.3 × 0.3

Data collection
Diffractometer	Bruker APEXII diffractometer
Absorption correction	Multi-scan [SORTAV (Blessing, 1995) and SADABS (Bruker, 2007)]
T_min, T_max	0.603, 0.746
No. of measured, independent and observed [I > 2σ(I)] reflections	2467, 1036, 1020
R_int	0.017
(sin θ/λ)_max (Å⁻¹)	0.703

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.025, 0.055, 1.20
No. of reflections	1036
No. of parameters	107
No. of restraints	3
Δρ_max, Δρ_min (e Å⁻³)	0.62, −0.54
Absolute structure	Flack (1983), 336 Friedel pairs
Absolute structure parameter	0.08 (10)

Computer programs: APEX2 (Bruker, 2007), SAINT (Bruker, 2007), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), XP in SHELXTL (Sheldrick, 2008) and SLANT PLANE in WinGX (Farrugia, 1999), SHELXTL (Sheldrick, 2008).

Comparison of bond lengths and angles (Å, °) top

	This work	McCarron & Calabrese (1993)
P1—O1	1.531 (5)	1.533 (3)
P1—O2ⁱ	1.541 (5)	1.540 (3)
Mg(Nb)1—O1ⁱⁱⁱ	2.116 (5)	2.124 (3)
Mg(Nb)1—O3	2.059 (3)	2.069 (3)
Mg(Nb)1—O5ⁱⁱⁱ	1.888 (4)	1.890 (1)
O1—P1—O3	110.9 (2)	111.5 (1)
O3—P1—O2ⁱ	108.0 (2)	107.9 (2)
O5—Mg(Nb)1—O1ⁱⁱⁱ	173.55 (14)	173.4 (1)
O5—Mg(Nb)1—O2ⁱⁱⁱ	173.74 (14)	173.4 (1)
O4—Mg(Nb)1—O3	163.53 (10)	163.1 (2)

Symmetry codes: (i) -y+2, x, z+1/4; (iii) -y+1, x, z+1/4].

Footnotes

†Dedicated to Professor Nikolay S. Slobodyanik on the occasion of his 65^th birthday.

References

Alekseeva, O. A., Sorokina, N. I., Verin, I. A., Losevskaya, T. Yu., Voronkova, V. I., Yanovski, V. K. & Simonov, V. I. (2003). Crystallogr. Rep. 48, 205–211. Web of Science CrossRef CAS Google Scholar
Authier, A. (2003). International Tables for Crystallography, Vol. D. Dordrecht, Kluwer. Google Scholar
Babaryk, A. A., Zatovsky, I. V., Baumer, V. N., Slobodyanik, N. S. & Domasevitch, K. V. (2007b). Acta Cryst. C63, i105–i108. Web of Science CrossRef IUCr Journals Google Scholar
Babaryk, A. A., Zatovsky, I. V., Baumer, V. N., Slobodyanik, N. S., Nagorny, P. G. & Shishkin, O. V. (2007a). J. Solid State Chem. 180, 1990–1997. Web of Science CrossRef CAS Google Scholar
Belokoneva, E. L., Knight, K. S., David, W. I. F. & Mill, B. V. (1997). J. Phys. Condens. Matter, 9, 3833–3855. CrossRef CAS Web of Science Google Scholar
Blessing, R. H. (1987). Crystallogr. Rev. 1, 3–58. CrossRef Google Scholar
Blessing, R. H. (1989). J. Appl. Cryst. 22, 396–397. CrossRef Web of Science IUCr Journals Google Scholar
Blessing, R. H. (1995). Acta Cryst. A51, 33–38. CrossRef CAS Web of Science IUCr Journals Google Scholar
Bruker (2007). APEX2, SAINT and SADABS. BrukerAXS Inc, Madison, Wisconsin, USA. Google Scholar
Delarue, P., Lecompte, C., Jannin, M., Marnier, G. & Menaert, B. (1999). J. Phys. Condens. Matter, 11, 4123–4134. Web of Science CrossRef CAS Google Scholar
Dudka, A. P., Verin, I. A., Molchanov, V. N., Blombegr, M. K., Alekseeva, O. A., Sorokina, N. I., Novikova, N. E. & Simonov, V. I. (2005). Crystallogr. Rep. 50, 36–41. Web of Science CrossRef CAS Google Scholar
Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837–838. CrossRef CAS IUCr Journals Google Scholar
Flack, H. D. (1983). Acta Cryst. A39, 876–881. CrossRef CAS Web of Science IUCr Journals Google Scholar
Flack, H. D. & Bernardinelli, G. (2000). J. Appl. Cryst. 33, 1143–1148. Web of Science CrossRef CAS IUCr Journals Google Scholar
McCarron, E. M. III & Calabrese, J. C. (1993). J. Solid State Chem. 102, 354–361. CrossRef CAS Web of Science Google Scholar
Norberg, S. T. & Ishizawa, N. (2005). Acta Cryst. C61, i99–i102. Web of Science CrossRef CAS IUCr Journals Google Scholar
Nordborg, J. (2000). Acta Cryst. C56, 518–520. CrossRef CAS IUCr Journals Google Scholar
Peuchert, U., Bohatý, L. & Fröhlich, R. (1995). Acta Cryst. C51, 1719–1721. CrossRef CAS Web of Science IUCr Journals Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Streltsov, V. A., Ishizawa, N. & Kishimoto, S. (1998). J. Synchrotron Rad. 5, 1309–1316. Web of Science CrossRef CAS IUCr Journals Google Scholar