K0.53Mn2.37Fe1.24(PO4)3

Mourad Hidouri; Mongi Ben Amara

doi:10.1107/S1600536810051238

. 2010 Dec 11;67(Pt 1):i1. doi: 10.1107/S1600536810051238

K_0.53Mn_2.37Fe_1.24(PO₄)₃

Mourad Hidouri ^a,^*, Mongi Ben Amara ^a

PMCID: PMC3050314 PMID: 21522509

Abstract

During an attempt to crystallize potassium manganese diiron phosphate KMnFe₂(PO₄)₃ by the flux method, a new phase, potassium dimanganese iron triphosphate, K_0.53Mn_2.37Fe_1.24(PO₄)₃, was isolated. This phase, whose composition was confirmed by ICP analysis, is isotypic with the alluaudite-like phosphates, thus it exhibits the (A2)(A′2)(A1)(A′1)(A′′1)(M1)(M2)₂(PO₄)₃ general formula. The site occupancies led to the following cation distribution: 0.53 K on A′2 (site symmetry 2), 0.31 Mn on A′′1, 1.0 Mn on M1 (site symmetry 2) and (0.62 Fe + 0.38 Mn) on M2. The structure is built up from infinite chains of edge-sharing M1O₆ and M2O₆ octahedra. These chains run along [10 Inline graphic ] and are connected by two different PO₄ tetrahedra, one of which exhibits 2 symmetry. The resulting three-dimensional framework delimits large tunnels parallel to [001], which are partially occupied by the K⁺ and Mn²⁺ cations.

Related literature

For the alluaudite structure, see: Fisher (1955 ▶); Moore (1971 ▶); Chouaibi et al. (2001 ▶); Corbin et al. (1986 ▶); Lee & Ye (1997 ▶); Hidouri et al. (2003 ▶, 2004 ▶, 2008 ▶) Antenucci et al. (1993 ▶, 1995 ▶); For P—O distances, see: Baur (1974 ▶). For bond-valence sums, see: Brown & Altermatt (1985 ▶). For ionic radii, see: Shannon (1976 ▶).

Experimental

Crystal data

K_0.53Mn_2.37Fe_1.24(PO₄)₃
M _r = 505.28
Monoclinic,
a = 12.272 (2) Å
b = 12.606 (2) Å
c = 6.416 (4) Å
β = 114.87 (2)°
V = 900.5 (6) Å³
Z = 4
Mo Kα radiation
μ = 6.07 mm⁻¹
T = 293 K
0.43 × 0.09 × 0.02 mm

Data collection

Enraf–Nonius TurboCAD-4 diffractometer
Absorption correction: refined from ΔF (Parkin et al., 1995 ▶) T _min = 0.42, T _max = 0.81
1754 measured reflections
1308 independent reflections
1047 reflections with I > 2σ(I)
R _int = 0.042
2 standard reflections every 120 min intensity decay: 1%

Refinement

R[F ² > 2σ(F ²)] = 0.037
wR(F ²) = 0.089
S = 1.07
1308 reflections
102 parameters
2 restraints
Δρ_max = 0.78 e Å⁻³
Δρ_min = −0.68 e Å⁻³

Data collection: CAD-4 EXPRESS (Enraf–Nonius, 1994 ▶); cell refinement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms & Wocadlo, 1995 ▶); program(s) used to solve structure: SIR92 (Altomare et al., 1993 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: SHELXTL (Sheldrick, 2008 ▶); software used to prepare material for publication: SHELXL97.

Supplementary Material

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536810051238/br2150sup1.cif

e-67-000i1-sup1.cif^{(15.2KB, cif)}

Structure factors: contains datablocks I. DOI: 10.1107/S1600536810051238/br2150Isup2.hkl

e-67-000i1-Isup2.hkl^{(63.4KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

Comment

The term alluaudite is referred to both natural and synthetic phosphates of compositions (Na⁺)(Na⁺, Ca²⁺)(M²⁺)(Fe²⁺,Fe³⁺)₂(PO₄)₃ where M²⁺ is a divalent cation. The first detailed structural description was reported in 1971 by Moore (Moore, 1971) who proposed the structural formula (X2)(X1)(M1)(M2)₂(PO₄)₃. X1 and X2 are cationic sites available to large monovalent and divalent cations such as Na⁺ and Ca²⁺ while M1 and M2 are octahedral sites containing a distribution of divalent and trivalent cations of moderate size such as Mn²⁺, Fe²⁺ or Fe³⁺. More recently, detailed structural analysis of several alluaudites demonstrated that the X2 site has two distinct positions labeled A2 (0, 0, 0) and A'2 (0,~0, 1/4) in a tunnel at (0, 0, z) and X1 has three distinct positions, labeled A1 (1/2, 0, 0), A'1 (0,~1/2, 1/4) and A''1 (x, y, z) in a tunnel at (1/2, 0, z). The general formula of Moore was then reformulated as [(A2)(A'2)][(A1)(A'1)(A''1)](M1)(M2)₂(PO₄)₃. The crystal structure consists of M2₂O₁₀ bioctahedral units of edge-sharing M2O₆ octahedra, sharing opposite edges with M1O₆ octahedra that form zigzag chains of a sequence –M(2)—M(2)—M(1)-, running along the [1 0 - 1] direction. Adjacent chains are linked by the phosphate tetrahedra leading to what have been described as "pleated sheets" perpendicular to the [0 1 0] direction (Fig. 1(b)). These sheets are connected by the phosphate groups giving rise to a three-dimensional framework with two sets of tunnels parallel to [001] (Fig. 1(b)).

The title phase K_0.53Mn_2.37Fe_1.24(PO₄)₃ was isolated during an attempt to synthesize KMnFe₂(PO₄)₃ and its structure has shown to be of the alluaudite type. The site occupancy factors indicated to the following cation distribution: 0.53 K on A'2, 0.31 Mn on A''1, 1.0 Mn on M1 and (0.62 Fe + 0.38 Mn) on M2. The partial occupancy of the large A sites has already been observed in several alluaudites being attributed to the great flexibility of these sites which allows them to be filled totally, partially or left vacant without significant influence on the alluaudite framework. Assuming a maximum gap in the cation-oxygen distances, the envronment of the A'2 site (figure 2) consists of eight oxygen atoms forming what has been called by Moore as a gable desphenoid (Moore, 1971). That of the A''1 site (figure 2) onsists of five O atoms forming a distorted trigonal bipyramid. The fivefold coordination of this site which is, to the best of our knowledge, observed for the first time in an alluaudite-like compound can be attributed to the small size of the Mn²⁺ cation. Both the M1 and M2 sites are octahedrally coordinated (figure 2). From the M1—O distances and cis O—M1—O angles, one can deduce that the M1O₆ octahedron is strongly distorted. However, the mean M1—O> mean distance of 2.238 Å is close to that 2.23 Å predicted by Shannon for octahedral Mn²⁺ cations (Shannon, 1976). The M2—O distances and cis O—M2—O angles show the M2O₆ octahedron to be less distorted than M1O₆. The <M(2)—O> mean distance (2.068 Å) is between 2.03 Å and 2.23 Å, calculated by Shannon (Shannon, 1976) for the Fe³⁺ and Mn²⁺ cations, respectively. This result confirms the presence of both atoms on the M(2) site. The PO₄ tetrahedra have classical P—O distances with an overall value of 1.537 Å close to that 1.537 Å, assigned by Baur for the monophosphate groups (Baur, 1974). The Bond Valence Sums (BVS) were calculated for all cationic sites by the Brown and Altermatt method (Brown et al., 1985). The analysis of the sums for the M2 site, around Fe³⁺ and Mn²⁺ led to valence sums of 2.87 and 2.64, respectively which corresponds to occupation numbers of 0.71 and 0.29, very close to the x-ray values of 0.62 and 0.38. The sum around Mn3 is 1.34 and around K is 0.42. These are poor because of the partial occupancy but unfortunately these values cannot be used to estimate the occupancy. The sums around P1 and P2 of 4.92 and 4.98, respectively around the O sites (from 1.91 to 2.12) are consistent with the predicted ones of 5 for P and 2 for O. In summery, the valence calculation results gave a good confirmation of the structure, including the assigned oxidation states which cannot be determined by x-ray analysis.

Experimental

Single crystals of the title phase were extracted from a mixture of nominal composition KMnFe₂(PO₄)₃. The latter was prepared by the flux method starting from a mixture of 2.042 g of KNO₃, 2.589 g of Mn(NO₃)₂.6H₂O, 8.245 g of Fe(NO₃)₃.9H₂O, 4.002 g of (NH₄)₂HPO₄ and 0.719 g of MoO₃. These reactants were dissolved in nitric acid and the solution obtained was dried for 24 h at 353 K. The obtained dry residue was ground in an agate mortar to ensure its best homogeneity, then heated in a platinum crucible to 673 K for 24 h in order to remove the decomposition products: NH₃ and H₂O. The sample was then reground, melted at 1173 K for 1 h and subsequently cooled at a 10 °.h^-1 rate to 673 K. The final product was washed with warm water in order to dissolve the flux. From the mixture, dark brown and hexagonally shaped crystals were extracted. Their analysis using ICP confirmed the presence of only K, Mn, Fe and P in atomic ratio of 0.53:2.37:1.24:3, in accordance with the K_0.53Mn_2.37Fe_1.24(PO₄)₃ composition.