inorganic compounds
K_{0.53}Mn_{2.37}Fe_{1.24}(PO_{4})_{3}
^{a}Faculté des Sciences de Monastir, 5019 Monastir, Tunisia
^{*}Correspondence email: mourad_hidouri@yahoo.fr
During an attempt to crystallize potassium manganese diiron phosphate KMnFe_{2}(PO_{4})_{3} by the method, a new phase, potassium dimanganese iron triphosphate, K_{0.53}Mn_{2.37}Fe_{1.24}(PO_{4})_{3}, was isolated. This phase, whose composition was confirmed by ICP analysis, is isotypic with the alluauditelike phosphates, thus it exhibits the (A2)(A′2)(A1)(A′1)(A′′1)(M1)(M2)_{2}(PO_{4})_{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 edgesharing M1O_{6} and M2O_{6} octahedra. These chains run along [10] and are connected by two different PO_{4} tetrahedra, one of which exhibits 2 symmetry. The resulting threedimensional framework delimits large tunnels parallel to [001], which are partially occupied by the K^{+} and Mn^{2+} 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 bondvalence sums, see: Brown & Altermatt (1985). For ionic radii, see: Shannon (1976).
Experimental
Crystal data

Data collection: CAD4 EXPRESS (Enraf–Nonius, 1994); cell CAD4 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.
Supporting information
https://doi.org//10.1107/S1600536810051238/br2150sup1.cif
contains datablocks I, global. DOI:Structure factors: contains datablock I. DOI: https://doi.org//10.1107/S1600536810051238/br2150Isup2.hkl
Single crystals of the title phase were extracted from a mixture of nominal composition KMnFe_{2}(PO_{4})_{3}. The latter was prepared by the
method starting from a mixture of 2.042 g of KNO_{3}, 2.589 g of Mn(NO_{3})_{2}.6H_{2}O, 8.245 g of Fe(NO_{3})_{3}.9H_{2}O, 4.002 g of (NH_{4})_{2}HPO_{4} and 0.719 g of MoO_{3}. 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 then heated in a platinum crucible to 673 K for 24 h in order to remove the decomposition products: NH_{3} and H_{2}O. 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 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_{0.53}Mn_{2.37}Fe_{1.24}(PO_{4})_{3} composition.The term alluaudite is referred to both natural and synthetic phosphates of compositions (Na^{+})(Na^{+}, Ca^{2+})(M^{2+})(Fe^{2+},Fe^{3+})_{2}(PO_{4})_{3} where M^{2+} is a divalent cation. The first detailed structural description was reported in 1971 by Moore (Moore, 1971) who proposed the ~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}(PO_{4})_{3}. The consists of M2_{2}O_{10} bioctahedral units of edgesharing M2O_{6} octahedra, sharing opposite edges with M1O_{6} 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 threedimensional framework with two sets of tunnels parallel to [001] (Fig. 1(b)).
(X2)(X1)(M1)(M2)_{2}(PO_{4})_{3}. X1 and X2 are cationic sites available to large monovalent and divalent cations such as Na^{+} and Ca^{2+} while M1 and M2 are octahedral sites containing a distribution of divalent and trivalent cations of moderate size such as Mn^{2+}, Fe^{2+} or Fe^{3+}. 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,The title phase K_{0.53}Mn_{2.37}Fe_{1.24}(PO_{4})_{3} was isolated during an attempt to synthesize KMnFe_{2}(PO_{4})_{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 cationoxygen 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 alluauditelike compound can be attributed to the small size of the Mn^{2+} 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 M1O_{6} 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 Mn^{2+} cations (Shannon, 1976). The M2—O distances and cis O—M2—O angles show the M2O_{6} octahedron to be less distorted than M1O_{6}. 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 Fe^{3+} and Mn^{2+} cations, respectively. This result confirms the presence of both atoms on the M(2) site. The PO_{4} 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 Fe^{3+} and Mn^{2+} 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 xray 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 xray analysis.
For the alluaudite structure, see: Fisher (1955); Moore (1971); Chouaibi et al. (20016); Corbin et al. (19867); Lee & Ye (19974); Hidouri et al. (20031, 20042, 20083). For related literature [on what subject?], see: Antenucci et al. (1993, 1995); For P—O distances, see: Baur (1974). For bondvalence sums, see: Brown & Altermatt (1985). For ionic radii, see: Shannon (1976).
For related literature, see: Hidouri et al. (2004, 2008).
Data collection: CAD4 EXPRESS (Enraf–Nonius, 1994); cell
CAD4 EXPRESS (Enraf–Nonius, 1994); 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 (Sheldrick, 2008).K_{0.53}Mn_{2.37}Fe_{1.24}(PO_{4})_{3}  F(000) = 957 
M_{r} = 505.28  D_{x} = 3.727 Mg m^{−}^{3} 
Monoclinic, C2/c  Mo Kα radiation, λ = 0.71073 Å 
Hall symbol: C 2yc  Cell parameters from 25 reflections 
a = 12.272 (2) Å  θ = 9.8–14.4° 
b = 12.606 (2) Å  µ = 6.07 mm^{−}^{1} 
c = 6.416 (4) Å  T = 293 K 
β = 114.87 (2)°  Hexagonal, brown 
V = 900.5 (6) Å^{3}  0.43 × 0.09 × 0.02 mm 
Z = 4 
Enraf–Nonius TurboCAD4 diffractometer  1047 reflections with I > 2σ(I) 
Radiation source: finefocus sealed tube  R_{int} = 0.042 
Graphite monochromator  θ_{max} = 30.0°, θ_{min} = 2.4° 
non–profiled ω/2θ scans  h = −17→15 
Absorption correction: part of the (Parkin et al., 1995)  model (ΔF) k = 0→17 
T_{min} = 0.42, T_{max} = 0.81  l = 0→9 
1754 measured reflections  2 standard reflections every 120 min 
1308 independent reflections  intensity decay: 1% 
Refinement on F^{2}  Primary atom site location: structureinvariant direct methods 
Leastsquares matrix: full  Secondary atom site location: difference Fourier map 
R[F^{2} > 2σ(F^{2})] = 0.037  w = 1/[σ^{2}(F_{o}^{2}) + (0.0451P)^{2}] where P = (F_{o}^{2} + 2F_{c}^{2})/3 
wR(F^{2}) = 0.089  (Δ/σ)_{max} = 0.001 
S = 1.07  Δρ_{max} = 0.78 e Å^{−}^{3} 
1308 reflections  Δρ_{min} = −0.68 e Å^{−}^{3} 
102 parameters  Extinction correction: SHELXL97 (Sheldrick, 2008) 
2 restraints  Extinction coefficient: 0.0009 (4) 
K_{0.53}Mn_{2.37}Fe_{1.24}(PO_{4})_{3}  V = 900.5 (6) Å^{3} 
M_{r} = 505.28  Z = 4 
Monoclinic, C2/c  Mo Kα radiation 
a = 12.272 (2) Å  µ = 6.07 mm^{−}^{1} 
b = 12.606 (2) Å  T = 293 K 
c = 6.416 (4) Å  0.43 × 0.09 × 0.02 mm 
β = 114.87 (2)° 
Enraf–Nonius TurboCAD4 diffractometer  1047 reflections with I > 2σ(I) 
Absorption correction: part of the (Parkin et al., 1995)  model (ΔF) R_{int} = 0.042 
T_{min} = 0.42, T_{max} = 0.81  2 standard reflections every 120 min 
1754 measured reflections  intensity decay: 1% 
1308 independent reflections 
R[F^{2} > 2σ(F^{2})] = 0.037  102 parameters 
wR(F^{2}) = 0.089  2 restraints 
S = 1.07  Δρ_{max} = 0.78 e Å^{−}^{3} 
1308 reflections  Δρ_{min} = −0.68 e Å^{−}^{3} 
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 
Refinement. Refinement of F^{2} against ALL reflections. The weighted Rfactor wR and goodness of fit S are based on F^{2}, conventional Rfactors R are based on F, with F set to zero for negative F^{2}. The threshold expression of F^{2} > σ(F^{2}) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. Rfactors based on F^{2} are statistically about twice as large as those based on F, and R factors based on ALL data will be even larger. 
x  y  z  U_{iso}*/U_{eq}  Occ. (<1)  
K  0.0000  −0.0116 (2)  0.2500  0.0229 (7)  0.531 (5) 
Mn1  0.0000  0.26376 (7)  0.2500  0.0156 (2)  
Fe2  0.22489 (4)  0.15521 (4)  0.13977 (8)  0.01004 (16)  0.6217 (6) 
Mn2  0.22489 (4)  0.15521 (4)  0.13977 (8)  0.01004 (16)  0.3783 (7) 
Mn3  −0.02360 (17)  0.49774 (17)  0.0393 (4)  0.0209 (5)  0.3064 (12) 
P1  0.0000  0.28642 (11)  −0.2500  0.0096 (3)  
O11  0.0479 (2)  0.2160 (2)  −0.0331 (4)  0.0138 (5)  
O12  −0.0942 (3)  0.3615 (2)  −0.2314 (6)  0.0251 (7)  
P2  0.24436 (8)  0.10810 (7)  0.63905 (15)  0.0093 (2)  
O21  0.2272 (2)  0.1782 (2)  0.8220 (4)  0.0156 (6)  
O22  0.1735 (3)  0.1629 (2)  0.4048 (4)  0.0169 (6)  
O23  0.3783 (2)  0.1019 (2)  0.6976 (6)  0.0255 (7)  
O24  0.1930 (2)  −0.0019 (2)  0.6349 (5)  0.0178 (6) 
U^{11}  U^{22}  U^{33}  U^{12}  U^{13}  U^{23}  
K  0.0093 (10)  0.0218 (13)  0.0260 (13)  0.000  −0.0040 (9)  0.000 
Mn1  0.0115 (4)  0.0202 (4)  0.0154 (4)  0.000  0.0058 (3)  0.000 
Fe2  0.0065 (2)  0.0138 (3)  0.0065 (3)  0.00016 (18)  −0.00043 (19)  0.0008 (2) 
Mn2  0.0065 (2)  0.0138 (3)  0.0065 (3)  0.00016 (18)  −0.00043 (19)  0.0008 (2) 
Mn3  0.0156 (12)  0.0164 (9)  0.0165 (11)  −0.0002 (9)  −0.0070 (7)  0.0014 (9) 
P1  0.0060 (5)  0.0140 (6)  0.0038 (5)  0.000  −0.0028 (4)  0.000 
O11  0.0087 (11)  0.0193 (13)  0.0066 (11)  −0.0027 (10)  −0.0033 (9)  0.0028 (10) 
O12  0.0149 (13)  0.0245 (15)  0.0301 (17)  0.0025 (12)  0.0037 (13)  −0.0119 (14) 
P2  0.0065 (4)  0.0123 (4)  0.0050 (4)  0.0004 (3)  −0.0016 (3)  0.0012 (3) 
O21  0.0128 (12)  0.0235 (14)  0.0078 (11)  −0.0030 (11)  0.0017 (10)  −0.0038 (11) 
O22  0.0244 (14)  0.0141 (14)  0.0048 (11)  0.0003 (11)  −0.0010 (10)  −0.0007 (10) 
O23  0.0114 (13)  0.0189 (15)  0.047 (2)  0.0003 (11)  0.0132 (14)  −0.0018 (15) 
O24  0.0127 (12)  0.0184 (14)  0.0171 (13)  −0.0013 (11)  0.0012 (11)  0.0030 (11) 
K—O24  2.608 (3)  Mn1—O22^{i}  2.315 (3) 
K—O24^{i}  2.608 (3)  Mn1—O22  2.315 (3) 
K—O24^{ii}  2.767 (3)  Fe2—O24^{ii}  1.970 (3) 
K—O24^{iii}  2.767 (3)  Fe2—O12^{x}  2.027 (3) 
K—O11^{iv}  2.869 (4)  Fe2—O22  2.049 (3) 
K—O11^{v}  2.869 (4)  Fe2—O21^{xi}  2.071 (3) 
K—O22  2.929 (3)  Fe2—O11  2.123 (3) 
K—O22^{i}  2.929 (3)  Fe2—O21^{vi}  2.167 (3) 
Mn3—O23^{vi}  2.253 (4)  P1—O12^{xii}  1.537 (3) 
Mn3—O23^{vii}  2.256 (4)  P1—O12  1.537 (3) 
Mn3—O12^{viii}  2.294 (4)  P1—O11^{xii}  1.544 (3) 
Mn3—O12  2.335 (4)  P1—O11  1.544 (3) 
Mn3—O23^{ix}  2.400 (4)  P2—O24  1.519 (3) 
Mn1—O23^{vi}  2.189 (3)  P2—O23  1.526 (3) 
Mn1—O23^{vii}  2.189 (3)  P2—O22  1.547 (3) 
Mn1—O11  2.215 (3)  P2—O21  1.553 (3) 
Mn1—O11^{i}  2.215 (3)  
O24—K—O24^{i}  174.63 (16)  O23^{vi}—Mn1—O11  86.39 (11) 
O24—K—O24^{ii}  73.24 (8)  O23^{vii}—Mn1—O11  118.95 (12) 
O24^{i}—K—O24^{ii}  106.42 (8)  O23^{vi}—Mn1—O11^{i}  118.95 (12) 
O24—K—O24^{iii}  106.42 (8)  O23^{vii}—Mn1—O11^{i}  86.39 (11) 
O24^{i}—K—O24^{iii}  73.24 (8)  O11—Mn1—O11^{i}  148.43 (15) 
O24^{ii}—K—O24^{iii}  172.95 (16)  O23^{vi}—Mn1—O22^{i}  159.42 (10) 
O24—K—O11^{iv}  114.87 (10)  O23^{vii}—Mn1—O22^{i}  85.06 (10) 
O24^{i}—K—O11^{iv}  70.34 (8)  O11—Mn1—O22^{i}  90.70 (10) 
O24^{ii}—K—O11^{iv}  87.10 (9)  O11^{i}—Mn1—O22^{i}  71.87 (10) 
O24^{iii}—K—O11^{iv}  99.26 (10)  O23^{vi}—Mn1—O22  85.06 (10) 
O24—K—O11^{v}  70.34 (8)  O23^{vii}—Mn1—O22  159.42 (10) 
O24^{i}—K—O11^{v}  114.87 (10)  O11—Mn1—O22  71.87 (10) 
O24^{ii}—K—O11^{v}  99.26 (10)  O11^{i}—Mn1—O22  90.70 (10) 
O24^{iii}—K—O11^{v}  87.10 (9)  O22^{i}—Mn1—O22  113.36 (14) 
O11^{iv}—K—O11^{v}  52.22 (11)  O24^{ii}—Fe2—O12^{x}  94.65 (12) 
O24—K—O22  53.32 (8)  O24^{ii}—Fe2—O22  86.05 (12) 
O24^{i}—K—O22  121.80 (11)  O12^{x}—Fe2—O22  109.44 (13) 
O24^{ii}—K—O22  57.49 (9)  O24^{ii}—Fe2—O21^{xi}  101.92 (12) 
O24^{iii}—K—O22  116.43 (11)  O12^{x}—Fe2—O21^{xi}  87.13 (12) 
O11^{iv}—K—O22  144.11 (8)  O22—Fe2—O21^{xi}  161.16 (11) 
O11^{v}—K—O22  122.58 (8)  O24^{ii}—Fe2—O11  101.11 (11) 
O24—K—O22^{i}  121.80 (11)  O12^{x}—Fe2—O11  162.62 (11) 
O24^{i}—K—O22^{i}  53.32 (8)  O22—Fe2—O11  79.18 (11) 
O24^{ii}—K—O22^{i}  116.43 (11)  O21^{xi}—Fe2—O11  82.52 (11) 
O24^{iii}—K—O22^{i}  57.49 (9)  O24^{ii}—Fe2—O21^{vi}  174.59 (11) 
O11^{iv}—K—O22^{i}  122.58 (8)  O12^{x}—Fe2—O21^{vi}  81.71 (11) 
O11^{v}—K—O22^{i}  144.11 (8)  O22—Fe2—O21^{vi}  91.37 (11) 
O22—K—O22^{i}  82.66 (13)  O21^{xi}—Fe2—O21^{vi}  81.97 (11) 
O23^{vi}—Mn3—O23^{vii}  75.91 (14)  O11—Fe2—O21^{vi}  83.04 (10) 
O23^{vi}—Mn3—O12^{viii}  84.68 (13)  O12^{xii}—P1—O12  104.0 (3) 
O23^{vii}—Mn3—O12^{viii}  121.62 (16)  O12^{xii}—P1—O11^{xii}  107.33 (17) 
Mn1^{viii}—Mn3—O12  76.0 (3)  O12—P1—O11^{xii}  114.23 (15) 
O23^{vi}—Mn3—O12  94.38 (14)  O12^{xii}—P1—O11  114.23 (15) 
O23^{vii}—Mn3—O12  79.85 (14)  O12—P1—O11  107.33 (17) 
O12^{viii}—Mn3—O12  157.20 (11)  O11^{xii}—P1—O11  109.7 (2) 
Mn1^{viii}—Mn3—O23^{ix}  69.7 (3)  O24—P2—O23  110.71 (16) 
O23^{vi}—Mn3—O23^{ix}  157.58 (11)  O24—P2—O22  109.32 (15) 
O23^{vii}—Mn3—O23^{ix}  123.93 (15)  O23—P2—O22  111.67 (18) 
O12^{viii}—Mn3—O23^{ix}  91.62 (14)  O24—P2—O21  110.22 (17) 
O12—Mn3—O23^{ix}  80.58 (13)  O23—P2—O21  108.50 (16) 
O23^{vi}—Mn1—O23^{vii}  78.61 (15)  O22—P2—O21  106.32 (16) 
Symmetry codes: (i) −x, y, −z+1/2; (ii) x, −y, z−1/2; (iii) −x, −y, −z+1; (iv) −x, −y, −z; (v) x, −y, z+1/2; (vi) −x+1/2, −y+1/2, −z+1; (vii) x−1/2, −y+1/2, z−1/2; (viii) −x, −y+1, −z; (ix) x−1/2, y+1/2, z−1; (x) x+1/2, −y+1/2, z+1/2; (xi) x, y, z−1; (xii) −x, y, −z−1/2. 
Experimental details
Crystal data  
Chemical formula  K_{0.53}Mn_{2.37}Fe_{1.24}(PO_{4})_{3} 
M_{r}  505.28 
Crystal system, space group  Monoclinic, C2/c 
Temperature (K)  293 
a, b, c (Å)  12.272 (2), 12.606 (2), 6.416 (4) 
β (°)  114.87 (2) 
V (Å^{3})  900.5 (6) 
Z  4 
Radiation type  Mo Kα 
µ (mm^{−}^{1})  6.07 
Crystal size (mm)  0.43 × 0.09 × 0.02 
Data collection  
Diffractometer  Enraf–Nonius TurboCAD4 
Absorption correction  Part of the refinement model (ΔF) (Parkin et al., 1995) 
T_{min}, T_{max}  0.42, 0.81 
No. of measured, independent and observed [I > 2σ(I)] reflections  1754, 1308, 1047 
R_{int}  0.042 
(sin θ/λ)_{max} (Å^{−}^{1})  0.703 
Refinement  
R[F^{2} > 2σ(F^{2})], wR(F^{2}), S  0.037, 0.089, 1.07 
No. of reflections  1308 
No. of parameters  102 
No. of restraints  2 
Δρ_{max}, Δρ_{min} (e Å^{−}^{3})  0.78, −0.68 
Computer programs: CAD4 EXPRESS (Enraf–Nonius, 1994), XCAD4 (Harms & Wocadlo, 1995), SIR92 (Altomare et al., 1993), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).
References
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The term alluaudite is referred to both natural and synthetic phosphates of compositions (Na^{+})(Na^{+}, Ca^{2+})(M^{2+})(Fe^{2+},Fe^{3+})_{2}(PO_{4})_{3} where M^{2+} 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}(PO_{4})_{3}. X1 and X2 are cationic sites available to large monovalent and divalent cations such as Na^{+} and Ca^{2+} while M1 and M2 are octahedral sites containing a distribution of divalent and trivalent cations of moderate size such as Mn^{2+}, Fe^{2+} or Fe^{3+}. 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}(PO_{4})_{3}. The crystal structure consists of M2_{2}O_{10} bioctahedral units of edgesharing M2O_{6} octahedra, sharing opposite edges with M1O_{6} 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 threedimensional framework with two sets of tunnels parallel to [001] (Fig. 1(b)).
The title phase K_{0.53}Mn_{2.37}Fe_{1.24}(PO_{4})_{3} was isolated during an attempt to synthesize KMnFe_{2}(PO_{4})_{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 cationoxygen 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 alluauditelike compound can be attributed to the small size of the Mn^{2+} 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 M1O_{6} 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 Mn^{2+} cations (Shannon, 1976). The M2—O distances and cis O—M2—O angles show the M2O_{6} octahedron to be less distorted than M1O_{6}. 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 Fe^{3+} and Mn^{2+} cations, respectively. This result confirms the presence of both atoms on the M(2) site. The PO_{4} 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 Fe^{3+} and Mn^{2+} 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 xray 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 xray analysis.