Single Crystal X-Ray Structural Investigation of Alluaudite Related Monophosphate Na2FeMn2(PO4)3

The compound Na2FeMn2(PO4)3 has been successfully isolated with the alluaudite structural type. Accurate single crystal x-ray diffraction has allowed solving the structure with reliability factors of R1 and Rw equal to 0.0322 and 0.0790 respectively. It was found that the symmetry is monoclinic with a space group of C2/c and lattice parameters: a = 12.180(2) Å, b = 12.660(2) Å, c = 6.500(2) Å, = 114.528(3)(°), unit cell volume = 911.8(3) Å, Z = 8 and dcal.=3.618 g.cm. Three-dimensional network is formed by the [MnO6] octahedra linked in pairs to form Mn-based octahedral dimers: ([Mn2O10]). Each dimer shares six vertices with six tetrahedra [P(2)O4] to form sheets within the plane (100). The latter are connected by tetrahedra [P(1)O4] delimiting cages and tunnels which house either Fe or Na cations. Each [FeO6] octahedron is linked to two [Mn2O10] dimers belonging to two adjacent sheets to form mixed Fe-Mn chains of the type: Fe Mn Mn Fe Mn Mn Fe ..., running along the direction [101].

In recent years, this family of minerals has been subject to various crystal-chemical analysis and new compounds have been synthesized. For e x a m p l e H a t e r t e t a l . [ 6 -8 ] h a v e r e f i n e d t h e structures from powder x-ray diffraction data of various phases by Reitveld method. It is also worth to mention the structural refinements reported by Daidouch et al. [9,10] using also powder diffraction patterns: XRD for the compounds Na 2 FeMn 2 (PO 4 ) 3 and NaAgFeMn 2 (PO 4 ) 3 and neutron for Ag 2 FeMn 2 (PO 4 ) 3 , by Daidouh et al. These Authors have found a rather high reliability factors of order of 9, 11 respectively for the pure silver and sodium phase and even a higher value of 15.4 for NaAgFeMn 2 (PO 4 ) 3 .
The title compound and related solid solutions have been more recently investigated for Li-Na batteries applications and some structural refinements were made by the same Authors, using powder Rietveld method [11,12]. The reliability Daidouch et al. [9,10]. To the best of our knowledge no single crystal structural resolution of the sodium iron manganese alluaudite phosphates has been yet performed. The purpose of the present work is to give reliable and accurate structural data from x-ray structural determination on single crystal of Na 2 FeMn 2 (PO 4 ) 3 .

Experimental
The title compound has been isolated from the melt of a stoichiometric mixture of the starting materials Bi 2 O 3 , MnCO 3 , Fe 2 O 3 and NH 4 H 2 PO 4 , taken in the following molar ratio: 1:2:1:8. The melt was slowly cooled down to room temperature. Pure purple single crystals have been collected from the water washed solid preparation. As the melt is achieved within a porcelain crucible, the latter has served as reservoir of sodium which has not been added within the starting compounds mixture. X-ray crystallography has shown that the XRD patterns of ground crystals can be totally indexed iso-typically with the alluaudite structural type. After evidence of alluaudite-like single crystals in the cooled melt, we have considered worth to resolve the crystal structure in order to get more information about the atomic arrangement within the lattice and check more particularly the originality of the chemical bonds in this newly synthesized material.
Therefore, a high quality single crystal of Na 2 FeMn 2 (PO 4 ) 3 with a needle like and dimension of (0.02 x 0.02 x 0.07) mm 3 was selected under optical microscope before to be mounted on a goniometric head using a glass fiber to fix the crystal. The diffraction intensities were collected at 293K using a Nonius four-circle diffractometer equipped with a Kappa CCD and a molybdenum anode giving x-ray wavelenth (K (Mo ) = 0.71073 Å.

Structure Refinement
Before any further analysis, the diffraction patterns have been corrected for: i) Lorentz factor; ii) polarization; iii) absorption effect [13]. The struc tu re o f Na 2 FeMn 2 (PO 4 ) 3 was determined by the direct method using the program SIR 97 [14] and refined by the method of least squares using the program SHELX-97 [15]. The refinement of this structure was made from1053 independent reflections (I> 2 (I)) over 4250 recorded reflections. The reliability factors obtained after refinement of all parameters as well as isotropic (& anisotropic) temperature factors were found to be e q u a l t o R = 3 % a n d R w = 8 % . T a b l e 1 summarizes the crystallography data and the conditions of diffraction intensities recording. The atomic coordinates and the corresponding isotropic temperature factors are given in Table 2 while  Table 3 recapitulates the main interatomic distances and bond angles of Na 2 FeMn 2 (PO 4 ) 3 structure.  __________________________________________________________________________ Table 3 Main inter-atomic distances (Å) and bond angles (°) of Na 2 FeMn 2 (PO 4 ) 3 structure Projections of the structure of Na 2 FeMn 2 (PO 4 ) 3 along various directions are given in Figs.1-3. Sodium ions occupy large sites X (1) and X (2) while the sites M (1) and M(2) are fully occupied by transition element cations Fe 3+ a n d M n 2+ respectively. As evidenced from these different views, the lattice of Na 2 FeMn 2 (PO 4 ) 3 can be seen as made of sheets (Fig. 2) (Fig. 3). Each dimmer is sharing six vertices with six neighbour [P(2)O 4 ] tetrahedra (Fig. 2) to form foils like planes parallel to (100). As shown in Figs. 1 & 3, the connexions by tetrahedra [P(1)O 4 ] allows to delimit cages and tunnels housing Fe 3+ a n d N a + respectively. These values are consistent with those generally given in the literature [6 -10]. Although, the main P O distance is the same in the two tetrahedra (), it is worth to mention that P O distance is more distorted within [P(2)O 4 ] than in [P(1)O 4 ], which shows that the mechanical tensions are more stronger within the layers parallel to (100), than between these sheets.   Regarding the sites M(1) and M(2) fully occupied by Fe(III) and Mn(II) respectively, the Fe 3+ environment is less distorted (three different Fe O bond lengths: 2.286, 2.320 and 2?329Å) than that of Mn 2+ , where all Mn O bonds are different (Table3). This is also consistent with higher tensions within the layers than between the layers as Mn O bonds constitute the layers while Fe 3+ ions link between the layers.
It also appears that as expected, phosphorus develops a rather strong covalent bond within the oxygen tetrahedral environment in both sites as the mean P O distance deduced from the present structure study ( Table 3) is lower that the sum of ionic radii: < P O > = 1.538 Å while [r(P 5+ ) + r(O 2-)] = 1.57 Å, according the ionic radii values due to Shannon and Prewitt [16].