Copper(II) manganese(II) ortho­phosphate, Cu_0.5Mn_2.5(PO₄)₂

The title compound, Cu_0.5Mn_2.5(PO₄)₂, is a copper-manganese phosphate solid solution with the graftonite-type structure, viz. (Mn,Fe,Ca,Mg)₃(PO₄)₂. The structure has three distinct metal cation sites, two of which are occupied by Mn^II and one of which accommodates Cu^II. Incorporation of Cu^II into the structure distorts the coordination geometry of the metal cation site from five-coordinate square-pyramidal towards four-coordinate flattened tetra­hedral, and serves to contract the structure principally along the c axis.

S1. Comment

The copper–manganese phosphate solid solution, Cu_0.15Mn_2.85(PO₄)₂, with the graftonite-type structure, (Mn,Fe,Ca,Mg)₃(PO₄)₂ (Calvo, 1968), was first prepared by Nord (1982) via a reaction between the end members, Cu₃(PO₄)₂ and Mn₃(PO₄)₂, at 1073 K for one month in an evacuated silica-glass ampoule. The product material was analysed using powder X-ray diffraction, and the monoclinic unit-cell parameters at 298 K were reported as a = 8.803 (6), b = 11.454 (9) and c = 6.267 (5) Å, β = 99.00 (6)°, and V = 624.1 (12) ų. No information was obtained concerning the location of Cu^II within the structure. Apparently, Nord was unable to increase the solubility of Cu₃(PO₄)₂ within Mn₃(PO₄)₂ beyond 5 mol% under these preparative conditions, and he noted that `the low solubility of Cu^II in Mn/Cu orthophosphate is curious, considering this ion's large tendency to enter into very distorted environments'. We have been able to prepare a significantly more copper-rich composition, namely Cu_0.5Mn_2.5(PO₄)₂, corresponding to approximately 17 mol% Cu^II (in terms of the metal content), via a reaction between the constituent oxides at 1223 K under argon.

Since Fe and Mn are known to substitute for one another within the alluaudite family of minerals, represented by the general formula (Na,Ca,□)₂(Fe,Mn)₃(PO₄)₃ (Moore & Ito, 1979), it seemed reasonable to attempt preparation of the Mn analogue of the known Cu-containing alluaudite-type phase, Cu_1.35Fe₃(PO₄)₃ (Warner et al., 1993). However, attempts to prepare the composition Cu_1.5Mn₃(PO₄)₃ yielded a material for which the powder X-ray diffraction pattern showed no resemblance to that of the alluaudite structure, and instead indicated the presence of a major phase with the graftonite-type structure. Consequently, a single crystal was selected for further analysis.

The single-crystal analysis confirmed the graftonite-type structure of the title compound, Cu_0.5Mn_2.5(PO₄)₂. It consists of three different cation polyhedra linked through edge- and corner-sharing of O atoms from the PO₄^3- groups (Figs. 1 and 2). The Cu^II content is accommodated at the Mn2/Cu2 site. Atom Mn1 occupies an essentially six-coordinate distorted octahedral environment, with Mn1—O bond distances in the range 2.0923 (16)–2.4125 (17) Å (Table 1). A further contact of 2.7617 (17) Å is made to atom O3, which led Calvo (1968) to describe the coordination geometry of Mn1 as an `irregular pentagonal bipyramid'. Atom Mn3 occupies a five-coordinate environment with a geometry that approximates square-pyramidal (Addison parameter τ = 0.38; Addison et al., 1984), with atom O2<sup>vi</sup> at the apical position [symmetry code: (vi) 1 + x, y, z]. The Mn3—O bond distances are in the range 2.0359 (16)–2.1398 (16) Å (Table 1). For the mixed cation site, Mn2/Cu2, atom Mn2 has a distorted square-pyramidal geometry (τ = 0.18) made up of atoms O3, O4, O6<sup>iv</sup>, O7<sup>ii</sup> and O8<sup>v</sup> [symmetry codes: (ii) 1 - x, -y, 1 - z; (iv) x, y, -1 + z; (v) -x, -y, 1 - z], with O7<sup>ii</sup> at the apical position. The Mn2—O distances are in the range 1.885 (3)–2.491 (2) Å (Table 1). The Cu2 site lies close to that of Mn2, but shifted away from atom O3 to give a Cu2—O3 distance of 2.913 (2) Å. Thus, the coordination environment of Cu2 resembles more closely a flattened tetrahedron, with the bond distances to O4, O6<sup>iv</sup>, O7<sup>ii</sup> and O8<sup>v</sup> in the range 1.955 (2)–1.993 (2) Å. The incorporation of Cu<sup>II</sup> at the Mn2 site is consistent with the statement made by Calvo (1968) that `the coordination number of site II can be changed homogeneously from four to five by altering the chemical composition'.

The graftonite-type crystal structure of the end member manganese orthophosphate, Mn₃(PO₄)₂, has not been published to date, although two other crystal modifications have been described for this compound (Stephens & Calvo, 1969; El Bali et al., 2000; Massa et al., 2005). The unit-cell parameters for graftonite-type Mn₃(PO₄)₂ have been reported by Calvo (1968) to be a = 8.80 (1), b = 11.45 (2) and c = 6.25 (5) Å, β = 98.3 (2)°, and V = 623 (1) ų. Thus, the incorporation of 17 mol% Cu^II into Mn₃(PO₄)₂ in the title compound reduces the unit-cell volume by approximately 13 ų compared with Mn₃(PO₄)₂, which is slightly more than that observed for the incorporation of an equivalent mole fraction of Co^II (approximately 10 ų) or Fe^II (approximately 8 ų) (Nord, 1982), consistent with the expected cation sizes. The largest change in the unit-cell parameters compared with Mn₃(PO₄)₂ is observed for the c axis, which lies approximately parallel to the O4—Cu2/Mn2—O6^iv bonds (Fig. 1). This indicates that the change in coordination geometry from five-coordinate for Mn^II towards four-coordinate for Cu^II serves to pull together the O4 and O6 corners of neighbouring Mn1 polyhedra, thereby contracting the structure along this direction.

It is interesting to note that it has not been possible, so far, to prepare a copper–manganese phosphate with the alluaudite-type structure, nor indeed a copper–iron phosphate with the graftonite-type structure. It seems likely that the relative stability of Mn^II over Mn^III favours the coexistence of Cu^II and Mn^II in the title compound, compared with the requirement for mixed valency (Mn^II/Mn^III) within an alluaudite-type phase.

Related literature

For related literature, see: Addison et al. (1984); Calvo (1968); El Bali, Boukhari, Glaum, Gerk & Maass (2000); Massa et al. (2005); Moore & Ito (1979); Nord (1982); Stephens & Calvo (1969); Warner et al. (1993).

S2. Experimental

An attempt was made to prepare a 10 g specimen of the hypothetical copper–manganese orthophosphate Cu_1.5Mn₃(PO₄)₃, using a method similar to that described by Warner et al. (1993) for preparation of the analogous alluaudite-type phase, Cu_1.35Fe₃(PO₄)₃. It was considered plausible that the following reaction might occur at high temperature: 0.75Cu₂O + 1.5Mn₂O₃ + 1.5P₂O₅ → Cu_1.5Mn₃(PO₄)₃ + 0.5O₂(g). The chemical reagents were purchased from Sigma–Aldrich. Appropriate amounts of Cu₂O (97%), Mn₂O₃ (99%) and NH₄H₂PO₄ (99.99+%), corresponding to the composition Cu_1.5Mn₃(PO₄)₃, were ground together in an agate pestle and mortar. This mixture was placed in an alumina boat (SRX61, Almath Ltd), which was contained within a larger alumina boat (SRX110, Almath Ltd) in order to reduce the risk of spillage, and then heated under a flowing atmosphere of argon (99.99%, 0.5 l min^-1) in a Lenton LTF16/50/180 tube furnace (with appropriate gas flow attachments) to 673 K at a rate of 300 K h^-1. After 5 h at 673 K, the furnace was heated further to 1223 K at a rate of 300 K h^-1. After 7 h at 1223 K, the furnace was cooled to ambient temperature at a rate of 300 K/h^-1. Crystals of the title compund, Cu_0.5Mn_2.5(PO₄)₂, were formed exclusively? Was the intended product formed at all? Was a mix of several compounds obtained? How was the title compound separated?

S3. Refinement

Refinement of the structure with all metal sites assigned as Cu produces unreasonably small displacement ellipsoids for all P and O atoms. Refinement with all metal sites assigned as Mn provides an acceptable result for all P and O atoms and for atoms Mn1 and Mn3. For the Mn2 site, however, isotropic refinement as a single Mn atom leaves two peaks of ca 14 e Å^-3 in the difference electron density either side of the atom. Anisotropic refinement gives a prolate ellipsoid for Mn2 that appears significantly smaller than those of Mn1 and Mn3, and peaks of ca 2 e Å^-3 remain in the difference electron density. Refinement of the site as a single Cu atom is not possible, since it results in several non-positive definite atoms and a dramatic increase of the R factors. Refinement as a mixed Mn/Cu site, with the two atoms constrained to lie at the same position, does not provide any significant improvement over refinement solely as Mn, and the best result is obtained by allowing the Mn and Cu atoms to refine freely with site-occupancy factors constrained to sum to unity. The refined site-occupancy factors of 0.489 (9):0.511 (9) for Mn2:Cu2 do not differ significantly from 0.5, and they were therefore constrained to 0.5 for the final cycles of refinement. There is no clear evidence for any Cu content at the Mn1 or Mn3 sites, although trace occupancy cannot be excluded. The labelling scheme chosen for the atoms corresponds to that used by Calvo (1968), although the positions of the atoms within the asymmetric unit have been changed in order to minimize as far as possible the number of symmetry operators required in Fig. 1 and Table 1.

Computing details

Data collection: APEX2 (Bruker Nonius, 2004); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT (Bruker, 2003); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and ATOMS (Dowty, 1997).

Figures

	Fig. 1. The coordination environments of the three metal cation sites, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Atoms Mn2 and Cu2 have site-occupancy factor 0.5. (Symmetry codes correspond to those in Table 1.)
	Fig. 2. A polyhedral representation of the structure, viewed approximately along the c axis. In the electronic version of the journal, Mn1 polyhedra are red, Mn2/Cu2 polyhedra yellow, Mn3 polyhedra blue and PO4 polyhedra grey.

Copper(II) manganese(II) orthophosphate

Crystal data

Cu0.5Mn2.5(PO4)2	F(000) = 684
Mr = 359.06	Dx = 3.908 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 7320 reflections
a = 8.8428 (3) Å	θ = 2.9–28.4°
b = 11.5331 (4) Å	µ = 7.35 mm^−1
c = 6.0539 (2) Å	T = 298 K
β = 98.712 (2)°	Block, orange
V = 610.28 (4) Å^3	0.15 × 0.10 × 0.05 mm
Z = 4

Data collection

Bruker Nonius X8 APEXII CCD area-detector diffractometer	1519 independent reflections
Radiation source: fine-focus sealed tube	1429 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.026
thin–slice ω and ϕ scans	θmax = 28.4°, θmin = 3.5°
Absorption correction: multi-scan (SADABS; Sheldrick, 2003)	h = −11→11
Tmin = 0.467, Tmax = 0.696	k = −15→15
11353 measured reflections	l = −7→8

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.019	Secondary atom site location: difference Fourier map
wR(F^2) = 0.050	w = 1/[σ^2(Fo^2) + (0.0252P)^2 + 0.7868P] where P = (Fo^2 + 2Fc^2)/3
S = 1.09	(Δ/σ)max < 0.001
1519 reflections	Δρmax = 0.47 e Å^−3
127 parameters	Δρmin = −0.63 e Å^−3

Special details

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 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 (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Mn1	0.44619 (4)	0.11505 (3)	0.83631 (6)	0.01687 (9)
Mn2	0.2268 (2)	0.07582 (16)	0.3304 (4)	0.0148 (3)	0.50
Cu2	0.19592 (18)	0.04626 (13)	0.3257 (3)	0.0125 (2)	0.50
Mn3	0.86759 (4)	0.30779 (3)	0.61972 (5)	0.01218 (9)
P1	0.59366 (5)	0.13888 (5)	0.39187 (8)	0.00892 (11)
P2	0.10749 (6)	0.08236 (5)	0.80618 (8)	0.01092 (11)
O1	0.57187 (17)	0.06845 (13)	0.1746 (2)	0.0133 (3)
O2	−0.01553 (17)	0.17335 (14)	0.8278 (3)	0.0164 (3)
O3	0.45012 (19)	0.20849 (14)	0.4172 (3)	0.0191 (3)
O4	0.20291 (18)	0.11644 (15)	0.6247 (3)	0.0196 (3)
O5	0.72889 (19)	0.22124 (15)	0.3750 (3)	0.0194 (3)
O6	0.22673 (18)	0.07638 (16)	1.0179 (3)	0.0209 (4)
O7	0.63048 (17)	0.06365 (13)	0.6033 (2)	0.0131 (3)
O8	0.03078 (17)	−0.03736 (13)	0.7549 (2)	0.0150 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Mn1	0.01465 (17)	0.01020 (17)	0.02495 (18)	−0.00058 (12)	0.00043 (13)	−0.00299 (12)
Mn2	0.0161 (8)	0.0201 (9)	0.0080 (4)	0.0054 (5)	0.0007 (5)	−0.0001 (5)
Cu2	0.0097 (5)	0.0200 (7)	0.0074 (3)	0.0055 (4)	0.0005 (3)	0.0003 (4)
Mn3	0.01220 (15)	0.01308 (16)	0.01137 (15)	−0.00434 (11)	0.00213 (11)	−0.00239 (11)
P1	0.0089 (2)	0.0075 (2)	0.0106 (2)	−0.00080 (17)	0.00200 (17)	0.00017 (17)
P2	0.0093 (2)	0.0121 (3)	0.0114 (2)	0.00237 (18)	0.00160 (17)	−0.00030 (18)
O1	0.0164 (7)	0.0110 (7)	0.0121 (7)	0.0005 (6)	0.0012 (5)	−0.0013 (5)
O2	0.0144 (7)	0.0161 (8)	0.0190 (7)	0.0065 (6)	0.0030 (6)	0.0011 (6)
O3	0.0209 (8)	0.0149 (8)	0.0237 (8)	0.0082 (6)	0.0107 (6)	0.0037 (6)
O4	0.0159 (7)	0.0242 (9)	0.0200 (8)	−0.0012 (6)	0.0072 (6)	0.0048 (6)
O5	0.0199 (8)	0.0214 (8)	0.0169 (7)	−0.0131 (7)	0.0032 (6)	−0.0001 (6)
O6	0.0125 (7)	0.0331 (10)	0.0161 (7)	0.0059 (7)	−0.0016 (6)	−0.0060 (7)
O7	0.0146 (7)	0.0127 (7)	0.0117 (7)	−0.0002 (6)	0.0009 (5)	0.0018 (5)
O8	0.0159 (7)	0.0122 (7)	0.0165 (7)	0.0000 (6)	0.0013 (6)	−0.0001 (6)

Geometric parameters (Å, º)

Mn1—O1^i	2.2425 (15)	P1—O3	1.5291 (16)
Mn1—O1^ii	2.1226 (15)	P1—O5	1.5426 (16)
Mn1—O3^iii	2.0923 (16)	P1—O7	1.5397 (15)
Mn1—O4	2.3314 (17)	P2—O2	1.5312 (16)
Mn1—O6	2.4125 (17)	P2—O4	1.5346 (16)
Mn1—O7	2.3861 (15)	P2—O6	1.5331 (16)
Mn2—O3	2.491 (2)	P2—O8	1.5490 (16)
Mn2—O4	1.885 (3)	O1—Mn1^ii	2.1226 (15)
Mn2—O6^iv	1.891 (3)	O1—Mn1^iv	2.2424 (15)
Mn2—O7^ii	2.047 (3)	O2—Mn3^ix	2.1597 (16)
Mn2—O8^v	2.301 (2)	O2—Mn3^x	2.1893 (16)
Cu2—O7^ii	1.987 (2)	O3—Mn1^xi	2.0923 (16)
Cu2—O6^iv	1.955 (2)	O5—Mn3^xi	2.1398 (16)
Cu2—O4	1.975 (2)	O6—Mn2^i	1.891 (3)
Cu2—O8^v	1.993 (2)	O6—Cu2^i	1.955 (2)
Mn3—O2^vi	2.1597 (16)	O7—Mn2^ii	2.047 (3)
Mn3—O2^vii	2.1892 (16)	O7—Cu2^ii	1.987 (2)
Mn3—O5	2.0359 (16)	O8—Mn2^v	2.301 (2)
Mn3—O5^iii	2.1398 (16)	O8—Cu2^v	1.993 (2)
Mn3—O8^viii	2.0896 (15)	O8—Mn3^xii	2.0896 (15)
P1—O1	1.5331 (15)
O3^iii—Mn1—O1^ii	167.31 (6)	O1—P1—O7	113.45 (9)
O3^iii—Mn1—O1^i	91.78 (6)	O3—P1—O5	110.30 (10)
O1^ii—Mn1—O1^i	79.28 (6)	O1—P1—O5	105.51 (9)
O3^iii—Mn1—O4	95.92 (6)	O7—P1—O5	109.49 (9)
O1^ii—Mn1—O4	86.04 (6)	O2—P2—O6	111.53 (9)
O1^i—Mn1—O4	142.37 (6)	O2—P2—O4	111.13 (9)
O3^iii—Mn1—O7	113.04 (6)	O6—P2—O4	103.13 (9)
O1^ii—Mn1—O7	77.81 (5)	O2—P2—O8	109.44 (9)
O1^i—Mn1—O7	101.07 (5)	O6—P2—O8	110.62 (9)
O4—Mn1—O7	109.44 (5)	O4—P2—O8	110.88 (9)
O3^iii—Mn1—O6	93.39 (6)	P1—O1—Mn1^ii	123.65 (9)
O1^ii—Mn1—O6	76.58 (6)	P1—O1—Mn1^iv	129.57 (9)
O1^i—Mn1—O6	82.01 (6)	Mn1^ii—O1—Mn1^iv	100.72 (6)
O4—Mn1—O6	60.84 (5)	P2—O2—Mn3^ix	136.52 (10)
O7—Mn1—O6	153.13 (6)	P2—O2—Mn3^x	124.43 (9)
O4—Mn2—O6^iv	164.03 (15)	Mn3^ix—O2—Mn3^x	99.02 (6)
O4—Mn2—O7^ii	99.43 (12)	P1—O3—Mn1^xi	118.15 (9)
O6^iv—Mn2—O7^ii	96.06 (12)	P1—O3—Mn2	107.46 (10)
O4—Mn2—O8^v	90.65 (10)	Mn1^xi—O3—Mn2	124.31 (9)
O6^iv—Mn2—O8^v	85.76 (10)	P2—O4—Mn2	139.97 (12)
O7^ii—Mn2—O8^v	116.92 (9)	P2—O4—Cu2	127.37 (11)
O4—Mn2—O3	81.17 (9)	P2—O4—Mn1	99.58 (8)
O6^iv—Mn2—O3	95.11 (9)	Mn2—O4—Mn1	106.96 (9)
O7^ii—Mn2—O3	89.75 (9)	Cu2—O4—Mn1	113.52 (9)
O8^v—Mn2—O3	153.14 (12)	P1—O5—Mn3	129.89 (9)
O6^iv—Cu2—O4	144.21 (10)	P1—O5—Mn3^xi	119.10 (9)
O6^iv—Cu2—O7^ii	96.02 (10)	Mn3—O5—Mn3^xi	104.73 (7)
O4—Cu2—O7^ii	98.47 (10)	P2—O6—Mn2^i	137.07 (12)
O6^iv—Cu2—O8^v	93.21 (10)	P2—O6—Cu2^i	128.87 (11)
O4—Cu2—O8^v	97.83 (10)	P2—O6—Mn1	96.29 (8)
O7^ii—Cu2—O8^v	137.37 (9)	Mn2^i—O6—Mn1	125.32 (9)
O5—Mn3—O8^viii	149.52 (7)	Cu2^i—O6—Mn1	134.85 (9)
O5—Mn3—O5^iii	95.79 (6)	P1—O7—Cu2^ii	126.76 (10)
O8^viii—Mn3—O5^iii	97.67 (6)	P1—O7—Mn2^ii	130.73 (11)
O5—Mn3—O2^vi	104.76 (7)	P1—O7—Mn1	105.91 (8)
O8^viii—Mn3—O2^vi	105.09 (6)	Cu2^ii—O7—Mn1	127.23 (8)
O5^iii—Mn3—O2^vi	75.08 (6)	Mn2^ii—O7—Mn1	122.37 (9)
O5—Mn3—O2^vii	76.54 (6)	P2—O8—Cu2^v	119.25 (10)
O8^viii—Mn3—O2^vii	88.77 (6)	P2—O8—Mn3^xii	122.54 (9)
O5^iii—Mn3—O2^vii	172.29 (6)	Cu2^v—O8—Mn3^xii	113.98 (8)
O2^vi—Mn3—O2^vii	107.42 (6)	P2—O8—Mn2^v	127.25 (10)
O3—P1—O1	111.46 (9)	Mn3^xii—O8—Mn2^v	105.87 (8)
O3—P1—O7	106.66 (9)

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