Ca₃Mn₂O₇

The tricalcium dimanganese heptaoxide (Ca₃Mn₂O₇) member of the Ruddlesden–Popper series Ca_n+1Mn_nO_3n+1, i.e. with n = 2, was previously reported with an I-centred tetragonal lattice [a_t = 3.68 and c_t = 19.57 Å] by Fawcett, Sunstrom, Greenblatt, Croft & Ramanujachary [Chem. Mater. (1998), 10, 3643–3651]. It is now found to be orthorhombic, with an A-­centred lattice [a = 5.2347 (6), b = 5.2421 (2) and c = 19.4177 (19) Å]. The structure has been refined in space group A2₁am using X-ray single-crystal diffraction data and assuming the existence of twin domains related by the (10) plane. A comparison with the basic perovskite structure CaMnO₃ (n = ∞) is proposed.

S1. Comment

In order to complete magnetic and electrical measurements on the magnetoresistive manganese oxide perovskite families Ln_{1 - x}Ca_xMnO₃ (Ln is a rare earth), a structural study of these compounds has been developed, including the end compound CaMnO₃ (x = 1). From the same CaMnO₃ preparation, single crystals of a second compound were isolated, exhibiting cell parameters clearly different from those known for CaMnO₃ (Poeppelmeier et al., 1982; Taguchi et al., 1989; Aliaga et al., 2000). Scanning electron microscopy measurements, coupled with EDS (please define) analysis, clearly lead to a Ca₃Mn₂ cationic composition.

The cell parameters of Ca₃Mn₂O₇ are consistent with either an orthorhombic or a tetragonal lattice. They depend both on the a_p cubic parameter (a_p = 3.72 Å) of the basic CaMnO₃ perovskite cell, with CaMnO₃ representing the n = ∞ member of the Ruddlesden-Popper series (Ruddlesden & Popper, 1958), and on the face-centred cubic cell of CaO {a_O = 4.8 Å [Tanida & Kitamura, 1981; ref. 41–0421, International Centre for Diffraction Data (ICDD, 1999)]}. The parameters found in the present work differ from those previously published for Ca₃Mn₂O₇ (a_t ≈ a_p, c_t ≈ 4a_p + 2a_O) (MacChesney et al., 1967; Tanida & Kitamura, 1981; Fawcett et al., 1998) by the relation a ≈ b ≈ a_t2^1/2, c ≈ c_t.

The symmetry of the present crystal was carefully scrutinized from both Laue diagrams (precession camera) and the intensity distribution in the X-ray diffraction data. The actual Laue symmetry is mmm rather than 4/mmm, as shown by the Laue diffraction pattern, and is confirmed from the R_int values of 4.49% and 9.73% calculated assuming orthorhombic and tetragonal symmetry, respectively. Moreover, some significant reflections of the type hkl, where h + k = 2n + 1, were observed and cannot be explained in the tetragonal model.

The present reflection conditions are consistent with the centrosymmetric space group Amam, but a satisfactory R factor could not be obtained with this symmetry. A new solution could be initiated using the direct method calculation program SIR97 (Altomare et al., 1999) in the non-centrosymmetric space group A2₁am (No 36). The standard setting of this group is Cmc2₁, but we adopted the non-standard setting in order to keep the pseudo-tetragonal cell along the c axis. This space group has already been proposed for the Ca₃Ti₂O₇ structure by Elcombe et al. (1991) and for La_2–2xCa_{1 + 2x}Mn₂O₇ by Bendersky et al. (2001).

The atomic positions were refined to R = 0.0228 using the JANA2000 structural refinement program (Petříček & Dušek, 2000) with anisotropic displacement parameters for all the atoms and assuming the existence of twin domains related by the (110) plane, due to the similarity between the a and b parameters, with reference to a pseudo-tetragonal cell. The twin ratio was found to be equal to 0.18. This twin model leads to a significant improvement of the R-factor (0.0317 without a twin).

The corresponding structure, with Ca₁ and O₁ atoms in 4a crystallographic sites and the other atoms in 8 b sites, is shown in Fig. 1a. It consists of a stacking of two layers formed by corner-sharing MnO₆ octahedra, separated by a double Ca—O layer. This description is consistent with the usual description of the Ruddlesden-Popper Ca_n+1Mn_nO_3n+1 family, which can also be represented by the formula CaO[CaMnO₃]_n, where n is the number of MnO₆ octahedra layers.

Three types of polyhedra are present in this structure, one per cation, i.e. Mn⁴⁺, Ca₁²⁺ and Ca₂²⁺. The Mn⁴⁺ ions are octahedrally coordinated, and the Mn—O bond distances in the equatorial plane range from 1.856 (5) to 1.899 (5) Å, with apical distances of 1.904 (1) and 1.9193 (4) Å. The corresponding average Mn—O distance is 1.890 (3) Å. Angles within the MnO₆ octahedra range from 88.7 (1) to 92.0 (2)° for O—Mn—O with cis O atoms, and from 177.9 (2) to 178.4 (1)° for O—Mn—O with trans O atoms.

Comparing the MnO₆ octahedra in Ca₃Mn₂O₇ with those in CaMnO₃ (Poeppelmeier et al., 1982), we note that the Mn—O distances in the equatorial plane of the octahedra are shorter in Ca₃Mn₂O₇, while the apical distances are larger, leading to an elongated octahedron in Ca₃Mn₂O₇ instead of a compressed one in CaMnO₃ (1.865 Å for the apical distances, and 1.900 and 1.903 Å for the equatorial ones).

The atoms Ca₁²⁺, at z = 0 and z = 1/2, are 12-fold coordinated (usual perovskite coordination), while the atoms Ca₂²⁺ are ninefold coordinated. Both Ca₁ and Ca₂ atoms belong to similar CaO layers orthogonal to c. The O—O distances in these layers, represented by dashed lines in Fig. 1 b, clearly show the difference from tetragonal symmetry, due to the MnO₆ octahedral distortion and tilting, which are forbidden in tetragonal symmetry. This projection clearly shows the analogy with the actual Pnma symmetry of the CaMnO₃ structure.

The average Ca—O distances are 2.646 (4) Å for the Ca₁ polyhedra and 2.554 (3) Å for the Ca₂ polyhedra, whereas the average Ca—O distance in CaMnO₃ is 2.652 Å. Two short Ca₂—O distances (<2.3 Å) are observed (Table 1). The average Mn—O and Ca—O distances are in good agreement with those predicted by the ionic radii calculated by Shannon (1976), with r_Mn⁴⁺ = 0.53, r_Ca(1)²⁺ = 1.34, r_Ca(2)²⁺ = 1.18 and r_O^2- = 1.35 Å. Nevertheless, they are shorter than in the parent phase CaMnO₃, but longer than in CaO.

So, the present structure can be interpreted as the alternate stacking of reduced CaMnO₃-type layers and of expanded CaO-type layers. The principal difference from the structure described by Fawcett et al. (1998) results in Mn polyhedra having Mn—O distances differing by ±0.03 Å from those calculated using the Shannon radii, contrasting with apical Mn—O distance of 2.09 Å with the O atom directed towards the CaO layer. This could be related to the alternate tilt of MnO₆ octahedra, mainly around the x and z axes (Fig. 1a and b), of 6.8 and 8.3°, respectively, using the formulae of Elcombe et al. (1991). These tilt angles, characterized by Mn—O₁—Mn 166.5 (1), Mn—O₂—Mn 158.9 (2) and Mn—O₃—Mn 162.5 (2)°, are quite compatible with the corresponding angles in CaMnO₃.

Simulations of X-ray diffraction powder patterns with JANA2000 (Fig. 2) in both models show very small differences. This outlines, in the present case, the difficulty of refining the structure with standard X-ray powder diffraction patterns.

S2. Experimental

The initial sample preparation consisted of a mixture of CaO, prepared by decarbonation of CaCO₃ at 1273 K, and MnO₂ (Aldrich) in stoichiometric proportions, to produce CaMnO₃. The mixture was heated to 1273 K and then crushed three times in succession Query, so as to obtain a good sample homogeneity, and was then compressed in an isostatic press at 3 × 10 ⁷ Pa (is this the correct conversion from 3 tons/cm²? Are metric tonnes intended?) in the form of a rod 5 × 50 mm, before sintering at 1673 K for 12 h in air. Crystal growth was carried out in a four-mirror optical floating-zone furnace (Crystal Systems Inc. FZT 10000 H III P). The samples were set to rotate in opposite directions at 20 revolutions per minute and were grown in an oxygen flow at atmospheric pressure, at a feeding speed of 10 mm/h. It is noteworthy to state that the previous ceramic synthesis of Ca₃Mn₂O₇ could only be performed under a high pressure of oxygen (3200 psi; 1 psi ≈ 6.895 × 10 ³ Pa) (MacChesney et al., 1967).

Computing details

Data collection: CAD-4 Software (Enraf-Nonius, 1994); cell refinement: CAD-4 Software; data reduction: JANA2000 (Petříček & Dušek, 2000); program(s) used to solve structure: Please provide missing details; program(s) used to refine structure: JANA2000; molecular graphics: ATOMS (Dowty, 1997); software used to prepare material for publication: JANA2000.

Figures

	Fig. 1. a) The structure of Ca3Mn2O7 in the (100) plane. b) The O—O distances viewed along the c axis show the difference from tetragonal symmetry.
	Fig. 2. Simulated X-ray diffraction patterns for Ca3Mn2O7 in the tetragonal (x) and orthorhombic (—) models. The difference plot is represented at the bottom.

tricalcium dimanganese heptaoxide

Crystal data

Ca3Mn2O7	Dx = 4.266 Mg m^−3
Mr = 342.1	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, A21am	Cell parameters from 25 reflections
a = 5.2347 (6) Å	θ = 11.0–24.0°
b = 5.2421 (2) Å	µ = 7.61 mm^−1
c = 19.4177 (19) Å	T = 298 K
V = 532.83 (8) Å^3	Prism, black
Z = 4	0.12 × 0.07 × 0.02 mm
F(000) = 664

Data collection

Enraf-nonius CAD-4 diffractometer	Rint = 0.045
θ/2θ scans	θmax = 50.0°, θmin = 2.1°
Absorption correction: gaussian (JANA2000; Petříček & Dušek, 2000)	h = −11→11
Tmin = 0.619, Tmax = 0.865	k = −11→11
10766 measured reflections	l = −41→41
1516 independent reflections	3 standard reflections every 60 min
745 reflections with I > 3σ(I)	intensity decay: 0.2%

Refinement

Refinement on F	Weighting scheme based on measured s.u.'s w = 1/σ^2(F)
R[F^2 > 2σ(F^2)] = 0.023	(Δ/σ)max < 0.001
wR(F^2) = 0.015	Δρmax = 1.39 e Å^−3
S = 1.45	Δρmin = −1.13 e Å^−3
1516 reflections	Absolute structure: (Flack, 1983)
59 parameters	Absolute structure parameter: 0.45 (6)

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Mn	0.3033 (7)	0.2507 (3)	0.098160 (10)	0.0185 (4)
Ca1	0.8276 (8)	0.2505 (5)	0.0	0.0383 (10)
Ca2	0.7907 (8)	0.2457 (3)	0.18642 (2)	0.0386 (8)
O1	0.3031 (7)	0.2937 (4)	0.0	0.051 (5)
O2	0.0954 (9)	0.9618 (6)	0.08924 (8)	0.024 (4)
O3	0.0221 (9)	0.4649 (6)	0.10545 (9)	0.041 (5)
O4	0.3101 (10)	0.2176 (4)	0.19583 (5)	0.051 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Mn	0.00362 (9)	0.00376 (10)	0.00390 (6)	0.0004 (3)	0.00067 (11)	0.0001 (2)
Ca1	0.0071 (4)	0.0078 (2)	0.00703 (14)	−0.0025 (8)	0.0	0.0
Ca2	0.00889 (18)	0.00776 (19)	0.00520 (8)	−0.0017 (6)	0.00032 (16)	0.0007 (3)
O1	0.0106 (10)	0.0103 (12)	0.0045 (4)	0.0011 (16)	0.0	0.0
O2	0.0037 (8)	0.0049 (9)	0.0103 (6)	−0.0017 (5)	0.0010 (6)	−0.0019 (6)
O3	0.0066 (9)	0.0084 (11)	0.0121 (6)	0.0028 (6)	0.0013 (8)	−0.0007 (8)
O4	0.0093 (7)	0.0104 (11)	0.0044 (4)	−0.0003 (12)	0.0011 (9)	0.0007 (3)

Geometric parameters (Å, º)

Mn—O1	1.9193 (4)	Ca1—O3^iii	2.548 (3)
Mn—O2^i	1.873 (4)	Ca1—O3^v	2.996 (4)
Mn—O2^ii	1.900 (5)	Ca1—O3^viii	2.548 (3)
Mn—O3	1.857 (5)	Ca1—O3^ii	2.996 (4)
Mn—O3^ii	1.885 (4)	Ca2—O2^vi	2.884 (4)
Mn—O4	1.9048 (10)	Ca2—O2^ii	2.406 (3)
Ca1—O1	2.755 (5)	Ca2—O3^iii	2.293 (4)
Ca1—O1^iii	2.499 (5)	Ca2—O3^ii	2.598 (4)
Ca1—O1^iv	2.856 (3)	Ca2—O4	2.526 (4)
Ca1—O1^v	2.393 (3)	Ca2—O4^iii	2.730 (4)
Ca1—O2^vi	2.694 (4)	Ca2—O4^ix	2.2968 (11)
Ca1—O2^v	2.391 (3)	Ca2—O4^x	2.438 (2)
Ca1—O2^vii	2.694 (4)	Ca2—O4^ii	2.821 (2)
Ca1—O2^ii	2.391 (3)
O1—Mn—O2^i	90.16 (12)	O3^iii—Ca2—O4^x	121.90 (14)
O1—Mn—O2^ii	88.78 (14)	O3^iii—Ca2—O4^ii	61.68 (9)
O1—Mn—O3	90.24 (14)	O3^ii—Ca2—O4	62.55 (13)
O1—Mn—O3^ii	88.97 (13)	O3^ii—Ca2—O4^iii	127.68 (14)
O1—Mn—O4	178.15 (18)	O3^ii—Ca2—O4^ix	125.28 (13)
O2^i—Mn—O2^ii	89.15 (19)	O3^ii—Ca2—O4^x	130.52 (13)
O2^i—Mn—O3	92.0 (2)	O3^ii—Ca2—O4^ii	58.43 (8)
O2^i—Mn—O3^ii	177.9 (2)	O4—Ca2—O4^iii	169.74 (6)
O2^i—Mn—O4	91.69 (10)	O4—Ca2—O4	88.69 (10)
O2^ii—Mn—O2^i	89.15 (19)	O4—Ca2—O4^x	88.75 (11)
O2^ii—Mn—O3	178.48 (18)	O4—Ca2—O4^ii	95.13 (11)
O2^ii—Mn—O3^ii	88.9 (2)	O4^iii—Ca2—O4^ix	83.89 (10)
O2^ii—Mn—O4	91.27 (12)	O4^iii—Ca2—O4^x	84.23 (11)
O3—Mn—O3^ii	89.87 (19)	O4^iii—Ca2—O4^ii	90.77 (10)
O3—Mn—O4	89.67 (11)	O4^ix—Ca2—O4^x	90.40 (7)
O3^ii—Mn—O4	89.19 (11)	O4^ix—Ca2—O4^ii	81.40 (7)
O1—Ca1—O1^iii	170.09 (13)	O4^x—Ca2—O4^ii	170.82 (9)
O1—Ca1—O1^iv	92.14 (14)	Mn—O1—Mn^xi	166.51 (14)
O1—Ca1—O1^v	82.21 (14)	Mn—O1—Ca1^xii	89.42 (15)
O1—Ca1—O2^vi	124.39 (13)	Mn—O1—Ca1^xiii	83.26 (8)
O1—Ca1—O2^v	62.08 (14)	Mn—O1—Ca1^xiv	96.73 (8)
O1—Ca1—O3^iii	111.22 (14)	Ca1^xii—O1—Ca1	170.09 (12)
O1—Ca1—O3^v	55.05 (11)	Ca1^xii—O1—Ca1^xiii	87.38 (13)
O1^iii—Ca1—O1^iv	97.77 (14)	Ca1^xii—O1—Ca1^xiv	98.27 (16)
O1^iii—Ca1—O1^v	87.87 (15)	Ca1—O1—Ca1^xiii	82.71 (13)
O1^iii—Ca1—O2^vi	62.14 (13)	Ca1—O1—Ca1^xiv	91.64 (15)
O1^iii—Ca1—O2^v	123.25 (14)	Ca1^xiii—O1—Ca1^xiv	174.4 (2)
O1^iii—Ca1—O3^iii	64.03 (13)	Ca1^xiv—O1—Ca1^xii	98.27 (16)
O1^iv—Ca1—O1	92.14 (14)	Ca1^xiv—O1—Ca1	91.64 (15)
O1^iv—Ca1—O1^v	174.4 (2)	Mn^xv—O2—Mn^xvi	159.0 (2)
O1^iv—Ca1—O2^vi	57.47 (10)	Mn^xv—O2—Ca1^xvii	84.70 (14)
O1^iv—Ca1—O2^v	60.81 (10)	Mn^xv—O2—Ca1^xiv	98.5 (2)
O1^iv—Ca1—O3^iii	117.30 (10)	Mn^xv—O2—Ca2^xvii	81.00 (14)
O1^iv—Ca1—O3^v	118.26 (12)	Mn^xv—O2—Ca2^xvi	92.64 (16)
O1^v—Ca1—O2^vi	126.07 (13)	Mn^xvi—O2—Ca1^xvii	88.24 (18)
O1^v—Ca1—O2^v	115.94 (14)	Mn^xvi—O2—Ca1^xiv	101.67 (15)
O1^v—Ca1—O3^iii	65.23 (10)	Mn^xvi—O2—Ca2^xvii	78.33 (15)
O1^v—Ca1—O3^v	58.27 (10)	Mn^xvi—O2—Ca2^xvi	90.31 (14)
O2^vi—Ca1—O2^v	117.98 (14)	Ca1^xvii—O2—Ca1^xiv	93.43 (9)
O2^vi—Ca1—O2^vii	80.06 (13)	Ca1^xvii—O2—Ca2^xvii	80.98 (12)
O2^vi—Ca1—O2^ii	62.42 (14)	Ca1^xvii—O2—Ca2^xvi	168.37 (15)
O2^vi—Ca1—O3^iii	61.51 (11)	Ca1^xiv—O2—Ca2^xvii	174.41 (15)
O2^vi—Ca1—O3^v	175.61 (12)	Ca1^xiv—O2—Ca2^xvi	98.16 (17)
O2^vi—Ca1—O3^viii	123.8 (2)	Ca2^xvii—O2—Ca2^xvi	87.43 (9)
O2^vi—Ca1—O3^ii	96.77 (8)	Mn—O3—Mn^xvi	162.6 (2)
O2^v—Ca1—O2^ii	92.87 (16)	Mn—O3—Ca1^xii	89.36 (14)
O2^v—Ca1—O3^iii	172.2 (2)	Mn—O3—Ca1^xiv	79.97 (15)
O2^v—Ca1—O3^v	57.67 (11)	Mn—O3—Ca2^xii	99.67 (16)
O2^v—Ca1—O3^viii	79.98 (11)	Mn—O3—Ca2^xvi	88.29 (19)
O2^v—Ca1—O3^ii	117.1 (2)	Mn^xvi—O3—Ca1^xii	92.62 (16)
O2^ii—Ca1—O3^iii	79.98 (11)	Mn^xvi—O3—Ca1^xiv	83.06 (13)
O3^iii—Ca1—O3^v	122.88 (13)	Mn^xvi—O3—Ca2^xii	97.3 (2)
O3^iii—Ca1—O3^viii	106.95 (16)	Mn^xvi—O3—Ca2^xvi	84.98 (14)
O3^iii—Ca1—O3^ii	56.25 (13)	Ca1^xii—O3—Ca1^xiv	83.32 (9)
O3^v—Ca1—O3^ii	86.23 (12)	Ca1^xii—O3—Ca2^xii	96.83 (16)
O2^vi—Ca2—O2^ii	59.24 (13)	Ca1^xii—O3—Ca2^xvi	163.76 (16)
O2^vi—Ca2—O3^iii	61.18 (12)	Ca1^xiv—O3—Ca2^xii	179.6 (2)
O2^vi—Ca2—O3^ii	101.81 (9)	Ca1^xiv—O3—Ca2^xvi	80.44 (12)
O2^vi—Ca2—O4	124.60 (12)	Ca2^xii—O3—Ca2^xvi	99.41 (9)
O2^vi—Ca2—O4^iii	57.66 (11)	Mn—O4—Ca2^xii	84.81 (11)
O2^vi—Ca2—O4^ix	132.06 (15)	Mn—O4—Ca2	86.65 (11)
O2^vi—Ca2—O4^x	60.78 (8)	Mn—O4—Ca2^xviii	169.33 (13)
O2^vi—Ca2—O4^ii	122.47 (12)	Mn—O4—Ca2^xix	90.88 (8)
O2^ii—Ca2—O3^iii	85.02 (11)	Mn—O4—Ca2^xvi	81.02 (8)
O2^ii—Ca2—O3^ii	63.86 (12)	Ca2^xii—O4—Ca2	169.74 (7)
O2^ii—Ca2—O4	66.88 (14)	Ca2^xii—O4—Ca2^xii	91.02 (11)
O2^ii—Ca2—O4^iii	116.84 (15)	Ca2^xii—O4—Ca2^xix	90.40 (11)
O2^ii—Ca2—O4^ix	146.87 (16)	Ca2^xii—O4—Ca2^xvi	84.60 (10)
O2^ii—Ca2—O4^x	68.05 (9)	Ca2—O4—Ca2^xviii	96.39 (11)
O2^ii—Ca2—O4^ii	121.13 (10)	Ca2—O4—Ca2^xix	95.41 (12)
O3^iii—Ca2—O3^ii	65.07 (14)	Ca2—O4—Ca2^xvi	88.46 (10)
O3^iii—Ca2—O4	127.21 (15)	Ca2^xviii—O4—Ca2^xix	98.99 (7)
O3^iii—Ca2—O4^iii	63.05 (15)	Ca2^xviii—O4—Ca2^xvi	88.82 (7)
O3^iii—Ca2—O4^ix	128.10 (16)	Ca2^xix—O4—Ca2^xvi	170.82 (7)

Symmetry codes: (i) x, y−1, z; (ii) x+1/2, −y+1, z; (iii) x+1, y, z; (iv) x+1/2, −y, −z; (v) x+1/2, −y+1, −z; (vi) x+1, y−1, z; (vii) x+1, y−1, −z; (viii) x+1, y, −z; (ix) x+1/2, −y+1/2, −z+1/2; (x) x+1/2, −y, z; (xi) x, y, −z; (xii) x−1, y, z; (xiii) x−1/2, −y, −z; (xiv) x−1/2, −y+1, −z; (xv) x, y+1, z; (xvi) x−1/2, −y+1, z; (xvii) x−1, y+1, z; (xviii) x−1/2, −y+1/2, −z+1/2; (xix) x−1/2, −y, z.