Monoclinic La_1−xBa_xMnO₃ (x = 0.185) at 160 K

Single-crystal X-ray diffraction has shown that lanthanum barium manganese trioxide, La_0.815Ba_0.185MnO₃, is monoclinic (I2/c) below a first-order phase transition at 187.1 (3) K. This result differs from the Pbnm symmetry usually assigned to colossal magnetoresistance oxides, A_1−xA′_xMnO₃ with x ≃ 0.2, which adopt a distorted perovskite-type crystal structure. The Mn atom lies on an inversion center, the disordered Li/Ba site is on a twofold axis and one of the two independent O atoms also lies on a twofold axis.

S1. Comment

La_0.815Ba_0.185MnO₃ is one of the manganese oxides in which colossal magnetoresistance (CMR) was found (Jonker & van Santen, 1950; van Santen & Jonker, 1950). These compounds exhibit various superstructures on the basis of tilting of octahedra (Glazer, 1972). Accordingly, rhombohedral, orthorhombic and monoclinic symmetries have been found in A_{1 − x} A'_xMnO₃ systems (A = La, Nd, Pr and Sm, and A' = Ca, Ba and Sr) with x ≈ 0.2 (Goodenough, 2004).

Dabrowski et al. (1998) have reported the results of X-ray powder diffraction on vacancy-free La_{1 − x}Ba_xMnO₃ ceramic compounds with 0.1 < x < 0.24. A t room temperature, these authors found orthorhombic Pbnm symmetry for x = 0.1 and rhombohedral R-3c symmetry for x = 0.14–0.24. For x = 0.12, the sample contained both phases. Arkhipov et al. (2000) reported the temperature dependence of the lattice parameters of La_0.8Ba_0.2MnO₃, also employing X-ray powder diffraction. According to their work, orthorhombic Pbnm symmetry is found at temperatures of less than 185 K, whereas a phase with R-3c symmetry is stable for temperatures higher than 196 K, with a first-order structural phase transition at T_c = 190.5 K.

Our investigations confirm the rhombohedral phase at high temperatures as well as the occurrence of a first-order phase transition at T_c = 187.1 (3) K, determined on cooling. However, we have found a structure with monoclinic I2/c symmetry for the low-temperature phase.

Both the rhombohedral and the monoclinic phases of La_0.815Ba_0.185MnO₃ are distorted perovskite-type structures composed of corner-linked MnO₆ octahedra with La/Ba cations lying in the 12-fold coordinated cavities (Fig.1). The tilting of the octahedra occurs in the same direction for both but with different magnitude, as described by the Mn—O—Mn angle, which is only one [164.7 (1)°] in the rhombohedral phase, whereas it splits into two in the monoclinic phase (Table 1). The tilting pattern is expressed as a⁻a⁻a⁻ and a⁻b⁻b⁻ for rhombohedral R-3c and monoclinic I2/c, respectively, which is significantly different from a⁺a⁻a⁻ expected for the orthorhombic Pbnm (Glazer, 1972). Distortions of the octahedra in the two structures are also different, as described by the Mn—O distances and by the O—Mn—O angles, which are 1.9742 (2) Å and 89.1 (1)°, respectively, in the rhombohedral phase whereas they both split into three different values in the monoclinic one.

A monoclinic I2/c structure was first reported for the compound La_0.788Sr_0.212Mn_0.958O₃ (Tamazyan et al., 2002). Compared with the La/Sr analogue, which exhibits almost equal Mn—O—Mn angles, La_0.815Ba_0.185MnO₃ has two different Mn—O—Mn angles (Table 1). The Mn—O bonds have almost equal lengths indicating a very small coherent Jahn-Teller distortion, as was also found for the La/Sr compound. The degree of shear distortion of the MnO₆ octahedra is smaller in the La/Ba compound, as shown by the smaller deviations of O—Mn—O bond angles from 90°. The effect of the larger cation radius is evidenced by larger lattice parameters as well as by different A—O (A = La, Ba or La, Sr) distances than in La_0.788Sr_0.212Mn_0.958O₃.

We report here the second finding of a monoclinic structure for the class of perovskite-type compounds A_{1 − x}A'_xMnO₃ with x ≈ 0.2. In light of this result, the (x, T) phase diagrams of these systems would need to be revised (Zhou & Goodenough, 2001).

S2. Experimental

Single crystals of nominal composition La_0.8Ba_0.2MnO₃ were grown by the non-crucible floating zone technique (Mukovskii et al., 2001). Electron microprobe analysis revealed a La:Ba:Mn ratio of 0.815 (10):0.185 (6):0.996 (8). The nominal and real Ba contents differ as a result of a strong evaporation of barium during melting.

S3. Refinement

A piece of about 0.13 × 0.06 × 0.05 mm was cut from the crystalline material and used for single-crystal X-ray diffraction. We found a crystal structure with space group R-3c at room temperature (T = 296 K), in accordance with previous studies (Arkhipov et al., 2000). At 160 K, the diffraction peaks appeared to be split in ω scans. The centering of 25 reflections showed an eightfold pseudo-cubic supercell 2a_c × 2a_c × 2a_c (the subscript c refers to the primitive cubic perovskite unitcell) with a = 7.830 (3) Å b = 7.802 (3) Å, c = 7.832 (2) Å, α = 90.12 (2)° β = 90.56 (3)° and γ = 90.13 (3)°. The distortions from cubic lattice symmetry indicate that the true lattice is 2^1/2a_c × 2^1/2a_c × 2a_c with either orthorhombic Pbnm or Imcm, or monoclinic I2/c symmetries (see Fig. 1 of Tamazyan et al., 2002). The splitting of reflections can be explained by twinning. Because the transition is first-order, every symmetry operator of the m3m point group that is not part of the crystal class (mmm or 2/m) may become a twinning operator (Tamazyan et al., 2002) and any orientation of the low-temperature structure within the hypothetical cubic perovskite lattice may occur. This is confirmed by the orientations of the five twin domains (out of a total of 12) with significant volume fractions (Table 2). Among them, two pairs of domains are related by a fourfold axis. The characteristic n-fold splitting of the pseudocubic (hh0)_c and (hhh)_c reflections is identified by means of measured ω–θ sections and compared with the number of maxima expected for different symmetries (Tamazyan et al., 2002). Fig. 2 shows that the (333)_c reflection is split into three peaks in accordance with monoclinic symmetry and at variance with orthorhombic symmetry. Twin matrices applied to the Miller indices (hkl are multiplied from the left) are the following: M1 = (1 0 0|0 1 0|0 0 1), M2 = (1/2 − 1/2 1/2|-1/2 1/2 1/2|-1 − 1 0), M3 = (1/2 1/2 1/2|-1/2 − 1/2 1/2|1 − 1 0), M4 = (−1/2 − 1/2 1/2|-1/2 − 1/2 − 1/2|1 − 1 0), M5 = (0 − 1 0|1 0 0|0 0 1). Structure refinements against all reflections led to R(obs) = 2.70, 3.23 and 3.46% for I2/c, Pbnm and Imcm, respectively. Additional tests were made by computing partial R values with the contributions of superlattice reflections only, which led to R(obs)/R(all) = 8.50/17.7, 11.93/74.1 and 18.27/31.1 for I2/c, Pbnm and Imcm, respectively. Measured intensity data show 45 observed reflections violating the I-centering. However, they are weak and they mainly belong to {110} in the eightfold pseudocubic superlattice. Such reflections can be explained as produced by λ/2 radiation since {220} reflections are very strong. The particularly high partial R(all) value for Pbnm demonstrates that the observed reflections violating the I-centering are not a result of structural effects. Because many studies of orthorhombic and monoclinic manganites report lattice parameters with a > b, we performed additional refinements in this setting and we checked the possibility of β < 90°. However, the splitting of the reflections prevented the unambiguous assignment of correct values to a and b (a > b or a < b). The best fit to the diffraction data was obtained in the monoclinic setting with a > b. This choice is confirmed by the observed maxima positions in the ω–θ section in Fig. 2, since the middle position of the strongest peak is only possible by assuming a > b (by assuming a < b, the strongest peak would have occurred on the right side at higher theta). In the difference Fourier map the largest residuals are located 0.4 (s.u.?) and 0.6 (s.u.?) Å, respectively, from the La/Ba atom.

Computing details

Data collection: DATCOL in CAD-4 Software (Enraf–Nonius, 1989); cell refinement: SETANG LS in CAD-4 Software; data reduction: HELENA (Spek, 1997) and HABITUS (Herrendorf & Bärnighausen, 1997); program(s) used to solve structure: program (reference?); program(s) used to refine structure: Jana2000 (Petricek et al., 2000); molecular graphics: DIAMOND (Brandenburg, 2005); software used to prepare material for publication: Jana2000 (Petricek et al., 2000).

Figures

	Fig. 1. A perspective view of the monoclinic crystal structure of La0.815Ba0.185MnO3.
	Fig. 2. The ω–θ profile of the (333)c reflection. (a) A three-dimensional plot showing both the Kα1 and Kα2 peaks. Miller indices with subscripts referring to the five twin domains given in Table 2 are indicated. (b) A contour plot of the Kα1 peak.

lanthanum barium manganese trioxide

Crystal data

Ba0.185La0.815MnO3	F(000) = 423.2
Mr = 241.6	Dx = 6.706 Mg m^−3
Monoclinic, I2/c	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -I 2yc	Cell parameters from 25 reflections
a = 5.564 (2) Å	θ = 18.4–29.8°
b = 5.510 (2) Å	µ = 22.37 mm^−1
c = 7.802 (3) Å	T = 160 K
β = 90.18 (3)°	Rectangular prism, translucent dark brown
V = 239.19 (14) Å^3	0.13 × 0.06 × 0.05 mm
Z = 4

Data collection

Nonius MACH3 diffractometer	1184 reflections with I > 3σ(I)
Radiation source: Rotating Anode	Rint = 0.000
Graphite monochromator	θmax = 39.9°, θmin = 4.5°
ω scans	h = −9→9
Absorption correction: ψ scan (HABITUS; Herrendorf & Bärnighausen, 1997)	k = −10→10
Tmin = 0.193, Tmax = 0.327	l = −9→14
1567 measured reflections	3 standard reflections every 60 min
1567 independent reflections	intensity decay: none

Refinement

Refinement on F	Weighting scheme based on measured s.u.'s w = 1/(σ^2(F) + 0.0001F^2)
R[F^2 > 2σ(F^2)] = 0.027	(Δ/σ)max = 0.0001
wR(F^2) = 0.033	Δρmax = 3.02 e Å^−3
S = 1.68	Δρmin = −2.99 e Å^−3
1567 reflections	Extinction correction: B-C type 1 Gaussian isotropic (Becker & Coppens, 1974)
31 parameters	Extinction coefficient: 0.0028 (1)

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Mn	0	0	0	0.00250 (15)
La	0	0.49717 (5)	0.75	0.00399 (7)	0.815
Ba	0	0.4972	0.75	0.00399 (7)	0.185
O1	0	0.0569 (7)	0.25	0.0107 (10)
O2	0.2513 (7)	0.2495 (4)	−0.0298 (4)	0.0117 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Mn	0.0018 (3)	0.0038 (2)	0.0020 (3)	0.0002 (2)	0.0013 (2)	−0.00013 (19)
La	0.00413 (12)	0.00502 (12)	0.00281 (13)	0	0.00096 (9)	0
Ba	0.00413 (12)	0.00502 (12)	0.00281 (13)	0	0.00096 (9)	0
O1	0.012 (2)	0.0134 (15)	0.0064 (16)	0	−0.0003 (13)	0
O2	0.0131 (15)	0.0111 (12)	0.0110 (12)	−0.0043 (9)	0.0009 (11)	0.0004 (9)

Geometric parameters (Å, º)

Mn—O1	1.9755 (7)	La—O2^xi	2.932 (3)
Mn—O1^i	1.9755 (7)	La—O2^xii	2.947 (3)
Mn—O2	1.975 (3)	La—O2^xiii	2.594 (3)
Mn—O2^i	1.975 (3)	O1—O2	2.805 (4)
Mn—O2^ii	1.968 (3)	O1—O2^xiv	2.784 (4)
Mn—O2^iii	1.968 (3)	O1—O2^i	2.782 (4)
La—O1^iv	2.8014 (5)	O1—O2^ix	2.793 (4)
La—O1^v	2.8014 (5)	O1—O2^x	2.805 (4)
La—O1^vi	3.053 (4)	O1—O2^ii	2.784 (4)
La—O1^vii	2.457 (4)	O1—O2^xv	2.782 (4)
La—O2^viii	2.599 (3)	O1—O2^iii	2.793 (4)
La—O2^iv	2.932 (3)	O2—O2^ii	2.794 (3)
La—O2^vii	2.947 (3)	O2—O2^xvi	2.794 (3)
La—O2^ix	2.594 (3)	O2—O2^iii	2.782 (6)
La—O2^x	2.599 (3)	O2—O2^xvii	2.782 (6)
O1—Mn—O1^i	180	O2^ix—La—O2^viii	82.86 (10)
O2—Mn—O2^i	180	O2^ix—La—O2^iv	90.40 (9)
O2^ii—Mn—O2^iii	180	O2^ix—La—O2^vii	173.68 (9)
O1—Mn—O2	90.46 (12)	O2^ix—La—O2^x	64.79 (11)
O1—Mn—O2^i	89.54 (12)	O2^ix—La—O2^xi	119.35 (11)
O1—Mn—O2^ii	89.81 (12)	O2^ix—La—O2^xii	60.15 (8)
O1—Mn—O2^iii	90.19 (12)	O2^ix—La—O2^xiii	116.80 (9)
O1^i—Mn—O2	89.54 (12)	O2^x—La—O2^viii	116.65 (8)
O1^i—Mn—O2^i	90.46 (12)	O2^x—La—O2^iv	60.33 (8)
O1^i—Mn—O2^ii	90.19 (12)	O2^x—La—O2^vii	118.87 (11)
O1^i—Mn—O2^iii	89.81 (12)	O2^x—La—O2^ix	64.79 (11)
O2—Mn—O2^ii	90.25 (14)	O2^x—La—O2^xi	173.23 (9)
O2—Mn—O2^iii	89.75 (14)	O2^x—La—O2^xii	90.83 (9)
O2^i—Mn—O2^ii	89.75 (14)	O2^x—La—O2^xiii	82.86 (10)
O2^i—Mn—O2^iii	90.25 (14)	O2^xi—La—O2^viii	60.33 (8)
O1^iv—La—O1^v	166.51 (12)	O2^xi—La—O2^iv	123.37 (7)
O1^iv—La—O1^vi	96.75 (8)	O2^xi—La—O2^vii	56.48 (10)
O1^iv—La—O1^vii	83.25 (8)	O2^xi—La—O2^ix	119.35 (11)
O1^iv—La—O2^viii	126.44 (9)	O2^xi—La—O2^x	173.23 (9)
O1^iv—La—O2^iv	58.52 (8)	O2^xi—La—O2^xii	95.91 (9)
O1^iv—La—O2^vii	58.06 (8)	O2^xi—La—O2^xiii	90.40 (9)
O1^iv—La—O2^ix	126.40 (9)	O2^xii—La—O2^viii	118.87 (11)
O1^iv—La—O2^x	61.93 (10)	O2^xii—La—O2^iv	56.48 (10)
O1^iv—La—O2^xi	114.25 (9)	O2^xii—La—O2^vii	123.47 (7)
O1^iv—La—O2^xii	114.69 (9)	O2^xii—La—O2^ix	60.15 (8)
O1^iv—La—O2^xiii	61.95 (10)	O2^xii—La—O2^x	90.83 (9)
O1^v—La—O1^iv	166.51 (12)	O2^xii—La—O2^xi	95.91 (9)
O1^v—La—O1^vi	96.75 (8)	O2^xii—La—O2^xiii	173.68 (9)
O1^v—La—O1^vii	83.25 (8)	O2^xiii—La—O2^viii	64.79 (11)
O1^v—La—O2^viii	61.93 (10)	O2^xiii—La—O2^iv	119.35 (11)
O1^v—La—O2^iv	114.25 (9)	O2^xiii—La—O2^vii	60.15 (8)
O1^v—La—O2^vii	114.69 (9)	O2^xiii—La—O2^ix	116.80 (9)
O1^v—La—O2^ix	61.95 (10)	O2^xiii—La—O2^x	82.86 (10)
O1^v—La—O2^x	126.44 (9)	O2^xiii—La—O2^xi	90.40 (9)
O1^v—La—O2^xi	58.52 (8)	O2^xiii—La—O2^xii	173.68 (9)
O1^v—La—O2^xii	58.06 (8)	Mn—O1—Mn^x	161.7 (2)
O1^v—La—O2^xiii	126.40 (9)	Mn—O1—La^xviii	89.11 (3)
O1^vi—La—O1^iv	96.75 (8)	Mn—O1—La^xix	88.76 (3)
O1^vi—La—O1^v	96.75 (8)	Mn—O1—La^vi	80.87 (12)
O1^vi—La—O1^vii	180	Mn—O1—La^vii	99.13 (12)
O1^vi—La—O2^viii	58.33 (6)	Mn^x—O1—Mn	161.7 (2)
O1^vi—La—O2^iv	118.31 (5)	Mn^x—O1—La^xviii	88.76 (3)
O1^vi—La—O2^vii	118.26 (5)	Mn^x—O1—La^xix	89.11 (3)
O1^vi—La—O2^ix	58.40 (6)	Mn^x—O1—La^vi	80.87 (12)
O1^vi—La—O2^x	58.33 (6)	Mn^x—O1—La^vii	99.13 (12)
O1^vi—La—O2^xi	118.31 (5)	La^xviii—O1—La^xix	166.51 (17)
O1^vi—La—O2^xii	118.26 (5)	La^xviii—O1—La^vi	83.25 (8)
O1^vi—La—O2^xiii	58.40 (6)	La^xviii—O1—La^vii	96.75 (8)
O1^vii—La—O1^iv	83.25 (8)	La^xix—O1—La^xviii	166.51 (17)
O1^vii—La—O1^v	83.25 (8)	La^xix—O1—La^vi	83.25 (8)
O1^vii—La—O1^vi	180	La^xix—O1—La^vii	96.75 (8)
O1^vii—La—O2^viii	121.67 (6)	La^vi—O1—La^xviii	83.25 (8)
O1^vii—La—O2^iv	61.69 (5)	La^vi—O1—La^xix	83.25 (8)
O1^vii—La—O2^vii	61.74 (5)	La^vi—O1—La^vii	180
O1^vii—La—O2^ix	121.60 (6)	La^vii—O1—La^xviii	96.75 (8)
O1^vii—La—O2^x	121.67 (6)	La^vii—O1—La^xix	96.75 (8)
O1^vii—La—O2^xi	61.69 (5)	La^vii—O1—La^vi	180
O1^vii—La—O2^xii	61.74 (5)	Mn—O2—Mn^xvi	166.44 (17)
O1^vii—La—O2^xiii	121.60 (6)	Mn—O2—La^xx	93.66 (14)
O2^viii—La—O2^iv	173.23 (9)	Mn—O2—La^xix	85.11 (9)
O2^viii—La—O2^vii	90.83 (9)	Mn—O2—La^vii	84.54 (12)
O2^viii—La—O2^ix	82.86 (10)	Mn—O2—La^ix	95.31 (10)
O2^viii—La—O2^x	116.65 (8)	Mn^xvi—O2—Mn	166.44 (17)
O2^viii—La—O2^xi	60.33 (8)	Mn^xvi—O2—La^xx	94.97 (10)
O2^viii—La—O2^xii	118.87 (11)	Mn^xvi—O2—La^xix	85.09 (12)
O2^viii—La—O2^xiii	64.79 (11)	Mn^xvi—O2—La^vii	85.15 (9)
O2^iv—La—O2^viii	173.23 (9)	Mn^xvi—O2—La^ix	93.98 (14)
O2^iv—La—O2^vii	95.91 (9)	La^xx—O2—La^xix	173.23 (13)
O2^iv—La—O2^ix	90.40 (9)	La^xx—O2—La^vii	89.17 (10)
O2^iv—La—O2^x	60.33 (8)	La^xx—O2—La^ix	97.14 (10)
O2^iv—La—O2^xi	123.37 (7)	La^xix—O2—La^xx	173.23 (13)
O2^iv—La—O2^xii	56.48 (10)	La^xix—O2—La^vii	84.09 (8)
O2^iv—La—O2^xiii	119.35 (11)	La^xix—O2—La^ix	89.60 (10)
O2^vii—La—O2^viii	90.83 (9)	La^vii—O2—La^xx	89.17 (10)
O2^vii—La—O2^iv	95.91 (9)	La^vii—O2—La^xix	84.09 (8)
O2^vii—La—O2^ix	173.68 (9)	La^vii—O2—La^ix	173.68 (13)
O2^vii—La—O2^x	118.87 (11)	La^ix—O2—La^xx	97.14 (10)
O2^vii—La—O2^xi	56.48 (10)	La^ix—O2—La^xix	89.60 (10)
O2^vii—La—O2^xii	123.47 (7)	La^ix—O2—La^vii	173.68 (13)
O2^vii—La—O2^xiii	60.15 (8)

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