The structure of dilanthanum ruthenium pentoxide was solved by powder neutron diffraction at room temperature and 1.5 K. High-temperature La
2RuO
5 crystallizes in the monoclinic space group
P2
1/
c. Upon cooling, the sample undergoes a phase transition to the triclinic low-temperature form (space group
P). This transition leads to pronounced changes in the Ru-O-Ru bond distances, resulting in a dimerization of the ruthenium ions.
Supporting information
Polycrystalline La2RuO5 was prepared from La2O3 and RuO2. La2O3 was dried at 1173 K for 6 h prior to use. The thoroughly ground stoichiometric mixture was heated in an alumina crucible at 1423 K for 48 h with one intermediate grinding. Phase purity was checked by preliminary X-ray diffraction measurements.
To reduce the number of free parameters, isotropic displacement parameters were used for all atoms. Attempts to refine the displacement parameters for the various O-atom positions independently led to unreasonable values for some of the atoms (especially for the lt phase) and only slightly reduced the residual parameters. We therefore decided to use one common displacement parameter for all O atoms. Becuase there was a strong correlation between refinement parameters, it was furthermore necessary to use a common displacement parameter for the La atoms in the lt phase.
For both compounds, data collection: SINQ Instrument Control System (SICS) (reference?); cell refinement: Fullprof2000 (Rodríguez-Carvajal, 1990); data reduction: SINQ Instrument Control System (SICS); program(s) used to solve structure: program (reference?); program(s) used to refine structure: Fullprof2000; molecular graphics: ATOMS for Windows (Dowty, 1995); software used to prepare material for publication: program (reference?).
(HT) dilanthanum ruthenium pentoxide
top
Crystal data top
La2RuO5 | Z = 4 |
Mr = 458.89 | Dx = 7.285 Mg m−3 |
Monoclinic, P21/c | Neutron radiation, λ = 1.4935 Å |
Hall symbol: -P 2ybc | µ = 0.29 mm−1 |
a = 9.1850 (4) Å | T = 293 K |
b = 5.8294 (2) Å | Particle morphology: irregular powder |
c = 7.9552 (3) Å | black |
β = 100.79 (2)° | cylinder, 35 × 8 mm |
V = 418.42 (4) Å3 | |
Data collection top
SINQ HRPT diffractometer | Scan method: Stationary detector |
Ge (533) monochromator | Absorption correction: for a cylinder mounted on the ϕ axis (Rodríguez-Carvajal, 1990) |
Specimen mounting: vanadium can | Tmin = ?, Tmax = ? |
Data collection mode: transmission | |
Refinement top
Refinement on Inet | 1604 data points |
Least-squares matrix: full with fixed elements per cycle | Profile function: Thompson-Cox-Hastings pseudo-Voigt |
Rp = 0.023 | 46 parameters |
Rwp = 0.029 | Weighting scheme based on measured s.u.'s |
Rexp = 0.018 | (Δ/σ)max = 0.0001 |
R(F) = 0.026 | Background function: 5th order polynomial. |
χ2 = 2.592 | Preferred orientation correction: none |
Crystal data top
La2RuO5 | V = 418.42 (4) Å3 |
Mr = 458.89 | Z = 4 |
Monoclinic, P21/c | Neutron radiation, λ = 1.4935 Å |
a = 9.1850 (4) Å | µ = 0.29 mm−1 |
b = 5.8294 (2) Å | T = 293 K |
c = 7.9552 (3) Å | cylinder, 35 × 8 mm |
β = 100.79 (2)° | |
Data collection top
SINQ HRPT diffractometer | Scan method: Stationary detector |
Specimen mounting: vanadium can | Absorption correction: for a cylinder mounted on the ϕ axis (Rodríguez-Carvajal, 1990) |
Data collection mode: transmission | Tmin = ?, Tmax = ? |
Refinement top
Rp = 0.023 | χ2 = 2.592 |
Rwp = 0.029 | 1604 data points |
Rexp = 0.018 | 46 parameters |
R(F) = 0.026 | |
Special details top
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
La1 | 0.1709 (3) | 0.2512 (7) | 0.5383 (4) | 0.0048 (5)* | |
La2 | 0.4466 (3) | 0.7638 (8) | 0.1197 (3) | 0.0056 (6)* | |
Ru1 | 0.1497 (4) | 0.7522 (11) | 0.2878 (6) | 0.0049 (5)* | |
O1 | 0.2767 (5) | 0.4881 (7) | 0.2803 (6) | 0.0096 (3)* | |
O2 | 0.3114 (5) | −0.0349 (9) | 0.3713 (6) | 0.0096 (3)* | |
O3 | 0.0225 (5) | 0.0433 (10) | 0.2925 (8) | 0.0096 (3)* | |
O4 | −0.1482 (6) | 0.3324 (7) | 0.4632 (8) | 0.0096 (3)* | |
O5 | 0.4225 (6) | 0.3365 (8) | 0.6024 (7) | 0.0096 (3)* | |
Geometric parameters (Å, º) top
Ru1—O1 | 1.939 (7) | Ru1—O4ii | 2.048 (8) |
Ru1—O2i | 1.954 (7) | Ru1—O4iii | 2.044 (8) |
Ru1—O3i | 2.065 (8) | Ru1iv—Ru1v | 3.975 (8) |
Ru1—O3ii | 2.004 (7) | Ru1v—Ru1iii | 3.978 (7) |
| | | |
O1—Ru1—O2i | 95.2 (4) | O3i—Ru1—O3ii | 95.1 (5) |
O1—Ru1—O3i | 177.2 (5) | O3i—Ru1—O4ii | 86.0 (4) |
O1—Ru1—O3ii | 87.2 (4) | O3i—Ru1—O4iii | 94.2 (4) |
O1—Ru1—O4ii | 92.6 (4) | O3ii—Ru1—O4ii | 88.1 (4) |
O1—Ru1—O4iii | 87.2 (4) | O3ii—Ru1—O4iii | 90.8 (5) |
O2i—Ru1—O3i | 82.5 (4) | O4ii—Ru1—O4iii | 178.9 (6) |
O2i—Ru1—O3ii | 177.3 (5) | Ru1iv—O3—Ru1v | 155.4 (5) |
O2i—Ru1—O4ii | 93.0 (4) | Ru1v—O4—Ru1iii | 152.8 (4) |
O2i—Ru1—O4iii | 88.1 (4) | | |
Symmetry codes: (i) x, y+1, z; (ii) −x, y+1/2, −z+1/2; (iii) −x, −y+1, −z+1; (iv) x, y−1, z; (v) −x, y−1/2, −z+1/2. |
(LT) dilanthanum ruthenium pentoxide
top
Crystal data top
La4Ru2O10 | V = 415.45 (13) Å3 |
Mr = 917.78 | Z = 2 |
Triclinic, P1 | Dx = 7.337 Mg m−3 |
Hall symbol: -P 1 | Neutron radiation, λ = 1.4935 Å |
a = 9.1614 (8) Å | µ = 0.30 mm−1 |
b = 5.8075 (5) Å | T = 2 K |
c = 7.9584 (8) Å | Particle morphology: irregular powder |
α = 89.78 (8)° | black |
β = 101.00 (8)° | cylinder, 35 × 8 mm |
γ = 91.76 (8)° | |
Data collection top
SINQ HRPT diffractometer | Scan method: Stationary detector |
Ge (533) monochromator | Absorption correction: for a cylinder mounted on the ϕ axis (Rodríguez-Carvajal, 1990) |
Specimen mounting: vanadium can | Tmin = ?, Tmax = ? |
Data collection mode: transmission | |
Refinement top
Refinement on Inet | 3208 data points |
Least-squares matrix: full with fixed elements per cycle | Profile function: Thompson-Cox-Hastings pseudo-Voigt |
Rp = 0.019 | 68 parameters |
Rwp = 0.023 | Weighting scheme based on measured s.u.'s |
Rexp = 0.014 | (Δ/σ)max = 0.0001 |
R(F) = 0.018 | Background function: 5th order polynomial. |
χ2 = 2.624 | Preferred orientation correction: none |
Crystal data top
La4Ru2O10 | γ = 91.76 (8)° |
Mr = 917.78 | V = 415.45 (13) Å3 |
Triclinic, P1 | Z = 2 |
a = 9.1614 (8) Å | Neutron radiation, λ = 1.4935 Å |
b = 5.8075 (5) Å | µ = 0.30 mm−1 |
c = 7.9584 (8) Å | T = 2 K |
α = 89.78 (8)° | cylinder, 35 × 8 mm |
β = 101.00 (8)° | |
Data collection top
SINQ HRPT diffractometer | Scan method: Stationary detector |
Specimen mounting: vanadium can | Absorption correction: for a cylinder mounted on the ϕ axis (Rodríguez-Carvajal, 1990) |
Data collection mode: transmission | Tmin = ?, Tmax = ? |
Refinement top
Rp = 0.019 | χ2 = 2.624 |
Rwp = 0.023 | 3208 data points |
Rexp = 0.014 | 68 parameters |
R(F) = 0.018 | |
Special details top
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
La1 | 0.1721 (5) | 0.2469 (6) | 1.0389 (5) | 0.0010 (2)* | |
La1A | 0.1682 (4) | 0.2441 (6) | 0.5386 (5) | 0.0010 (2)* | |
La2 | 0.4476 (4) | −0.2691 (7) | 0.6212 (5) | 0.0010 (2)* | |
La2A | 0.5538 (4) | 0.2417 (7) | 0.8825 (5) | 0.0010 (2)* | |
Ru1 | 0.1474 (6) | −0.2549 (10) | 0.7935 (7) | 0.0022 (4)* | |
Ru1A | 0.1511 (6) | 0.7481 (9) | 1.2872 (7) | 0.0022 (4)* | |
O1 | 0.2803 (6) | −0.0041 (8) | 0.7786 (7) | 0.0027 (2)* | |
O1A | 0.2797 (6) | 0.4827 (9) | 1.2773 (7) | 0.0027 (2)* | |
O2 | 0.3084 (6) | −0.0424 (9) | 0.3668 (7) | 0.0027 (2)* | |
O2A | 0.3141 (6) | 0.5214 (9) | 0.8734 (6) | 0.0027 (2)* | |
O3 | −0.0136 (5) | −0.0251 (10) | 0.6940 (7) | 0.0027 (2)* | |
O3A | 0.0156 (6) | 0.4727 (9) | 0.7892 (7) | 0.0027 (2)* | |
O4 | −0.1364 (6) | 0.1678 (10) | 0.9656 (7) | 0.0027 (2)* | |
O4A | 0.1574 (6) | 0.6582 (9) | 0.5378 (7) | 0.0027 (2)* | |
O5 | 0.4219 (6) | 0.1592 (10) | 1.1031 (7) | 0.0027 (2)* | |
O5A | 0.4262 (5) | 0.3232 (9) | 0.6031 (6) | 0.0027 (2)* | |
Geometric parameters (Å, º) top
Ru1—O1 | 1.888 (8) | Ru1A—O3iv | 2.097 (8) |
Ru1—O2Ai | 2.042 (8) | Ru1A—O3Aiv | 1.969 (8) |
Ru1—O3 | 2.062 (8) | Ru1A—O4iv | 2.049 (8) |
Ru1—O3Ai | 1.957 (8) | Ru1A—O4Av | 2.050 (8) |
Ru1—O4ii | 2.007 (8) | Ru1—Ru1Aiv | 4.045 (8) |
Ru1—O4Ai | 2.118 (8) | Ru1vi—Ru1Aiv | 3.868 (8) |
Ru1A—O1A | 1.979 (8) | Ru1ii—Ru1Aiv | 3.923 (8) |
Ru1A—O2iii | 1.873 (8) | Ru1vi—Ru1Avii | 4.036 (8) |
| | | |
O1—Ru1—O3 | 84.1 (4) | O1A—Ru1A—O3iv | 178.0 (5) |
O1—Ru1—O2Ai | 93.5 (4) | O1A—Ru1A—O3Aiv | 85.3 (4) |
O1—Ru1—O3Ai | 174.4 (6) | O1A—Ru1A—O4iv | 94.4 (4) |
O1—Ru1—O4Ai | 88.6 (4) | O2iii—Ru1A—O4Av | 87.2 (4) |
O1—Ru1—O4ii | 91.0 (5) | O3iv—Ru1A—O4Av | 92.0 (4) |
O2Ai—Ru1—O3 | 175.1 (5) | O3Aiv—Ru1A—O4Av | 91.1 (4) |
O3—Ru1—O3Ai | 97.3 (5) | O4iv—Ru1A—O4Av | 177.7 (5) |
O3—Ru1—O4Ai | 87.3 (4) | O2iii—Ru1A—O3iv | 85.2 (4) |
O3—Ru1—O4ii | 91.9 (4) | O2iii—Ru1A—O3Aiv | 178.2 (6) |
O2Ai—Ru1—O3Ai | 84.7 (4) | O2iii—Ru1A—O4iv | 94.9 (5) |
O2Ai—Ru1—O4Ai | 88.4 (4) | O3iv—Ru1A—O3Aiv | 94.3 (5) |
O2Ai—Ru1—O4ii | 92.4 (4) | O3iv—Ru1A—O4iv | 87.5 (4) |
O3Ai—Ru1—O4Ai | 86.0 (4) | O3Aiv—Ru1A—O4iv | 86.8 (4) |
O3Ai—Ru1—O4ii | 94.4 (5) | Ru1—O3—Ru1Aiv | 153.2 (5) |
O4ii—Ru1—O4Ai | 179.1 (5) | Ru1vi—O3A—Ru1Aiv | 160.2 (6) |
O1A—Ru1A—O4Av | 86.1 (4) | Ru1ii—O4—Ru1Aiv | 150.6 (5) |
O1A—Ru1A—O2iii | 95.1 (4) | Ru1vi—O4A—Ru1Avii | 151.0 (5) |
Symmetry codes: (i) x, y−1, z; (ii) −x, −y, −z+2; (iii) x, y+1, z+1; (iv) −x, −y+1, −z+2; (v) x, y, z+1; (vi) x, y+1, z; (vii) x, y, z−1. |
Experimental details
| (HT) | (LT) |
Crystal data |
Chemical formula | La2RuO5 | La4Ru2O10 |
Mr | 458.89 | 917.78 |
Crystal system, space group | Monoclinic, P21/c | Triclinic, P1 |
Temperature (K) | 293 | 2 |
a, b, c (Å) | 9.1850 (4), 5.8294 (2), 7.9552 (3) | 9.1614 (8), 5.8075 (5), 7.9584 (8) |
α, β, γ (°) | 90, 100.79 (2), 90 | 89.78 (8), 101.00 (8), 91.76 (8) |
V (Å3) | 418.42 (4) | 415.45 (13) |
Z | 4 | 2 |
Radiation type | Neutron, λ = 1.4935 Å | Neutron, λ = 1.4935 Å |
µ (mm−1) | 0.29 | 0.30 |
Specimen shape, size (mm) | Cylinder, 35 × 8 | Cylinder, 35 × 8 |
|
Data collection |
Diffractometer | SINQ HRPT diffractometer | SINQ HRPT diffractometer |
Specimen mounting | Vanadium can | Vanadium can |
Data collection mode | Transmission | Transmission |
Scan method | Stationary detector | Stationary detector |
Absorption correction | For a cylinder mounted on the ϕ axis (Rodríguez-Carvajal, 1990) | – |
2θ values (°) | 2θfixed = ? | 2θfixed = ? |
|
Refinement |
R factors and goodness of fit | Rp = 0.023, Rwp = 0.029, Rexp = 0.018, R(F) = 0.026, χ2 = 2.592 | Rp = 0.019, Rwp = 0.023, Rexp = 0.014, R(F) = 0.018, χ2 = 2.624 |
No. of data points | 1604 | 3208 |
No. of parameters | 46 | 68 |
No. of restraints | ? | ? |
Selected geometric parameters (Å, º) for (HT) topRu1—O1 | 1.939 (7) | Ru1—O4ii | 2.048 (8) |
Ru1—O2i | 1.954 (7) | Ru1—O4iii | 2.044 (8) |
Ru1—O3i | 2.065 (8) | Ru1iv—Ru1v | 3.975 (8) |
Ru1—O3ii | 2.004 (7) | Ru1v—Ru1iii | 3.978 (7) |
| | | |
O1—Ru1—O2i | 95.2 (4) | O3i—Ru1—O3ii | 95.1 (5) |
O1—Ru1—O3i | 177.2 (5) | O3i—Ru1—O4ii | 86.0 (4) |
O1—Ru1—O3ii | 87.2 (4) | O3i—Ru1—O4iii | 94.2 (4) |
O1—Ru1—O4ii | 92.6 (4) | O3ii—Ru1—O4ii | 88.1 (4) |
O1—Ru1—O4iii | 87.2 (4) | O3ii—Ru1—O4iii | 90.8 (5) |
O2i—Ru1—O3i | 82.5 (4) | O4ii—Ru1—O4iii | 178.9 (6) |
O2i—Ru1—O3ii | 177.3 (5) | Ru1iv—O3—Ru1v | 155.4 (5) |
O2i—Ru1—O4ii | 93.0 (4) | Ru1v—O4—Ru1iii | 152.8 (4) |
O2i—Ru1—O4iii | 88.1 (4) | | |
Symmetry codes: (i) x, y+1, z; (ii) −x, y+1/2, −z+1/2; (iii) −x, −y+1, −z+1; (iv) x, y−1, z; (v) −x, y−1/2, −z+1/2. |
Selected geometric parameters (Å, º) for (LT) topRu1—O1 | 1.888 (8) | Ru1A—O3iv | 2.097 (8) |
Ru1—O2Ai | 2.042 (8) | Ru1A—O3Aiv | 1.969 (8) |
Ru1—O3 | 2.062 (8) | Ru1A—O4iv | 2.049 (8) |
Ru1—O3Ai | 1.957 (8) | Ru1A—O4Av | 2.050 (8) |
Ru1—O4ii | 2.007 (8) | Ru1—Ru1Aiv | 4.045 (8) |
Ru1—O4Ai | 2.118 (8) | Ru1vi—Ru1Aiv | 3.868 (8) |
Ru1A—O1A | 1.979 (8) | Ru1ii—Ru1Aiv | 3.923 (8) |
Ru1A—O2iii | 1.873 (8) | Ru1vi—Ru1Avii | 4.036 (8) |
| | | |
O1—Ru1—O3 | 84.1 (4) | O1A—Ru1A—O3iv | 178.0 (5) |
O1—Ru1—O2Ai | 93.5 (4) | O1A—Ru1A—O3Aiv | 85.3 (4) |
O1—Ru1—O3Ai | 174.4 (6) | O1A—Ru1A—O4iv | 94.4 (4) |
O1—Ru1—O4Ai | 88.6 (4) | O2iii—Ru1A—O4Av | 87.2 (4) |
O1—Ru1—O4ii | 91.0 (5) | O3iv—Ru1A—O4Av | 92.0 (4) |
O2Ai—Ru1—O3 | 175.1 (5) | O3Aiv—Ru1A—O4Av | 91.1 (4) |
O3—Ru1—O3Ai | 97.3 (5) | O4iv—Ru1A—O4Av | 177.7 (5) |
O3—Ru1—O4Ai | 87.3 (4) | O2iii—Ru1A—O3iv | 85.2 (4) |
O3—Ru1—O4ii | 91.9 (4) | O2iii—Ru1A—O3Aiv | 178.2 (6) |
O2Ai—Ru1—O3Ai | 84.7 (4) | O2iii—Ru1A—O4iv | 94.9 (5) |
O2Ai—Ru1—O4Ai | 88.4 (4) | O3iv—Ru1A—O3Aiv | 94.3 (5) |
O2Ai—Ru1—O4ii | 92.4 (4) | O3iv—Ru1A—O4iv | 87.5 (4) |
O3Ai—Ru1—O4Ai | 86.0 (4) | O3Aiv—Ru1A—O4iv | 86.8 (4) |
O3Ai—Ru1—O4ii | 94.4 (5) | Ru1—O3—Ru1Aiv | 153.2 (5) |
O4ii—Ru1—O4Ai | 179.1 (5) | Ru1vi—O3A—Ru1Aiv | 160.2 (6) |
O1A—Ru1A—O4Av | 86.1 (4) | Ru1ii—O4—Ru1Aiv | 150.6 (5) |
O1A—Ru1A—O2iii | 95.1 (4) | Ru1vi—O4A—Ru1Avii | 151.0 (5) |
Symmetry codes: (i) x, y−1, z; (ii) −x, −y, −z+2; (iii) x, y+1, z+1; (iv) −x, −y+1, −z+2; (v) x, y, z+1; (vi) x, y+1, z; (vii) x, y, z−1. |
Among the various modifications of the perovskite structure, compounds belonging to the so-called [110] phases are quite special. In these oxides, the three-dimensional perovskite framework can be considered to be cut along the [110] direction, resulting in blocks of different possible thicknesses. Many of the [110] phases have the general composition AnBnO3n + 2, in which n represents the number of BO6-octahedra within the blocks (Lichtenberg et al., 2001). The thinnest possible blocks correspond to zigzag chains of single BO6 octahedra, which are isolated by the A-type cations. This arrangement is found, for example, in LaTaO4 (n = 2, corresponding to La2Ta2O8) (Cava and Roth, 1981).
The title compound is closely related to the AnBnO3n + 2 family of oxides. It can formally be described as an n = 2 member, in which the perovskite slabs are separated by one additional AO– unit. Fig. 1 shows the structural relationship between the cubic perovskite archetype structure, La2RuO5, and the [110] phases. La2RuO5 was discovered independently by two groups. Boullay et al. (2003) published an ab-initio structural determination based on powder X-ray diffraction data, while Khalifah et al. (2002) reported electrical and magnetic properties. In the latter paper, a structural phase transition at 160 K was also described. This transition is accompanied by strong changes in the magnetic susceptibility and electrical resistivity. Khalifah et al. (2002) used powder neutron diffraction for their investigations, but unfortunately no structural details were given and to the best of our knowledge this information has not been published. In the course of our own research on the physical properties of ruthenates, we came across the need for structural data of both the high-temperature (ht) and low-temperature (lt) modifications of La2RuO5. As a starting model for ht-La2RuO5, the atomic coordinates given by Boullay et al. (2003) were used. For the triclinic lt-phase, possible new positions were generated using the program PowderCell (Kraus & Nolze, 1996). Figs. 2 and 3 show the results of the refinements. A graphical presentation of the crystal structures at 293 and 1.5 K is given in Fig. 4, while Fig. 5 shows the local Ru—O—Ru bonding arrangement within the zigzag chains.
In the high-temperature modification, the La1 ions are coordinated by nine oxygen ions, with bond distances ranging from 2.325 (6) to 2.968 (6) Å. The coordination geometry is rather irregular and cannot be described in terms of a simple polyhedron. For the La2 ions, an irregular ninefold O-atom coordination is observed, with bond distances ranging from 2.342 (7) to 2.836 (6) Å. It is noteworthy that the shortest La—O distances are found for atom O5, i.e. the oxygen ions in between the perovskite blocks. The Ru/O moieties can be described as slightly distorted octahedra. The difference between the longest and shortest bonds is 0.13 (1) Å. The O—Ru—O bond angles range from 82.5 (4) to 95.2 (4)°. The Ru—Ru distances in the zigzag chains are, within experimental error, identical to the Ru—Ru distances along the crystallographic c axis. Additionally, the Ru1—O3—Ru1 (zigzag chain) and Ru1—O4—Ru1 (along c) bond angles are very similar [155.4 (5) and 152.8 (4)°, respectively].
For the low-temperature modification, the coordination geometries change significantly, although the dimension of the unit cell remains very similar. For atoms La1 and La1A the bond lengths lie in the ranges 2.320 (7)–3.054 (6) Å and 2.352 (6)–3.000 (6) Å, respectively. Interestingly, the La1—O1A bond becomes rather short [2.381 (7) Å]. The interatomic distances for atoms La2 and La2A are 2.332 (6)–2.820 (7) and 2.346 (7)–2.834 (7) Å, respectively. The most interesting changes concern the ruthenium–oxygen coordination. Within the zigzag chains, the Ru1—O3A—Ru1A distance was found to be 0.23 (2) Å shorter than the Ru1—O3–Ru1A distance. A similar, although less pronounced, effect was found for the distances parallel to the c direction; here, the Ru1—O4—Ru1A distance is 0.12 (2) Å shorter than the Ru1–O4A–Ru1A distance. These changes in the interatomic distances can be described as a dimerization of the ruthenium ions, which apparently occurs both within the zigzag chains and along c. In addition, the bond angles differ significantly in the ht and lt modifications. The Ru—O–Ru angles within the zigzag chains are 153.2 (5)° for Ru1—O3—Ru1A and 160.2 (6)° for Ru1—O3A—Ru1A, respectively. The corresponding angles along the c axis (Ru1—O4—Ru1A and Ru1—O4A—Ru1A), on the other hand, are almost identical (151°). It is worth noting that within the zigzag chains the shorter Ru—Ru distance is accompanied by a bond angle closer to 180°. Both effects are expected to increase the superexchange interaction between these neighbouring ruthenium ions. Calculations of the electronic band structure based on our Rietveld results are currently under progress and results will be reported elsewhere. N·B. Should the bond lengths/angles involving La atoms have been included in cif?