Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536805040973/wm6116sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S1600536805040973/wm6116Isup2.hkl |
Crystals were grown by the flux method. The reagent grade powders of Gd2O3 (Wako Pure Chemical Industries Ltd., 078–04562, 3.56 mmol), RuO2 (Japan Pure Chemical Co., 083278, 2.38 mmol) and SrCl2 flux (Wako Wako Pure Chemical Industries, Ltd., 197–04205, 21.39 mmol) were mixed together, placed in a 30 ml platinum crucible and heated in air to 1373 K in 12 h. The crucible was then kept at that temperature for 6 h, cooled to 973 K at 5 K h−1, and then cooled to room temperature in the switched-off furnace. The product was leached out with warm water to dissolve the flux. The yield was 95.7% in weight. The crystals are black with dimensions of 10–100 µm. Most crystals grown in the present experiment had the shape of a parallelepiped prism elongated along the c axis. The side faces of the prism were composed of {120} and the top and bottom of {101}. Considering the number and strength of bonds that intersect the crystal faces, it is reasonable to assume that the {120} side faces are a set of planes running through the [Gd3O] matrix. No heavy elements other than Gd and Ru were found by energy dispersive spectroscopy (Jeol, JSM-6100) attached to the scanning electron microscope (Jeol, JED-2001). Most crystals had relatively high crystallinity and were identified as the P21nb modification.
The unit cell was chosen so that the a and c axes correspond to those of the Cmcm modification given by Bontchev et al. (2000). This resulted in a non-standard setting of the space group Pna21 (No. 33). The superstructure reflections that double the b axis were relatively strong and easy to discern. The structure was solved by direct methods (Sheldrick, 1997). No reasonable solutions were obtained assuming the centrosymmetric space group Pmnb, which has the same extinction rules as the non-centrosymmetric space group P21nb. The refinement of structure models in Pmnb derived from the final P21nb model converged with R values higher than 0.12, leaving negative or extraordinarily large ADP values for several atoms. The absence of mirror planes perpendicular to the a axis in the structure was justified from the octahedral tilting as discussed in the Comment. A stoichiometric composition was assumed in the refinement since population analysis resulted in site occupation factors of 0.96 (1)–1.03 (1) for all Gd and Ru atom sites. The x parameter of Ru1 was fixed at 0 to define the origin in the least-squares procedure. Refinements assuming anisotropic displacement parameters for O atoms were tried in vain. The absolute structure parameter (Flack, 1983) was refined to 0.50 (2), suggesting a presence of inversion related twin components with nearly equal volumes in the crystal. This value did not vary significantly over several different crystals sampled from the same batch. The highest peak and the deepest hole in the final difference Fourier map are located 0.53 Å from Gd5 and 0.39 Å from Gd6, respectively.
Data collection: RAPID-AUTO (Rigaku, 1999); cell refinement: RAPID-AUTO; data reduction: Xtal3.7.2 DIFDAT SORTRF and ADDREF (Hall et al., 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: Xtal3.7.2 CRYLSQ; molecular graphics: ATOMS (Dowty, 2005); software used to prepare material for publication: Xtal3.7.2 CIFIO.
Gd3O7Ru | F(000) = 2336 |
Mr = 684.82 | Dx = 7.882 Mg m−3 |
Orthorhombic, P21nb | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: P -2bc 2a | Cell parameters from 11591 reflections |
a = 10.644 (5) Å | θ = 3–70° |
b = 14.685 (6) Å | µ = 36.58 mm−1 |
c = 7.384 (3) Å | T = 293 K |
V = 1154.2 (9) Å3 | Prism, black |
Z = 8 | 0.10 × 0.05 × 0.04 mm |
Rigaku R-AXIS RAPID image-plate diffractometer | 3921 reflections with F > 3σ(F) |
Detector resolution: 0.1 pixels mm-1 | Rint = 0.062 |
ω scans | θmax = 50°, θmin = 3.1° |
Absorption correction: numerical (NUMABS; Higashi, 2000) | h = −22→22 |
Tmin = 0.087, Tmax = 0.183 | k = −31→31 |
95408 measured reflections | l = −15→10 |
5726 independent reflections |
Refinement on F | Weighting scheme based on measured s.u.'s |
Least-squares matrix: full | (Δ/σ)max = 0.001 |
R[F2 > 2σ(F2)] = 0.047 | Δρmax = 17.86 e Å−3 |
wR(F2) = 0.035 | Δρmin = −17.04 e Å−3 |
S = 2.38 | Extinction correction: Zachariasen (1967), Eq22 p292 "Cryst. Comp." Munksgaard 1970 |
3921 reflections | Extinction coefficient: 135 (7) |
129 parameters | Absolute structure: Flack (1983) |
0 restraints | Absolute structure parameter: 0.50 (2) |
0 constraints |
Gd3O7Ru | V = 1154.2 (9) Å3 |
Mr = 684.82 | Z = 8 |
Orthorhombic, P21nb | Mo Kα radiation |
a = 10.644 (5) Å | µ = 36.58 mm−1 |
b = 14.685 (6) Å | T = 293 K |
c = 7.384 (3) Å | 0.10 × 0.05 × 0.04 mm |
Rigaku R-AXIS RAPID image-plate diffractometer | 5726 independent reflections |
Absorption correction: numerical (NUMABS; Higashi, 2000) | 3921 reflections with F > 3σ(F) |
Tmin = 0.087, Tmax = 0.183 | Rint = 0.062 |
95408 measured reflections |
R[F2 > 2σ(F2)] = 0.047 | 0 restraints |
wR(F2) = 0.035 | Δρmax = 17.86 e Å−3 |
S = 2.38 | Δρmin = −17.04 e Å−3 |
3921 reflections | Absolute structure: Flack (1983) |
129 parameters | Absolute structure parameter: 0.50 (2) |
x | y | z | Uiso*/Ueq | ||
Gd1 | −0.2193 (2) | 0.02324 (6) | −0.23773 (9) | 0.0041 (4) | |
Gd2 | −0.2309 (2) | 0.22229 (7) | 0.24574 (10) | 0.0056 (5) | |
Gd3 | −0.2819 (2) | 0.27495 (7) | −0.25656 (10) | 0.0061 (4) | |
Gd4 | −0.2722 (2) | −0.02921 (6) | 0.25127 (8) | 0.0040 (4) | |
Gd5 | 0.0025 (2) | 0.38728 (4) | 0.00302 (12) | 0.0088 (3) | |
Gd6 | −0.0102 (2) | 0.36318 (4) | −0.49721 (12) | 0.0077 (3) | |
Ru1 | 0.00000 | 0.12357 (10) | −0.0059 (2) | 0.0030 (3) | |
Ru2 | −0.00151 (11) | 0.12258 (10) | −0.4978 (2) | 0.0032 (3) | |
O1 | 0.0017 (12) | 0.0831 (7) | −0.2500 (9) | 0.0049 (18)* | |
O2 | −0.0137 (12) | 0.1683 (8) | 0.2503 (10) | 0.006 (2)* | |
O3 | 0.1264 (10) | 0.2179 (8) | −0.4561 (15) | 0.010 (2)* | |
O4 | −0.3654 (12) | 0.1036 (8) | 0.2371 (12) | 0.005 (2)* | |
O5 | 0.1282 (11) | 0.0338 (9) | −0.5394 (15) | 0.011 (2)* | |
O6 | −0.1317 (13) | 0.3812 (9) | −0.2358 (13) | 0.006 (2)* | |
O7 | −0.1290 (9) | 0.2188 (7) | −0.4572 (13) | 0.0053 (16)* | |
O8 | −0.1343 (13) | −0.1408 (8) | 0.2631 (12) | 0.006 (2)* | |
O9 | −0.1265 (9) | 0.0297 (7) | −0.5301 (13) | 0.0055 (16)* | |
O10 | −0.3704 (14) | 0.1319 (9) | −0.2421 (13) | 0.007 (2)* | |
O11 | −0.1056 (9) | 0.0177 (6) | 0.0324 (13) | 0.0060 (16)* | |
O12 | 0.1052 (10) | 0.2343 (7) | −0.0526 (14) | 0.013 (2)* | |
O13 | 0.1606 (10) | 0.0638 (7) | 0.0412 (14) | 0.016 (2)* | |
O14 | −0.1465 (9) | 0.1939 (6) | −0.0529 (13) | 0.0111 (18)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
Gd1 | 0.0037 (5) | 0.0041 (3) | 0.0046 (4) | 0.0016 (3) | −0.0004 (3) | 0.0016 (2) |
Gd2 | 0.0035 (5) | 0.0060 (3) | 0.0073 (5) | 0.0014 (3) | 0.0000 (2) | 0.0004 (2) |
Gd3 | 0.0055 (5) | 0.0059 (3) | 0.0070 (4) | −0.0003 (3) | 0.0016 (2) | 0.0004 (2) |
Gd4 | 0.0061 (5) | 0.0025 (3) | 0.0035 (4) | 0.0030 (3) | −0.0014 (2) | −0.0003 (2) |
Gd5 | 0.0077 (3) | 0.0139 (3) | 0.0047 (2) | 0.0053 (3) | 0.0009 (3) | 0.0017 (3) |
Gd6 | 0.0030 (3) | 0.0150 (3) | 0.0049 (2) | 0.0000 (3) | 0.0006 (3) | −0.0013 (3) |
Ru1 | 0.0032 (3) | 0.0044 (2) | 0.0013 (3) | 0.0012 (4) | −0.0001 (4) | −0.0008 (3) |
Ru2 | 0.0042 (3) | 0.0023 (2) | 0.0030 (3) | −0.0010 (4) | 0.0002 (4) | 0.0015 (3) |
Gd1i—O10i | 2.266 (14) | Gd4—O1i | 2.533 (13) |
Gd1i—O6 | 2.293 (14) | Gd5—O6 | 2.271 (12) |
Gd1i—O13ii | 2.318 (11) | Gd5—O8ii | 2.296 (11) |
Gd1i—O11i | 2.334 (10) | Gd5—O10i | 2.334 (12) |
Gd1i—O9i | 2.376 (10) | Gd5—O4 | 2.418 (11) |
Gd1i—O5 | 2.458 (12) | Gd5—O9 | 2.510 (10) |
Gd1i—O1i | 2.513 (13) | Gd5—O12 | 2.532 (11) |
Gd2—O4 | 2.257 (13) | Gd5—O5 | 2.548 (12) |
Gd2—O8 | 2.260 (13) | Gd6—O6 | 2.339 (11) |
Gd2—O3 | 2.303 (11) | Gd6—O10i | 2.343 (12) |
Gd2—O12 | 2.381 (11) | Gd6—O8ii | 2.366 (11) |
Gd2—O14 | 2.417 (10) | Gd6—O4i | 2.368 (11) |
Gd2—O2 | 2.444 (13) | Gd6—O7 | 2.487 (10) |
Gd2—O7 | 2.448 (10) | Gd6—O11ii | 2.500 (9) |
Gd3—O6 | 2.239 (14) | Gd6—O3 | 2.600 (12) |
Gd3—O10 | 2.305 (14) | Ru1ii—O1ii | 1.898 (7) |
Gd3—O7 | 2.350 (10) | Ru1ii—O14ii | 1.902 (10) |
Gd3—O14 | 2.399 (10) | Ru1ii—O11ii | 1.939 (9) |
Gd3—O3 | 2.426 (11) | Ru1ii—O13ii | 1.953 (11) |
Gd3—O12ii | 2.498 (11) | Ru1ii—O12ii | 2.004 (11) |
Gd3—O2ii | 2.605 (13) | Ru1ii—O2ii | 2.008 (8) |
Gd4—O4 | 2.190 (13) | Ru2—O9 | 1.919 (10) |
Gd4—O8 | 2.202 (13) | Ru2—O1 | 1.920 (7) |
Gd4—O13i | 2.331 (11) | Ru2—O5 | 1.924 (12) |
Gd4—O5i | 2.378 (11) | Ru2—O3 | 1.977 (11) |
Gd4—O9 | 2.401 (10) | Ru2—O2ii | 1.981 (8) |
Gd4—O11 | 2.496 (9) | Ru2—O7 | 1.982 (10) |
O1ii—Ru1ii—O14ii | 90.3 (5) | O12ii—Ru1ii—O2ii | 86.4 (5) |
O1ii—Ru1ii—O11ii | 83.9 (4) | O9—Ru2—O1 | 85.2 (5) |
O1ii—Ru1ii—O13ii | 91.1 (5) | O9—Ru2—O5 | 89.8 (5) |
O1ii—Ru1ii—O12ii | 94.9 (5) | O9—Ru2—O2ii | 94.5 (5) |
O14ii—Ru1ii—O11ii | 89.2 (4) | O9—Ru2—O7 | 92.9 (4) |
O14ii—Ru1ii—O12ii | 89.2 (4) | O1—Ru2—O5 | 86.2 (5) |
O14ii—Ru1ii—O2ii | 86.3 (5) | O1—Ru2—O3 | 93.1 (5) |
O11ii—Ru1ii—O13ii | 97.0 (4) | O1—Ru2—O7 | 94.8 (5) |
O11ii—Ru1ii—O2ii | 94.7 (4) | O5—Ru2—O3 | 90.6 (5) |
O13ii—Ru1ii—O12ii | 84.6 (4) | O5—Ru2—O2ii | 97.3 (5) |
O13ii—Ru1ii—O2ii | 92.4 (5) |
Symmetry codes: (i) x+1/2, −y+1/2, z+1/2; (ii) x, y+1/2, −z+1/2. |
Experimental details
Crystal data | |
Chemical formula | Gd3O7Ru |
Mr | 684.82 |
Crystal system, space group | Orthorhombic, P21nb |
Temperature (K) | 293 |
a, b, c (Å) | 10.644 (5), 14.685 (6), 7.384 (3) |
V (Å3) | 1154.2 (9) |
Z | 8 |
Radiation type | Mo Kα |
µ (mm−1) | 36.58 |
Crystal size (mm) | 0.10 × 0.05 × 0.04 |
Data collection | |
Diffractometer | Rigaku R-AXIS RAPID image-plate diffractometer |
Absorption correction | Numerical (NUMABS; Higashi, 2000) |
Tmin, Tmax | 0.087, 0.183 |
No. of measured, independent and observed [F > 3σ(F)] reflections | 95408, 5726, 3921 |
Rint | 0.062 |
(sin θ/λ)max (Å−1) | 1.078 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.047, 0.035, 2.38 |
No. of reflections | 3921 |
No. of parameters | 129 |
Δρmax, Δρmin (e Å−3) | 17.86, −17.04 |
Absolute structure | Flack (1983) |
Absolute structure parameter | 0.50 (2) |
Computer programs: RAPID-AUTO (Rigaku, 1999), RAPID-AUTO, Xtal3.7.2 DIFDAT SORTRF and ADDREF (Hall et al., 2003), SHELXS97 (Sheldrick, 1997), Xtal3.7.2 CRYLSQ, ATOMS (Dowty, 2005), Xtal3.7.2 CIFIO.
Gd1i—O10i | 2.266 (14) | Gd4—O1i | 2.533 (13) |
Gd1i—O6 | 2.293 (14) | Gd5—O6 | 2.271 (12) |
Gd1i—O13ii | 2.318 (11) | Gd5—O8ii | 2.296 (11) |
Gd1i—O11i | 2.334 (10) | Gd5—O10i | 2.334 (12) |
Gd1i—O9i | 2.376 (10) | Gd5—O4 | 2.418 (11) |
Gd1i—O5 | 2.458 (12) | Gd5—O9 | 2.510 (10) |
Gd1i—O1i | 2.513 (13) | Gd5—O12 | 2.532 (11) |
Gd2—O4 | 2.257 (13) | Gd5—O5 | 2.548 (12) |
Gd2—O8 | 2.260 (13) | Gd6—O6 | 2.339 (11) |
Gd2—O3 | 2.303 (11) | Gd6—O10i | 2.343 (12) |
Gd2—O12 | 2.381 (11) | Gd6—O8ii | 2.366 (11) |
Gd2—O14 | 2.417 (10) | Gd6—O4i | 2.368 (11) |
Gd2—O2 | 2.444 (13) | Gd6—O7 | 2.487 (10) |
Gd2—O7 | 2.448 (10) | Gd6—O11ii | 2.500 (9) |
Gd3—O6 | 2.239 (14) | Gd6—O3 | 2.600 (12) |
Gd3—O10 | 2.305 (14) | Ru1ii—O1ii | 1.898 (7) |
Gd3—O7 | 2.350 (10) | Ru1ii—O14ii | 1.902 (10) |
Gd3—O14 | 2.399 (10) | Ru1ii—O11ii | 1.939 (9) |
Gd3—O3 | 2.426 (11) | Ru1ii—O13ii | 1.953 (11) |
Gd3—O12ii | 2.498 (11) | Ru1ii—O12ii | 2.004 (11) |
Gd3—O2ii | 2.605 (13) | Ru1ii—O2ii | 2.008 (8) |
Gd4—O4 | 2.190 (13) | Ru2—O9 | 1.919 (10) |
Gd4—O8 | 2.202 (13) | Ru2—O1 | 1.920 (7) |
Gd4—O13i | 2.331 (11) | Ru2—O5 | 1.924 (12) |
Gd4—O5i | 2.378 (11) | Ru2—O3 | 1.977 (11) |
Gd4—O9 | 2.401 (10) | Ru2—O2ii | 1.981 (8) |
Gd4—O11 | 2.496 (9) | Ru2—O7 | 1.982 (10) |
Symmetry codes: (i) x+1/2, −y+1/2, z+1/2; (ii) x, y+1/2, −z+1/2. |
A series of Ln3MO7 crystals composed of a trivalent lanthanide (Ln) and pentavalent transition metal (M) oxides is structurally characterized by the presence of infinite single chains of corner-linked [MO6] octahedra embedded in the matrix of Ln and O atoms. This series has attracted attention in recent decades because of its interesting electric and magnetic properties.
Single crystals of Gd3RuO7, a member of the Ln3MO7 series, were grown by Bontchev et al. (2000) and reported to crystallize in the orthorhombic space group Cmcm at room temperature. The electric conductivity measurement of the Gd3RuO7 ceramic sample revealed the Motto variable-range hopping conduction of localized carriers along the [RuO6] chains (Bontchev et al., 2000). Harada & Hinatsu (2002) reported the first-order phase transition of Gd3RuO7 at 382 K in addition to low-temperature transitions at 9.5 and 15 K. They also suggested a possible appearance of a monoclinic modification below 382 K for Gd3RuO7. In the course of a systematic study of the Ln3RuO7 series, we encountered a new polymorph of Gd3RuO7, which is neither the orthorhombic Cmcm modification nor the predicted monoclinic one. It crystallizes in the non-centrosymmetric space group P21nb with a doubled b axis compared with that of the Cmcm modification.
The structure of the P21nb modification comprises two crystallographically independent Ru atom sites, Ru1 and Ru2, whereas there is only one Ru site in the Cmcm modification. The [Ru1O6] and [Ru2O6] octahedra are connected with each other by sharing corners to form isolated single chains along the c axis, as shown in Fig. 1. The [RuO6] octahedra are tilted about the a axis with an Ru1—O1—Ru2 angle of 144.1 (6)° and an Ru1—O2—Ru2 angle of 140.2 (6)°, resulting in two different Ru1–Ru2 intermetallic distances of 3.632 (3) and 3.752 (3) Å, respectively. The mean values are very close to those (142.6° and 3.690 Å) reported for the Cmcm modification. In the P21nb modification, however, another tilt system exists and distinguishes the two modifications. This second tilt occurs only in [Ru1O6] octahedra with a magnitude of approximately 13° about the axes close to the c direction. [Ru2O6] has no such tilt system. The second tilt clearly breaks the pseudo-mirror planes at x ≈ 0 and 1/2, discarding the possibility of the centrosymmetric space group Pmnb for the present polymorph.
The Ru—O bond lengths of the [RuO6] octahedron in the Cmcm modification do not differ significantly (i.e. 1.948 Å × 2 along the chain and 1.940 Å × 4 for the others). This high regularity breaks down in the P21nb modification. Here the Ru—O bond lengths of the double-tilted [Ru1O6] octahedron are grouped into three (i.e. ~1.90 Å × 2, ~1.95 Å × 2 and ~2.01 Å × 2), while those of the single-tilted [Ru2O6] octahedron are grouped into two (i.e. ~1.92 Å × 3 and ~1.98 Å × 3). The splitting of the Ru—O bonds suggests that the energy level degeneracy of Ru is removed considerably in the P21nb modification compared with that in Cmcm.
The title compound contains six crystallographically independent Gd atoms coordinated by seven O atoms each, with interatomic distances ranging from 2.190 (13) to 2.605 (13) Å (Table 1). Atoms Gd1, Gd2, Gd3 and Gd4 are related to the Gd2 site in the Cmcm modification, while atoms Gd5 and Gd6 are related to the Gd1 site according to the notation given by Bontchev et al. (2000). The Gd1 atom in the Cmcm modification has a coordination number (CN) of 8, with four short Gd—O (2.34 Å) and four long Gd—O (2.72 Å) bonds. In contrast, atoms Gd5 and Gd6 in the P21nb modification have a CN of 7, with bond distances distributed in shorter ranges of 2.271 (12)–2.548 (12) Å for Gd5 and 2.339 (11)–2.600 (12) Å for Gd6. On the other hand, atoms Gd1, Gd2, Gd3 and Gd4 in the P21nb modification have CN of 7, with Gd—O bond lengths similar to those of Gd2 in the Cmcm modification.
The tilting of the [Ru1O6] octahedra about the c axis in the P21nb modification is closely related to the coordination of Gd5 and Gd6 lying near the same (002) plane, as illustrated in Fig. 2. The dashed lines represent lengths of 3.28–3.68 Å, being much longer than the other six Gd—O bonds (three for each Gd with 2.49–2.60 Å) near the plane. On the other hand, the [RuO6] octahedra in the Cmcm modification have no tilt about the c axis, as shown in Fig. 3. A l l four long Gd—O bonds observed in this modification lie near the (002) plane with the same length of 2.72 Å.
These facts are suggestive when one considers a possible phase transition between the P21nb and Cmcm modifications. The formation of octahedral tilt about the c axis in the P21nb modification can be described by three steps: (1) systematic small displacements of atoms Gd5 and Gd6 along the direction close relative to the b axis, (2) bond breaking of Gd—O, i.e. Ru1—O—Gd → Ru1—O···Gd (two breaks per [Ru1O6] octahedron), and (3) small rotation of [Ru1O6] octahedra about the c axis in order to balance with the two missing Gd—O bonds near the (002) plane. These three steps are geometrically connected to each other and probably occur simultaneously in reality. Therefore it is less meaningful to persist their sequence or the cause/effect relation. However, it is noteworthy that several different structures could be derived from the transition depending on the direction of Gd atom displacement vectors if they are energetically comparable to those that occur in the ideal P21nb modification.
Actually we have observed rather large residual electron densities of about +/-17 e Å−3 in the direction of the b axis near atoms Gd5 and Gd6. This suggests a possible existence of local disorder of Gd atoms in the crystal structure. In addition, the structure refinement always suffered from the presence of inversion twins with nearly equal volumes. This may suggest an occurrence of any systematic misorientations of the Gd displacements near the composition plane of the twins.
Bontchev et al. (2000) grew crystals of the Cmcm modification by cooling the melt from 1573 to 1273 K at the rate of 1 K h−1 using the same SrCl2 flux as we did. On the other hand, we grew the crystals by cooling the melt from 1373 to 973 K at a rate of 5 K h−1 with subsequent cooling in the switched-off furnace. This suggests that the present P21nb polymorph might be a low-temperature modification with respect to the Cmcm polymorph. The decrease in the CN of Gd5 and Gd6 of the P21nb modification from 8 to 7, and a resultant shrinkage of the mean Gd—O bond length, is conformable with this assumption. Further studies to investigate the possible high temperature phase transition of the compound are being in progress.
The present structure belongs to the noncentrosymmetric space group P21nb which is polar along the a axis. The [Gd3O] matrix is supposed to be much insulative compared with the conductive [RuO6] chains along the c axis. Thus the crystal may be a candidate for a ferroelectric with a spontaneous polarization along the a axis.