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Logo of actae2this articlesearchopen accesssubmitActa Crystallographica Section E: Crystallographic CommunicationsActa Crystallographica Section E: Crystallographic Communications
Acta Crystallogr E Crystallogr Commun. 2017 July 1; 73(Pt 7): 975–979.
Published online 2017 June 7. doi:  10.1107/S2056989017008271
PMCID: PMC5499272

The phase transition of rubidium hydrogen carbonate, RbHCO3


Rubidium hydrogen carbonate, RbHCO3, features an order/disorder phase transition at T C = 245 K from the high-temperature (HT) disordered C2/m modification to the low-temperature (LT) C-1 modification. The crystal structures are characterized by [HCO3]2 2− pairs of hydrogen carbonate groups connected by strong hydrogen bonding. The [HCO3]2 2− pairs are connected by Rb+ cations into a three-dimensional network. In HT-RbHCO3, the hydrogen atom is disordered. In LT-RbHCO3, ordering of the hydrogen atom leads to a translation­engleiche symmetry reduction of index 2. The lost reflections and rotations are retained as twin operations.

Keywords: crystal structure, order/disorder transition, hydrogen carbonate

Chemical context  

The crystal chemistry of partially protonated oxoanions of main group elements [e.g. hydrogen carbonates, (di)hydrogen phosphates, hydrogen sulfates etc] is characterized by the formation of strong hydrogen bonds. Topologically, the hydrogen-bonding network may lead to isolated units (e.g. pairs in KHCO3; Thomas et al., 1974  ), infinite chains (e.g. NaHCO3; Sass & Scheuerman, 1962  ) or two-dimensional networks (e.g. CsH2PO4; Uesu & Kobayashi, 1976  ). Compounds with such extended hydrogen-bonded network structures may be useful as proton conductors (Kim et al., 2015  ).

In many cases, at higher temperatures, the hydrogen atoms are dynamically disordered between the connected oxoanions. On cooling, the disorder is ‘frozen’, resulting in a reduction of symmetry (order/disorder phase transition). Such phase transitions are of technological importance, for example in the KH2PO4 (KDP) family of compounds and therefore have been studied extensively. At high temperatures, these compounds exist in a paraelectric tetra­gonal phase. On cooling below T C, they order into ortho­rhom­bic ferroelectrics. This kind of phase transition is likewise of theoretical inter­est, because it allows the study of proton quantum dynamics (Fillaux et al., 2008  ).

From a crystallographic point of view, these phase trans­itions offer the potential to study group/subgroup relationships (Müller, 2013  ). Moreover, in the case of a reduction of point symmetry, the lost symmetry is typically retained as a twin operation, leading to inter­esting as well as challenging problems.

A well known example of a hydrogen-bonding order/disorder transition is potassium hydrogen carbonate, KHCO3 (Kashida & Yamamoto, 1990  ). Above T C = 318 K, it crystallizes in a monoclinic C2/m phase featuring disorder of the hydrogen atom (Fillaux et al., 2008  ). On cooling, it transforms into an ordered P21/a phase (Thomas et al., 1974  ). Rubidium hydrogen carbonate RbHCO3 shows an analogous phase transition at T C = 245 K, which has been thoroughly studied by NMR spectroscopy (Odin, 2004  ). The published structural data, on the other hand, leave much to be desired. A structure model of the high-temperature (HT) modification in the C2 space group has been provided by Kim (1969  ). The structure was later redetermined by Cirpus (1997  ), establishing the correct space group C2/m and isotypism with KHCO3. The lattice metrics of the low-temperature (LT) modification were identified as triclinic by Müller & Roth (2005  ). Although a model was refined by these authors, structural data were not deposited. To fill this gap, in this communication we report detailed structural data of the LT modification of RbHCO3 which were derived from a twinned crystal. We also redetermined the structure of the HT modification. The phase transition is discussed in detail and contrasted to the structural changes observed in KHCO3.

An external file that holds a picture, illustration, etc.
Object name is e-73-00975-scheme1.jpg

Structural commentary  


The structure model of HT-RbHCO3 (C2/m) is in good agreement with that of Cirpus (1997  ). The crystal structures of HT-RbHCO3 (C2/m) and LT-RbHCO3 (CAn external file that holds a picture, illustration, etc.
Object name is e-73-00975-efi1.jpg) are closely related. The central building blocks are pairs of HCO3 anion groups, which are connected by strong hydrogen bonds (Fig. 1  , Tables 1  and 2  ). The prime cause for the order/disorder phase transition is the dynamic behaviour of the protons in these pairs. In HT-RbHCO3, they are dynamically disordered, resulting in a short [C—O1: 1.237 (4) Å] and two symmetry-equivalent inter­mediate [2×C—O2: 1.307 (3) Å] C—O bonds. The [HCO3]2 2− pair accordingly possesses 2/m point group symmetry. On cooling, the protons cannot overcome the tunneling barrier and are attached to distinct O atoms. In consequence, the point group symmetry of the [HCO3]2 2− pair is reduced to An external file that holds a picture, illustration, etc.
Object name is e-73-00975-efi1.jpg. There are two short [C—O1: 1.241 (4) Å, C—O3: 1.270 (4) Å] and one longer [C—O2: 1.349 (4) Å] bond, as is characteristic for partially hydrogenated oxoanions. In both cases, the HCO3 group is flat [distance of the C atom to the plane defined by the three O atoms: 0.000 (4) Å (HT) and 0.007 (3) Å (LT)], in accordance with literature data (Zemann, 1981  ).

Figure 1
[HCO3]2 2− pair in LT-RbHCO3 connected by strong hydrogen bonding. C and O atoms are represented by grey and red ellipsoids drawn at the 75% probability levels and H atoms by white spheres of arbitrary radius. Hydrogen bonding is indicated by ...
Table 1
Hydrogen-bond geometry (Å, °) for HT-RbHCO3
Table 2
Hydrogen-bond geometry (Å, °) for LT-RbHCO3

The [HCO3]2 2− pairs are connected by the Rb+ cations into a three-dimensional network (Figs. 2  and 3  ). The Rb+ cations are located on the reflection plane of the C2/m group (HT-RbHCO3) or on general positions (LT-RbHCO3). They are connected to six carbonate groups, two of which coordinate in a bidentate manner, the four others via one O atom (Fig. 4  ). Thus, in total, the Rb+ cations are coordinated by eight O atoms with bond lengths in the ranges 2.869 (3)–3.0662 (12) Å (HT-RbHCO3) and 2.865 (3)–3.101 (2) Å (LT-RbHCO3).

Figure 2
The crystal structures of (a) HT-RbHCO3, (b) LT-RbHCO3 and (c) LT-KHCO3 viewed down [001]. Rb/K atoms are represented by purple spheres, O atoms by red spheres, H atoms by colourless spheres and CO3 2− groups by yellow triangles.
Figure 3
The crystal structure of HT-RbHCO3 viewed down [010]. Atoms as in Fig. 2  , C atoms are grey.
Figure 4
Coordination environment of the Rb+ cations in (a) HT-RbHCO3 and (b) LT-RbHCO3. Atoms as in Fig. 1  , Rb+ cation are purple.

Symmetry reduction and relationship to KHCO3  

Whereas the HT-KHCO3 and HT-RbHCO3 phases are isotypic, the corresponding LT phases are not. To understand the different behaviour on cooling, it is useful to consider the structures as being made up of layers of [HCO3]2 2− pairs parallel to (100). In the HT phases, these layers possess the p12/m1 layer group symmetry. On cooling, owing to the ordering of the protons, the m [010] operation is lost. All [HCO3]2 2− pairs are rotated in the same direction about [001], resulting in layers with p An external file that holds a picture, illustration, etc.
Object name is e-73-00975-efi1.jpg symmetry.

Thus, the lower symmetry layers may appear in one out of two orientations with respect to the [010] direction. In the LT-RbHCO3 phase, all layers feature the same orientation. Adjacent layers are related by translation symmetry (as in the C2/m HT phase) and therefore the translation lattice is retained. The symmetry loss concerns only the point symmetry. Since one out of two symmetry operations is retained (viz. the translations and inversions), the symmetry reduction is of the t2 kind, where t stands for translationengleiche and 2 for the index of An external file that holds a picture, illustration, etc.
Object name is e-73-00975-efi1.jpg in 2/m (Müller, 2013  ).

In the LT-KHCO3 phase, on the other hand, layers feature alternating orientation with respect to [010]. The layers are split in two sets of translationally equivalent layers. The translation lattice is therefore reduced by an index of two, which here corresponds to a change of the Bravais lattice (mC to mP) while retaining the volume of the cell in the (more convenient but non-standard) centred setting. In return, adjacent layers are related by an a glide reflection. Thus, the point symmetry 2/m is retained. The symmetry reduction is therefore of the k2 kind, where k stands for klassengleiche (Müller, 2013  ).

The structural relationships of the HT and LT phases of RbHCO3 and KHCO3 are represented in a Bärnighausen family tree in Fig. 5  . The atomic labelling and coordinates of the KHCO3 modifications were adapted from the original literature (Fillaux et al., 2008  ; Thomas et al., 1974  ) to be comparable to the data presented here. Note that the fractional coordinates of all four phases depicted in Fig. 5  are remarkably similar.

Figure 5
Bärnighausen family tree (Bärnighausen, 1980  ) representing the symmetry reduction from the HT-KHCO3 and RbHCO3 modifications to their LT modifications. Coordinates of the KHCO3 modifications were adapted from Fillaux et al. (2008 ...

The atoms on the reflection planes in the HT phases are located on general positions in the LT phases. The O2 atom, which is located on a general position in the HT phase, is split into two positions in the LT phase. In contrast, the position of the H atom, which is also located on a general position in the HT modification, is not split. Instead, its occupancy is raised from 0.5 to 1.


Phase transitions are one of the classical causes of twinning (Hahn & Klapper, 2006  ; Stöger et al., 2016  ). If the point symmetry of the structure is reduced, the lost operations may be retained as twin operations. Indeed, the crystals of RbHCO3 were all systematically twinned below T C. The crystal under investigation was made up of two domains related by m [010] and equivalently 2[010], which corresponds precisely to the second coset in the coset decomposition of An external file that holds a picture, illustration, etc.
Object name is e-73-00975-efi1.jpg in 2/m. The twin volume ratio was determined by the TWINABS software as 51.3:48.7, which compares well to the volume ratio obtained from the (abandoned, see Section 3.3) refinement against HKLF 5 style data [52.0:48.0 (4)].

Since the transformation into the triclinic C An external file that holds a picture, illustration, etc.
Object name is e-73-00975-efi1.jpg LT phase results in a substantial increase of the γ angle to γ = 92.748 (9)°, the diffraction spots at higher k indices are clearly separated (Fig. 6  ). Such a twin cannot be treated as a twin by pseudo-merohedry. The α angle, on the other hand, deviates only slightly from the monoclinic metrics [α = 89.343 (4)°]. The lattice of the layers therefore is pseudo-rectangular, which is consistent with the crystallo-chemical considerations above.

Figure 6
Part of the hk2 plane in reciprocal space of LT-RbHCO3 reconstructed from CCD data. The reciprocal basis vectors of the two twin domains are indicated.

In KHCO3, the HT and LT phase feature the same point symmetry 2/m. Stacking faults therefore do not result in twinning but in anti­phase domains (Wondratschek & Jeitschko, 1976  ). These kinds of domains are significantly more difficult to qu­antify using X-ray diffraction.


Synthesis and crystallization  

Large crystals of RbHCO3 were grown by dissolving commercial ‘Rb2CO3’ (actually the sesquihydrate according to powder X-ray diffraction) in a small qu­antity of water followed by evaporation of the solution overnight at ca 295 K.

Data collection  

Crystals were cut to sizes suitable for single crystal diffraction with a razor blade. Abrupt cooling of the crystals to below the phase-transition temperature by immersion into a cooled N2 stream led to fourfold splitting of reflections as described by Müller & Roth (2005  ). Data reduction was successful using four orientation matrices and a reasonable structure model could be obtained. Nevertheless, the quality of the refinement was deemed not optimal (notably, the hydrogen atoms could not be located). From structural reasoning, only two domains are expected (see Section 2.2). The higher number of domains was therefore attributed to a cracking of the crystal under thermal stress. Therefore, a data collection was first performed above T C at 270 K. Then, the crystal was slowly (2 K h−1) cooled to 200 K and a full sphere of reciprocal space was collected with fine slicing. The first scan was discarded because it contained distinct reflections from the HT phase as well as two LT domains. The data set obtained from the remaining scans featured only the two expected LT twin domains.

Data processing  

Data of the HT modification was subjected to routine processing using SAINT and SADABS (Bruker, 2016  ). For the LT phase, reflections of both domains were separated and reduced to intensity data using overlap information. An absorption correction was applied using the TWINABS (Bruker, 2016  ) software. This software outputs ‘detwinned’ conventional data (HKLF 4 style), usually used for structure solution and data with overlap information (HKLF 5 style). Surprisingly, the detwinned data set resulted in significantly better refinements. Not only were the residuals lower by two percentage points, additionally only in the detwinned data could the hydrogen atoms be located and refined. Therefore the discussion is based on the refinement using the detwinned data set.

Structure solution and refinement  

An initial model of the HT modification was adapted from the data of Cirpus (1997  ). The structure of the LT modification was solved using the dual-space approach implemented in SHELXT (Sheldrick, 2015  ). Atomic coordinates and labelling were adapted to be analogous to those of the HT modification. The non-standard C An external file that holds a picture, illustration, etc.
Object name is e-73-00975-efi1.jpg setting of the P An external file that holds a picture, illustration, etc.
Object name is e-73-00975-efi1.jpg space group was chosen to facilitate comparison with the HT modification [lattice basis transformation from P An external file that holds a picture, illustration, etc.
Object name is e-73-00975-efi1.jpg to C An external file that holds a picture, illustration, etc.
Object name is e-73-00975-efi1.jpg: (a C, b C, c C) = (b P + 2c P, −b P, a P)]. The structure models were refined against F 2 with JANA2006 (Petříček et al., 2014  ). The hydrogen atoms were located in difference Fourier maps and the O—H distances restrained to 0.850 (1) Å. Crystal data, data collection and structure refinement details are summarized in Table 3  .

Table 3
Experimental details

Supplementary Material

Crystal structure: contains datablock(s) HT_RbHCO3, LT_RbHCO3, global. DOI: 10.1107/S2056989017008271/pk2601sup1.cif

Structure factors: contains datablock(s) HT_RbHCO3. DOI: 10.1107/S2056989017008271/pk2601HT_RbHCO3sup2.hkl

Structure factors: contains datablock(s) LT_RbHCO3. DOI: 10.1107/S2056989017008271/pk2601LT_RbHCO3sup3.hkl

CCDC references: 1554022, 1554021

Additional supporting information: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

(HT_RbHCO3)   Crystal data

RbHCO3F(000) = 272
Mr = 146.5Dx = 2.897 Mg m3
Monoclinic, C2/mMo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yCell parameters from 1945 reflections
a = 14.807 (3) Åθ = 2.8–32.6°
b = 5.8216 (12) ŵ = 14.54 mm1
c = 4.0217 (9) ÅT = 270 K
β = 104.321 (5)°Rod, colourless
V = 335.91 (12) Å30.43 × 0.18 × 0.09 mm
Z = 4

(HT_RbHCO3)   Data collection

Bruker KAPPA APEXII CCD diffractometer544 reflections with I > 3σ(I)
Radiation source: X-ray tubeRint = 0.051
ω– and [var phi]–scansθmax = 33.0°, θmin = 2.8°
Absorption correction: multi-scan (SADABS; Bruker, 2016)h = −22→20
Tmin = 0.05, Tmax = 0.27k = −8→8
3067 measured reflectionsl = −6→6
672 independent reflections

(HT_RbHCO3)   Refinement

Refinement on F20 constraints
R[F2 > 2σ(F2)] = 0.029H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.066Weighting scheme based on measured s.u.'s w = 1/(σ2(I) + 0.0004I2)
S = 1.26(Δ/σ)max = 0.011
672 reflectionsΔρmax = 0.86 e Å3
32 parametersΔρmin = −0.96 e Å3
1 restraint

(HT_RbHCO3)   Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2)

xyzUiso*/UeqOcc. (<1)
Rb0.66119 (3)00.29537 (9)0.02714 (13)
O10.6878 (2)0.51.0996 (8)0.0324 (10)
O20.57663 (15)0.3079 (3)0.7353 (5)0.0304 (7)
C0.6159 (3)0.50.8641 (9)0.0210 (10)
H0.522 (2)0.328 (12)0.608 (16)0.04 (2)*0.5

(HT_RbHCO3)   Atomic displacement parameters (Å2)

Rb0.0363 (3)0.02090 (17)0.0221 (2)00.00320 (15)0
O10.0244 (16)0.0332 (14)0.0343 (16)0−0.0030 (13)0
O20.0332 (13)0.0173 (8)0.0342 (11)0.0018 (7)−0.0040 (9)−0.0016 (7)
C0.0231 (19)0.0186 (14)0.0217 (17)00.0062 (14)0

(HT_RbHCO3)   Geometric parameters (Å, º)

Rb—O1i2.869 (3)Rb—O2vi3.001 (2)
Rb—O1ii3.046 (3)Rb—Ciii3.370 (2)
Rb—O1iii3.0662 (12)Rb—Civ3.370 (2)
Rb—O1iv3.0662 (12)O1—C1.237 (4)
Rb—O2iv2.908 (2)O2—C1.307 (3)
Rb—O2v2.908 (2)O2—H0.85 (4)
Rb—O23.001 (2)
O1i—Rb—O2iv138.83 (5)Rbvii—O1—C172.6 (3)
O1i—Rb—O281.45 (7)Rb—O2—Rbviii85.75 (5)
O1i—Rb—O2v138.83 (5)Rb—O2—C122.4 (2)
O1i—Rb—O2vi81.45 (7)Rb—O2—H102 (4)
O2iv—Rb—O285.75 (6)Rbviii—O2—C98.99 (16)
O2iv—Rb—O2v76.10 (5)Rbviii—O2—H135 (4)
O2iv—Rb—O2vi131.47 (6)C—O2—H113 (5)
O2—Rb—O2v131.47 (6)O1—C—O2121.17 (15)
O2—Rb—O2vi73.35 (6)O1—C—O2ix121.17 (15)
O2v—Rb—O2vi85.75 (6)O2—C—O2ix117.7 (3)

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

(HT_RbHCO3)   Hydrogen-bond geometry (Å, º)

O2—H···O2x0.85 (4)1.75 (4)2.571 (3)162 (6)

Symmetry code: (x) −x+1, y, −z+1.

(LT_RbHCO3)   Crystal data

RbHCO3Z = 4
Mr = 146.5F(000) = 272
Triclinic, C1Dx = 2.908 Mg m3
Hall symbol: -C 1Mo Kα radiation, λ = 0.71073 Å
a = 14.945 (3) ÅCell parameters from 1945 reflections
b = 5.8212 (9) Åθ = 2.8–32.6°
c = 3.9699 (6) ŵ = 14.60 mm1
α = 89.343 (4)°T = 200 K
β = 104.096 (4)°Rod, colourless
γ = 92.748 (9)°0.43 × 0.18 × 0.09 mm
V = 334.59 (9) Å3

(LT_RbHCO3)   Data collection

Bruker KAPPA APEXII CCD diffractometer1029 reflections with I > 3σ(I)
Radiation source: X-ray tubeRint = 0.059
ω– and [var phi]–scansθmax = 32.7°, θmin = 2.8°
Absorption correction: multi-scan (TWINABS; Bruker, 2016)h = −6→22
Tmin = 0.05, Tmax = 0.27k = −8→8
8005 measured reflectionsl = −6→5
1215 independent reflections

(LT_RbHCO3)   Refinement

Refinement on F20 constraints
R[F2 > 2σ(F2)] = 0.038H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.075Weighting scheme based on measured s.u.'s w = 1/(σ2(I) + 0.0004I2)
S = 1.50(Δ/σ)max = 0.018
1215 reflectionsΔρmax = 1.88 e Å3
50 parametersΔρmin = −1.81 e Å3
1 restraint

(LT_RbHCO3)   Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2)

Rb0.66221 (2)−0.00995 (4)0.29542 (8)0.01812 (10)
O10.68811 (18)0.4885 (4)1.0957 (7)0.0232 (8)
O20.57544 (19)0.2938 (4)0.7298 (7)0.0218 (8)
O30.57832 (19)0.6790 (4)0.7345 (6)0.0219 (8)
C0.6160 (2)0.4961 (5)0.8620 (8)0.0153 (9)
H0.529 (2)0.326 (7)0.570 (10)0.037 (13)*

(LT_RbHCO3)   Atomic displacement parameters (Å2)

Rb0.02362 (19)0.01426 (16)0.01495 (15)0.00035 (9)0.00184 (11)0.00014 (9)
O10.0190 (14)0.0226 (11)0.0236 (13)0.0006 (8)−0.0032 (10)0.0001 (8)
O20.0227 (14)0.0118 (10)0.0266 (13)0.0014 (8)−0.0022 (11)−0.0019 (8)
O30.0256 (14)0.0128 (10)0.0223 (12)0.0003 (8)−0.0033 (10)0.0007 (7)
C0.0155 (16)0.0151 (13)0.0155 (14)−0.0005 (10)0.0047 (12)−0.0003 (9)

(LT_RbHCO3)   Geometric parameters (Å, º)

Rb—O1i3.031 (2)Rb—O3v2.944 (3)
Rb—O1ii3.015 (3)Rb—Civ3.324 (3)
Rb—O1iii2.865 (3)Rb—Ci3.408 (3)
Rb—O1iv3.101 (2)O1—C1.241 (4)
Rb—O2i2.926 (2)O2—C1.349 (4)
Rb—O23.027 (3)O2—H0.85 (3)
Rb—O3iv2.885 (2)O3—C1.270 (4)
O1i—Rb—O1ii73.01 (7)O3iv—Rb—O3v85.85 (7)
O1i—Rb—O1iii94.75 (6)Rbvi—O1—Rbii106.99 (8)
O1i—Rb—O2i43.56 (6)Rbvi—O1—Rbiii85.25 (6)
O1i—Rb—O270.67 (7)Rbvi—O1—C96.77 (18)
O1i—Rb—O3iv116.17 (7)Rbii—O1—Rbiii84.89 (7)
O1i—Rb—O3v144.31 (8)Rbii—O1—C103.4 (2)
O1ii—Rb—O1iii84.89 (8)Rbiii—O1—C170.4 (3)
O1ii—Rb—O2i81.38 (7)Rb—O2—Rbvi83.64 (6)
O1ii—Rb—O2140.08 (6)Rb—O2—C122.1 (2)
O1ii—Rb—O3iv80.76 (7)Rb—O2—H97 (3)
O1ii—Rb—O3v141.28 (7)Rbvi—O2—C98.99 (17)
O1iii—Rb—O2i138.29 (6)Rbvi—O2—H149 (3)
O1iii—Rb—O282.17 (7)C—O2—H107 (3)
O1iii—Rb—O3iv139.80 (7)Rbvii—O3—Rbviii85.85 (6)
O1iii—Rb—O3v82.33 (7)Rbvii—O3—C121.9 (2)
O2i—Rb—O283.64 (7)Rbviii—O3—C98.73 (18)
O2i—Rb—O3iv76.08 (6)O1—C—O2117.2 (3)
O2i—Rb—O3v130.12 (8)O1—C—O3125.2 (3)
O2—Rb—O3iv130.57 (7)O2—C—O3117.6 (3)
O2—Rb—O3v73.70 (7)

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

(LT_RbHCO3)   Hydrogen-bond geometry (Å, º)

O2—H···O3ix0.85 (3)1.75 (3)2.582 (3)165 (4)

Symmetry code: (ix) −x+1, −y+1, −z+1.


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Articles from Acta Crystallographica Section E: Crystallographic Communications are provided here courtesy of International Union of Crystallography