Crystal structure of K0.75[FeII3.75FeIII1.25(HPO3)6]·0.5H2O, an open-framework iron phosphite with mixed-valent FeII/FeIII ions

Larrea, E.S.; Mesa, J.L.; Legarra, E.; Aguayo, A.T.; Arriortua, M.I.

doi:10.1107/S2056989015024007

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Volume 72| Part 1| January 2016| Pages 63-65

doi:10.1107/S2056989015024007

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Crystal structure of K_0.75[Fe^II_3.75Fe^III_1.25(HPO₃)₆]·0.5H₂O, an open-framework iron phosphite with mixed-valent Fe^II/Fe^III ions

Edurne S. Larrea,^a ^* José Luis Mesa,^b Estibaliz Legarra,^c Andrés Tomás Aguayo ^d and Maria Isabel Arriortua ^a,^c

^aDpto. Mineralogía y Petrología, Universidad del País Vasco, UPV/EHU, Sarrina s/n, 48940 Leioa, Spain, ^bDpto. Química Inorgánica, Universidad del País Vasco, UPV/EHU, Sarrina s/n, 48940 Leioa, Spain, ^cBasque Center for Materials, Applications & Nanostructures (BCMaterials), Parque Tecnológico de Zamudio, Camino de Ibaizabal, Edificio 500-1°, 48160 Derio, Spain, and ^dDpto. Ingeniería Química, Universidad del País Vasco, UPV/EHU, Sarrina s/n, 48940 Leioa, Spain
^*Correspondence e-mail: edurne.serrano@ehu.eus

Edited by M. Weil, Vienna University of Technology, Austria (Received 8 September 2015; accepted 14 December 2015; online 1 January 2016)

Single crystals of the title compound, potassium hexaphosphitopentaferrate(II,III) hemihydrate, K_0.75[Fe^II_3.75Fe^III_1.25(HPO₃)₆]·0.5H₂O, were grown under mild hydrothermal conditions. The crystal structure is isotypic with Li_1.43[Fe^II_4.43Fe^III_0.57(HPO₃)₆]·1.5H₂O and (NH₄)₂[Fe^II₅(HPO₃)₆] and exhibits a [Fe^II_3.75Fe^III_1.25(HPO₃)₆]^0.75− open framework with disordered K⁺ (occupancy 3/4) as counter-cations. The anionic framework is based on (001) sheets of two [FeO₆] octahedra (one with point group symmetry 3.. and one with point group symmetry .2.) linked along [001] through [HPO₃]²⁻ oxoanions. Each sheet is constructed from 12-membered rings of edge-sharing [FeO₆] octahedra, giving rise to channels with a radius of ca 3.1 Å where the K⁺ cations and likewise disordered water molecules (occupancy 1/4) are located. O⋯O contacts between the water molecule and framework O atoms of 2.864 (5) Å indicate hydrogen-bonding interactions of medium strength. The infrared spectrum of the compound shows vibrational bands typical for phosphite and water groups. The Mössbauer spectrum is in accordance with the presence of Fe^II and Fe^III ions.

Keywords: crystal structure; open-framework structure; hydrothermal synthesis; mixed-valent Fe^II/Fe^III compound; isotypism.

CCDC reference: 1442401

1. Chemical context

Open-framework materials have been a major research topic in materials science during the last decades because of their potential applications (Barrer, 1982 ; Wilson et al., 1982 ; Davis, 2002 , Adams & Pendlebury, 2011 ). Many efforts have been made to obtain porous materials using different oxoanions in combination with metals (Yu & Xu, 2010 ). The use of structure-directing agents or templates, not only organic but also inorganic, has also been extended in order to achieve this purpose. In this context, a new porous mixed-valent Fe^II/Fe^III phosphitoferrate with lithium cations and an open-framework structure, Li_1.43[Fe^II_4.43Fe^III_0.57(HPO₃)₆]·1.5H₂O, has been reported (Chung et al., 2011 ). This structure presents channels of ca 5.5 Å diameter along the [001] direction in which water molecules and lithium ions are located. The same type of framework but with Fe^II cations and with ammonium counter-anions was reported recently for (NH₄)₂[Fe^II₅(HPO₃)₆] (Berrocal et al., 2014 ).

Here we report on the synthesis and crystal structure of isotypic K_0.75[Fe^II_3.75Fe^III_1.25(HPO₃)₆]·0.5H₂O resulting from the replacement of lithium/ammonium by potassium. The iron cations in this compound are again in a mixed valence oxidation state of +II and +III.

2. Structural commentary

The asymmetric unit of K_0.75[Fe^II_3.75Fe^III_1.25(HPO₃)₆]·0.5H₂O (Fig. 1) contains two Fe sites on special positions (6f and 4d) with site symmetries of .2. and .3., respectively, three O sites, one P site and one H site. In addition, disordered sites associated with a water molecule (O1W) and the potassium counter-cation are present. The crystal structure is made up of two types of [FeO₆] octahedra linked via edge-sharing into sheets parallel to (001). These sheets consist of 12-membered rings formed by six [Fe1O₆] octahedra and six [Fe2O₆] octahedra. In one of the FeO₆ octahedra (Fe1), the Fe—O bond lengths range from 2.046 (2) to 2.179 (2) Å while in the [Fe2O₆] octahedron, a more uniform bond-length distribution from 2.134 (2) to 2.143 (2) is observed. In order to assign the content of Fe^II and Fe^III on these sites, a Mössbauer spectrum was recorded (Fig. 2). Three different components were observed, two doublets, corresponding to Fe^II cations, and a third doublet, corresponding to Fe^III cations. The determined Fe^II/Fe^III ratio is 3.1, in good agreement with the formula. According to bond-valence calculations (Brown, 2002 ), a clear assignment of which of the two iron sites carries the Fe^III cations cannot be made. The calculated bond-valence sum for site Fe1 assuming Fe^II is 2.213 valence units (v.u.), while assuming Fe^III gives 2.367. Corresponding values for the Fe2 site are 2.014 v.u. assuming Fe^II and 2.155 assuming Fe^III. The O—Fe—O bond angles of the two [FeO₆] octahedra are in the range between 78.10 (8) and 102.63 (7)° for cis- and between 175.77 (11) and 163.23 (8)° for the trans-angles.

Figure 1
Asymmetric unit of K_0.75[Fe^II_3.75Fe^III_1.25(HPO₃)₆]·0.5H₂O with displacement parameters drawn at the 50% probability level.

Figure 2
Mössbauer spectrum of the title compound showing the presence of Fe^II and Fe^III. The fit was made with the NORMOS program (Brand et al., 1983

The (001) iron oxide sheets are linked through phosphite groups whereby six anions share the innermost oxygen atoms of each ring (Fig. 3), forming 12-membered channels extending along [001]. The channels have a radius of about 3.1 Å. The P—O bond lengths of the anion range from 1.529 (2) to 1.541 (2) Å and are comparable with those of the two isotypic structures. The P—H distance in the title compound is 1.29 (4) Å, and the O—P—O bond angles range from 110.24 (11) to 114.32 (11)°.

Figure 3
Crystal structure of K_0.75[Fe^II_3.75Fe^III_1.25(HPO₃)₆]·0.5H₂O in polyhedral representation, in a projection along [001]. Colour code: Fe1O₆ octahedra are blue, Fe2O₆ octahedra are magenta, HPO₃ tetrahedra are orange, O atoms are red and K⁺ ions are grey. Hydrogen-bonding interactions between O1 from the framework and O1W are shown with dashed lines.

The disordered potassium cations and water molecules are located on special positions in the twelve-membered channels of the framework with site symmetries of 32. and 3.., respectively. The occupancy factors are 0.75 for potassium and 0.25 for the water molecule. Although the hydrogen atoms of the water molecule could not be located, the O⋯O distance of 2.864 (5) Å between the water O1W atom and the O1 atom of the framework indicates possible hydrogen-bonding interactions of medium strength. Because the O1W site is located on a threefold rotation axis, three hydrogen bonds with the inorganic skeleton with an angle of 113.42 (5)° are possible.

3. Synthesis and characterization

K_0.75[Fe^II_3.75Fe^III_1.25(HPO₃)₆]·0.5H₂O was synthesized under mild hydrothermal conditions and autogeneous pressure (10–20 bar at 343 K). The reaction mixture was prepared from 30 ml water, 2 ml of hypophosphorous acid, 0.17 mmol of KOH and 0.37 mmol of FeCl_3··6H₂O. The mixture had a pH value of ≃ 3.0. The reaction mixture was sealed in a polytetrafluoroethylene (PTFE)-lined steel pressure vessel, which was maintained at 343 K for five days. This procedure allowed the formation of single crystals of the title compound with a dark green to black colour.

The IR spectrum (see supporting information for this submission) shows typical bands corresponding to the stretching and deformation mode of the water molecules at 3235 and 2410 cm⁻¹, respectively. The spectrum also shows the stretching and deformation modes of the P—H bond at 1750 cm⁻¹. The bands corresponding to the symmetric (ν_s) and antisymmetric (ν_as) stretching vibrational modes of the (PO₃) groups appear at 930 and 1151 cm⁻¹, whereas the symmetric (δ_s) and antisymmetric (δ_as) deformation modes of this group are centred at 450 and 590 cm⁻¹ (Nakamoto, 1997 ; Chung et al., 2011).

Thermogravimetric analysis of the title compound (see supporting information for this submission) shows a first mass-loss process of 1.05% between room temperature and 498 K. This mass loss corresponds to the removal of water (theoretical value: 1.13%). Between 498 K and 673 K, another mass loss of 0.45% takes place which could not be assigned to a chemical reaction. This second process is followed by a third continuous process associated with a considerable gain of mass due to the oxidation of the compound.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. H atoms of the water molecule were not modelled. The hydrogen atom of the phosphite group was located in a difference density map and was refined without any constraint. Potassium and water oxygen sites are located in the channels. The occupancy factors of both atoms were initially set taking into account the previous characterization (themogravimetric measurement, Mössbauer spectrum fit). Some trials to refine the occupancy factors of these atoms were made. However, the results were very similar to those initially set, with a slight increase of reliability factors. Therefore, for the final model the occupancy factors were fixed at 0.75 for the K1 and at 0.25 for the O1W site.

Table 1
Experimental details

Crystal data
Chemical formula	K_0.75[Fe^II_3.75Fe^III_1.25(HPO₃)₆]·0.5H₂O
M_r	797.45
Crystal system, space group	Trigonal, P $[\overline{3}]$ c1
Temperature (K)	100
a, c (Å)	10.1567 (5), 9.2774 (6)
V (Å³)	828.82 (8)
Z	2
Radiation type	Mo Kα
μ (mm⁻¹)	5.14
Crystal size (mm)	0.29 × 0.05 × 0.04

Data collection
Diffractometer	Agilent SuperNova
Absorption correction	Analytical (CrysAlis PRO; Agilent, 2014 )
T_min, T_max	0.423, 0.845
No. of measured, independent and observed [I > 2σ(I)] reflections	5132, 647, 618
R_int	0.026
(sin θ/λ)_max (Å⁻¹)	0.664

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.026, 0.061, 1.19
No. of reflections	647
No. of parameters	54
H-atom treatment	All H-atom parameters refined
Δρ_max, Δρ_min (e Å⁻³)	0.8, −0.50
Computer programs: CrysAlis PRO (Agilent, 2014), OLEX2 (Dolomanov, 2009 ), SHELXL2014 (Sheldrick, 2015 ), DIAMOND (Brandenburg, 2001 ) and WinGX (Farrugia, 2012 ).

Supporting information

CCDC reference: 1442401

Crystal structure: contains datablocks I, 4R. DOI: 10.1107/S2056989015024007/wm5216sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S2056989015024007/wm5216Isup2.hkl

Supporting information file. DOI: 10.1107/S2056989015024007/wm5216Isup4.tif

Supporting information file. DOI: 10.1107/S2056989015024007/wm5216Isup5.tif

checkCIF report

Computing details top

Data collection: CrysAlis PRO (Agilent, 2014); cell refinement: CrysAlis PRO (Agilent, 2014); data reduction: CrysAlis PRO (Agilent, 2014); program(s) used to solve structure: OLEX2 (Dolomanov, 2009); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: WinGX (Farrugia, 2012).

Potassium hexaphosphitopentaferrate(II,III) hemihydrate top

Crystal data top

K_0.75[FeII3.75FeIII_1.25(HPO₃)₆]·0.5H₂O	D_x = 3.195 Mg m⁻³
M_r = 797.45	Mo Kα radiation, λ = 0.71073 Å
Trigonal, P3c1	Cell parameters from 3002 reflections
Hall symbol: -P 3 2"c	θ = 2.3–28.0°
a = 10.1567 (5) Å	µ = 5.14 mm⁻¹
c = 9.2774 (6) Å	T = 100 K
V = 828.82 (8) Å³	Prism, black
Z = 2	0.29 × 0.05 × 0.04 mm
F(000) = 779

Data collection top

Agilent SuperNova diffractometer	647 independent reflections
Radiation source: Nova (Mo) X-ray micro-source	618 reflections with I > 2σ(I)
Multilayer optics monochromator	R_int = 0.026
Detector resolution: 16.2439 pixels mm^-1	θ_max = 28.2°, θ_min = 2.3°
ω scans	h = −13→12
Absorption correction: analytical (CrysAlis Pro; Agilent, 2014)	k = −9→13
T_min = 0.423, T_max = 0.845	l = −12→10
5132 measured reflections

Refinement top

Refinement on F²	Primary atom site location: iterative
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.026	Hydrogen site location: difference Fourier map
wR(F²) = 0.061	All H-atom parameters refined
S = 1.19	w = 1/[σ²(F_o²) + (0.0254P)² + 2.2546P] where P = (F_o² + 2F_c²)/3
647 reflections	(Δ/σ)_max = 0.015
54 parameters	Δρ_max = 0.8 e Å⁻³
0 restraints	Δρ_min = −0.50 e Å⁻³

Special details top

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > 2σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	U_iso*/U_eq	Occ. (<1)
Fe1	0.62108 (5)	0	0.25	0.00709 (16)
Fe2	0.6667	0.3333	0.33159 (7)	0.00659 (18)
P1	0.70307 (8)	0.11196 (8)	0.58780 (7)	0.00748 (18)
O3	0.6916 (2)	0.1569 (2)	0.4316 (2)	0.0110 (4)
O2	0.3954 (2)	−0.1473 (2)	0.3120 (2)	0.0095 (4)
O1	0.8210 (2)	0.1342 (2)	0.1437 (2)	0.0124 (4)
K1	1	0	0.25	0.0247 (5)	0.75
O1W	1	0	0.063 (2)	0.033 (4)	0.25
H1	0.645 (4)	−0.033 (4)	0.592 (4)	0.011 (9)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Fe1	0.0070 (2)	0.0067 (3)	0.0075 (3)	0.00336 (14)	−0.00016 (10)	−0.00031 (19)
Fe2	0.0062 (2)	0.0062 (2)	0.0074 (3)	0.00310 (11)	0	0
P1	0.0072 (3)	0.0087 (3)	0.0068 (3)	0.0042 (3)	0.0006 (2)	0.0003 (2)
O3	0.0112 (10)	0.0147 (10)	0.0083 (9)	0.0075 (8)	0.0018 (7)	0.0024 (8)
O2	0.0091 (9)	0.0079 (9)	0.0113 (9)	0.0041 (8)	0.0023 (7)	−0.0004 (7)
O1	0.0153 (10)	0.0095 (10)	0.0140 (10)	0.0073 (9)	−0.0003 (8)	0.0002 (8)
K1	0.0151 (7)	0.0151 (7)	0.0439 (15)	0.0075 (3)	0	0
O1W	0.018 (5)	0.018 (5)	0.062 (12)	0.009 (2)	0	0

Geometric parameters (Å, º) top

Fe1—O1	2.046 (2)	P1—H1	1.29 (4)
Fe1—O1ⁱ	2.046 (2)	O2—P1^vii	1.534 (2)
Fe1—O2	2.096 (2)	O2—Fe2^viii	2.134 (2)
Fe1—O2ⁱ	2.096 (2)	O1—P1^ix	1.529 (2)
Fe1—O3ⁱ	2.179 (2)	O1—K1	2.935 (2)
Fe1—O3	2.179 (2)	K1—O1^x	2.935 (2)
Fe1—K1	3.8486 (6)	K1—O1^xi	2.935 (2)
Fe2—O2ⁱ	2.134 (2)	K1—O1^xii	2.935 (2)
Fe2—O2ⁱⁱ	2.134 (2)	K1—O1^xiii	2.935 (2)
Fe2—O2ⁱⁱⁱ	2.134 (2)	K1—O1ⁱ	2.935 (2)
Fe2—O3	2.143 (2)	K1—Fe1^xii	3.8486 (6)
Fe2—O3^iv	2.143 (2)	K1—Fe1^x	3.8486 (6)
Fe2—O3^v	2.143 (2)	K1—K1^xiv	4.6387 (3)
P1—O1^vi	1.529 (2)	K1—K1^xv	4.6387 (3)
P1—O2^vii	1.534 (2)	O1W—O1W^xv	1.17 (4)
P1—O3	1.541 (2)

O1—Fe1—O1ⁱ	97.50 (12)	O1^xi—K1—O1^xii	63.23 (8)
O1—Fe1—O2	167.07 (8)	O1^x—K1—O1^xiii	63.23 (8)
O1ⁱ—Fe1—O2	89.81 (8)	O1^xi—K1—O1^xiii	109.30 (4)
O1—Fe1—O2ⁱ	89.81 (8)	O1^xii—K1—O1^xiii	78.54 (8)
O1ⁱ—Fe1—O2ⁱ	167.07 (8)	O1^x—K1—O1	109.30 (4)
O2—Fe1—O2ⁱ	85.10 (11)	O1^xi—K1—O1	78.54 (8)
O1—Fe1—O3ⁱ	90.97 (8)	O1^xii—K1—O1	109.30 (4)
O1ⁱ—Fe1—O3ⁱ	91.82 (8)	O1^xiii—K1—O1	171.12 (8)
O2—Fe1—O3ⁱ	78.11 (8)	O1^x—K1—O1ⁱ	78.54 (8)
O2ⁱ—Fe1—O3ⁱ	98.73 (7)	O1^xi—K1—O1ⁱ	109.30 (4)
O1—Fe1—O3	91.82 (8)	O1^xii—K1—O1ⁱ	171.12 (8)
O1ⁱ—Fe1—O3	90.97 (8)	O1^xiii—K1—O1ⁱ	109.30 (4)
O2—Fe1—O3	98.73 (7)	O1—K1—O1ⁱ	63.23 (8)
O2ⁱ—Fe1—O3	78.11 (8)	O1^x—K1—Fe1^xii	140.73 (4)
O3ⁱ—Fe1—O3	175.77 (11)	O1^xi—K1—Fe1^xii	31.62 (4)
O1—Fe1—K1	48.75 (6)	O1^xii—K1—Fe1^xii	31.62 (4)
O1ⁱ—Fe1—K1	48.75 (6)	O1^xiii—K1—Fe1^xii	94.44 (4)
O2—Fe1—K1	137.45 (6)	O1—K1—Fe1^xii	94.44 (4)
O2ⁱ—Fe1—K1	137.45 (6)	O1ⁱ—K1—Fe1^xii	140.73 (4)
O3ⁱ—Fe1—K1	92.11 (5)	O1^x—K1—Fe1^x	31.62 (4)
O3—Fe1—K1	92.11 (5)	O1^xi—K1—Fe1^x	140.73 (4)
O2ⁱ—Fe2—O2ⁱⁱ	85.13 (8)	O1^xii—K1—Fe1^x	94.44 (4)
O2ⁱ—Fe2—O2ⁱⁱⁱ	85.13 (8)	O1^xiii—K1—Fe1^x	31.62 (4)
O2ⁱⁱ—Fe2—O2ⁱⁱⁱ	85.13 (8)	O1—K1—Fe1^x	140.73 (4)
O2ⁱ—Fe2—O3	78.10 (8)	O1ⁱ—K1—Fe1^x	94.44 (4)
O2ⁱⁱ—Fe2—O3	93.44 (7)	Fe1^xii—K1—Fe1^x	120
O2ⁱⁱⁱ—Fe2—O3	163.22 (8)	O1^x—K1—Fe1	94.44 (4)
O2ⁱ—Fe2—O3^iv	163.23 (8)	O1^xi—K1—Fe1	94.44 (4)
O2ⁱⁱ—Fe2—O3^iv	78.10 (8)	O1^xii—K1—Fe1	140.73 (4)
O2ⁱⁱⁱ—Fe2—O3^iv	93.44 (7)	O1^xiii—K1—Fe1	140.73 (4)
O3—Fe2—O3^iv	102.63 (7)	O1—K1—Fe1	31.62 (4)
O2ⁱ—Fe2—O3^v	93.44 (7)	O1ⁱ—K1—Fe1	31.62 (4)
O2ⁱⁱ—Fe2—O3^v	163.22 (8)	Fe1^xii—K1—Fe1	120
O2ⁱⁱⁱ—Fe2—O3^v	78.10 (8)	Fe1^x—K1—Fe1	120
O3—Fe2—O3^v	102.63 (7)	O1^x—K1—K1^xiv	109.64 (4)
O3^iv—Fe2—O3^v	102.63 (7)	O1^xi—K1—K1^xiv	70.36 (4)
O1^vi—P1—O2^vii	112.13 (11)	O1^xii—K1—K1^xiv	109.64 (4)
O1^vi—P1—O3	114.32 (11)	O1^xiii—K1—K1^xiv	70.36 (4)
O2^vii—P1—O3	110.24 (11)	O1—K1—K1^xiv	109.64 (4)
O1^vi—P1—H1	105.9 (16)	O1ⁱ—K1—K1^xiv	70.36 (4)
O2^vii—P1—H1	105.6 (16)	Fe1^xii—K1—K1^xiv	90
O3—P1—H1	108.1 (16)	Fe1^x—K1—K1^xiv	90
P1—O3—Fe2	135.47 (12)	Fe1—K1—K1^xiv	90
P1—O3—Fe1	123.82 (12)	O1^x—K1—K1^xv	70.36 (4)
Fe2—O3—Fe1	98.26 (8)	O1^xi—K1—K1^xv	109.64 (4)
P1^vii—O2—Fe1	127.45 (12)	O1^xii—K1—K1^xv	70.36 (4)
P1^vii—O2—Fe2^viii	130.31 (12)	O1^xiii—K1—K1^xv	109.64 (4)
Fe1—O2—Fe2^viii	101.19 (8)	O1—K1—K1^xv	70.36 (4)
P1^ix—O1—Fe1	129.84 (13)	O1ⁱ—K1—K1^xv	109.64 (4)
P1^ix—O1—K1	124.86 (11)	Fe1^xii—K1—K1^xv	90
Fe1—O1—K1	99.63 (7)	Fe1^x—K1—K1^xv	90
O1^x—K1—O1^xi	171.12 (8)	Fe1—K1—K1^xv	90
O1^x—K1—O1^xii	109.30 (4)	K1^xiv—K1—K1^xv	180

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

Acknowledgements

We gratefully acknowledge financial support of this work by the Ministerio de Economía y Competitividad (MAT2013–42092-R), the Gobierno Vasco (IT-630–13 and SAI12/82) and the University of the Basque Country (UFI-11/15). The authors also thank the technicians of SGIker (UPV/EHU), Dr J. Sangüesa, Dr Leire San Felices and Dr A. Larrañaga for the X-ray diffraction measurements. ESL thanks the Basque Government for her postdoctoral contract.

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