Crystal structure of dicaesium strontium hexa­cyanidoferrate(II), Cs2Sr[Fe(CN)6], from laboratory X-ray powder data

Massoni, N.; Moloney, M.P.; Grandjean, A.; Misture, S.T.; Loye, H.-C. zur

doi:10.1107/S2056989020006660

research communications

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ISSN: 2056-9890

Volume 76| Part 6| June 2020| Pages 900-904

https://doi.org/10.1107/S2056989020006660

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Crystal structure of dicaesium strontium hexacyanidoferrate(II), Cs₂Sr[Fe(CN)₆], from laboratory X-ray powder data

Nicolas Massoni,^a ^* Mícheál P. Moloney,^a Agnès Grandjean,^a Scott T. Misture ^b and Hans-Conrad zur Loye ^c

^aCEA, DES, ISEC, DE2D, Univ Montpellier, Marcoule, France, ^bKazuo Inamori School of Engineering, Alfred University, Alfred, NY, 14802, USA, and ^cCenter for Hierarchical Waste Form Materials, Columbia, SC, 29208, USA
^*Correspondence e-mail: nicolas.massoni@cea.fr

Edited by M. Weil, Vienna University of Technology, Austria (Received 15 April 2020; accepted 18 May 2020; online 22 May 2020)

Ferrocyanides with general formula A^I_xB^II_y[Fe(CN)₆], where A and B are cations, are thought to accept many substitutions on the A and B positions. In this communication, the synthesis and crystal structure of Cs₂Sr[Fe(CN)₆] are reported. The latter was obtained from K₂Ba[Fe(CN)₆] particles, put in contact with caesium and strontium ions. Hence, a simultaneous ion-exchange mechanism (Cs for K, Sr for Ba) occurs to yield Cs₂Sr[Fe(CN)₆]. The synthesis protocol shows that K₂BaFe(CN)₆ particles can be used for the simultaneous trapping of radioactive caesium and strontium nuclides in water streams. Cs₂Sr[Fe(CN)₆] adopts the cryolite structure type and is isotypic with the known compound Cs₂Na[Mn(CN)₆] [dicaesium sodium hexacyanidomanganate(III)]. The octahedrally coordinated Sr and Fe sites both are located on inversion centres, and the eightfold-coordinated Cs site on a general position.

Keywords: crystal structure; X-ray powder diffraction; caesium strontium hexaferrocyanate; cryolite-type structure.

CCDC reference: 2004551

1. Chemical context

Ferrocyanides (FCN), such as Prussian blue, were discovered almost 300 years ago. The attractive properties of these materials for batteries and decontamination processes have ensured that FCNs remain an active research topic (Haas, 1993 ; Paolella et al., 2017 ). In particular, potassium copper FCN is currently being investigated for the purification of ¹³⁷Cs-contaminated water streams through partial exchange with potassium (Haas, 1993; Mimura et al., 1997 ). To the best of our knowledge, however, using FCNs to extract strontium either alone or with caesium has never been considered before. In the framework of the Center for Hierarchical Waste Forms (CHWM), an Energy Frontier Research Center (EFRC) funded by the US Department of Energy, we have been working on potassium copper FCN as an efficient K-ionic exchanger to capture ¹³⁷Cs and serve as a waste containment matrix (zur Loye et al., 2018 ). In this context, we have synthesized a cesium strontium FCN to study its efficiency in immobilizing both ⁹⁰Sr and ¹³⁷Cs, two radionuclides that are in most cases found together in radioactive water streams. Caesium strontium FCNs appear to be poorly described in the ICDD 2020 PDF4+ powder diffraction database (Gates-Rector & Blanton, 2019 ). Since the synthesized phase Cs₂Sr[Fe(CN)₆] did not match with existing entries, we decided to characterize the structure and we report the results herein.

2. Structural commentary

Cs₂Sr[Fe(CN)₆] is isotypic with Cs₂Na[Mn(CN)₆] (Ziegler et al., 1989 ). As shown in Table 1, the lattice parameters of Cs₂Sr[Fe(CN)₆] are slightly greater than those of Cs₂Na[Mn(CN)₆], but the cell volumes differ by less than 0.3%. The crystal structure adopts the cryolite structure type and comprises a framework of corner-sharing [Sr(CN)₆] (dark green in Fig. 1) and [Fe(CN)₆] octahedra (brown in Fig. 1). Both types of octahedra exhibit site symmetry $[\overline{1}]$ , with Sr situated on Wyckoff position 2 c, and Fe on 2 a. In the voids of this framework, Cs sites (light green in Fig. 1) have a distorted square-antiprismatic environment with four C and four N atoms as ligands. The substitution of manganese by iron in Cs₂Na[Mn(CN)₆] can be explained by the similar ionic radii of the two elements: r_Mn(III) = 0.58 Å and r_Fe(II) = 0.61 Å (Shannon, 1976 ). For the substitution of sodium by strontium, the ionic radii differ more substantially: r_Na(I) = 1.02 Å and r_Sr(II) = 1.18 Å. The two crystal structures were compared numerically using COMPSTRU (de la Flor et al., 2016 ). The structure similarity index Δ was calculated to be 0.009 (Bergerhoff et al., 1999 ). However, since only a few parameters (11) were refined and many parameters kept fixed in the refinement, the similarity index is not reliable.

Table 1
Comparison of lattice parameters (Å, °), selected bond lengths (Å) and volumes (Å³) for Cs₂Sr[Fe(CN)₆] and Cs₂Na[Mn(CN)₆]

	Cs₂Sr[Fe(CN)₆]		Cs₂Na[Mn(CN)₆]
a, b, c	7.6237 (2), 7.7885 (2), 10.9600 (3)	a, b, c	7.597 (1), 7.806 (1), 10.950 (1)
α, β, γ	90, 90.4165 (19), 90	α, β, γ	90, 90.07 (1), 90
V	650.76 (4)	V	649.36

Cs polyhedron volume*	44.56	Cs polyhedron volume*	44.46
Cs—C1	3.603 (4)	Cs—C1	3.6291 (5)
Cs—N1	3.348 (4)	Cs—N1	3.3688 (5)
Cs—C1ⁱ	3.628 (3)	Cs—C1ⁱ	3.5936 (5)
Cs—N1ⁱ	3.5232 (18)	Cs—N1ⁱ	3.4920 (5)
Cs—C2ⁱⁱ	3.592 (6)	Cs—C2ⁱⁱ	3.5831 (4)
Cs—N2ⁱⁱ	3.367 (6)	Cs—N2ⁱⁱ	3.3839 (3)
Cs—C3ⁱⁱⁱ	3.711 (5)	Cs—C3ⁱⁱⁱ	3.7044 (4)
Cs—N3ⁱⁱⁱ	3.274 (6)	Cs—N3ⁱⁱⁱ	3.2840 (3)
Sr octahedron volume*	20.49	Na octahedron volume*	20.45
Fe octahedron volume*	10.49	Mn octahedron volume*	10.47
Atomic bond lengths were not compared since the positions of C, N, Fe and Sr were not refined. Volumes were calculated by VESTA* V3.2.1 (Momma & Izumi, 2011 ). [Symmetry codes: (i) −x + $[{1\over 2}]$ , y − $[{1\over 2}]$ , −z + $[{1\over 2}]$ ; (ii) x + 1, y, z; (iii) x + $[{1\over 2}]$ , −y + $[{1\over 2}]$ , z + $[{1\over 2}]$ .]

Figure 1
Polyhedral plot of the cryolite-type Cs₂Sr[Fe(CN)₆] crystal structure.

3. Database survey

Ferrocyanides have rather complex structures. The ICDD 2020 PDF4+ database (Gates-Rector & Blanton, 2019) contains records of about 1521 phases with the general A^I_xB^II_y[Fe(CN)₆] ferrocyanide formula in which A and B are cations, with no constraints on the A:B ratio. As shown in Fig. 2, the studied sample contained only Cs, Sr, Fe, C and N. Hence, we focused on ferrocyanides with A^I = Cs and B^II = Sr, for which only three phases have been reported, however with poorly described crystal data. The Cs₂Sr[Fe(CN)₆] phase studied by Kuznetsov et al. (1970 ) is reported to crystallize in the tetragonal crystal system with a ranging from 10.72 to 10.89 Å and c from 10.75 to 10.99 Å (PDF00-024-0293 and PDF00-24-0294). The entry for the third phase (PDF00-048-1203), the hydrated ferrocyanide CsSr[Fe(CN)₆]·3H₂O reported by Slivko et al. (1988 ), is comprised only of reflections without further crystal data given. None of these PDF cards matched the X-ray diffraction pattern of the studied sample. As shown in Fig. 2a,b, the cubic crystal habit revealed by SEM measurements hints at a crystal structure with cubic symmetry, but the number of reflections is not consistent with such a highly symmetrical crystal system. The whole pattern can be described by a monoclinic cell and the experimental data are well reproduced by adjusting the reflections from the Cs₂Na[Mn(CN)₆] phase (Ziegler et al., 1989; PDF 04-012-3126). The Cs₂Sr[Fe(CN)₆] crystal structure was refined from that of Cs₂Na[Mn(CN)₆] assuming complete substitution of manganese by iron and sodium by strontium. As described above, the ionic radii of the corresponding metals are close enough for these substitutions to be possible.

Figure 2
(a) and (b) SEM-backscattered electron images of Cs₂Sr[Fe(CN)₆]. (c) EDS spectrum of Cs₂Sr[Fe(CN)₆].

4. Synthesis and crystallization

All solutions were prepared using Millipore water. Cs₂Sr[Fe(CN)₆] (Cs₂SrHCF) particles were not prepared directly by adding Sr and Cs salts to K₄[Fe(CN)₆]·3H₂O. Although it was found that Cs₂SrHCF could be prepared directly by adding aqueous Sr(NO₃)₂ to a K₄[Fe(CN)₆]·3H₂O/CsNO₃ solution, the yield was extremely poor (≤ 1%). Instead, an ion-exchange reaction was initiated by adding a mixed Sr(NO₃)₂/CsNO₃ solution to K₂Ba[Fe(CN)₆] particles, thereby simultaneously substituting barium for strontium and potassium for cesium. This simple approach, using K₂Ba[Fe(CN)₆]·2.6H₂O (K₂BaHCF) as an intermediate compound, allowed 1:1 amounts of Cs₂SrHCF to be produced from K₂BaHCF by ion exchange.

Briefly, the K₂BaHCF itself was prepared by adding a 1.5 M solution of Ba(NO₃)₂ to a 1 M solution of K₄[Fe(CN)₆]·3H₂O as described by Padigi et al. (2015 ). Once prepared, K₂BaHCF was collected by centrifugation, washed and dried. Its chemical composition (K, Fe and Ba) and water content, respectively, were determined by inductively coupled plasma (ICP) analysis and thermogravimetric analysis (TGA). The dried K₂BaHCF particles redispersed readily in water, producing a clear, slightly yellow dispersion. Cs₂SrHCF forms immediately as a milky white precipitate (Fig. 3a) upon adding the mixed CsNO₃/Sr(NO₃)₂ solution to the clear yellow K₂BaHCF dispersion. To ensure complete substitution, 2.2 moles of CsNO₃ and 1.1 moles of Sr(NO₃)₂ were added for every mole of K₂BaHCF present. After being left to mix for 1 h, the formed Cs₂SrHCF was collected by centrifugation, washed and dried. The chemical composition (Fe and Sr) of the powder was determined by ICP analysis while the Cs content was determined by atomic absorption spectroscopy (AAS). An initial characterization of the Cs₂SrHCF powder was carried out by TGA, UV–Vis and FT–IR spectroscopy. The UV–Vis spectrum of the Cs₂SrHCF (Fig. 3b) confirmed that the [Fe(CN)₆] moiety was maintained with only slight decreases in the wavelengths of the various absorption peaks (Gray & Beach, 1963 ). The FT–IR spectra of Cs₂SrHCF and K₂BaHCF are shown in Fig. 3c. While a δ(HOH) signal is observed for K₂BaHCF at 1611 cm⁻¹ along with ν(OH) signals at 3527 cm⁻¹ and 3601 cm⁻¹, no such signals were detected for Cs₂SrHCF. This absence of water was confirmed by TGA, which showed no mass loss between 30 and 400°C (Fig. S1 in the supporting information. The largest change was in the ν(M—N) stretching mode, which shifted from 421 cm⁻¹ ν(Ba—N) to 439 cm⁻¹ ν(Sr—N) (Fig. 3d).

Figure 3
(a) Pictures of a clear yellow K₂Ba[Fe(CN)₆]·2.6H₂O dispersion before (left) and immediately after (right) the addition of Cs/Sr. (b) UV–Vis spectra of K₂Ba[Fe(CN)₆]·2.6H₂O (black) and Cs₂Sr[Fe(CN)₆] (red). Both spectra show the characteristic features of the [Fe(CN)₆] moiety (Gray & Beach, 1963

). (c) FT–IR spectra of K₂BaHCF (black) and Cs₂SrHCF (red). All peaks shift to higher frequencies after ion exchange. The arrows indicate the δ(HOH) signal at 1611 cm⁻¹ and ν(OH) signals at 3527 cm⁻¹ and 3601 cm⁻¹ observed for K₂BaHCF but not for Cs₂SrHCF. These signals indicate the presence of structural water, which is common in hexacyanidoferrate particles. (d) Enlarged view of the FT–IR spectra in the 400–650 cm⁻¹ range.

5. Refinement

Crystal data and details of the data collection and structure refinement methods are summarized in Table 2. The observed and calculated intensities are shown in Fig. 4 along with the difference pattern. For the refinement of Cs₂Sr[Fe(CN)₆], atomic positions of the Cs₂Na[Mn(CN)₆] phase (Ziegler et al., 1989) and the given individual isotropic displacement parameters were used. All occupancies were set to unity because of the experimentally determined composition. Except for cesium, all displacement parameters were kept fixed because otherwise some became negative. The positions of the nitrogen and carbon atoms were also kept fixed. Since iron and strontium atoms are in special positions, only the lattice parameters, the position of the cesium atom and its U_iso value were refined, together with three profile parameters. The residual electron density is about is 6.06 e Å⁻³ at a distance of 0.71 Å from Cs.

Table 2
Experimental details

Crystal data
Chemical formula	Cs₂Sr[Fe(CN)₆]
M_r	565.4
Crystal system, space group	Monoclinic, P2₁/n
Temperature (K)	293
a, b, c (Å)	7.6237 (2), 7.7885 (2), 10.9600 (3)
β (°)	90.4165 (19)
V (Å³)	650.76 (4)
Z	2
Radiation type	Cu Kα₁, λ = 1.540562, 1.544390 Å
Specimen shape, size (mm)	Flat sheet, 25 × 25

Data collection
Diffractometer	Panalytical XPert MPD Pro
Specimen mounting	Packed powder pellet
Data collection mode	Reflection
Scan method	Step
2θ values (°)	2θ_min = 10.023 2θ_max = 130.010 2θ_step = 0.017

Refinement
R factors and goodness of fit	R_p = 0.031, R_wp = 0.043, R_exp = 0.025, R(F) = 0.101, χ² = 2.993
No. of parameters	11
Coordinates from an isotypic compound. Computer programs: Data Collector (Panalytical, 2011 ), JANA2006 (Petříček et al., 2014 ), VESTA (Momma & Izumi, 2011) and publCIF (Westrip, 2010 ).

Figure 4
Observed and calculated X-ray powder diffraction intensities for Cs₂Sr[Fe(CN)₆].

Supporting information

CCDC reference: 2004551

Crystal structure: contains datablocks cacesio, I. DOI: https://doi.org/10.1107/S2056989020006660/wm5555sup1.cif

TG curves of Cs2Sr[Fe(CN)6] and K2Ba[Fe(CN)6].2.6H2O. DOI: https://doi.org/10.1107/S2056989020006660/wm5555sup2.png

checkCIF report

Computing details top

Data collection: Data Collector (Panalytical, 2011); cell refinement: JANA2006 (Petříček et al., 2014); data reduction: JANA2006 (Petříček et al., 2014); program(s) used to solve structure: coordinates from isotypic compound; program(s) used to refine structure: JANA2006 (Petříček et al., 2014); molecular graphics: VESTA (Momma & Izumi, 2011); software used to prepare material for publication: publCIF (Westrip, 2010).

Dicaesium strontium hexacyanidoferrate top

Crystal data top

Cs₂Sr[Fe(CN)₆]	F(000) = 504
M_r = 565.4	y
Monoclinic, P2₁/n	D_x = 2.885 Mg m⁻³
a = 7.6237 (2) Å	Cu Kα₁ radiation, λ = 1.540562, 1.544390 Å
b = 7.7885 (2) Å	T = 293 K
c = 10.9600 (3) Å	Particle morphology: plate-like
β = 90.4165 (19)°	brown
V = 650.76 (4) Å³	flat_sheet, 25 × 25 mm
Z = 2	Specimen preparation: Prepared at 1873 K and 100 kPa, cooled at 30 K min⁻¹

Data collection top

Panalytical XPert MPD Pro diffractometer	Data collection mode: reflection
Radiation source: sealed X-ray tube	Scan method: step
Specimen mounting: packed powder pellet	2θ_min = 10.023°, 2θ_max = 130.010°, 2θ_step = 0.017°

Refinement top

R_p = 0.031	11 parameters
R_wp = 0.043	0 restraints
R_exp = 0.025	26 constraints
R(F) = 0.101	Weighting scheme based on measured s.u.'s
6881 data points	(Δ/σ)_max = 0.012
Profile function: Lorentzian	Background function: Manual background

Special details top

Refinement. The Platon test reports 21 Alerts level C. They could be gathered in three groups for explanation :

i)17 C-Alerts (out of 21) are "missing esd on x,y,z coordinates of N and C atoms. This is normal since these positions were not refined. Hence no esd was calculated by Jana2006.

ii)3 C-Alerts (out of 21) are due to a slighlty larger Fourier difference density than allowed by CheckCIF. I can not enhance the quality of the data so no reduction of this value can be done.

iii) The last C-Alert is about a "calc. and reported Sum Formula which differ. I did not find the origin of the Alert.

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

	x	y	z	U_iso*/U_eq
Cs	0.5080 (4)	−0.04022 (18)	0.2497 (7)	0.0410 (6)*
Sr	0	0	0.5	0.024*
Fe	0	0	0	0.019*
C1	0.0488	0.0087	0.178	0.029*
N1	0.0754	0.0169	0.2809	0.052*
C2	−0.2128	0.1444	0.022	0.03*
N2	−0.3347	0.2296	0.0368	0.048*
C3	0.1449	0.2097	−0.0251	0.029*
N3	0.2281	0.3301	−0.0406	0.046*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cs	0.0576 (17)	0.0343 (13)	0.0345 (16)	−0.016 (4)	0.0050 (17)	−0.008 (4)

Geometric parameters (Å, º) top

Cs—C1	3.603 (3)	Sr—N3ⁱ	2.4965 (1)
Cs—C1ⁱ	3.628 (2)	Sr—N3^vi	2.4965 (1)
Cs—N1	3.348 (3)	Fe—C1	1.9845 (1)
Cs—N1ⁱ	3.5232 (18)	Fe—C1^vii	1.9845 (1)
Cs—C2ⁱⁱ	3.592 (6)	Fe—C2	1.9901 (1)
Cs—N2ⁱⁱ	3.367 (6)	Fe—C2^vii	1.9901 (1)
Cs—C3ⁱⁱⁱ	3.711 (5)	Fe—C3	1.9920 (1)
Cs—N3ⁱⁱⁱ	3.274 (6)	Fe—C3^vii	1.9920 (1)
Sr—N1	2.4767 (1)	C1—N1	1.1462 (1)
Sr—N1^iv	2.4767 (1)	C2—N2	1.1543 (1)
Sr—N2^v	2.4857 (1)	C3—N3	1.1455 (1)
Sr—N2ⁱⁱⁱ	2.4857 (1)

Csⁱ—Cs—Cs^viii	89.42 (6)	N1—Cs—N3ⁱⁱⁱ	111.23 (16)
Csⁱ—Cs—Cs^ix	88.75 (4)	N1ⁱ—Cs—C2ⁱⁱ	115.67 (16)
Csⁱ—Cs—Cs^x	178.16 (6)	N1ⁱ—Cs—N2ⁱⁱ	127.6 (2)
Csⁱ—Cs—Cs^xi	94.04 (12)	N1ⁱ—Cs—C3ⁱⁱⁱ	142.4 (2)
Csⁱ—Cs—Cs^xii	93.31 (12)	N1ⁱ—Cs—N3ⁱⁱⁱ	130.34 (17)
Csⁱ—Cs—Sr	57.71 (7)	C2ⁱⁱ—Cs—N2ⁱⁱ	18.74 (3)
Csⁱ—Cs—Srⁱⁱ	128.17 (12)	C2ⁱⁱ—Cs—C3ⁱⁱⁱ	91.10 (5)
Csⁱ—Cs—Srⁱ	55.62 (7)	C2ⁱⁱ—Cs—N3ⁱⁱⁱ	89.11 (6)
Csⁱ—Cs—Sr^viii	125.77 (11)	N2ⁱⁱ—Cs—C3ⁱⁱⁱ	85.88 (4)
Csⁱ—Cs—C1	52.06 (4)	N2ⁱⁱ—Cs—N3ⁱⁱⁱ	89.50 (5)
Csⁱ—Cs—C1ⁱ	39.87 (4)	C3ⁱⁱⁱ—Cs—N3ⁱⁱⁱ	17.47 (3)
Csⁱ—Cs—N1	52.61 (4)	Cs^xiii—Sr—Cs	108.15 (11)
Csⁱ—Cs—N1ⁱ	35.27 (4)	Cs^xiii—Sr—Csⁱ	67.33 (6)
Csⁱ—Cs—C2ⁱⁱ	135.00 (18)	Cs^xiii—Sr—Cs^viii	71.79 (7)
Csⁱ—Cs—N2ⁱⁱ	134.15 (18)	Cs^xiii—Sr—Cs^iv	71.85 (11)
Csⁱ—Cs—C3ⁱⁱⁱ	133.13 (17)	Cs^xiii—Sr—Cs^xii	180
Csⁱ—Cs—N3ⁱⁱⁱ	135.35 (19)	Cs^xiii—Sr—Cs^xiv	108.21 (7)
Cs^viii—Cs—Cs^ix	178.16 (6)	Cs^xiii—Sr—Cs^vi	112.67 (6)
Cs^viii—Cs—Cs^x	88.75 (4)	Cs^xiii—Sr—N1	67.84 (8)
Cs^viii—Cs—Cs^xi	84.87 (11)	Cs^xiii—Sr—N1^iv	112.16 (8)
Cs^viii—Cs—Cs^xii	84.16 (11)	Cs^xiii—Sr—N2^v	55.97 (3)
Cs^viii—Cs—Sr	51.21 (7)	Cs^xiii—Sr—N2ⁱⁱⁱ	124.03 (3)
Cs^viii—Cs—Srⁱⁱ	121.54 (11)	Cs^xiii—Sr—N3ⁱ	137.39 (4)
Cs^viii—Cs—Srⁱ	123.59 (12)	Cs^xiii—Sr—N3^vi	42.61 (4)
Cs^viii—Cs—Sr^viii	53.50 (6)	Cs—Sr—Csⁱ	68.79 (6)
Cs^viii—Cs—C1	40.22 (4)	Cs—Sr—Cs^viii	73.18 (7)
Cs^viii—Cs—C1ⁱ	126.52 (9)	Cs—Sr—Cs^iv	180
Cs^viii—Cs—N1	37.42 (4)	Cs—Sr—Cs^xii	71.85 (11)
Cs^viii—Cs—N1ⁱ	124.14 (8)	Cs—Sr—Cs^xiv	106.82 (7)
Cs^viii—Cs—C2ⁱⁱ	98.28 (9)	Cs—Sr—Cs^vi	111.21 (6)
Cs^viii—Cs—N2ⁱⁱ	79.54 (8)	Cs—Sr—N1	41.51 (8)
Cs^viii—Cs—C3ⁱⁱⁱ	73.00 (7)	Cs—Sr—N1^iv	138.49 (8)
Cs^viii—Cs—N3ⁱⁱⁱ	90.47 (9)	Cs—Sr—N2^v	105.29 (4)
Cs^ix—Cs—Cs^x	93.09 (6)	Cs—Sr—N2ⁱⁱⁱ	74.71 (4)
Cs^ix—Cs—Cs^xi	95.39 (13)	Cs—Sr—N3ⁱ	52.52 (6)
Cs^ix—Cs—Cs^xii	95.81 (13)	Cs—Sr—N3^vi	127.48 (6)
Cs^ix—Cs—Sr	127.44 (12)	Csⁱ—Sr—Cs^viii	109.57 (11)
Cs^ix—Cs—Srⁱⁱ	59.69 (7)	Csⁱ—Sr—Cs^iv	111.21 (6)
Cs^ix—Cs—Srⁱ	55.27 (7)	Csⁱ—Sr—Cs^xii	112.67 (6)
Cs^ix—Cs—Sr^viii	127.94 (12)	Csⁱ—Sr—Cs^xiv	70.43 (11)
Cs^ix—Cs—C1	138.10 (8)	Csⁱ—Sr—Cs^vi	180
Cs^ix—Cs—C1ⁱ	51.68 (4)	Csⁱ—Sr—N1	61.18 (7)
Cs^ix—Cs—N1	140.75 (6)	Csⁱ—Sr—N1^iv	118.82 (7)
Cs^ix—Cs—N1ⁱ	54.06 (4)	Csⁱ—Sr—N2^v	36.60 (5)
Cs^ix—Cs—C2ⁱⁱ	83.13 (9)	Csⁱ—Sr—N2ⁱⁱⁱ	143.40 (5)
Cs^ix—Cs—N2ⁱⁱ	101.85 (11)	Csⁱ—Sr—N3ⁱ	70.10 (5)
Cs^ix—Cs—C3ⁱⁱⁱ	108.22 (12)	Csⁱ—Sr—N3^vi	109.90 (5)
Cs^ix—Cs—N3ⁱⁱⁱ	90.74 (10)	Cs^viii—Sr—Cs^iv	106.82 (7)
Cs^x—Cs—Cs^xi	85.93 (12)	Cs^viii—Sr—Cs^xii	108.21 (7)
Cs^x—Cs—Cs^xii	86.37 (12)	Cs^viii—Sr—Cs^xiv	180
Cs^x—Cs—Sr	120.84 (11)	Cs^viii—Sr—Cs^vi	70.43 (11)
Cs^x—Cs—Srⁱⁱ	52.95 (7)	Cs^viii—Sr—N1	51.05 (8)
Cs^x—Cs—Srⁱ	125.72 (13)	Cs^viii—Sr—N1^iv	128.95 (8)
Cs^x—Cs—Sr^viii	52.97 (6)	Cs^viii—Sr—N2^v	124.75 (7)
Cs^x—Cs—C1	126.21 (7)	Cs^viii—Sr—N2ⁱⁱⁱ	55.25 (7)
Cs^x—Cs—C1ⁱ	141.80 (10)	Cs^viii—Sr—N3ⁱ	122.47 (4)
Cs^x—Cs—N1	125.56 (5)	Cs^viii—Sr—N3^vi	57.53 (4)
Cs^x—Cs—N1ⁱ	146.56 (9)	Cs^iv—Sr—Cs^xii	108.15 (11)
Cs^x—Cs—C2ⁱⁱ	45.51 (8)	Cs^iv—Sr—Cs^xiv	73.18 (7)
Cs^x—Cs—N2ⁱⁱ	45.53 (9)	Cs^iv—Sr—Cs^vi	68.79 (6)
Cs^x—Cs—C3ⁱⁱⁱ	46.01 (8)	Cs^iv—Sr—N1	138.49 (8)
Cs^x—Cs—N3ⁱⁱⁱ	44.55 (9)	Cs^iv—Sr—N1^iv	41.51 (8)
Cs^xi—Cs—Cs^xii	166.72 (4)	Cs^iv—Sr—N2^v	74.71 (4)
Cs^xi—Cs—Sr	123.46 (9)	Cs^iv—Sr—N2ⁱⁱⁱ	105.29 (4)
Cs^xi—Cs—Srⁱⁱ	126.26 (10)	Cs^iv—Sr—N3ⁱ	127.48 (6)
Cs^xi—Cs—Srⁱ	59.14 (9)	Cs^iv—Sr—N3^vi	52.52 (6)
Cs^xi—Cs—Sr^viii	50.43 (7)	Cs^xii—Sr—Cs^xiv	71.79 (7)
Cs^xi—Cs—C1	75.93 (12)	Cs^xii—Sr—Cs^vi	67.33 (6)
Cs^xi—Cs—C1ⁱ	108.98 (13)	Cs^xii—Sr—N1	112.16 (8)
Cs^xi—Cs—N1	94.11 (14)	Cs^xii—Sr—N1^iv	67.84 (8)
Cs^xi—Cs—N1ⁱ	90.80 (13)	Cs^xii—Sr—N2^v	124.03 (3)
Cs^xi—Cs—C2ⁱⁱ	43.39 (9)	Cs^xii—Sr—N2ⁱⁱⁱ	55.97 (3)
Cs^xi—Cs—N2ⁱⁱ	41.07 (9)	Cs^xii—Sr—N3ⁱ	42.61 (4)
Cs^xi—Cs—C3ⁱⁱⁱ	125.88 (9)	Cs^xii—Sr—N3^vi	137.39 (4)
Cs^xi—Cs—N3ⁱⁱⁱ	130.40 (11)	Cs^xiv—Sr—Cs^vi	109.57 (11)
Cs^xii—Cs—Sr	52.97 (8)	Cs^xiv—Sr—N1	128.95 (8)
Cs^xii—Cs—Srⁱⁱ	55.18 (8)	Cs^xiv—Sr—N1^iv	51.05 (8)
Cs^xii—Cs—Srⁱ	133.85 (9)	Cs^xiv—Sr—N2^v	55.25 (7)
Cs^xii—Cs—Sr^viii	116.50 (7)	Cs^xiv—Sr—N2ⁱⁱⁱ	124.75 (7)
Cs^xii—Cs—C1	100.15 (13)	Cs^xiv—Sr—N3ⁱ	57.53 (4)
Cs^xii—Cs—C1ⁱ	83.65 (13)	Cs^xiv—Sr—N3^vi	122.47 (4)
Cs^xii—Cs—N1	81.64 (13)	Cs^vi—Sr—N1	118.82 (7)
Cs^xii—Cs—N1ⁱ	101.55 (14)	Cs^vi—Sr—N1^iv	61.18 (7)
Cs^xii—Cs—C2ⁱⁱ	131.46 (10)	Cs^vi—Sr—N2^v	143.40 (5)
Cs^xii—Cs—N2ⁱⁱ	128.90 (10)	Cs^vi—Sr—N2ⁱⁱⁱ	36.60 (5)
Cs^xii—Cs—C3ⁱⁱⁱ	43.02 (8)	Cs^vi—Sr—N3ⁱ	109.90 (5)
Cs^xii—Cs—N3ⁱⁱⁱ	42.36 (10)	Cs^vi—Sr—N3^vi	70.10 (5)
Sr—Cs—Srⁱⁱ	108.15 (15)	N1—Sr—N1^iv	180
Sr—Cs—Srⁱ	113.18 (5)	N1—Sr—N2^v	90.4970 (12)
Sr—Cs—Sr^viii	104.58 (4)	N1—Sr—N2ⁱⁱⁱ	89.5030 (12)
Sr—Cs—C1	47.71 (5)	N1—Sr—N3ⁱ	90.1559 (18)
Sr—Cs—C1ⁱ	80.76 (9)	N1—Sr—N3^vi	89.8441 (18)
Sr—Cs—N1	29.36 (6)	N1^iv—Sr—N2^v	89.5030 (12)
Sr—Cs—N1ⁱ	88.41 (8)	N1^iv—Sr—N2ⁱⁱⁱ	90.4970 (12)
Sr—Cs—C2ⁱⁱ	149.38 (7)	N1^iv—Sr—N3ⁱ	89.8441 (18)
Sr—Cs—N2ⁱⁱ	130.69 (6)	N1^iv—Sr—N3^vi	90.1559 (18)
Sr—Cs—C3ⁱⁱⁱ	78.33 (11)	N2^v—Sr—N2ⁱⁱⁱ	180
Sr—Cs—N3ⁱⁱⁱ	88.72 (14)	N2^v—Sr—N3ⁱ	89.952 (2)
Srⁱⁱ—Cs—Srⁱ	114.82 (5)	N2^v—Sr—N3^vi	90.048 (2)
Srⁱⁱ—Cs—Sr^viii	105.80 (4)	N2ⁱⁱⁱ—Sr—N3ⁱ	90.048 (2)
Srⁱⁱ—Cs—C1	154.37 (18)	N2ⁱⁱⁱ—Sr—N3^vi	89.952 (2)
Srⁱⁱ—Cs—C1ⁱ	91.92 (10)	N3ⁱ—Sr—N3^vi	180
Srⁱⁱ—Cs—N1	136.3 (2)	C1—Fe—C1^vii	180
Srⁱⁱ—Cs—N1ⁱ	105.47 (9)	C1—Fe—C2	90.5021 (17)
Srⁱⁱ—Cs—C2ⁱⁱ	84.45 (6)	C1—Fe—C2^vii	89.4979 (17)
Srⁱⁱ—Cs—N2ⁱⁱ	94.42 (6)	C1—Fe—C3	90.4341 (12)
Srⁱⁱ—Cs—C3ⁱⁱⁱ	48.54 (6)	C1—Fe—C3^vii	89.5659 (12)
Srⁱⁱ—Cs—N3ⁱⁱⁱ	31.08 (4)	C1^vii—Fe—C2	89.4979 (17)
Srⁱ—Cs—Sr^viii	109.57 (15)	C1^vii—Fe—C2^vii	90.5021 (17)
Srⁱ—Cs—C1	86.72 (10)	C1^vii—Fe—C3	89.5660 (12)
Srⁱ—Cs—C1ⁱ	50.37 (4)	C1^vii—Fe—C3^vii	90.4341 (12)
Srⁱ—Cs—N1	99.12 (9)	C2—Fe—C2^vii	180
Srⁱ—Cs—N1ⁱ	33.14 (5)	C2—Fe—C3	90.387 (2)
Srⁱ—Cs—C2ⁱⁱ	84.36 (13)	C2—Fe—C3^vii	89.613 (2)
Srⁱ—Cs—N2ⁱⁱ	94.50 (16)	C2^vii—Fe—C3	89.613 (2)
Srⁱ—Cs—C3ⁱⁱⁱ	163.24 (9)	C2^vii—Fe—C3^vii	90.387 (2)
Srⁱ—Cs—N3ⁱⁱⁱ	145.89 (8)	C3—Fe—C3^vii	180
Sr^viii—Cs—C1	77.56 (8)	Cs—C1—Cs^viii	99.91 (7)
Sr^viii—Cs—C1ⁱ	158.40 (18)	Cs—C1—Fe	112.69 (12)
Sr^viii—Cs—N1	86.35 (7)	Cs—C1—N1	68.07 (12)
Sr^viii—Cs—N1ⁱ	140.12 (19)	Cs^viii—C1—Fe	103.06 (12)
Sr^viii—Cs—C2ⁱⁱ	44.81 (5)	Cs^viii—C1—N1	75.62 (12)
Sr^viii—Cs—N2ⁱⁱ	26.11 (2)	Fe—C1—N1	178.6228 (6)
Sr^viii—Cs—C3ⁱⁱⁱ	77.49 (3)	Cs—N1—Cs^viii	107.31 (6)
Sr^viii—Cs—N3ⁱⁱⁱ	88.16 (4)	Cs—N1—Sr	109.13 (13)
C1—Cs—C1ⁱ	91.87 (6)	Cs—N1—C1	93.41 (13)
C1—Cs—N1	18.52 (2)	Cs^viii—N1—Sr	95.82 (12)
C1—Cs—N1ⁱ	84.74 (7)	Cs^viii—N1—C1	86.01 (12)
C1—Cs—C2ⁱⁱ	112.70 (17)	Sr—N1—C1	155.5560 (16)
C1—Cs—N2ⁱⁱ	97.66 (14)	Cs^xiii—C2—Fe	110.22 (4)
C1—Cs—C3ⁱⁱⁱ	109.87 (10)	Cs^xiii—C2—N2	69.56 (4)
C1—Cs—N3ⁱⁱⁱ	126.34 (13)	Fe—C2—N2	178.6331 (10)
C1ⁱ—Cs—N1	89.26 (7)	Cs^xiii—N2—Sr^xv	117.29 (5)
C1ⁱ—Cs—N1ⁱ	18.369 (10)	Cs^xiii—N2—C2	91.71 (5)
C1ⁱ—Cs—C2ⁱⁱ	127.70 (13)	Sr^xv—N2—C2	150.6891 (7)
C1ⁱ—Cs—N2ⁱⁱ	143.06 (18)	Cs^xvi—C3—Fe	120.47 (5)
C1ⁱ—Cs—C3ⁱⁱⁱ	124.0 (2)	Cs^xvi—C3—N3	59.13 (5)
C1ⁱ—Cs—N3ⁱⁱⁱ	113.08 (18)	Fe—C3—N3	179.4092 (8)
N1—Cs—N1ⁱ	87.85 (6)	Cs^xvi—N3—Sr^viii	106.31 (4)
N1—Cs—C2ⁱⁱ	127.37 (16)	Cs^xvi—N3—C3	103.40 (4)
N1—Cs—N2ⁱⁱ	110.12 (12)	Sr^viii—N3—C3	150.1635 (8)
N1—Cs—C3ⁱⁱⁱ	96.46 (12)

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

Comparison of lattice parameters (Å, °), selected bond lengths (Å) and volumes (Å³) for Cs₂Sr[Fe(CN)₆] and Cs₂Na[Mn(CN)₆] top

	Cs₂Sr[Fe(CN)₆]		Cs₂Na[Mn(CN)₆]
a, b, c	7.6237 (2), 7.7885 (2), 10.9600 (3)	a, b, c	7.597 (1), 7.806 (1), 10.950 (1)
α, β, γ	90, 90.4165 (19), 90	α, β, γ	90, 90.07 (1), 90
V	650.76 (4)	V	649.36

Cs polyhedron volume*	44.56	Cs polyhedron volume*	44.46
Cs—C1	3.603 (4)	Cs—C1	3.6291 (5)
Cs—N1	3.348 (4)	Cs—N1	3.3688 (5)
Cs—C1ⁱ	3.628 (3)	Cs—C1ⁱ	3.5936 (5)
Cs—N1ⁱ	3.5232 (18)	Cs—N1ⁱ	3.4920 (5)
Cs—C2ⁱⁱ	3.592 (6)	Cs—C2ⁱⁱ	3.5831 (4)
Cs—N2ⁱⁱ	3.367 (6)	Cs—N2ⁱⁱ	3.3839 (3)
Cs—C3ⁱⁱⁱ	3.711 (5)	Cs—C3ⁱⁱⁱ	3.7044 (4)
Cs—N3ⁱⁱⁱ	3.274 (6)	Cs—N3ⁱⁱⁱ	3.2840 (3)
Sr octahedron volume*	20.49	Na octahedron volume*	20.45
Fe octahedron volume*	10.49	Mn octahedron volume*	10.47

*Atomic bond lengths were not compared since the positions of C, N, Fe and Sr were not refined. Volumes were calculated by VESTA V3.2.1 (Momma & Izumi, 2011). [Symmetry codes: (i) -x+1/2, y - 1/2, -z + 1/2; (ii) x + 1, y, z; (iii) x + 1/2, -y + 1/2, z + 1/2.]

Funding information

Funding for this research was provided by: the Center for Hierarchical Waste Forms (CHWM), an Energy Frontier Research Center funded by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0016574.

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