Cs₂Zn(CN)₄: a first example of a non-cyano spinel of composition A₂M(CN)₄ with A=alkali metal and M=group 12 metal

1 Introduction

Despite their pronounced dumbbell-like shape compounds with CN⁻ or C₂²⁻ anions show a high tendency to crystallize in archetypal, sometimes distorted structure types very frequently found in oxides and fluorides, e. g. rock-salt (KCN [1], CaC₂ [2]), anti-fluorite (Na₂C₂ [3]), ReO₃ (Ga(CN)₃ [4]), anti-cuprite (Zn(CN)₂ [5]), elpasolite/cryolite (Cs₂LiM(CN)₆ with M=Mn, Fe, Co [6]), or spinel-type structures. Already in 1906, K₂Zn(CN)₄, Tl₂Zn(CN)₄, K₂Cd(CN)₄, K₂Hg(CN)₄, and Tl₂Hg(CN)₄ were recognized as compounds crystallizing with a cubic symmetry [7], and in 1922 the respective potassium compounds were unambiguously assigned to the spinel-type structure crystallizing in the cubic space group Fd3̅m (no. 227, Z=8) [8], [9], [10]. Later on Na₂Zn(CN)₄, Rb₂Zn(CN)₄, and Rb₂Cd(CN)₄ were added to this class of compounds [11], which is sometimes termed the “cyano spinel” type. Some of these cyano spinels show phase transitions and a distorted rhombohedral variant (R3̅c, Z=4) was found to be a high-pressure phase of K₂Zn(CN)₄ [12] and a low-temperature phase of K₂Hg(CN)₄ [13]. For the latter it was shown that this phase transition is accompanied by a change from paraelastic (cubic) to ferroelastic (rhombohedral) behavior [13]. For Rb₂Hg(CN)₄ already at room temperature the slightly distorted rhombohedral phase is found, which transforms to the undistorted cubic spinel structure at approx. T=398 K losing its ferroelastic properties [14]. Ternary cyanides with the general composition A₂M(CN)₄ with A=alkali metal and M=Zn, Cd are interesting starting materials for the respective acetylides A₂M(C₂H)₄ [15]. In our own studies, we came across Cs₂Zn(CN)₄, whose diffraction pattern did not show any similarity with a cubic or distorted rhombohedral spinel-type structure. In the following, we present the elucidation of its crystal structure and show that Cs₂Zn(CN)₄ crystallizes in a new and unprecedented structure type not related to the spinel structural family.

2 Experimental section

2.8 Rietveld refinement

Rietveld refinements were carried out with Gsas [26], [27]. The unit cell obtained with Jana2006 [24] and the positional parameters of Cs, Zn, C1, N1, C2, and N2 obtained with Endeavour [25] were used as a starting model for the refinement. The C–N bond lengths were fixed using the soft constraint C–N=1.15(2) Å. In the final refinement cycles all atoms were refined with isotropic temperature factors (U_iso), with the U_iso variables of the light atoms C and N constrained to one common value. Thus, in the final refinement cycles 36 variables were refined: a, b, c, β, zero shift, scale, two profile parameters (Pseudo-Voigt function), nine background parameters (Chebyshev function), 16 positional parameters, and three isotropic displacement parameters. With these considerations a stable refinement leading to a smooth convergence was achieved. Selected details of the crystal structure, the measurement and the refinement are summarized in Table 1, the resulting Rietveld fit is given in Fig. 1.

Table 1:

Selected crystallographic data and some refinement details of the synchrotron powder diffraction measurement of Cs₂Zn(CN)₄.

	Cs2Zn(CN)4
Formula Weight/g·mol−1	435.27
Space group, Z	C2/c (no. 15), Z=4
a/Å	14.4641(6)
b/Å	9.1914(4)
c/Å	8.5614(3)
β/deg	109.629(2)
V/Å3	1072.0(1)
Rp (with/without background)	0.1023/0.0921
wRp (with/without background)	0.1359/0.2123
RBragg	0.0361
χ2 (goodness of fit)	0.94
Data points	12 251
No. of refined parameters	36
No. of reflections	288
No. of restraints	3a
Background	Chebyshev function (9 terms)
Data range; step size/deg	2.0≤2θ≤26.5; 0.002
Instrument; detector	ESRF, BM31; 6 scintillation detectors
Radiation; wavelength λ /Å	synchrotron radiation, 0.49890
CCDC deposition numberb	1956613

1. ^aN1–C1: 1.15(2) Å; N2–C2: 1.15(2) Å; U_iso(N1)=U_iso(C1)=U_iso(N2)=U_iso(C2). ^bCCDC 1956613 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

Fig. 1:

Rietveld refinement of the synchrotron diffraction pattern of Cs₂Zn(CN)₄ (Swiss-Norwegian beamline BM31, ESRF, Grenoble, France; λ=0.49890 Å; T=295(2) K; quartz capillary: ∅=0.7 mm). Experimental data points (dark blue crosses), calculated profile (red solid line), background (blue solid line), and difference curve (turquoise curve below) are shown. Vertical red bars mark the positions of Bragg reflections of Cs₂Zn(CN)₄.

3 Results and discussion

From an aqueous solution containing Zn(CN)₂ and CsCN a crystalline powder of Cs₂Zn(CN)₄ precipitated, from which its crystal structure could be elucidated in a quite straight-forward manner (see Experimental Section). Already from the resulting diffraction pattern it was obvious that Cs₂Zn(CN)₄ does not crystallize in a cubic or slightly distorted spinel-type variant. The resulting crystal structure is depicted in Figs. 2 and 3, atomic coordinates and selected interatomic distances and angles are listed in Table 2. As found in all cyano spinels A₂B(CN)₄ reported up to now, the B²⁺ cation (here: Zn²⁺) is coordinated tetrahedrally by four CN⁻ groups with the carbon end pointing towards Zn²⁺ [9]. The resulting [Zn(CN)₄]²⁻ unit is shown in Fig. 2 (left). The C–N bond lengths were fixed to 1.15 Å in the refinement (refined to 1.155(5) Å in K₂Zn(CN)₄ from neutron powder diffraction data [9]), as such distances of weak scatterers like carbon and nitrogen cannot reliably be obtained from X-ray powder diffraction data. The C–Zn–C angles within the [Zn(CN)₄]²⁻ tetrahedron are close to the ideal angle of 109.5° (106(1)–111(1)°, Table 2 b)) and the Zn–C–N angles deviate slightly from linearity (170(3)° and 166(3)°, Table 2b)). Slight deviations from linearity are also found in the rhombohedral variant of K₂Zn(CN)₄ (∠Zn–C2–N2=177.45° [12]), whereas in the cubic structure they are restricted to 180° for symmetry reasons. In Cs₂Zn(CN)₄ two crystallographically distinct CN⁻ anions are found. It is somewhat surprising that the Zn–C bond lengths differ by 0.13 Å: Zn–C1=1.84(2) Å (2×) vs. Zn–C2=1.97(4) Å (2×). For cubic K₂Zn(CN)₄ Zn–C=2.018(4) Å (4×) [9] and for rhombohedral K₂Zn(CN)₄ the bond lengths Zn–C1=2.028 Å and Zn–C2=2.002 Å (3×) [12] have been reported. As the coordination spheres around ⁻C1≡N1 and ⁻C2≡N2 do not show distinct differences, it must be assumed that the large differences of the Zn–C1 and Zn–C2 bond lengths in Cs₂Zn(CN)₄ are an artefact of the refinement from X-ray powder diffraction data. The large standard deviations support this assumption. However, in Tl₂Zn(CN)₄ a similar spread of Zn–C bond lengths was found (1.97–2.06 Å [28]) so that these deviations are not unprecedented. The distortions within the ZnC₄ tetrahedron of Cs₂Zn(CN)₄ were calculated using the Continuous Shape Measures (CShM) approach by Llunell et al. [29], [30]. A low CShM_T-4 value of 0.153 points to a tetrahedron with small distortions.

Fig. 2:

Left: coordination sphere around Zn²⁺ with the atomic numbering scheme in the crystal structure of Cs₂Zn(CN)₄. Atoms not being part of the asymmetric unit are not numbered and drawn in a slightly transparent mode. Right: View of the unit cell of Cs₂Zn(CN)₄ using the same color code as in the figure on the left.

Fig. 3:

Left: coordination sphere around Cs⁺ in the crystal structure of Cs₂Zn(CN)₄. For side-on coordinating CN⁻ anions their center of gravity was used to draw the polyhedron around Cs⁺. Right: Projection of the crystal structure of Cs₂Zn(CN)₄ in a view along [010]. The same color code as in Fig. 2 was used.

In Fig. 2 (right) it is shown how layers of [Zn(CN)₄]²⁻ tetrahedra are separated by Cs⁺ cations in Cs₂Zn(CN)₄. This is also obvious from the projection of the crystal structure along [010] shown in Fig. 3 (right).

In Fig. 3 (left) the coordination sphere around a Cs⁺ cation is depicted. Each Cs⁺ cation is surrounded by eight CN⁻ anions. Six of them coordinate in a side-on mode and two end-on via the nitrogen atom. The respective Cs–N bond lengths start at 3.25(3) Å and the Cs–C bond lengths at 3.58(3) Å. These CN⁻ groups stem from six different [Zn(CN)₄]²⁻ tetrahedra so that two tetrahedra coordinate with two of their CN⁻ groups in a chelating fashion. This coordination of the Cs⁺ cation by eight CN⁻ groups is different from that in the cyano spinels, where a coordination of the A⁺ cation by only six CN⁻ anions is found. To characterize the Cs(CN)₈ polyhedron in more detail, the end-on coordinating CN⁻ groups are reduced to the coordinating nitrogen atoms and the side-on coordinating anions to their center of gravity. The resulting polyhedron is emphasized in Fig. 3 (left). Using the Continuous Shape Measures (CShM) approach [29], [30] with these eight nodes the lowest CShM value is calculated for a square antiprism with CShM_SAPR-8=2.786. As the second best value (CShM_TDD-8=4.027) for a trigonal dodecahedron is significantly larger, the Cs(CN)₈ polyhedron in Cs₂Zn(CN)₄ is best described by a square antiprism.

The composition of Cs₂Zn(CN)₄ has been further corroborated by an elemental analysis (see experimental section) and IR/Raman spectroscopic investigations (Figure S1, Supporting Information available online). The distinctive C≡N stretching vibration is found at 2148 cm⁻¹ (IR) and 2146 cm⁻¹ (Raman), respectively. This is in very good agreement with the results reported in the literature on comparable compounds [31] as well as our own measurements on Rb₂Zn(CN)₄ (2149 cm⁻¹, IR, Figure S1 (left), Supporting Information available online).

In Fig. 4 the results of a DSC/TGA measurement, heating Cs₂Zn(CN)₄ under inert conditions up to 1000°C, are shown. At approx. T=380°C an endothermal signal is observed followed by a mass loss of 67% indicating the decomposition of the material. In Figure S2 (Supporting Information available online) the results of another DSC/TGA measurement, heating Cs₂Zn(CN)₄ under inert conditions up to 500°C and then cooling down to room temperature, are shown. The endothermal event (~380°C) is reversible with a hysteresis (approx. 300°C upon cooling). The observed minor mass loss of ~1% might be attributed to some humidity on the surface of the sample. The material obtained after heating to 500°C was analyzed by X-ray powder diffraction and compared with the starting material. The results are shown in Figure S3 (Supporting Information available online). Cs₂Zn(CN)₄ is obtained in a mainly unchanged form. After the heating procedure the crystallinity of the material had slightly decreased leading to a reduced signal-to-noise ratio, but no additional signals pointing to possible decomposition products are visible.

Fig. 4:

DSC (red) and TGA curves (blue) of Cs₂Zn(CN)₄ upon heating to T=1000°C under inert conditions.

According to the cubic to rhombohedral phase transitions found in some of the cyano spinels we speculated that a similar phase transition might occur in Cs₂Zn(CN)₄ at approx. T=380°C. To corroborate this assumption we recorded temperature-dependent synchrotron powder diffraction data at the ESRF (Swiss-Norwegian beamline). The resulting patterns are shown in Figure S4 (Supporting Information available online). At 400 and 450°C – temperatures above the endothermal event observed in the DSC measurements – the patterns indicate a completely amorphous material. Upon cooling, the resulting diffraction patterns show a different appearance and an increased crystallinity as indicated by an improved signal-to-noise ratio. This is very obvious from Figure S5 (Supporting Information available online), where the patterns obtained at 200°C upon heating and cooling are compared. However, when trying to index the resulting “new” diffraction patterns the same monoclinic unit cell was obtained. Visual inspection of the capillary indicated the formation of a solidified melt, i.e. Cs₂Zn(CN)₄ melts at 380°C and solidifies at approx. 300°C upon cooling. With the small focus of the synchrotron beam the resulting crystallites lead to strong preferred orientation effects explaining the “wrong”, i.e. changed intensities upon cooling. The melting of the material also explains the small mass losses observed in the TGA measurements (Figure S2, Supporting Information available online), as some of the material might evaporate after melting. In Table S1 (Supporting Information available online) the results of the Rietveld (upon heating) and Le-Bail fits (upon cooling) are summarized. The calculated unit cell volumes are plotted in Fig. 5 against the temperature. The expected linear increase of the unit cell volume with increasing temperature as well as the good agreement of the volumes obtained upon heating and cooling indicate that Cs₂Zn(CN)₄ did not decompose. Obviously, no phase transition occurred at 380°C, but instead a reversible melting and recrystallization of the material took place. We were unable to isolate single crystals of Cs₂Zn(CN)₄ from the solidified melt, but we are optimistic that under improved crystallization conditions this will be feasible in the future.

Fig. 5:

Unit cell volume of Cs₂Zn(CN)₄ as obtained from synchrotron powder diffraction data (SNBL beamline, ESRF) upon heating (orange squares) and cooling (transparent blue squares). The calculated error bars are smaller than the symbol used for the figure.

4 Conclusion

Using high-resolution synchrotron powder diffraction data (Swiss-Norwegian beamline, ESRF, Grenoble, France) we were able to solve and refine the crystal structure of Cs₂Zn(CN)₄. In contrast to all other known cyanides of composition A₂B(CN)₄ with A=Na–Cs, Tl and B=Zn, Cd, Hg, no cubic or distorted rhombohedral spinel-type structure with A⁺ in octahedral and B²⁺ in tetrahedral voids is found, but a new monoclinic structure type (C2/c, Z=4) with B²⁺ still in tetrahedral voids, but Cs⁺ in a square antiprismatic coordination of CN⁻ anions. Upon heating no phase transition is observed, but at approx. 380°C a melting of Cs₂Zn(CN)₄ occurs. We were unable to isolate single crystals from the solidified melt suitable for an X-ray single crystal structure analysis, but in the future we will try to optimize the crystallization to obtain such single crystals.

In Table 3 the ratios of the ionic radii of A⁺ (coordination number: VI) and B²⁺ (coordination number: IV) according to Shannon [32] are listed for different cyanides A₂B(CN)₄. It is well-known that the cubic spinel-type structure can accommodate metal cations A and B with a wide range of different radii by adapting the sizes of its tetrahedral and octahedral voids. However, there seems to be a limit for this flexibility. Among all compounds listed in Table 3, Cs₂Zn(CN)₄ shows the highest ratio of r(A⁺) to r(B²⁺). Obviously, this ratio is too large to be adapted by the cubic spinel-type structure and accordingly a new structure type is found with a higher coordination number of Cs⁺ (coordination number: VIII). For lower r(A⁺):r(B²⁺) ratios the rhombohedrally distorted variant seems to become more likely. For example, for K₂Hg(CN)₄ with the lowest ratio the rhombohedral variant is found as a low-temperature phase and for Rb₂Hg(CN)₄ with the small value of 1.51 the rhombohedral variant is already stable at room temperature. Since it has been argued that this cubic to rhombohedral transition cannot be understood in terms of “steric interactions but must be a dynamic effect” [13], it seems to be even more interesting to clarify whether the missing members in Table 3 follow our simple “size argument”. Accordingly, we expect Na₂Cd(CN)₄ and Na₂Hg(CN)₄ to crystallize in the rhombohedral variant (R3̅c, Z=4).

5 Supporting information

IR and Raman spectra, DSC/TGA curve for heating and cooling in the range RT to 500°C, a comparison of X-ray powder diffraction patterns obtained as-synthesized and after heating to 500°C in the DSC/TG, temperature-dependent synchrotron powder diffraction patterns, a comparison of synchrotron powder diffraction patterns obtained at T=200°C and results of the Rietveld and Le-Bail fits of the temperature-dependent synchrotron powder diffraction patterns of Cs₂Zn(CN)₄ are given as Supplementary Material available online (DOI: 10.1515/znb-2019-0159).