A new tilt and an old twist on the nickel arsenide structure-type: synthesis and characterisation of the quaternary transition-metal cyanamides A2MnSn2(NCN)6 (A = Li and Na)

Alex J. Corkett *a and Richard Dronskowski ab
aChair of Solid-State and Quantum Chemistry, Institute of Inorganic Chemistry, RWTH Aachen University, 52056 Aachen, Germany. E-mail: alexander.corkett@ac.RWTH-aachen.de
bHoffmann Institute of Advanced Materials, Shenzhen Polytechnic, 7098 Liuxian Blvd, Nanshan District, Shenzhen, China

Received 26th July 2019 , Accepted 29th August 2019

First published on 30th August 2019

In this work, we describe the synthesis and structure of the quaternary transition-metal cyanamides Na2MnSn2(NCN)6 and Li2MnSn2(NCN)6. These phases crystallise isotypically in layered structures, with P[3 with combining macron]1m symmetry, that comprise hexagonal close-packed arrays of NCN2− anions with metal cations in 5/6 of the octahedral holes, thereby reflecting low-symmetry modifications of the hierarchical [NiAs]-type MNCN structure. The distinct coordination requirements of the metal cations template an ordered decoration across the octahedral sites with corundum-like [Sn2(NCN)3]2+ layers alternating with [A2Mn(NCN)3]2− layers which resemble a portion of the Li2Zr(NCN)3 structure. This motif is also mirrored in the form of the NCN2− anions which adopt N–C[triple bond, length as m-dash]N2− cyanamide shapes with clear single- and triple-bond character. Distortion-mode analysis reveals the importance of K1 octahedral twist and K2 cyanamide tilt displacements in stabilising these phases, the latter of which is only accessible because of the extended nature of the NCN2− anion. These are the first examples of non-binary transition-metal cyanamides to be discovered and this study highlights how the additional flexibility of the NCN2− anion affords a novel structure-type not observed in oxide chemistry.


In recent years, the physical properties of solid-state transition-metal carbodiimides with the general formula Mx(NCN)y (M = Cr–Cu) have been exploited to great effect, particularly with respect to their electrochemical and photocatalytic activities.1–4 These binary phases are well thought of as nitrogen-containing analogues of MO oxides generated through the isovalent replacement of O2− by the extended carbodiimide (linear N[double bond, length as m-dash]C[double bond, length as m-dash]N moieties with D∞h symmetry) or cyanamide (less symmetric N–C[triple bond, length as m-dash]N2− moieties) anions, resulting in phases with enhanced covalent character. The preparation of such materials is, however, a considerable synthetic challenge and eventually came to fruition only after their existence as metastable, but nonetheless inert phases, had been predicted by first-principles electronic-structure calculations.5 A natural development is to extend this family to include ternary and higher-order phases, thereby mimicking the structural and compositional versatility of oxides and potentially leading to new or enhanced properties. However, before exploring this idea further it is expeditious to first consider the structure and bonding in Mx(NCN)y binaries.

MnNCN was the first non-d10 transition-metal carbodiimide to be discovered, and its crystal structure is characterised by cubic close-packed (ccp) layers of NCN2− with Mn2+ in the octahedral holes (Fig. 1(a)).6 It is therefore isotypic to the high-temperature structure of NaN3 and well understood in terms of a rhombohedrally distorted [NaCl]-type rock-salt structure (i.e. MnO). A similar, but topologically distinct structure is adopted by MNCN binaries with M = Fe, Co and Ni.7,8 In this case the divalent metals retain an octahedral coordination, however, the alternate hexagonal close-packed (hcp) array of NCN2− results in extended [NiAs]-type structures (Fig. 1(b)). Cr2(NCN)3, the only example of a trivalent transition-metal carbodiimide discovered to date,9 crystallises like its oxide counterpart Cr2O3 in a [corundum]-type motif which is well thought of as a vacancy-ordered derivative of [NiAs] with ⅔ of the octahedral sites occupied in each layer (Fig. 1(c)). The structures of these simple binary transition-metal carbodiimides are therefore very similar to those of their oxide analogues, with a propensity for close-packed anionic arrays.

image file: c9dt03062j-f1.tif
Fig. 1 Crystal structures and stacking patterns of the binary Mx(NCN)y transition-metal carbodiimides: [NaCl]-type MNCN (a), [NiAs]-type MNCN (b) and [corundum]-type M2(NCN)3 (c).

With a view to preparing cation-ordered transition-metal carbodiimides, it is encouraging that low-symmetry modifications of these close-packed structures are adopted by a number of ternary carbodiimides with the general formula AxMy(NCN)x+y (A = alkali metal, M = main-group or rare-earth metal). For example, the LiM(NCN)2 family (M = Al, In, Y and Yb) crystallises in cation-ordered [NiAs]-type structures with 1D chains of like cations in the orthorhombic sub-group Pbcn,10,11 whilst the incorporation of tetravalent metals has been demonstrated in A2Sn(NCN)3 phases (A = Li and Na) which crystallise in the orthorhombic subgroup Pnna and Li2M(NCN)3 phases (M = Hf and Zr) with R[3 with combining macron]c symmetry.12 In addition, M(NCN)2 phases (M = Hf and Zr) have been prepared with structures closely related to LiM(NCN)2 but with 50% ordered vacancies which template an open wine-rack-like framework.13

Motivated by this recent work we sought to prepare ternary transition-metal carbodiimides with the general formula MM′(NCN)3 which combine divalent transition-metal (M) and tetravalent main-group metal cations (M′). MnSn(NCN)3 was selected as an initial synthetic target since its oxide analogue MnSnO3 is known to adopt both ilmenite and LiNbO3 structures (i.e. cation-ordered derivatives of corundum), depending on the synthetic conditions employed.14,15 MnSn(NCN)3 was targeted via a solid-state metathesis (SSM) with A2NCN (A = Li or Na) as the carbodiimide source. However, as this work will go on to describe the unexpected incorporation of the alkali–metal cations resulted instead in the serendipitous discovery of the A2MnSn2(NCN)6 family (A = Li and Na), the first examples of quaternary cyanamides whose structure and composition are unique to this branch of solid-state chemistry.


Synthesis of A2MnSn2(NCN)6 (A = Li and Na)

As outlined in the introduction, initial synthetic efforts were focused on targeting the ternary carbodiimide MnSn(NCN)3 by a solid-state metathesis (SSM) between SnF4, MnF2 and Li2NCN (prepared as described in ref. 16) according to eqn (1). A stoichiometric ratio of the reagents was homogenized in an agate pestle and mortar and loaded into an open glass capillary (8 mm). The sample was then transferred to a glass ampoule and heated to 500 °C under flowing argon for 18 h, with heating and cooling rates of 2 °C min−1.
3Li2NCN + MnF2 + SnF4 → MnSn(NCN)3 + 6LiF(1)

Preliminary PXRD analysis of the resultant yellow product revealed the presence of multiple phases including the metathesis salt LiF, SnO and SnO2. In addition, a series of reflections were observed that could be indexed to a hexagonal unit cell with lattice parameters of a = 5.85976(6) Å and c = 9.54660(16) Å, thereby representing a [(210), (1[1 with combining macron]0), (001)] expansion of the aristotypic [NiAs]-type MNCN structure. An equivalent reaction utilising Na2NCN (prepared according to ref. 17) as the carbodiimide source yielded a similar mixture of products with NaF as the metathesis salt and a hexagonal phase with slightly larger lattice parameters (a = 5.95857(2) Å and c = 9.65989(7) Å). This result is suggestive of the incorporation of the Na+ (r[6] = 1.02 Å) and Li+ (0.76 Å) cations into quaternary hexagonal phases with the formula AxMnySnz(NCN)x+y+2z).18 Indeed, there is a precedence for the unintentional inclusion of alkali metals during SSM reactions which dictates the use of ZnNCN rather than Li2NCN in the synthesis of M(NCN)2 phases (M = Hf and Zr), thereby avoiding the formation of Li2M(NCN)3 ternaries.13 However, in this instance we sought to identify and optimise the synthetic route to these novel quaternary phases through the variation of the reactant stoichiometry. This was achieved, first in the case of the Li analogue, through the reaction of Li2NCN, SnF4 and MnF2 in a 6[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 molar ratio according to eqn (2), thereby inferring the formation of a quaternary carbodiimide with the composition Li2MnSn2(NCN)6.

6Li2NCN + MnF2 + 2SnF4 → Li2MnSn2(NCN)6 + 10LiF(2)

A 250 mg stoichiometric mixture of the reagents was thoroughly ground in an argon-filled glovebox, before being transferred to an open glass ampoule and heated, under flowing argon, to 475 °C for 1 h with heating and cooling rates of 2 °C min−1. The yellow product was subsequently air-exposed and PXRD data collected which revealed the Li2MnSn2(NCN)6 targeted phase, LiF metathesis salt and trace amounts of SnO2 (3.0(8) wt%). Equivalent conditions were employed for the preparation of the Na-analogue using Na2NCN as the carbodiimide source which yielded a phase pure sample of yellow Na2MnSn2(NCN)6. Remarkably, Li2MnSn2(NCN)6 and Na2MnSn2(NCN)6 are both stable to washing with dilute sulphuric acid which affords the removal of the co-produced LiF and NaF metathesis salts.

PXRD analysis

Powder X-ray diffraction (PXRD) data were collected on washed samples of Li2MnSn2(NCN)6 and Na2MnSn2(NCN)6 using a STOE STADI-P powder diffractometer with a flat sample holder (Cu Kα1, linear PSD, 2θ range 5–120° with individual steps of 0.01°). Rietveld analysis was undertaken using the refinement suite GSAS with the EXPGUI interface.19 In the final cycles of least-squares refinement lattice parameters, fractional coordinates and isotropic thermal displacement parameters were refined for both phases as described in the main body of the text. In the Li2MnSn2(NCN)6 refinement, a secondary phase was introduced to model additional reflections from trace amounts of SnO2 (3.0(8) wt%). Full crystallographic details of both phases may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, on quoting the deposition numbers CSD-1942601 for Na2MnSn2(NCN)6 and CSD-1942600 for Li2MnSn2(NCN)6.

Infrared measurements

The infrared (IR) spectra of Li2MnSn2(NCN)6, Na2MnSn2(NCN)6, and [NaCl]-type MnNCN were measured on an ALPHA II FT-IR-Spectrometer (Bruker).

Results and discussion

Structural characterisation of A2MnSn2(NCN)6 (A = Li and Na)

The PXRD patterns of Li2MnSn2(NCN)6 and Na2MnSn2(NCN)6 were indexed isotypically to primitive hexagonal unit cells that may be related to those of the binary [NiAs]-type MNCN phases by a [(210), (1[1 with combining macron]0), (001)] expansion. The absence of any reflection conditions yields the rather ambiguous extinction group P_[thin space (1/6-em)]_[thin space (1/6-em)]_ which affords 16 possible space groups. Structural determination was therefore approached by considering sub-groups of P63/mmc, i.e. that of the MNCN aristotype. In addition, the presence of strong (001) reflections, along with the unit cell expansion within the basal plane intimate the presence of both inter- and intra-layer cation-order across the octahedral metal sites, respectively. Taking into account sub-groups of P63/mmc that permit such ordering schemes narrows the potential options for A2MnSn2(NCN)6 phases to the trigonal space groups P[3 with combining macron]1m and P312. In the first instance P[3 with combining macron]1m was selected over P312 as it has the higher symmetry and is centrosymmetric.

A P[3 with combining macron]1m starting model was therefore generated for Na2MnSn2(NCN)6 from an [NiAs]-type MNCN supercell using the web-based program ISODISTORT,20 with Mn at the 1b, Na at the 2d and Sn at the 2c positions, to reflect the high degree of layered cation ordering inferred from the strong (001) reflection. In initial structural refinements, fractional coordinates and isotropic thermal displacement parameters were permitted to refine with a single thermal parameter for all C and N atoms. This resulted in an excellent agreement between the observed and calculated data as seen in the Rietveld fit to PXRD data presented in Fig. 2.

image file: c9dt03062j-f2.tif
Fig. 2 Rietveld fit of Na2MnSn2(NCN)6 to PXRD data, showing observed (red), calculated (black) and difference (blue) intensities. Bragg positions of Na2MnSn2(NCN)6 (green) are denoted by vertical markers.

An equivalent structural model was generated for Li2MnSn2(NCN)6 by replacing Na with Li at the 2d position and adding an additional restraint to describe the thermal parameters of Li and Sn with a single value. The resultant Rietveld fit to PXRD data is shown in Fig. S1. Crystallographic data for Na2MnSn2(NCN)6 and Li2MnSn2(NCN)6 are presented in Tables 1 and S1, respectively, with selected bond lengths and angles for both given in Table 2.

Table 1 Crystallographic data and fractional coordinates for Na2MnSn2(NCN)6. Standard deviations are given in parentheses
Atom   x y z U iso (102 Å2)
Trigonal, P[3 with combining macron]1m (no. 162), Z = 1, a = 5.95857(2) Å, c = 9.65989(7) Å; Rwp = 5.33%, Rp = 4.11%, χ2 = 1.357, 32 variables.
Na 2d ½ 1.28(11)
Mn 1b 0 0 ½ 2.31(7)
Sn 2c 0 2.34(2)
C 6k 0.6513(9) 0 0.2527(5) 1.78(9)
N1 6k 0.6933(7) 0 0.3685(4) 1.78(9)
N2 6k 0.5963(7) 0 0.1218(4) 1.78(9)

Table 2 Selected bond lengths and angles for Na2MnSn2(NCN)6 and Li2MnSn2(NCN)6. Standard deviations are given in parentheses
Bond lengths (Å) Na2MnSn2(NCN)6 Li2MnSn2(NCN)6
A–N1 [6×] 2.429(2) 2.333(4)
Mn–N1 [6×] 2.226(4) 2.200(7)
Sn–N2 [6×] 2.161(2) 2.130(4)
C–N1 1.146(6) 1.166(10)
C–N2 1.307(6) 1.311(10)
Bond angles (°)
N1–Mn–N1 180 180
89.35(12), 90.65(12) 86.5(2), 93.5(2)
N1–A–N1 173.52(15) 175.6(3)
80.22(18), 89.81(12), 95.15(10) 80.5(3), 87.0(2), 96.3(2)
N2–Sn–N2 160.6(2) 161.7(4)
74.53(15), 93.19(11), 102.2(2) 74.9(2), 93.4(2), 101.2(4)
N1–C–N2 178.1(6) 178.9(13)

The crystal structures of A2MnSn2(NCN)6 quaternaries are typical of carbodiimide/cyanamide compounds in that they comprise layers of metal cations which alternate with NCN2− ions (Fig. 3(a)). In this instance an hcp arrangement of cyanamide is present with metal cations in the octahedral holes similar to that in the aristotypic [NiAs]-type MNCN structure. However, as in M(NCN)2 (M = Zr or Hf) binaries,13 the cyanamide anions in these A2MnSn2(NCN)6 phases are tilted away from the stacking direction, in this case the [001], to facilitate the inclusion of cation vacancies. These vacancies are accommodated in the Sn sheets, with the nominal composition [Sn2(NCN)3]2+ which form [corundum]- or honeycomb-like rings of edge-sharing SnN6 octahedra (Fig. 3(b)). These alternate with [A2Mn(NCN)3]2− sheets that resemble a portion of the Li2Zr(NCN)3 structure (Fig. 3(c)).12

image file: c9dt03062j-f3.tif
Fig. 3 Crystal structure of A2MnSn2(NCN)6 phases (a), layers of SnN6 octahedra (b), layers of MnN6 and AN6 octahedra (c), and coordination environment of NCN2− (d).

The SnN6 octahedra have Sn–N bond lengths of 2.161(2) Å and 2.130(4) Å in Na and Li analogues, respectively, very similar to the average distances in Li2Sn(NCN)3 (2.185 Å) and Na2Sn(NCN)3 (2.175 Å),12 and therefore consistent with the presence of tetravalent Sn. The SnN6 octahedra, with 32 symmetry, are heavily distorted in both phases with N–Sn–N bond angles in Na2MnSn2(NCN)6 ranging from 74.53(15)° to 102.2(2)°, as shown in Fig. 4(a). The AN6 octahedra have the same site symmetry but are far less distorted (Fig. 4(b)) and have Li–N (2.333(4) Å) and Na–N (2.429(2) Å) bond lengths congruent with the literature examples.12 The Mn atoms, by contrast, are located in almost regular octahedral sites of [3 with combining macron]m symmetry with Mn–N bond lengths of 2.226(4) Å and 2.200(7) Å, for Na and Li analogues, respectively, similar to those in [NaCl]-type MnNCN (2.262(2) Å).6

image file: c9dt03062j-f4.tif
Fig. 4 MN6 octahedra in Na2MnSn2(NCN)6 as viewed along the c axis for Sn (a), Na (b) and Mn (c).

A single crystallographically independent NCN2− unit is present in A2MnSn2(NCN)6 phases which are almost linear (N1–C–N2(Na) = 178.1(6)°, N1–C–N2(Li) = 178.9(13)°) and adopt a 5-fold coordination environment (Fig. 3(d)) which is somewhat distorted from the trigonal prismatic arrangement in [NiAs]-type MNCN phases. Two distinct C–N bond lengths are observed in Na (C–N1 = 1.146(6) and C–N2 = 1.307(6) Å) and Li (C–N1 = 1.166(10) and C–N2 = 1.311(10) Å) analogues, that are very similar to those in PbNCN (1.156(28) and 1.297(29) Å) and reflect N–C[triple bond, length as m-dash]N2− cyanamide forms with strong single and triple bond character, which is consistent with the distinct environments of the terminal nitrogen atoms.21 Further experimental insight into the character of the NCN2− anion is gleaned from IR data which are presented in Fig. 5 and Table S2, along with that of [NaCl]-type MnNCN by way of comparison.

image file: c9dt03062j-f5.tif
Fig. 5 IR spectra of Li2MnSn2(NCN)6, Na2MnSn2(NCN)6 and [NaCl] MnNCN.

The lower symmetry of the cyanamide moiety in A2MnSn2(NCN)6 phases is clearly reflected in the IR spectra which demonstrate a marked splitting of the asymmetric νas(NCN) (≈2000 cm−1) and deformation vibrations δ(NCN) (≈650 cm−1). In addition, strong symmetric vibrations νs(NCN) (≈1250 cm−1) are observed which are IR-forbidden for symmetric N[double bond, length as m-dash]C[double bond, length as m-dash]N, further supporting the crystallographic evidence for N–C[triple bond, length as m-dash]N2− cyanamide-shaped moieties.

Discussion and distortion mode analysis

Na2MnSn2(NCN)6 and Li2MnSn2(NCN)6 are the first examples of non-binary transition-metal carbodiimides to be discovered and intriguingly reflect a structure and composition not observed in oxide or sulphide chemistry. They crystallise in low-symmetry modifications of the aristotypic [NiAs]-type MNCN structure that combine vacancy- and cation-order across the octahedral metal sites, as seen in the symmetry tree developed in Fig. S2. A related structure is, however, adopted by the mineral rosiaite PbSb2O6[thin space (1/6-em)]22 and a broader family of antimonates M2+Sb2O6 (M2+ = Mg, Fe–Zn, Cd, Ca, Sr, Pb, Ba),23,24 arsenates M2+As2O6 (M2+ = Mn, Co, Ni, Cd, Hg, Pd),25–28 along with the ruthenate SrRu2O6,29 and the osmanate CaOs2O6.30 Indeed, these M2+M′5+2O6 phases also crystallise in trigonal structures with P[3 with combining macron]1m symmetry that feature a distorted hcp arrangement of anions. In these oxide phases, however, only ½ of the metal sites are occupied, with honeycomb-like layers of M′5+O6 octahedra, similar to the Sn layers in A2MnSn2(NCN)6, alternating with M2+ containing layers in which only ⅓ of the metal sites are occupied (Fig. 6(a)).
image file: c9dt03062j-f6.tif
Fig. 6 Crystal structure of PbSb2O6 highlighting PbO6 octahedra (purple) and SbO6 octahedra (yellow) (a) and results of distortion mode analysis (b). Crystal structure of A2MnSn2(NCN)6 (c) and results of distortion mode analysis for Na2MnSn2(NCN)6 (purple) and Li2MnSn2(NCN)6 (green) (d).

A closer comparison of MM′2O6 and A2MnSn2(NCN)6 structures reveals another key distinction between the two, namely the ability of the cyanamide anion to tilt away from the [001] stacking direction which may well explain the absence of A2M2+M′4+O6 oxides. To delve deeper into this idea, distortion-mode analysis was undertaken for Na2MnSn2(NCN)6, Li2MnSn2(NCN)6 and PbSb2O6 using the AMPLIMODES applet of the Bilbao crystallographic server to help better understand the importance of specific displacive modes in stabilising A2MnSn2(NCN)6 structures.31,32 The results of this analysis are summarized in Fig. 6(c) and (d) along with graphical descriptions of the displacive modes, with further details available in Tables S3–5.

Considering first the simpler case of PbSb2O6 it is found that only two displacive modes are required to describe the distortion from a hypothetical aristotype with P63/mmc symmetry to the P[3 with combining macron]1m PbSb2O6 hettotype. These are a Γ3+ mode which is a displacement of the divalent anion along the c axis, and the K1 mode which introduces a twisting of the octahedra at the 2c (vacant) and 2d (Sb) positions through anionic displacements within the basal plane (Fig. 6(b)). In PbSb2O6 these modes have a similar amplitude and their cooperative action yields a relatively open and flexible structure that affords the incorporation of a wide range of M2+ cations at the 1b position from Ni2+ (0.69 Å) to Ba2+ (1.35 Å).18 However, since it is only the action of the Γ3+ mode that removes the equivalence of the 2c and 2d positions they are strongly constrained to adopt similar coordination environments.

By contrast, in A2MnSn2(NCN)6 phases two additional displacive modes are present as a direct result of the extended nature of the NCN2− anion (Fig. 6(c)). The first is a Γ1+ symmetric stretching of NCN2− and the second, K2 mode, is a tilt of the NCN2− moiety away from the [001] direction (Fig. 6(d)). In both A2MnSn2(NCN)6 phases this K2 cyanamide tilt mode has the highest amplitude and its action combined with that of the K1 twist lend these structures a flexibility that allows them to bond to cations with quite different coordination requirements at either end, i.e. Sn4+ and Na+, see Fig. 4(a) and (b). An additional consequence of this cooperative distortion is the generation of voids at the 1a position (Fig. 6(c)), which dictates the overall 5/6 occupancy of the octahedral metal sites. It appears therefore that the K2 tilt mode is crucial in stabilising this novel [NiAs]-type derivative, and since such a displacement is not accessible to monoatomic oxide anions this helps to rationalise the non-occurrence of A2M2+M′4+O6 analogues.

The magnitude of the K2 tilt mode is also the main differentiator between the Na and Li analogues, being considerably greater in the former to accommodate the larger alkali–metal cation at the 2d position. This observation prompts comparison to ABO3 perovskite oxides which exhibit an extensive compositional flexibility as a result of framework distortions which afford the inclusion of A and B cations with a range of size ratios. In a similar manner it is believed that the octahedral twist and cyanamide tilt distortions discussed here may well support a broader family of A2MM′2(NCN)6 phases. Furthermore, this inherent flexibility coupled with the presence of alkali–metal cations (A) and redox-active M2+ transition-metal cations invites investigations into the potential for the chemical and electrochemical oxidative deintercalation of the alkali–metal species thereby leading to A2−xM2+xM′(NCN)6 (0 ≤ x ≤ 2) deintercalates.


Herein, we report two new quaternary A2MnSn2(NCN)6 phases (M = Li and Na) which were prepared by solid-state metathesis reactions. These are the first examples of non-binary transition-metal carbodiimides or cyanamides to be discovered which crystallise isotypically in low-symmetry modifications of the [NiAs]-type MNCN structure with P[3 with combining macron]1m symmetry. This new structure-type comprises corundum-like [Sn2(NCN)3]2+ layers which alternate with [A2Mn(NCN)3]2− layers that resemble a portion of the Li2Zr(NCN)3 structure. IR data confirm the crystallographic evidence for N–C[triple bond, length as m-dash]N2− cyanamide-shaped moieties with discrete single- and triple-bond character as a result of the distinct coordination environments of the terminal nitrogen atoms. Distortion-mode analysis reveals the importance of K1 octahedral twist and K2 cyanamide tilt displacements in creating voids at the 1a position and in generating distinct coordination environments at the 2c (A) and 2d (Sn) positions. This study highlights the flexibility of the cyanamide anion even within well-trodden framework of hexagonal close-packed anionic motifs to template novel compositions and it is believed that this inherent flexibility may yield a broader family of A2MM′2(NCN)6 materials whose physical properties await examination.

Conflicts of interest

There are no conflicts to declare.


We thank Mr B. Faßbänder for assistance with PXRD measurements and the Deutsche Forschungsgemeinschaft for their financial support.


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