The new sodium tellurido manganates(II) Na₂Mn₂Te₃, Na₂Mn₃Te₄, Na₂AMnTe₃ (A=K, Rb), and NaCsMnTe₂

2.1 Preparation and phase analysis

The new sodium tellurido manganates(II) were synthesized in corundum crucibles sealed in steel autoclaves under an argon atmosphere. Starting materials were the pure alkali metals (Na, K: Merck, >99%; Rb, Cs: 99.99%) MnTe (powder, ABCR GmbH, Karlsruhe, 99.9%) or elemental Mn as source of manganese and a corresponding amount of the elemental tellurium (shots, ABCR GmbH, Karlsruhe). Due to the violent reactions, cautious heating rates T˙↑ like e.g. 5 K h⁻¹ (between 80 and 110°C) were applied. Subsequently after reaching the maximum temperatures (T_max) of 600–1000°C, the samples were slowly cooled down to r.t. After the preparations, representative parts of the reguli were ground and sealed in capillaries with a diameter of 0.3 mm. X-ray powder diagrams were collected on transmission powder diffractometer systems (Stoe STADI-P with linear PSD or Dectris Mythen 1K strip detector, MoK_α radiation, graphite monochromator). For the phase analysis, the measured powder diagrams were compared to the calculated (program Lazy-Pulverix [29]) reflections of the target compounds and other known phases in the respective systems Na–(K/Rb/Cs)–Mn–Te.

In the ternary system Na–Mn–Te several samples were prepared primarily with the focus to obtain new mixed-valent Mn(II/III) compounds. Several samples of the overall composition Na₃Mn₄Te₆, which is one of the very rare mixed-valent tellurido manganates reported in the literature [24], were heated to different maximum temperatures. In a first attempt to reproduce this compound directly starting from the pure elements [46.3 mg (2.01 mmol) Na, 145.3 mg (2.645 mmol) Mn and 510.6 (4.002) Te] a maximum temperature of 900°C has been applied, which was reached after several heating steps [→2090→4110→100420→20480→250900 (T˙↑ in K h⁻¹ and T in °C)]. Subsequent cooling with rates of 250 (down to 700°C) and 20 K h⁻¹ down to r.t. yielded the new compound Na₂Mn₃Te₄ instead of Na₃Mn₄Te₆, together with small amounts of MnTe₂. Whereas an analogous sample with a maximum temperature of 700°C also gave this new phase, a further reduced T_max of 600°C led to the formation of the known mixed-valent compound Na₃Mn₄Te₆ (according to the phase analysis by powder X-ray diffraction together with NaTe₃, MnTe and MnTe₂). This critical maximum temperature agrees with the synthesis of Na₃Mn₄Te₆ as reported by Kim et al. [24], who obtained this phase keeping stoichiometric amounts of the elements in niobium ampules at 600°C for 1 week. The second pure sodium tellurido manganate, Na₂Mn₂Te₃, was obtained from a Te-rich sample of the composition Na₃MnTe₆, for which 38.1 mg (1.66 mmol) sodium, 104.5 mg (0.573 mmol) MnTe and 361.2 mg (2.831 mmol) tellurium were heated up with a similar temperature program as detailed above to a maximum temperature of 1000°C. The cooling rates were 250 K h⁻¹ to 800°C and finally again 20 K h⁻¹ to r.t. The powder diagram of this sample shows the additional formation of Na₃Mn₄Te₆ and, due to the high tellurium excess not surprising, NaTe₃. Both new sodium compounds form dark-red platelike crystals.

In attempts to obtain new chain/cluster compounds with compositions close to A₂MnQ₂ mixtures of alkali cations A of different size were used. The respective sample NaKMnTe₂ consisting of 43.2 mg (1.88 mmol) Na, 73.8 mg (1.89 mmol) K, 344.9 mg (1.890 mmol) MnTe and 239.6 mg (1.878 mmol) Te, was heated with 20 K h⁻¹ to 80°C and subsequently with 200 K h⁻¹ to a T_max of 760°C. Cooling with a rate of 10 K h⁻¹ to r.t. yielded the first specimen of the new compounds Na₂AMnTe₃. In accordance with the excess of potassium, the known K-rich phase K₂MnTe₂ [8] was identified as a by-product in the X-ray powder diagram of this sample. The analogous sample containing the heavier alkali element rubidium, NaRbMnTe₂, yielded only a mixture of already known ternary compounds (Rb₂MnTe₂ [8], Na₆MnTe₄ [13] and RbMnTe₂ [24]). The isotypic Rb analog Na₂RbMnTe₂ could finally be obtained in pure phase from a stoichiometric sample [224.9 mg (9.783 mmol) Na, 418.7 mg (4.899 mmol) Rb, 894.2 mg (4.899 mmol) MnTe and 1243.7 mg (9.747 mmol) Te] at a maximum temperature of 800°C, when applying a slow cooling rate of 5 K h⁻¹ between 650 and 100°C. Attempts to synthesize the cesium derivative of this type [sample Na₂CsMnTe₃: 40.9 mg (1.78 mmol) Na, 120.4 mg (0.906 mmol) Cs, 167.4 mg (0.917 mmol) MnTe and 234.4 mg (1.837 mmol) Te, T_max=800°C] yielded (together with Na₆MnTe₄) the new compound NaCsMnTe₂, which could be finally obtained in pure phase from a stoichiometric sample NaCsMnTe₂ [27.1 mg (1.18 mmol) Na, 151.7 mg (1.141 mmol) Cs, 208.0 mg (1.140 mmol) MnTe and 146.0 mg (1.144 mmol) Te] at the same maximum temperature (cooling rate T˙↓=10 K−1 from 650°C to r.t.). Similar to the pure sodium phases, the mixed Na-(K/Rb/Cs) tellurido manganates also form dark-red crystals, which are very sensitive against moisture.

2.2 Crystal structure determinations

For the single crystal structure analyses of the title compounds, the dark-red crystals were selected and fixed in loop sample holders (100 K measurements) or in glass capillaries (⊘<0.1 mm) under dried paraffin oil. Data sets were collected on a diffractometer equipped with a microsource and a CCD detector (Bruker APEX II Quazar, Mo radiation, 100 K) or on a sealed tube image plate detector system (Stoe IPDS-2, Ag radiation, r.t.).

Na₂Mn₂Te₃: The reflections on the diffraction images collected at 100 K could be indexed with a monoclinic C-centered lattice with lattice parameters resembling those of the sulfido and selenido compounds Na₂Mn₂Q₃. The additional reflection condition 0kl: k+l=2n is also compatible with the space group C2/c of these structures. The refinement of the positional and ADP parameters of the Na₂Mn₂S₃ structure model [16] using the program Shelxl-2013 [30] smoothly converged to a residual value R1 of 0.0225. The results of this refinement are collected in the Tables 1 (first row) and 2 [35].

Na₂Mn₃Te₄: The lattice parameters and the general reflection conditions of the single crystal data of Na₂Mn₃Te₄ allowed the assignment of the possible monoclinic space groups C2/m, Cm and C2. The structure solution by Direct Methods (program Shelxs-2013 [30]) was successful in the centrosymmetric space group C2/m and yielded one sodium, two Mn and two Te positions. The standardized (Structure Tidy [36]) parameters were refined as described for Na₂Mn₂Te₃ above and led again to a satisfactorily low R1 value of 0.0270. However, the refined ADP parameters of the octahedrally coordinated manganese atom Mn(1) at the origin of the unit cell describe an ellipsoid elongated towards the Te(2) ligands [max./min. r.m.s. values: 913(15) and 127(5) pm²]. A detailed analysis of the electron density distribution at this special position with the help of difference electron density maps calculated and visualized with the programs Jana2006 [37] and DrawXTL [38] (cf. Fig. 1a and b) indicated a small displacement of this Mn atom from its special position. Thus, equivalent alternatives for the structure refinement are (i) the application of a structure model in the acentric subgroup Cm together with a 1:1 inversion twin, (ii) the refinement of a Mn(1) split position in the centrosymmetric space group C2/m or (iii) an anharmonic treatment of the Mn(1) site applying a higher ADP tensor for this atom. Due to the only small deviation of Mn(1) from the special position 2a and the only marginal improvements of the R values and residual densities in the F_o–F_c map, the final structure refinement included herein (Tables 1 and 4], [[35]) is the simple harmonic anisotropic refinement in the centrosymmetric space group C2/m. The electron density minimum in the F_o–F_c map of −5.9 e⁻×10⁻⁶ pm⁻³, which lies in the direct vicinity of Mn(1), is a consequence of this choice. A similar small disorder of the octahedrally coordinated Mn(1) cations was very recently described for the isotypic selenido compound Na₂Mn₂Se₃ [17].

Fig. 1:

Crystal structure of Na₂Mn₂Te₃ (Na₂Mn₂S₃-type). (a) Perspective view of the unit cell with a single tellurido manganate sheet extended to the left and the Te nets to the right; (b) projection of the subfigure a along (100); (c) details of the layers of [MnTe₄] tetrahedra (cf. Table 3 for the interatomic distances; gray polyhedra: [MnTe₄] tetrahedra; yellow polyhedra: [NaTe_5,6] coordination polyhedra; red/yellow/green spheres: Mn/Na/Te; program DrawXTL [38]).

Na₂AMnTe₃: The diffraction images of crystals of the two compounds Na₂KMnTe₃ and Na₂RbMnTe₃ exhibit mmm Laue symmetry. The only additional reflection condition h0l: l=2n leads uniquely to the acentric space group Pmc2₁. The structure solution, using the methods and programs described above, yielded all 10 tellurium positions, the four manganese and the two heavier alkali sites. The positions of the overall five crystallographically different sodium cations were extracted from difference electron density maps. The conclusive Flack parameters support the non-centrosymmetric structure and the absence of twins. A noticeable Na–(K/Rb) (statistical) exchange at the overall seven cation positions is not observed. For the final refinement, the atomic coordinates were standardized and a consistent labeling of the manganese and tellurium atoms was applied (cf. Structure description). The final crystal data are collected in two intermediate columns of Table 1 and in Table 6 (see also [35]).

NaCsMnTe₂: The dark-red crystals of the Cs compound NaCsMnTe₂ also exhibit an orthorhombic, in this case small and C-centered, unit cell. The two further reflection conditions (0kl: l=2n and h0l: l=2n) allowed for two different space groups, the centrosymmetric Cccm and the acentric Ccc2 group. The structure was solved and completed (Na position) as described above in the space group Cccm. Similar to the K and Rb compounds Na₂AMnTe₃, the distribution of the two different alkali cations is distinct, hints to a statistic distribution of the cation positions are missing. The final parameters for the structure of NaCsMnTe₂, which represents a new structure type, are listed in the last column of Table 1 and in Table 8 [35].

3.1 The pure sodium tellurido manganates Na₂Mn₂Te₃ and Na₂Mn₃Te₄

The two new pure sodium tellurido manganates(II) Na₂Mn₂Te₃ and Na₂Mn₃Te₄ were obtained in the form of dark-red platelike crystals from melts of the three elements Na, Mn and Te or alternatively from Na, MnTe and Te. Na₂Mn₃Te₄ is formed only at maximum temperatures above 700°C. Samples of the target or closely related compositions heated to 600°C yielded the known mixed-valent phase Na₃Mn₄Te₆ [25] instead. Further details of the synthesis of the two sodium compounds are collected in the Experimental section.

Na₂Mn₂Te₃ crystallizes in the monoclinic space group C2/c and is isotypic to the analogous sulfido manganate Na₂Mn₂S₃ obtained (from sodium carbonate, sulfur and manganese), characterized and described by Klepp et al. as early as 1983 [16]. The isotypic selenido manganate Na₂Mn₂Se₃ [17] was obtained from Na, MnSe and Se at a maximum temperature of 850°C by Kim and Hughbanks also already 20 years ago [17]. Further compounds forming this structure type are not known. Different views of the crystal structure of Na₂Mn₂Te₃ are shown in Fig. 1. The layered tellurido manganate anion contains two crystallographically different manganese and three tellurium atoms, all at general Wyckoff positions 8f (Table 2). The Te atoms form undulated hexagonal close packed layers (3⁶ nets after Schläfli) which are running perpendicular to (100) (green nets at the right hand side of the structure drawing in Fig. 1a and b). The sheets are stacked in a hexagonal close packed (h.c.p.; |:AB:|) sequence. Between each second layer, ⅔ of the tetrahedral voids are occupied by Mn²⁺ cations resulting in layers [Mn₂Te₃] of edge and vertex-sharing [MnTe₄] tetrahedra. The occupation of the tetrahedral interstices results in ribbons of equally oriented pairs of tetrahedra (cf. gray bars with arrows in Fig. 1c). The chalcogenido manganate layers finally consist of units of four edge-sharing tetrahedra (blue dotted ellipse in Fig. 1c), which are further connected via the μ₃-Te(1) ligands. The common edges within the tetramers [Mn₄Te(2,3)₄Te(1)_6/3] are formed by Te(2) and Te(3). The shortest Mn–Mn contacts are lying within this tetramer and are 329.0 [Mn(1)–Mn(2)] and 323.9 pm [Mn(1)–Mn(1)] long. The Mn–Te distances (labeled a–h) in the tellurido manganate sheets are with 269.6–279.2 pm in the expected range (Table 3). They are shorter than the sum of Shannons radii (287 pm, [39]), but similar to the values found in other tellurido manganates(II), like e.g. in the ortho salts A₆MnTe₄ (275–282 pm, [13], [15]). The sodium cations occupy two special positions 4e [site symmetry 2; Na(1) and Na(2)] and one general position [Na(3)]. The Na⁺ cations at C₂ symmetric positions occupy (overall ⅔) of the octahedral voids in the interstitial layers between the tellurido manganate sheets. Similar to the Mn centered tetrahedra, [Na(1,2)Te₆] pairs of octahedra form ribbons, which are arranged – under the exclusion of face-sharing – between the ribbons of [MnTe₄] tetrahedra of the adjacent layers. The Na–Te distances within these octahedra are 320–335 pm (Table 3). A separated Na(1) octahedral coordination polyhedron is shown at the right hand side of in Fig. 1a and b. The Na(3) cations are located within the (expanded) Te₃-rings of the tellurium sheets and thus exhibit a five-fold coordination by Te²⁻, with Na–Te distances of 304–354 pm (cf. the second isolated polyhedron in Fig. 1a and b).

Even though the structure of the long-known compound Na₃Mn₄Te₆ shows some similarities with the new phase described above (monoclinic C lattice, h.c.p. substructure of Te, sheets of vertex- and edge-sharing [MnTe₄] tetrahedra), the connection of the [MnTe₄] tetrahedra within the sheets is fundamentally different. The two similar-looking compounds Na₂Mn₂Te₃ and Na₃Mn₄Te₆ show no crystallographic relation (cf. the discussion below) and are thus also not convertible by e.g. an electrochemical redox-induced Na⁺ exchange.

The monoclinic crystal structure of the second pure sodium title compound Na₂Mn₃Te₄ [28], which is isotypic to the very recently reported selenido analog [18], contains one Na, two Mn and two Te positions (Table 4). Similar to Na₂Mn₂Te₃, the structure exhibits tellurido manganate sheets, but in this case Mn(II) is both tetrahedrally and octahedrally coordinated, though it should be pointed out that the latter bonding situation is very rare in alkali sulfido/selenido/tellurido manganates. The Mn(1) atoms at the unit cell origin (2a, point group symmetry 2/m) are these octahedrally coordinated Mn²⁺ cations ([Mn(1)Te(1)_4/4Te(2)_2/3], blue polyhedra in Fig. 2). The [Mn(1)Te₆] octahedra are connected via opposite edges to form chains running along the b axis of the monoclinic cell. As expected, the Mn–Te bond lengths are with 293.3 (a) and 325.7 pm (b, Table 4) significantly larger than inside the [Mn^IITe₄] tetrahedra. However, as illustrated by the 99% ellipsoids of the refinement in the centrosymmetric space group C2/m [Mn(1) at 0,0,0, cf. Experimental section for details of the refinements] and the difference electron density map (Fig. 2a and b) Mn(1) is evidently slightly disordered around the 2a site (Δ≈25 pm), leading to a distortion of the octahedra towards a square pyramid and to two different distances b to the Te(2) octahedra tips of ≈300 and 350 pm. The very same situation has been described for the isotypic selenido compound Na₂Mn₃Se₄ [18]. The second type of Mn(II) cations in the structure of Na₂Mn₃Te₄ [Mn(2), Wyckoff site 4i, m symmetry] are tetrahedrally coordinated by four Te²⁻ anions ([MnTe(1)_2/4Te(2)_2/3], gray polyhedra in Fig. 2), whereby two tetrahedra form dinuclear units sharing a common edge. These dimers are again connected via four common corners to form strands, which are again running along b. The Mn(2)–Te bond lengths are again in the range common for tetrahedral building blocks (270.8–279.2 pm, Table 5). The ribbons of octahedra and tetrahedra are fused and form puckered layers [Mn₃Te₄]²⁻ in the b–c plane, which are |:AB:| stacked along [100]. Between the layers, Na⁺ cations (Wyckoff position 4i) are interspersed, which exhibit an octahedral coordination with very similar distances like in Na₂Mn₂Te₃ (d_Na−Te=317.3–356.9 pm, yellow polyhedra). In contrast to the simple Te²⁻ layers found in Na₂Mn₂Te₃ [3⁶ nets, (defect) CaAl₂Si₂-type] and the tellurido compounds with the larger A cations [4⁴ nets, ThCr₂Si₂-type, see below], the Te atoms in Na₂Mn₃Te₄ form 3³·4² nets: the planar nets running in the a–c plane at y=¼ and ¾ are marked by red arrows in Fig. 2c; the second set of slightly puckered similar nets along the diagonal are marked by blue arrows. The four-membered rings of these tellurium nets coincide with the ‘bases’ of the [Mn(1)Te₆] (blue layers) and the [NaTe₆] octahedra (red layers).

Fig. 2:

Crystal structure of Na₂Mn₃Te₄. a/b: details of the surrounding of Mn(1), with the difference electron density around Mn(1) plotted at a level of 3 (solid) and 15 (red net) e⁻×10⁻⁶ pm⁻³ (a) and difference electron density map with isolines of 1 e⁻×10⁻⁶ pm⁻³ (b). (c) Projection of the structure along c; (d) perspective view of the tellurido manganate sheet, with Mn ellipsoids at a level of 99% (cf. Fig. 1 for a detailed legend and Table 5 for interatomic distances).

Even though the structure type of the two compounds Na₂Mn₃Se₄ and Na₂Mn₃Te₄ [18] is singular, it is however closely related to the isopointal sodium sulfido metallate Na₂Cu₂ZrS₄ [40]: in the latter compound, the chalcogen packing as well as the arrangements of the [ZrS₆] and the [NaS₆] octahedra are similar. The only difference is the connection of the [Cu^IIS₄] tetrahedra, which form zweier single zig-zag chains along b sharing two adjacent edges, while Na₂Mn₃Se₄ and Na₂Mn₃Te₄ exhibit einer double chains of [Mn₂Q₆] dinuclear units.

3.2 Mixed tellurido manganates Na₂AMnTe₃ (A=K, Rb)

Despite its very simple overall composition A₃MnTe₃, which suggests the presence of Mn(III), the mixed tellurido manganates Na₂AMnTe₃ (A=K, Rb) exhibit an unexpectedly complex crystal structure. However, the bonding situation is closely related to that in the cobaltate Na₃CoS₃ [12], where Q–Q bonds are present and the metal cation is kept at an oxidation state of +II. For Na₃CoS₃, the polyanions formed are already complex, but the two manganates Na₂AMnTe₃ show an even more diverse structural chemistry. The new chiral orthorhombic structure type (space group Pmc2₁) consists of five crystallographically different sodium and two K/Rb cations (Table 6). The anionic building blocks contain four manganese and 10 tellurium sites, which are labelled according to their bonding features. The polyanions are best divided into two different layers coinciding with the mirror planes at x=½ (layers A) and x=0 (B). The layers consist of chains formed by two crystallographically different Mn(II) cations each.

The layers A are formed by chains of [Mn(1,2)Te₄] tetrahedra, alternately connected via common edges and corners (Fig. 3a). The chains of layer A are running along [001] around y≈½. Whereas Mn(1) is coordinated by monatomic tellurido ligands Te²⁻ only, the η₁ tellurido ligand bonded to Mn(2) is a ditelluride dumbbell [Te(21,22)₂]²⁻. The chains are composed of vertex-sharing dinuclear units [Te(2)_1/2Te(1)Mn(1)Te(12)₂Mn(2)(Te₂)Te(2)_1/2], which themselves consist of the edge-sharing tetrahedra [Mn(1)Te(1)Te_3/2] and [Mn(2)(Te)₂Te_3/2]. The overall formula of the chains within the layer A is thus ∞1[Mn2Te6]6− and therewith meets the composition of the whole compound. The sodium cations Na(1), which exhibit a trigonal bipyramidal coordination, are located at the same mirror plane.

Fig. 3:

Crystal structure of Na₂AMnTe₃. (a) Chains [Mn(1,2)₂Te₆]⁶⁻ (layer A) around x≈½. (b) Chains [Mn(3,4)₂Te₄]⁴⁻ and [Te₂]²⁻ dumbbells (layer B) around x≈0 and 1. (c) Cation nets [Rb and Na(3) to Na(5)] at x≈¼ and ¾. (d) Perspective view of the unit cell together with all cation coordination spheres (Rb, brown sticks) and polyhedra (Na, yellowish polyhedra; cf. Table 7 for interatomic distances and their labels).

The second mirror plane (at x=0, layers B, Fig. 3b) again contains angulated chains of [MnTe₄] tetrahedra also running along the c axis. In this case, the tetrahedra are connected via edges exclusively; their overall formula is thus ∞1[MnTe4/2]2−. These chains around y≈0 are staggered against the chains of the adjacent layers A. The mirror planes B are additionally occupied by [Te(31,32)₂]²⁻ dumbbells, which are not connected to manganese atoms, but fill the space between the [MnTe_4/2]²⁻ chains. Due to the ratio of these [Te₂]²⁻ dumbbells and the [MnTe_4/2]²⁻ tetrahedra of 1:2 the total composition and charge of this layer B [Mn₂Te₄]⁴⁻[Te₂]²⁻=[Mn₂Te₆]⁶⁻ is again similar to that of the overall formula. The cations Na(2) are also located at this mirror plane and are stuffed between the ditelluride dumbbells and the Te(¾) atoms of the chains. They show a pentagonal-bipyramidal 7-fold coordination by Te atoms (transparent yellow polyhedron in Fig. 3d).

The remaining alkali cations (the three sodium Na(3), Na(4), Na(5) and the two Rb/Cs cations) are located close to x=¼ (and ¾) and thus form only very slightly puckered nets with the Schläfli symbol 3.5.3.5. (Na nodes)+3².5.3.5. (Rb nodes) (3:2). The five-membered rings of this net of cations are centered by the Te(12) and Te(34) atoms forming those common edges of the [MnTe₄] tetrahedra, which are oriented perpendicular to the mirror planes. The three sodium cations exhibit distorted octahedral coordination spheres (yellow polyhedra in Fig. 3d) with Na–Te distances in the range 312–386 pm (Table 7). As expected from the increased ionic radii, the heavier alkali cations K⁺ and Rb⁺ are coordinated by eight [A(2): 7+1] tellurium atoms with larger distances of minimum 356 pm. The respective coordination spheres are shown by thin brown ‘bonds’ in Fig. 3d.

The Mn–Te bond lengths within the [MnTe₄] tetrahedra are again in the usual range between 269 and 282 pm. Compared to the pure Na compounds above, these Te-richer compounds exhibit μ₁- and μ₂-Te²⁻ ligands only and the tetrahedra are less connected with each other. In addition to the one or two Mn²⁺ cations, five to seven A⁺ cations round out the tellurium coordination spheres. The Te(22) and Te(32) atoms, which are not connected to manganese, show a 1 Te+8 A coordination sphere (Table 7), which is likewise observed in binary alkali (poly)tellurides (cf. discussions in [41], [42]). The Te–Te bonds within the dumbbells amount to 276/278 pm for the [Te₂] ligands and 280/284 pm for the ‘free’ ditelluride anions. These slightly decreased bond lengths of bonded dichalcogenid anions are similarly observed for other dichalcogenido metallates (like e.g. Na₃CoS₃ [12]) and are sufficiently discussed in the literature [43], [44].

3.3 NaCsMnTe₂

NaCsMnTe₂ crystallizes in a new, very simple, but nevertheless new, orthorhombic structure type (space group Cccm). The crystal parameters are collected in the last column of Table 1 and in Table 8, interatomic distances can be found in Table 9.

The compound exhibits the largest differences in the size of the two alkali cations, Na⁺ and Cs⁺. Consequently, the cations are again fully ordered at two crystallographically different positions with very diverse coordination numbers of four (tetrahedra, for Na⁺, transparent yellow polyhedra in Fig. 4) and eight (cubes, for Cs⁺). The tellurido manganate(II) polyanions are linear chains of edge-sharing tetrahedra [MnTe_4/2], which are running along the c axis. The Mn–Te distance of 277.2 pm is again approx. 10 pm shorter than the sum of Shannons radii (287 pm, [39]), but very similar to the bond lengths in the chains of the pure alkali compounds (K/Rb/Cs)₂MnTe₂ forming the K₂ZnO₂-type structure (e.g. Cs₂MnTe₂: d_Mn−Te=277.3 pm [8]). The bond angles Te–Mn–Te within the tetrahedra are 106.4–114.5° and the polyhedra are therewith slightly stretched along the chain direction. The [NaTe_4/2] tetrahedra are connected among each other in a similar manner, the Na–Te distance of 305.2 pm is also somewhat decreased with respect to the sum of the Shannon radii of 320 pm. Similar short values can be found e.g. for Na(3) with CN=5 in Na₂Mn₂Te₃ (303.7 pm, Table 2), whereas the Na–Te distances within octahedra are commonly somewhat larger (e.g. Na₂Mn₂Te₃: d_Na−Te=320–334 pm). Both chains of tetrahedra share the tellurium atoms and are thus connected to PbO-like layers [NaMnTe₂], which are also the building blocks in the prominent ThCr₂Si₂-type iron arsenido and chalcogenido superconductors and generally in many alkali-poorer chalcogenido-metallates with larger counter-cations K⁺ to Cs⁺. Analogously, the larger cation Cs⁺ in NaCsMnTe₂ occupies the alkali (Th in the aristotype) position between the layers. The pseudo-tetragonal orthorhombic unit cell, in which the a and the c axis are of comparable lengths, is a consequence of this structure relation. Not surprisingly, the new structure type can be crystallographically derived by a group subgroup relation [45], [46], [47], [48] from the BaAl₄/ThCr₂Si₂-type by firstly applying a t2 symmetry reduction from the space group I4/mmm of the aristotype to Fmmm (basis change: a′=a–b, b′=a+b). Secondly, a k2 symmetry reduction leads directly to the space group Cccm of NaCsMnTe₂ (cf. Fig. 5). Thereby the Ba/Th Wyckoff position 2a transforms to 4e (Cs), the 4d site [Cr/Al(1)/M in the chalcogenido metallates AM₂Q₂] splits into 4a (Na) and 4b (Mn), again without any free positional parameter. The 4e position of the aristotype (0,0,z) with a free z parameter and point group 4mm is transformed to the 8l position of the Te anions (∼¼, ∼¼+z, 0; cf. Table 8).

Fig. 4:

Crystal structure of NaCsMnTe₂ (cf. Table 9 for interatomic distances; caption of Fig. 1).

Fig. 5:

Structure relations (if possible, with a crystallographic group-subgroup relation) of Li/Na chalcogenido manganates basing on a hexagonal close packing of Q²⁻ (left, CaAl₂Si₂-aristotype) and (right) K/Rb/Cs-containing compounds related to the ThCr₂Si₂-aristotype (see text).

For the closely related Li/K sulfido manganate KLiMnS₂, Bronger et al. reported the statistic occupation of Li⁺ and Mn²⁺ at the Cr sites of the aristotype [49].

3.4 Classification of the title compounds within the family of alkali chalcogenido manganates

The five title compounds can be classified within the families of known alkali chalcogenido (excluding oxygen) manganates(II) and (III). To that, Table 10 contains the basic structure data of all compounds reported in the literature. The manganates can obviously be categorized according to the connectivity of the [MnQ₄] tetrahedra into (i) ‘ortho’ salts containing isolated tetrahedra, (ii) ‘ino’ compounds containing chains of edge-connected tetrahedra ∞1[MnQ4/2] and (iii) finally several types of ‘phyllo’ manganates of different types with layered manganate polyanions. The two new isotypic ‘phyllo’ compounds Na₂Mn₃Se₄ [18] and Na₂Mn₃Te₄ are the only examples of S/Se/Te manganates containing (besides a tetrahedral) an octahedrally coordinated Mn(II). Despite this general difference, the structure can be related to the remaining layered Li/Na manganates, if the compounds are categorized according to their basic Q packing (see below).

Table 10:

Summary of known alkali chalcogenido manganates.

Type	OS of Mn	General composition	Individual compound(s)	Reference	Structure type	Space group	Mn–Q distance/pm (CN)b		Building blocks, comments, etc.
Ortho-	2	A6MnQ4	(Na/K)6MnS4	[13], [14]	Na6ZnO4	P63mc	242.4–246.7		Isolated tetrahedra [MnQ4]
			(Na/K)6MnSe4	[13]	Na6ZnO4	P63mc	254.5–259.2
			(Na/K)6MnTe4	[13]	Na6ZnO4	P63mc	275.4–278.6
			(Rb/Cs)6MnS4	[15]	Na6ZnO4	P63mc	248.7–250.7
			(Rb/Cs)6MnSe4	[15]	Na6ZnO4	P63mc	260.7–263.0
			(Rb/Cs)6MnTe4	[15]	Na6ZnO4	P63mc	280.0–282.4
Ino-	2	A2MnQ2	(K/Rb/Cs)2MnS2	[8]	K2ZnO2	Ibam	243.5–247.4		Linear chains of [MnQ4/2] tetrahedra
			(K/Rb/Cs)2MnSe2	[8]	K2ZnO2	Ibam	256.2–257.6
			(K/Rb/Cs)2MnTe2	[8]	K2ZnO2	Ibam	275.2–277.3
			NaCsMnTe2	a	New	Cccm	cf. Table 9
			LiNaMnS2	[23]	CaAl2Si2	P3̅m1	(244.3–247.8)		Li/Mn statistically distributed
			Li(K/Rb/Cs)MnS2	[49]	ThCr2Si2	I4/mmm	(243.9–252.7)
		A3MnQ3	Na2(K/Rb)MnTe3	a	New	Pmc21	cf. Table 7		Te–Te bonds
Phyllo-	2	A2Mn2Q3	Na2Mn2S3	[16]	Na2Mn2S3	C2/c	239.3–246.5		Sheets of edge-sharing tetrahedra
			Na2Mn2Se3	[17]	Na2Mn2S3	C2/c	251.9–260.2
			Na2Mn2Te3	a	Na2Mn2S3	C2/c	269.6–279.2
		A2Mn3Q4	Na2Mn3Se4	[18]	Na2Mn3Te4	C2/m	252.2–254.9(4)		Sheets of [MnQ4] tetrahedra and [MnQ6] octahedra
							272.1–296.9(6)
			Na2Mn3Te4	a, [28]	Na2Mn3Te4	C2/m	272.3–272.9(4)
							272.1–296.9(6)
			K2Mn3S4	[19], [20]	Singular	P2/c	239.7–245.3		cf. Fig. 2
			(Rb/Cs)2Mn3S4	[50], [51], [52]	Cs2Mn3S4	Ibam	241.9–246.0		Defect variants of ThCr2Si2 type layers
			(Rb/Cs)2Mn3Se4	[18], [21]	Cs2Mn3S4	Ibam	254.1–257.9
			(Rb/Cs)2Mn3Te4	[21]	Cs2Mn3S4	Ibam	275.1–277.6
	2.25		Na3Mn4Te6	[24]	Singular	C2/m	277.0–280.6		Defect variants of the CaAl2Si2-layers
			NaMn1.56Te2	[24]	CaAl2Si2	P3̅m1	274.9–280.8
	3	AMnQ2	(Li/Na)MnSe2	[17]	LiMnSe2	P3m1	255.5–259.7		Ordered defect variants with CaAl2Si2 layers
			(Li/Na)MnTe2	[25]	LiMnSe2	P3m1	276.8–278.4
			(K/Rb/Cs)MnSe2	[17]	TlAgTe2	I4̅m2	256.9 (A=Rb)		Ordered defect variants with ThCr2Si2 layers
			(K/Rb/Cs)MnTe2	[24]	TlAgTe2	I4̅m2	277.7–278.2

1. ^aThis work. ^bDistances from derived coordinates in brackets. OS, (averaged) oxidation state.

1. For the lighter alkali elements sodium and potassium, the ortho metallate salts A₆M^IIQ₄ of the 3d elements Mn and Co, which strongly prefer the oxidation state +II in S, Se and Te compounds, are long-known [13], [14], [53], [54], [55], [56]. All compounds of this composition form the Na₆ZnO₄-type structure. While lithium salts of this type are missing, the structure family has recently been extended to the respective manganates(II) of rubidium and cesium, which are indeed isotypic to the Na/K salts [15]. Attempts to prepare other Rb/Cs metallate(II) salts of this type (i.e. Co, Zn, Cd compounds) are subject of ongoing work. The Na₆ZnO₄-type structure exhibits a h.c.p. arrangement of the chalcogenid ions and is crystallographically connected by a group-subgroup relation with the Mg-type (h.c.p.) structure (cf. discussion and Bärnighausen symmetry tree in [15]). In contrast to the less A-richer ‘ino’ and ‘phyllo’ manganates discussed below, also the larger Rb⁺ and Cs⁺ cations occupy (distorted) octahedral and even tetrahedral voids in the chalcogenide close packing.

   For some ‘ino’ and all ‘phyllo’ manganates(II) and (III), the basic chalcogenide packing is determined by the different coordination spheres of the A cations: the pure Li and Na salts can be derived from a hexagonal close packing of Q²⁻ (3⁶ nets), in which all cations (A⁺ and Mn²⁺) occupy tetrahedral and/or octahedral voids. Structures belonging to this family are collected in the symmetry tree depicted at the left hand side of Fig. 5. The aristotype for the layered compounds is CaAl₂Si₂ (or similarly La₂O₃). All salts containing K, Rb or Cs (optionally together with Li or Na) contain these larger cations in a cubic eight-fold coordination by Q. In these cases, the basic chalcogen arrangement corresponds to the nine-fold close packing [49] consisting of 4⁴ nets and ThCr₂Si₂/BaZn₂P₂ as aristotype (right hand side of Fig. 5).

2. Similar to the ortho salts, the sulfido, selenido and tellurido chain (‘ino’) manganates A₂MnQ₂, which are known for the heavier alkali elements K, Rb and Cs only [8], are all isotypic (K₂ZnO₂-type). The orthorhombic structure contains linear chains ∞1[MnQ4/2] separated by alkali cations, the latter with a distorted octahedral coordination by the chalcogen atoms. Despite the similar building blocks, A/M coordinations, unit cell dimensions and the same space group (Ibam), the K₂ZnO₂-type is not crystallographically related to the layered compounds of the ThCr₂Si₂-aristotype (e.g. Cs₂Mn₃S₄-type). In contrast, the new compound CsNaMnTe₂, which contains layers [NaMnTe₂] built up of two fused parallel [NaTe_4/2] and chains of [MnTe_4/2] tetrahedra, can be very simply derived from this aristotype (cf. structure description above and the right hand side symmetry tree in Fig. 5). For the ternary lithium K/Rb/Cs compounds of this type, a Li/Mn cation ordering, which leads to the subgroup Cccm, is not observed [49]. LiNaMnS₂, in which both A cations are small and which analogously belongs to the h.c.p. type family, also exhibits a statistical occupation of Li and Mn at the tetrahedral voids of the respective aristotype CaAl₂Si₂ (Fig. 5, left). The complex new structure of the two mixed compounds Na₂(K/Rb)MnTe₃, which exhibit the formula of a metallate(III) containing edge-sharing dimers, contains Mn^II besides ditelluride ligands in addition to Te²⁻ and thus represents a basically new class of chalcogenido manganates. Nevertheless, the structure – among other anions – contains chains similar to those of the common K₂ZnO₂-type structure. The strong buckling of the chains in Na₂(K/Rb)MnTe₃, which exhibit a similar charge as the linear chains in A₂MnQ₂, again shows, that pure packing effects determine the shape of this structure motif; electronic contributions, atom pairing, charge ordering or magnetic interactions are not the reason for the strong differences in the shape of chalcogenido metallate chains (cf. discussion for the mixed-valent ferrates in [9]).

3. The structures of the layered (‘phyllo’) lithium and sodium chalcogenido metallates are fundamentally different from those of the larger alkali cations. Li/Na: The layered manganates(II) with the smaller lithium or/and sodium cations can be derived from the trigonal layers of tetrahedra in the CaAl₂Si₂-type structure: whereas the mixed-valent manganat(II/III) NaMn_1.56Te₂ show a statistical occupation of the tetrahedral voids in the aristotype, the manganates(III) (Li/Na)MnSe₂ [17] and (Li/Na)MnTe₂ [25] exhibit polar layers, in which only ½ of the tetrahedra of each layer are occupied, crystallographically leading to the t2 subgroup P3m1 (Fig. 5, left). In the mixed-valent phase Na₃Mn₄Te₆ Mn²⁺ cations occupy ⅔ of the tetrahedral voids between each second pair of hexagonal close packed Te layers. The monoclinic structure of space group C2/m is thus again crystallographically related to the CaAl₂Si₂-aristotype (Fig. 5, left): the general t3 symmetry reduction from the trigonal (space group P3̅m1) to the monoclinic C-centered crystal system (C2/m) is followed by a tripling of the monoclinic b axis, which allows to create the ⅓ empty tetrahedral voids along [010]. The crystallographic a–b plane coincides with the manganate layers. In the likewise monoclinic C centered structure of Na₂Mn₂S₃ [16], Na₂Mn₂Se₃ [17] and the title compound Na₂Mn₂Te₃ a different ordering of a ⅔ occupation of the tetrahedral voids occurs. In this case, the unit cell contains two layers of tetrahedra along the stacking direction, which needs a doubling of c in the first step of a symmetry reduction. However, the observed C centering does not coincide with the former trigonal unit cell basis and there is consequently no direct group-subgroup relation between the CaAl₂Si₂ and the Na₂Mn₂S₃ structure. Only a metric transformation, which was already discussed in [16], can be quoted (cf. Fig. 5).

   For Na₂Mn₃Se₄ [18] and the tellurium analogue Na₂Mn₃Te₄, the Se/Te arrangement consists of 3⁶.4² nets (cf. Fig. 1 in Sec. 3.1). Therefore, these two special compounds, which contain [MnQ₆] octahedra in addition the the common [MnQ₄] tetrahedra, can be denoted ‘intermediates’ between the h.c.p./CaAl₂Si₂ (3⁶ nets) and the ThCr₂Si₂ (4⁴ nets) structure family.

   K-Cs: The layered manganates of the larger cations K⁺, Rb⁺ and Cs⁺ are ordered defect variants of the tetragonal ThCr₂Si₂/PbO-type layers. Comparable to Li/Na manganates(III), the removal of ½ of the tetrahedra of the aristotype (t2 symmetry reduction from I4/mmm to I4̅m2) leads to the TlAgTe₂-type structure of the manganates(III) AMnQ₂ (A=K, Rb, Cs; Q=Se, Te; [17], [24]). A chain-like occupation of the tetrahedral voids by Mn and Na leads to the tetragonal structure of CsNaMnTe₂ (cf. above and Fig. 5). The group-subgroup relations associated with the ¾ occupation of the tetrahedra in the Cs₂Mn₃S₄-type structure of the Rb and Cs salts A₂M₃Q₄, which have long been known for the whole chalcogen series S/Se/Te, were described by Bronger et al. in [21]. Equally, the symmetry relation of the singular monoclinic structure of the potassium analog K₂Mn₃S₄ to the Cs₂Mn₃S₄- and the ThCr₂Si₂-type was already pointed out by this group [19].