jeanflop/post_ocr_correction2 huggingface dataset:
des bandes occupées de la figure 5 a sont construites principalement à partir des orbitales 4d du niobium. Puisqu'il y a quatre atomes de niobium par maille, l'état d'oxydation approprié pour Nb est + III. Le caractère orbitalaire principal de ces bandes est donné par les orbitales cr+ et e de la figure 5 b. La population de recouvrement associée aux distances courtes Te-Te est pratiquement nulle. Toutes ces conclusions sont les mêmes quand les interactions intercouches sont prises en considération. En conséquence se trouve ainsi confirmé le formalisme d'oxydation pour Nb2SiTe4 : (Nb + III)(Si + II)(Te - I). CONCLUSION. La structure de Nb2SiTe4 démontre donc de façon convaincante que l'existence de distances courtes Te-Te n'est pas suffisante pour conduire à une vraie liaison anionique et donc à un transfert anion → cation. Bien que considérablement plus courts que la somme des rayons de van der Waals, ces contacts, quand ils sont localisés dans la couche 2D et parallèle à celle-ci, n'élèvent pas les niveaux orbitalaires suffisamment pour conduire à leur appauvrissement électronique. En comparaison avec d'autres tellurures [6]-[7] et la phase proche NbGeTe4 [3], il apparaît que de telles liaisons et transferts sont beaucoup plus aisément obtenus au travers d'orbitales 4p du tellure de chaque côté du gap de van der Waals avec 4p perpendiculaire au feuillet. 1. INTRODUCTION. — A thorough study of numerous MTe2 phases with a CdI2-like or pyrite-like structure led to the elaboration of a new classification that takes into account not only the phase dimensionality (2D or 3D in this case) but also the polymerization degree of the tellurium network [1]. So was introduced the occurrence of two new subfamilies, the polymeric CdI2-like and the polymeric pyrite-like ones. To assert the generality of the tellurium polymerization character that was found in those transition metal ditellurides, syntheses of new ternary tellurides were attempted in the M-A-Te systems (M = Ta, Nb, or V and A = Si, Ge or Sn). A new family was hit upon, M3ATe6 (M = Ta, Nb, or V and A = Si or Ge), its representative being Nb3Ge0.9Te6 [2].
travers d'orbitales 4p du tellure de chaque côté du gap de van der Waals avec 4p perpendiculaire au feuillet. 1. INTRODUCTION. — A thorough study of numerous MTe2 phases with a CdI2-like or pyrite-like structure led to the elaboration of a new classification that takes into account not only the phase dimensionality (2D or 3D in this case) but also the polymerization degree of the tellurium network [1]. So was introduced the occurrence of two new subfamilies, the polymeric CdI2-like and the polymeric pyrite-like ones. To assert the generality of the tellurium polymerization character that was found in those transition metal ditellurides, syntheses of new ternary tellurides were attempted in the M-A-Te systems (M = Ta, Nb, or V and A = Si, Ge or Sn). A new family was hit upon, M3ATe6 (M = Ta, Nb, or V and A = Si or Ge), its representative being Nb3Ge0.9Te6 [2]. This phase, which can also be formulated Ge0.3NbTe2, could have been expected to be made of a layered NbTe2 host with Ge ions inserted in the van der Waals gap. Actually, Ge0.3NbTe2 structure presents a tellurium stacking of the AA/BB type with empty sites between the A and B layers, a peculiar square coordination for Ge2+ atoms, and two kinds of Nb3+ atoms (alone or in Nb2 pairs). In addition, short Te-Te contacts (around 350 pm) indicate a charge transfer, with a partial oxidation state for tellurium anions confirmed by electronic band calculations [3], whereas short Nb-Nb and Nb-Ge distances (292.0 and 281.3 pm, respectively) imply intercationic bondings. The interlayer Te-Te contacts play a role similar to those of the polymeric CdI2-type ditellurides. Thus, the tellurium polymerization tendencies observed and demonstrated in binary phases was confirmed in ternary compounds. To better understand those highlighted features, we strove to extend the number of families of new ternary tellurides. In that quest we evidenced a new family, M2ATe4, in the same M-A-Te systems (M = Ta, Nb, or V and A = Si, Ge or Sn) and it is the crystal and electronic structures of one of the phases obtained, Nb2SiTe4, that
a peculiar square coordination for Ge2+ atoms, and two kinds of Nb3+ atoms (alone or in Nb2 pairs). In addition, short Te-Te contacts (around 350 pm) indicate a charge transfer, with a partial oxidation state for tellurium anions confirmed by electronic band calculations [3], whereas short Nb-Nb and Nb-Ge distances (292.0 and 281.3 pm, respectively) imply intercationic bondings. The interlayer Te-Te contacts play a role similar to those of the polymeric CdI2-type ditellurides. Thus, the tellurium polymerization tendencies observed and demonstrated in binary phases was confirmed in ternary compounds. To better understand those highlighted features, we strove to extend the number of families of new ternary tellurides. In that quest we evidenced a new family, M2ATe4, in the same M-A-Te systems (M = Ta, Nb, or V and A = Si, Ge or Sn) and it is the crystal and electronic structures of one of the phases obtained, Nb2SiTe4, that are reported in this article. 2. EXPERIMENTAL METHOD. — Synthesis and analysis. — The new ternary layered phase Nb2SiTe4 was obtained by heating the elements taken in stoichiometric proportion (2:1:4) at 950°C for 12 days. The sealed silica tube was heated at a rate of 100°C an hour and subsequently quenched to room temperature. Semi-quantitative SEM analyses that were performed on various single crystals agreed quite well with the title formula (Nb2Si1.2Te4.1) and were confirmed by the refinement results (vide infra). Unlike the preparation of Nb3Ge0.9Te6, no attack of the tube inner walls was noticed. Single crystal studies. Single crystals of Nb2SiTe4 were unraveled from a batch obtained though the aforementioned procedure. A small platelet-shaped suitable crystal was glued on top of a Lindemann fibre. Preliminary Weissenberg photographs indicated a monoclinic system, the extinction conditions suggesting C2/c or Cc as possible space groups. They also exhibited diffuse superstructure streaks doubling the b parameter (vide infra) and implying a lack of long-range order. The intensities of the subcell reflexions were measured on an Enraf Nonius CAD4 kappa diffractometer. MoKα radiation and a graphite monochromator were used. The cell parameters refined from 25 strong reflexions converged to a = 1,421.6(2) pm, b
quenched to room temperature. Semi-quantitative SEM analyses that were performed on various single crystals agreed quite well with the title formula (Nb2Si1.2Te4.1) and were confirmed by the refinement results (vide infra). Unlike the preparation of Nb3Ge0.9Te6, no attack of the tube inner walls was noticed. Single crystal studies. Single crystals of Nb2SiTe4 were unraveled from a batch obtained though the aforementioned procedure. A small platelet-shaped suitable crystal was glued on top of a Lindemann fibre. Preliminary Weissenberg photographs indicated a monoclinic system, the extinction conditions suggesting C2/c or Cc as possible space groups. They also exhibited diffuse superstructure streaks doubling the b parameter (vide infra) and implying a lack of long-range order. The intensities of the subcell reflexions were measured on an Enraf Nonius CAD4 kappa diffractometer. MoKα radiation and a graphite monochromator were used. The cell parameters refined from 25 strong reflexions converged to a = 1,421.6(2) pm, b = 394.40 (8) pm, c = 633.62 (13) pm, β = 97.97 (2)°, and V = 351.81 (12) x 106 pm3. Omitting Bravais systematically absent reflexions, 2,970 h k l were measured in the h/ h, 0/k and 1/1 reciprocal space. Because of the in-plane undefined shape of the crystal, a Ψ measurement on four reflexions (x close to 90°) was carried out for subsequent empirical absorption corrections. After the usual corrections (decay, absorption, and Lorentz-polarization) a set of 1,532 reflexions verified the I > 3 σ(1) condition. Equivalent reflexions were averaged according to the Laue symmetry 2/m (agreement factor for accepted reflexions: 0.021 on Fobs and 0.032 on I), yielding 425 independent h k l with I > 3 σ(1) for structural refinements. 3. STRUCTURE REFINEMENT. All structure refinements and calculations were carried out with the MOLEN chain programs [4]. Tellurium atom positions were easily unraveled from direct-method outputs while missing niobium and silicon locations were extracted from Fourier difference maps. Statistical occupancies had to be introduced for niobium and silicon atoms. Since the site occupations did not significantly depart from 1/2 (la level) and because they perfectly agreed (i) with the charge balance and (ii) with a doubling
a Ψ measurement on four reflexions (x close to 90°) was carried out for subsequent empirical absorption corrections. After the usual corrections (decay, absorption, and Lorentz-polarization) a set of 1,532 reflexions verified the I > 3 σ(1) condition. Equivalent reflexions were averaged according to the Laue symmetry 2/m (agreement factor for accepted reflexions: 0.021 on Fobs and 0.032 on I), yielding 425 independent h k l with I > 3 σ(1) for structural refinements. 3. STRUCTURE REFINEMENT. All structure refinements and calculations were carried out with the MOLEN chain programs [4]. Tellurium atom positions were easily unraveled from direct-method outputs while missing niobium and silicon locations were extracted from Fourier difference maps. Statistical occupancies had to be introduced for niobium and silicon atoms. Since the site occupations did not significantly depart from 1/2 (la level) and because they perfectly agreed (i) with the charge balance and (ii) with a doubling of the cell (vide infra), they were fixed to 0.5. Final full matrix refinements yielded a reliability factor R=3.0% (Rw = 3.4%) and a merit factor S=1.3 for 425 reflections and with 26 parameters (see Table I). Fourier difference maps did not reveal meaningful extrema. Abnormally high Atomic Displacement Parameters (ADP) are observed for tellurium atoms along the c direction (U33 > U11 or U22) in relation with the parameter doubling (vide infra) observed but not taken into account. In Table II and III are gathered all atomic positions and ADPs, respectively; in Table IV the main distances are summarized. 4. DISCUSSION. — Nb2SiTe4 structural skeleton is based upon an anionic AA/BB stacking very similar to that observed in Nb3GeTe6 [2]. It manages trigonal prismatic sites, on the one hand, and octahedral and tetrahedral sites, on the other hand, alternating along the piling direction. Cations fill up the prismatic layer only, leaving the octahedral/tetrahedral layer empty which, with the absence of inter-layer tellurium bonds (vide infra the electronic structure), constitutes a true van der Waals gap. Niobium atoms classically occupy the trigonal prismatic sites but, contrary to what is observed in Nb3Ge0.9Te6, they all pair up in Nb2Te8 biprism units
(ADP) are observed for tellurium atoms along the c direction (U33 > U11 or U22) in relation with the parameter doubling (vide infra) observed but not taken into account. In Table II and III are gathered all atomic positions and ADPs, respectively; in Table IV the main distances are summarized. 4. DISCUSSION. — Nb2SiTe4 structural skeleton is based upon an anionic AA/BB stacking very similar to that observed in Nb3GeTe6 [2]. It manages trigonal prismatic sites, on the one hand, and octahedral and tetrahedral sites, on the other hand, alternating along the piling direction. Cations fill up the prismatic layer only, leaving the octahedral/tetrahedral layer empty which, with the absence of inter-layer tellurium bonds (vide infra the electronic structure), constitutes a true van der Waals gap. Niobium atoms classically occupy the trigonal prismatic sites but, contrary to what is observed in Nb3Ge0.9Te6, they all pair up in Nb2Te8 biprism units (see Fig. 1). Silicon atoms adopt a peculiar environment, that is, a square coordination at the junction of two trigonal prisms, as germanium atoms do in Nb3Ge0.9 Te6. A meaningful, rational (i.e. alternate) occupancy of the cationic sites would imply a doubling of the cell along b, as Weissenberg photographs suggested, with an ordering between the double prisms enclosing Nb2 pairs and the double prisms housing Si atoms (see Fig. 2). As observed in Nb3Ge0.9 Te6, such an ordering should go along with a lengthening of the Te-Te diagonal in-plane edge of the Si-containing biprisms and a shrinking of the Te-Te diagonal in-plane edge of the Nb2-containing biprisms, that is, with a tilting of the biprisms and a correlative buckling of the Te rows along the b direction (see Fig. 3). Taking into account the mean structure only leads to a partial occupancy (50%) of the niobium and silicon sites and to an averaging out of the Tellurium positions. This implies (i) a mean diagonal in-plane Te-Te distance (360.1 pm) across Fig. 1. Ortep view of the projection in the (bc) plane of one layer of the Nb2SiTe4 structure. Niobium atoms pair up in biprisms (dNb-Nb=288.1 pm), whereas silicon atoms
suggested, with an ordering between the double prisms enclosing Nb2 pairs and the double prisms housing Si atoms (see Fig. 2). As observed in Nb3Ge0.9 Te6, such an ordering should go along with a lengthening of the Te-Te diagonal in-plane edge of the Si-containing biprisms and a shrinking of the Te-Te diagonal in-plane edge of the Nb2-containing biprisms, that is, with a tilting of the biprisms and a correlative buckling of the Te rows along the b direction (see Fig. 3). Taking into account the mean structure only leads to a partial occupancy (50%) of the niobium and silicon sites and to an averaging out of the Tellurium positions. This implies (i) a mean diagonal in-plane Te-Te distance (360.1 pm) across Fig. 1. Ortep view of the projection in the (bc) plane of one layer of the Nb2SiTe4 structure. Niobium atoms pair up in biprisms (dNb-Nb=288.1 pm), whereas silicon atoms are surrounded by a square of tellurium anions. Both cations only partially occupy (50%) their sites. Fig. 1. Vue Ortep de la projection dans le plan (bc) d'un feuillet de la structure Nb2SiTe4. Les atomes de niobium forment des paires dans des biprismes (dNb-Nb = 288.1 pm), tandis que les atomes de silicium sont dans des plans carrés. Les deux types de cations occupent statistiquement (50%) leur site. Fig. 2 Fig. 3 Fig. 2. — Schematic view of the expected in-plane biprism ordering. The ordering is easily obtained from Figure 1 by a doubling of the b cell parameter with the following physical restrictions: (i) both cations (niobium and silicon) are mutually exclusive and (ii) two Nb2Te8 biprisms cannot share a face. Fig. 2. — Vue schématique de la mise en ordre envisagée des biprismes dans les plans. La mise en ordre est facilement dérivée de la figure 1 en doublant le paramètre b avec les restrictions physiques suivantes : (i) les deux cations (niobium et silicium) s'excluent mutuellement, (ii) deux biprismes Nb2Te8 ne peuvent pas avoir de faces communes. Fig. 3. Distorted Nb2SiTe4 slab upon lengthening and shrinking of the Te-Te diagonal in-plane edge of Si and Nb2 double
atomes de silicium sont dans des plans carrés. Les deux types de cations occupent statistiquement (50%) leur site. Fig. 2 Fig. 3 Fig. 2. — Schematic view of the expected in-plane biprism ordering. The ordering is easily obtained from Figure 1 by a doubling of the b cell parameter with the following physical restrictions: (i) both cations (niobium and silicon) are mutually exclusive and (ii) two Nb2Te8 biprisms cannot share a face. Fig. 2. — Vue schématique de la mise en ordre envisagée des biprismes dans les plans. La mise en ordre est facilement dérivée de la figure 1 en doublant le paramètre b avec les restrictions physiques suivantes : (i) les deux cations (niobium et silicium) s'excluent mutuellement, (ii) deux biprismes Nb2Te8 ne peuvent pas avoir de faces communes. Fig. 3. Distorted Nb2SiTe4 slab upon lengthening and shrinking of the Te-Te diagonal in-plane edge of Si and Nb2 double prisms, respectively. Fig. 3. Distorsion du feuillet Nb2SiTe4 par allongement des distances Te-Te dans la diagonale parallèle au feuillet des biprismes contenant le silicium et rétrécissement de ces mêmes distances diagonales pour les biprismes contenant les paires Nb2. PLATE II/PLANCHE II (a) (b) Fig. 4. — Averaging [(a). (b)] of the Nb2SiTe4 hypothetical doubled cell that shows (i) the resulting statistical occupancy and (ii) the origin of high tellurium U33 ADPs. Fig. 4. — Moyenne [(a) —► (b)] de la maille hypothétique double de Nb2SiTe4 montrant (i) l'occupation statistique résultante et (ii) l'origine des valeurs élevées des PDAs U33 des atomes de tellure. (a) (b) Fig. 5. (a) Calculated band structure for the ordered Nb2SiTe4 slab considered in the study. The dashed line refers to the highest occupied level. Γ = (0,0), X = (a*/2,0), Y = (0, b'*/2), and M = (a*/2, b'*/2). (b) Low-lying d-block levels calculated for the Nb2Te8 double prisms of Nb2SiTe4 Fig. 5. (a) Structure de bandes calculé pour un feuillet ordonné de Nb2SiTe4. Les pointillés indiquent le niveau occupé le plus élevé. Γ = (0,0), X = (a*/2,0), Y = (0, b'*/2), et M = (a*/2, b'*/2). (b) Bas des niveaux à caractère d
the Nb2SiTe4 hypothetical doubled cell that shows (i) the resulting statistical occupancy and (ii) the origin of high tellurium U33 ADPs. Fig. 4. — Moyenne [(a) —► (b)] de la maille hypothétique double de Nb2SiTe4 montrant (i) l'occupation statistique résultante et (ii) l'origine des valeurs élevées des PDAs U33 des atomes de tellure. (a) (b) Fig. 5. (a) Calculated band structure for the ordered Nb2SiTe4 slab considered in the study. The dashed line refers to the highest occupied level. Γ = (0,0), X = (a*/2,0), Y = (0, b'*/2), and M = (a*/2, b'*/2). (b) Low-lying d-block levels calculated for the Nb2Te8 double prisms of Nb2SiTe4 Fig. 5. (a) Structure de bandes calculé pour un feuillet ordonné de Nb2SiTe4. Les pointillés indiquent le niveau occupé le plus élevé. Γ = (0,0), X = (a*/2,0), Y = (0, b'*/2), et M = (a*/2, b'*/2). (b) Bas des niveaux à caractère d calculés pour les biprismes Nb2Te8 de Nb2SiTe4. TABLE I Analytical and crystallographic data: parameters of the X-ray data collection and refinement. Données analytiques et cristallographiques : paramètres de l'enregistrement aux rayons X et de l'affinement. 1. Physical, crystallographic and analytical data Formula: Nb2SiTe4. Space group: C 2/c (# 15). Room temperature crystallographic constants: a= 1,421.6 (2) pm, β = 97.97 (1)°; b = 394.40 (8) pm, Z = 4; c = 633.62 (13) pm. Molecular weight: 724.3 g/mol Density (calc.): 13.674 Absorption factor: Il (MoKα) = 389.8 cm-1 2. Data collection Diffractometer: Enraf-Nonius CAD4 Temperature: 295 K Radiation: MoKα Monochromator: oriented graphite (002) Scan mode: co/2θ Scan angle: 0.8 + 0.35 tan (θ) Recording range: 1.5 35° h k l range: -22 22, 2 6, 10 10 Standard reflections: 80 0, 00 2, 020 (every 3,600 sec.) 3. Refinement conditions: Reflexions for crystal matrix orientation: 25 Recorded reflexions in the 1/2 space: 2,970 Independent reflexions with I/a (I) > 3: 425 Refined parameters: 26 Reliability factors: ~R = (|F.| - |Fc|)/|Fo| ~R = (|IFo - IFc|)/|Fo| Rw = 3.4%, S = 1.3 Extinction coefficient: 7.1 (6) x 10-7 Difference Fourier extrema: 2.0 x 10-6 e/pm3 -2.0x 10-6 e/pm3 TABLE II
temperature crystallographic constants: a= 1,421.6 (2) pm, β = 97.97 (1)°; b = 394.40 (8) pm, Z = 4; c = 633.62 (13) pm. Molecular weight: 724.3 g/mol Density (calc.): 13.674 Absorption factor: Il (MoKα) = 389.8 cm-1 2. Data collection Diffractometer: Enraf-Nonius CAD4 Temperature: 295 K Radiation: MoKα Monochromator: oriented graphite (002) Scan mode: co/2θ Scan angle: 0.8 + 0.35 tan (θ) Recording range: 1.5 35° h k l range: -22 22, 2 6, 10 10 Standard reflections: 80 0, 00 2, 020 (every 3,600 sec.) 3. Refinement conditions: Reflexions for crystal matrix orientation: 25 Recorded reflexions in the 1/2 space: 2,970 Independent reflexions with I/a (I) > 3: 425 Refined parameters: 26 Reliability factors: ~R = (|F.| - |Fc|)/|Fo| ~R = (|IFo - IFc|)/|Fo| Rw = 3.4%, S = 1.3 Extinction coefficient: 7.1 (6) x 10-7 Difference Fourier extrema: 2.0 x 10-6 e/pm3 -2.0x 10-6 e/pm3 TABLE II Refined atomic coordinates, equivalent isotropic Atomic Displacement Parameters (ADP), and their estimated standard deviations. Coordonnées atomiques affinées, Paramètres de Déplacement Atomique (PDA) équivalents, et leurs déviations standards estimées. K Atom x y z (x 10-4) t Te 0.13342 (3) 0.2830 (1) 0.29170 (8) 1.557 (8) 100% Nb 0.50034 (7) 0.3253 (3) 0.3964 (2) 0.45 (1) 50% Si 0.5 0.0 0.5 0.79 (8) 50% Isotopic equivalent ADP defined as Biso (pm2) = 4/3 Σi, I, P,7 a2 The biprisms as compared to those found in Nb3Ge0.9Te6 (412.1 and 333.8 pm across the silicon and niobium-containing biprisms, respectively) and (ii) an apparent, high thermal motion for tellurium atoms (large U33 ADP) corresponding to a static disorder as sketched on Figure 4 (from 4 a to 4 b). TABLE III Atomic Displacement Parameters (ADP) and their estimated standard deviations (pm2) (x 104). Paramètres de Déplacement Atomique (PDA) et leurs déviations standards estimées (pm2) (x 104). Atom U11 U22 U33 U12 U13 U23 Te 0.0076 (2) 0.0076 (2) 0.0432 (2) -0.0001 (2) 0.0006(2) -0.0012 (2) Nb 0.0074 (4) 0.0036 (4) 0.0061 (3) -0.0001 (3) 0.0012 (3) 0.0000 (3) Si 0.018 (2) 0.006(2) 0.007 (2) 0.001 (2) 0.003 (2) 0.000 (2) The expression of the
(3) 0.3964 (2) 0.45 (1) 50% Si 0.5 0.0 0.5 0.79 (8) 50% Isotopic equivalent ADP defined as Biso (pm2) = 4/3 Σi, I, P,7 a2 The biprisms as compared to those found in Nb3Ge0.9Te6 (412.1 and 333.8 pm across the silicon and niobium-containing biprisms, respectively) and (ii) an apparent, high thermal motion for tellurium atoms (large U33 ADP) corresponding to a static disorder as sketched on Figure 4 (from 4 a to 4 b). TABLE III Atomic Displacement Parameters (ADP) and their estimated standard deviations (pm2) (x 104). Paramètres de Déplacement Atomique (PDA) et leurs déviations standards estimées (pm2) (x 104). Atom U11 U22 U33 U12 U13 U23 Te 0.0076 (2) 0.0076 (2) 0.0432 (2) -0.0001 (2) 0.0006(2) -0.0012 (2) Nb 0.0074 (4) 0.0036 (4) 0.0061 (3) -0.0001 (3) 0.0012 (3) 0.0000 (3) Si 0.018 (2) 0.006(2) 0.007 (2) 0.001 (2) 0.003 (2) 0.000 (2) The expression of the general ADP is defined as: exp [-2 n2 (Utt h2 a*2 + U22 k2 b*2 + U33 l2 c*2 + 2 U12 h k a* b* + 2 U13 hla* c* + 2 U23 k b* c*)] TABLE IV Main interatomic distances (pm) in Nb2SiTe4. Principales distances interatomiques (pm) dans Nb2SiTe4. I 276.2 (1) I 277.1 (1) Nb-Te 295.3 (1) 295.8 (1) I 298.9 (1) V 299.8 (1) j 2x260.16 (6) Si-Te 2 x 260,34 (5) Nb-Nb 288.1 (2) Nb-Si 2x274.2 (1) Nb-Si 2x281.9 (1) I 360.11 (8) 360.09(8) Te-Te (II to the layer and <400) 387.54 (8) I 387.56 (8) 394.40 (7) Te-Te (1 to the layer and <400) 375.79 (8) (394.79 (8) Te-Te (through the van der Waals gap) 395.39 (8) 395.33 (8) The diffuse character of the superstructure streaks observed on Weissenberg photographs suggests stacking faults that destroy the out-of-plane long range order. Those defects probably correspond to sliding of the sheets. Considering, like in parent Nb3Ge0.9Te6 and many other silicon (or germanium-) containing chalcogenides, an oxidation state of II for Si and III for Nb in Nb2 groups, a straightforward charge balance is achieved with Te-III anions 2 1 e-. However, in view of short contact