On the Arrangement of Pentagonal Columns in Tetragonal Tungsten Bronze-Type Nb₁₈W₁₆O₉₃

Abstract

The evaluation of HAADF-STEM images of a sample with the composition Nb₁₈W₁₆O₉₃ provided new insights into its real structure. The basic structure comprises an intact octahedral framework of the tetragonal tungsten bronze (TTB) type. The partial occupation of the pentagonal tunnels (PT) by metal–oxygen strings determines the oxygen-to-metal ratio (O/ΣM with M = Nb,W). For a large area, the O/ΣM was determined to be 2.755, which is bigger than the value of Nb₁₈W₁₆O₉₃, which is O/ΣM = 2.735. To a large extent, the three-fold TTB superstructure of Nb₈W₉O₄₇ with a high oxygen content is present (O/ΣM = 2.765). In addition, a new four-fold TTB superstructure was found in small domains. Nb₁₂W₁₁O₆₃ with an O/ΣM = 2.739 obviously accommodates part of the sample’s metal excess compared to the stable phase Nb₈W₉O₄₇.

1. Introduction

At present, there is a broad interest in niobium tungsten oxides, since they have potential applications as high-performance anode materials in lithium-ion batteries. The recent worldwide research activities were induced in 2018 by a report of extremely fast reversible Li exchange and high cycling stability for the block-type phase Nb₁₆W₅O₅₅ and for a sample with the composition Nb₁₈W₁₆O₉₃; the structure of Nb₁₈W₁₆O₉₃ was found to be based on the tetragonal tungsten bronze (TTB) type [1]. This structure type comprises corner-sharing octahedra MO₆ that are arranged in such a way that trigonal, square, and pentagonal tunnels arise (Figure 1, bottom left). In the structure of Nb₈W₉O₄₇, one third of the pentagonal tunnels (PTs) are systematically occupied by metal–oxygen (MO) strings, resulting in a three-fold superstructure of the TTB type (Figure 1, top). The metal inside the PT has a pentagonal bi-pyramidal coordination by oxygen. The unit comprising this polyhedron and the five equatorially connected octahedra is designated a pentagonal column (PC) [2]. Two of such PCs are grouped into pairs by sharing a square of octahedra (so-called diamond link [2]). Four orientations of these PC pairs are possible in the TTB framework, and, at one time, two of them occur in each of the two unit cell orientations of the Nb₈W₉O₄₇ structure (Figure 1). The three-fold TTB superstructure realized in Nb₈W₉O₄₇ exhibits high thermodynamic stability, apparently because the interactions between the MO strings inside the TTB substructure are optimized in this arrangement. Iijima and Allpress postulated the following rules for the stable dense packing of PCs [3]: (i) the PTs directly adjacent to a PC (d_PC-PT ≈ 0.6–0.65 nm) remain empty; (ii) one of the two PTs with the next-nearest distance to a PC (d_PC-PT ≈ 0.9 nm) is occupied, forming a pair of diamond-linked PCs. These rules are strictly obeyed in the structure of Nb₈W₉O₄₇. Therefore, any deviation from the composition of M₁₇O₄₇ (M = Nb,W) leads to a less ordered real structure. The occupation of directly adjacent PTs, which breaches the first rule, has rarely been observed in TTB-type niobium tungsten oxides up to now. In contrast, the second rule is strictly valid for the composition of M₁₇O₄₇ only, and chains of diamond-linked PCs have frequently been observed in W-richer structural variants, such as Nb₆W₈O₃₉ (Figure 1) [4].

In 1979, Li-containing phases, such as Li_xNb₈W₉O₄₇ with x = 2 and 4, could be synthesized by the reaction of LiNbO₃ and WO₃ [5]. In 1998, the insertion of Li into Nb₈W₉O₄₇, by both electrochemical and chemical reactions, could be achieved [6]. A reversible electrochemical uptake of up to 20 Li ions per formula unit was reported for Nb₈W₉O₄₇ and for the not fully oxidized members of the solid solution series Nb_8−xW_9+xO₄₇ (1 ≤ x ≤ 6) [7]. As already mentioned above, the electrochemical investigations on TTB-type NbW oxides gained momentum after the study on Nb₁₈W₁₆O₉₃ had been published by Griffith et al. in 2018 [1]. Several follow-up investigations confirmed the outstanding electrochemical performance of samples with this composition [8,9,10,11,12,13]. The orthorhombic structure Nb₁₈W₁₆O₉₃ was proposed by Stephenson in 1968 to explain the split reflections observed in higher-order Laue zones of a twinned crystal with the composition Nb₁₂W₁₁O₆₃ (6Nb₂O₅:11WO₃) [14]. For this composition, congruent melting was found in the course of the exploration of the phase diagram Nb₂O₅/WO₃ by Roth and Waring [15]. The structure of the hypothetical Nb₁₈W₁₆O₉₃ could not be confirmed, neither by single-crystal X-ray diffraction nor by transmission electron microscopy studies [3,16,17]. As both the structural model of Nb₁₈W₁₆O₉₃ and the well-established phase Nb₈W₉O₄₇ are described by a three-fold TTB superstructure with similar symmetry and practically the same unit cell metric, the differentiation of the two structures by powder XRD is difficult, but possible, if a comparative Rietveld refinement is performed [18]. The difference between these structures is the occupation of PTs in the TTB host lattice, and, thus, the appropriate characterization method is to image the metal positions in the ab plane by high-resolution transmission electron microscopy (HRTEM) [3,19] or by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) [20,21,22]. No evidence for the hitherto assumed structural model for Nb₁₈W₁₆O₉₃ was found in the HAADF-STEM images of a sample with this composition. Instead, the observed occupation of the PTs points to a complex intergrowth of two TTB-type phases, as reported in this article.

2. Materials and Methods

2.1. Synthesis

The sample with composition Nb₁₈W₁₆O₉₃ was prepared by the solid-state reaction of Nb₂O₅ and WO₃ powders (molar ratio Nb₂O₅:WO₃ = 9:16). After grinding the powders in an agate mortar, the resulting mixture was pressed into pellets and then annealed in a Pt crucible covered with a Pt lid at 700 °C for 12 h followed by 1200 °C for 12 h. For details, see [12].

2.2. Electron Microscopy

Scanning transmission electron microscopy images (STEM) were recorded on a JEM-ARM300F (JEOL, Japan) using a high-angle annular dark field (HAADF) detector, as described elsewhere [12].

3. Results

The HAADF-STEM images of all the crystal fragments observed along the short crystallographic c axis reveal a defect-free TTB substructure (Figure 2a, Figure 3a and Figure 5a). A close examination of the images shows that most the PTs are empty, as recognized by five bright dots corresponding to a pentagon of octahedra MO₆ centered by a black patch. Similarly, filled PTs (PCs) are imaged as a pentagon of bright dots centered by a sixth bright dot (Figure 2a). The arrangement of the PCs in large areas corresponds to that of Nb₈W₉O₄₇ (Figure 1), e.g., in the upper half of Figure 2a. In the lower part of Figure 2a, a domain with a hitherto unobserved structure occurs, such as in the Nb₈W₉O₄₇ structure, where two differently oriented PC pairs are present (marked yellow and blue in Figure 2b,c). In this structure, they are oriented nearly perpendicular with respect to each other, so that two regularly arranged PCs are directly adjacent (red arrows in Figure 2d). While this close distance breaks the first rule proposed for a stable PC arrangement, it enables denser PC packing and, thus, a decrease in the oxygen/metal ratio O/ΣM (M = Nb,W). A four-fold TTB supercell with a = b = a_TTB, c = c_TTB describes this structure (Figure 2d). The metric of this cell is approximately tetragonal, but the peculiar PC arrangement decreases its symmetry to orthorhombic. In this cell, six of the sixteen PTs are occupied according to (MO)₆(M₁₀O₃₀)₄, or normalized to a single TTB unit (MO)_1.5(M₁₀O₃₀). Thus, the number of PCs is higher than in Nb₈W₉O₄₇, corresponding to (MO)_1.33(M₁₀O₃₀), which leads to a decreased ratio O/ΣM: 2.739 vs. 2.765. The general formula (MO)₆(M₁₀O₃₀)₄ corresponds to the composition Nb₁₂W₁₁O₆₃, which is that of the so-called 6:11 phase (6Nb₂O₅:11WO₃) [15]. Interestingly, the structural model contains TTB cells in which all four PTs are empty, as indicated by red frames in Figure 2d, which is an array that does not occur in the Nb₈W₉O₄₇ structure. These empty TTB units and the units in which two PTs are occupied (blue PC pairs) are alternatingly arranged in the structural model of Nb₁₂W₁₁O₆₃ (Figure 2d). The domains of Nb₈W₉O₄₇ and Nb₁₂W₁₁O₆₃ are intimately intergrown with each other by sharing a slab of PC pairs (yellow in Figure 2c) that run along the crystallographic a axis of Nb₈W₉O₄₇. The domain of the new structure is terminated on the left side by a defective area, which contains the blue PC pairs as a slab in a perpendicular orientation.

A larger area, which mainly contains the Nb₈W₉O₄₇ structure, is shown in Figure 3. Two distinct defects that are typical for this structure occur, namely, an out-of-phase boundary on the lower left side and rotational twinning by 90° on the right side. The three-fold TTB superstructure is perfectly ordered on the upper left side (Figure 3a). The inclusion of a row of five PC pairs with perpendicular orientation (blue) causes a shift of the adjacent domains of Nb₈W₉O₄₇ by 1/3 b (=b_TTB), with respect to each other (Figure 3b,c). In the lower part of the boundary, the structure is semiregular; the slabs of yellow PC pairs in the two domains are connected via a chain of four diamond-linked PCs, with each slab contributing two PCs. The slabs with green PC pairs are not connected; instead, they directly neighbor an isolated yellow PC.

This structure is depicted on the lower left side of Figure 4a. Four-membered PC chains had been observed before, e.g., in zigzag out-of-phase and twin boundaries in Nb₈W₉O₄₇ [3] and as a regular structural element in Nb₆W₈O₃₉ (Figure 1) [4]. At the top of this boundary, the occupation of PTs is less regular and includes a three-membered PC chain, in which the middle PC belongs to two perpendicularly oriented slabs of PC pairs (blue and green). Here, a small block of six PCs with perpendicular orientation (90° rotation twin) is incorporated (light blue area). Interestingly, this block connects two slabs of green PC pairs, and the two PCs at the top and at the bottom can be regarded as part of the small section of three PC pairs (light blue area) as well as part of the connected slabs of green PCs.

Another example for such rotational twinning appears on the right side of Figure 3, with the corresponding structural model shown in Figure 4b. In the upper part, slabs of blue and magenta PC pairs are connected to perpendicularly oriented slabs of green and yellow PC pairs. This arrangement is frequently observed at boundaries between rotational twin domains; the central PC of a triple PC chain belongs to the two perpendicular PC slabs. This is also the case for the isolated PC that is quasi an extension of the PC rows that are not directly connected. This frequently appearing array is marked by dotted lines in Figure 4b (top center) for the two units just described. It should be mentioned that such a connection of 90° rotational twin domains by three-membered PC chains was already observed in the first HRTEM investigation of this material in 1974 [3]. Similarly to here, an empty TTB cell and directly adjacent simultaneously occupied PTs were found.

As the HAADF-STEM method generates images with the intensity I increasing with the atomic number Z (I~Z²; Z contrast imaging [23,24]), the metal positions appear as bright dots, with their brightness increasing with the W content of the particular atomic column (Z_W = 74 > Z_Nb = 41) [19,21]. Thus, the observation of the relative brightness of the individual atomic columns in regions of constant thickness provides some information about the distribution of the metals (inset in Figure 3a). The relative darkness of the metal position inside the PTs indicates that it is preferentially occupied by Nb, as is also the case in W-richer samples [21]. Thus, the preference of Nb for this position, as it was found during the structure determination of Nb₈W₉O₄₇ from single-crystal X-ray diffraction data [18,25], has been preserved. This is also the case for a W-rich position that appears with high brightness (inset in Figure 3a).

The inclusion of structural units with directly adjacent PCs is a necessary means to account for the increased number of occupied PTs compelled by the decreased ratio O/ΣM. This observation is corroborated by determining the ratio O/ΣM of a rather large crystal fragment (Figure 5). As already observed in the fragments described above, this region comprises small domains with the structures of Nb₈W₉O₄₇ and Nb₁₂W₁₁O₆₃, which are intimately intergrown with each other and appear in different orientations. The occupied PTs in this large area were detected (Figure 5b) and counted. The whole area contains 1345 TTB cells, in which 1876 PTs are occupied. Therefore, the overall composition is (MO)₁₈₇₆(M₁₀O₃₀)₁₃₄₅, with a ratio O/ΣM = 2.755. This value is larger than that of the sample Nb₁₈W₁₆O₉₃, with O/ΣM = 2.735, but significantly smaller than that of Nb₈W₉O₄₇, with O/ΣM = 2.765.

4. Discussion

According to the evaluation of the HAADF-STEM images, the sample Nb₁₈W₁₆O₉₃ comprises an intact TTB framework that hosts intimately intergrown domains of the Nb₈W₉O₄₇ phase, with an increased amount of filled PTs. In the latter domains, the occupation of directly neighbored PTs by MO strings compensates for the decreased O/ΣM, as follows: (MO)_x(M₁₀O₃₀) with x > 4/3 compared to x = 4/3 in Nb₈W₉O₄₇. This is the case in the structure of Nb₁₂W₁₁O₆₃ (=(MO)₆(M₁₀O₃₀)₄ with x = 1.5) that has been observed in small domains in the sample Nb₁₈W₁₆O₉₃ for the first time. In fact, the ratio O/ΣM of Nb₁₂W₁₁O₆₃ (2.739) is close to that of Nb₁₈W₁₆O₉₃ (2.735). This dense occupation of PTs, violating the first rule for a stable array of PCs, is enforced by the decreased O content that must be somehow adjusted by structural adaptions. Nonetheless, the most stable PC arrangement, namely, that of Nb₈W₉O₄₇, is still the dominating structure in the sample Nb₁₈W₁₆O₉₃, and is present in many domains, with a typical diameter of several 10 nm. In contrast, the O-poorer domains are smaller and heavily disordered, with regular structures appearing as rather small inclusions only. Both the TTB superstructures Nb₈W₉O₄₇ and Nb₁₂W₁₁O₆₃ do occur in different orientation variants (Figure 5b), with the respective nanometer-sized domains coherently intergrown with each other. This flexibility is caused by the unaffected TTB-type framework, with the structural modifications realized by filling PTs in different ways.

Note that on the well-investigated W-rich side of Nb₈W₉O₄₇ (O/SM > 2.765 (=47:17)), the O surplus is accommodated by forming little ordered structures with a lower density of filled PTs. An example of this is the Nb₆W₈O₃₉ structure (O/ΣM = 2.786) with four-membered PC chains (Figure 1) [4], which are intergrown with domains, or even single units, of Nb₄W₇O₃₁ (O/ΣM = 2.818) [18,26]. It is important to state that the PCs in all TTB-based structures and variants tend to be connected to form pairs or chains via the diamond link, rather than being isolated or directly adjacent. In the structure Nb₁₂W₁₁O₆₃, corresponding to (MO)₆(M₁₀O₃₀)₄, diamond-linked PCs pairs are directly adjacent to each other, which might represent a moderately stable arrangement, as it is present as an intact structure in nanodomains.

5. Conclusions

The present HAADF-STEM investigation reveals that the sample Nb₁₈W₁₆O₉₃ cannot be considered as a separate phase. The arrangement of PCs embedded in a perfectly ordered TTB substructure corresponds, in wide areas, to that of the well-known and stable phase Nb₈W₉O₄₇, but the arrangement is only slightly ordered in small domains. No evidence for the existence of a PC array, as suggested by Stephenson for the structure of Nb₁₈W₁₆O₉₃ [14] could be found. In general, most PCs are connected via the diamond link to form pairs or, less frequently, three- or four-membered chains of PCs. The decreased O content compared to Nb₈W₉O₄₇ is structurally compensated by denser packing of PTs filled with MO strings. This unavoidably leads to the occupation of directly adjacent PTs, similarly to how they are presented in the structural model proposed for Nb₁₂W₁₁O₆₃. This structure only appears in the investigated sample in nanometer-sized domains, but future synthetic efforts might be successful in preparing the Nb₁₂W₁₁O₆₃ phase in pure form.