Hollandite tunnel structures

Fig. 7.4 Tunnel structures based on rutiles, (a) Rutile-like chains, showing alternately short and long metal-metal distances, as in MoOj. (h)-(

The astute reader will recognize that this compound is a metal-metal bonded adaptation of the well-known hollandite structure (19). Further, the metal cluster configuration is like that in CsNb Cl.. (20), though the latter compound possesses a double layer structure rather than the tunnel structure observed here. Finally these new structures provide only a glimpse of what is possible, and an excitement about new compounds of unusual structure and properties yet to come from studies of highly reduced ternary and quaternary refractory metal oxide systems. 

Generally, there are two major structural forms for these minerals chain or tunnel structures, and layer structures. All of these forms are comprised of MnOs octahedras. Water molecules and/or other cations (8) are ofien present at various sites in the structures. Mn oxides having a chain or tunnel structure include pyrolusite, ramsdellite, hollandite, romanechite, and todorokite. Typical structures for the chain or tunnel type Mn oxide mineral are presented in Figure 1. Lithiophorite, chalcophanite, and bimessite are examples of Mn oxide minerals havii a layer structure. Typical structural maps are shown in Figure 2. 

Hereinafter, the materials with a (2 x 2) tunnel structure will be referred to as hollandites with the identity of their counter cations. 

Hollandite- or cryptomelane-type manganese oxides have a one-dimensional tunnel structure comprised of tunnels with an almost quadratic cross section... 

Other complex oxides which are known or expected to exhibit high ionic conductivities at moderate temperature as a result of possessing tunnel structures are the hollandites, K Mg. Ti/ y x 0 and K A1 Ti, vO, the tungstates (K W O ), iobates... 

Fig. 6.2 Schematic view of the tunnel structure of Mn02 polymorph (a) pyrolusite p-Mn02 (Ti i) and (b) ramsdellite R-Mn02 (Tj 2), hollandite a-Mn02 (T2,2) and (c) nsutite y-Mn02 (intergrowth Ti.i + Ti 2)...

Figure 7. Crystal structures of (a) hollandite, (b) romanechite (psilomelane), and (c) todorokite. The structures arc shown as three-dimensional arrangements of the MnO() octahedra (the tunnel-tilling cations and water molecules, respectively, are not shown in these plots) and as projections along the short axis. Small, medium, and large circles represenl the manganese atoms, oxygen atoms, and the foreign cations or water molecules, respectively. Open circles, height z. = 0 fdled circles, height z = Vi.

Figure 2. In this low-symmetry, metal-metal bonded adaptation of the well-known hollandite structure the chains forming the four sides of each tunnel are related in pairs by P 1 symmetry.

The use of framework structures to minimize AH for alkali-ion electrolytes has been demonstrated to provide a means of opening up the bottlenecks to cation motion in a number of oxides (Goodenough, Hong and Kafalas, 1976). Framework structures may provide one-dimensional tunnels as in hollandite, two-dimensional transport in planes as in the )S-aluminas, or three-dimensional transport as in NASICON and LISICON. Since one-dimensional tunnels are readily blocked, the two-and three-dimensional conductors are the more interesting. 

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