BaxCsy [(Ti, Al)3+2x+y Ti4+8-2x-y] O16 Synroc-type hollandites - II. Structural chemistry

High-resolution transmission electron microscopy and selected-area electron diffraction show that all phases of the general formula [BaxCsy [(Al, Ti)3+2x + y Ti4+8-2x-y] O16, 1.08 ≼ x + y ≼ 1.51 have the hollandite-type substructure. These hollandites display commensurate and incommensurate superlattices owing to the ordered insertion of large cations (Ba2+, Cs+) into the (2, 2) tunnel interstices of the octahedral (Al, Ti) O6 framework. Multiplicity (m) of a supercell is defined as dsupercell divided by d002 for the subcell. Ordering may be one-dimensional, in which case the cation sequences between (2, 2) channels are independent, three-dimensional with lateral correlation between tunnels, or a combination of both. One-dimensional superstructures yield commensurate multiplicities of 4 in all phases except an aluminous caesium hollandite where m = 6. Three-dimensional superstructures are both incommensurate and commensurate, with 4.50 ≼ m ≼ 6.59. Multiplicities correlate directly with caesium content per formula unit, establishing a maximum in caesiumrich hollandites. Among barium (y = 0) and caesium endmembers, (x = 0) multiplicities increase modestly with increasing Al3+: (Al + Ti)3+ content. Superstructure dimensionality is largely determined by the nature and proportions of the trivalent species, rather than the tunnel cations; one-dimensional order is commonplace in hollandites rich in trivalent titanium but rare in aluminous hollandites. High-resolution electron microscopy supports the interpretation of incommensurate superstructures as fine-scale intergrowths of commensurate microdomains with m = 4, 5, 6 or 7. For aluminous hollandites, rare examples of structural modifications involving tunnels of different cross-sectional dimensions may be found, i. e. T(2, n), 1 ≼ n ≼ 3 intergrowths. As all specimens are sensitive to the electron beam, prolonged irradiation at high electron fluxes can initiate the transformation of single-crystal hollandite to single-crystal rutile. A mechanism for this transformation is proposed, whereby the hollandite crystals initially adjust their multiplicity to six. Growth fronts on {101}holl subsequently propagate through the crystals consuming hollandite and leaving rutile: the structure of the interface between the phases is believed to contain components of rutile possessing antiphase boundaries. In this reconstructive transformation, [100] of the newly formed rutile invariably lies almost parallel to [110] of the orig