No abstract

The members of the cyanotrichite group, cyanotrichite, carbonate-cyanotrichite, camerolaite and khaidarkanite, are closely related in terms of their structures, although the single-crystal structure of the latter is the only one to have been determined. Powder X-ray-diffraction data for all members can be indexed on the basis of a common monoclinic cell with a � 10.2, b � 2.9, c � 12.6 Å and b in the range 92.3-97.9° (a and c are interchanged in khaidarkanite owing to C-centering). The structures of cyanotrichite, carbonate-cyanotrichite and camerolaite can be rationalized in terms of that of khaidarkanite, with sulfate and carbonate taking the positions of AlF 3 groups. Ordering and disordering of the sites yields P-and C-centered cells, respectively, depending on composition. The analytical data originally reported for carbonate-cyanotrichite are re-interpreted in terms of Al(OH) 3 taking the place of AlF 3 groups in khaidarkanite. There is no evidence for solid solution between cyanotrichite and carbonate-cyanotrichite; it is evident that some carbonate-cyanotrichite specimens contain no sulfate. A range of other stoichiometries may be accommodated by the archetypical structural motif, and these remain to be discovered in nature. Other related species, including a Co-rich member of the group from the Grandview mine, Arizona, and a cyanotrichite-like phase from the Clara mine, Germany, have also been examined. These too conform to the structural relations that have been derived. Final resolution of the nature of individual members of the group awaits single-crystal structural studies, if and when suitable crystals become available, or if structure solution using powder X-ray diffraction data or ab initio methods becomes possible.

This investigation was aimed at identifying the influence of applied processing conditions, temperature and oxygen activity, on the segregation of Ta in Tadoped TiO 2 . Over the temperature range of 1173−1523 K, it has been observed that the tendency for Ta to segregate is greater under the application of oxidizing conditions (p(O 2 ) = 101 kPa) than that under reducing conditions (p(O 2 ) = ∼10 −10 Pa). This respectively manifests as an accumulation and depletion of Ta at the surface of Ta−TiO 2 under the respective applied oxygen activities. This behavior has been interpreted in terms of the relative activities of Ta at the surface and bulk during processing. The results indicate that it is possible to substantially alter the concentration of a donor dopant at the surface of TiO 2 despite maintaining the bulk dopant loading. In so doing, a TiO 2 -based homojunction is formed that can be engineered to favor charge separation, as demonstrated by preliminary surface photovoltage measurements. The optimization of this homojunction provides a promising approach for developing a novel photoelectrode material for solar-driven water splitting. This investigation has provided a fundamental basis for further investigations of compositionally graded TiO 2 -based materials for improved solar harvesting.

The crystal structures of two polymorphic forms of bariopharmacosiderite have been determined. Bariopharmacosiderite-C, from Robinson's Reef, Clunes, Victoria, Australia, (Ba 0.47 K 0.04 Na 0.02 )(Fe 3.97 Al 0.03 )[(As 0.72 P 0.28 )O 4 ] 3 (OH) 4 •2.52H 2 O, is cubic, space group P43m, a 7.942(1) Å, Z = 1, R = 0.089. Bariopharmacosiderite-Q, from the Sunny Corner mine, Sunny Corner, New South Wales, Australia, Ba 0.5 Fe 4 (OH) 4 (AsO 4 ) 3 •6.16H 2 O, is tetragonal, space group P42m, a 7.947(1), c 8.049(2) Å, Z = 1, R = 0.050. In the cubic polymorph, Ba ions are disordered over all faces of the unit cell, whereas in the tetragonal polymorph, Ba ions are centered on the 001 face. In both cases, the Ba ions are 12-coordinate, with eight bonds to arsenate oxygen atoms; four H 2 O groups complete the coordination sphere, with longer bonds to Ba in the cubic polymorph. Additional H 2 O groups are hydrogen-bonded to each other and to the H 2 O groups that coordinate Ba. Some of these exhibit "zeolitic" behavior. By analogy to the properties of synthetic pharmacoalumite, KAl 4 (AsO 4 ) 3 (OH) 4 •nH 2 O, the cubic polymorph appears to be more stable than the tetragonal one, although two further body-centered polymorphs are known. In addition, the crystal structure of natropharmacosiderite from the Gold Hill mine, Utah, has been determined. Natropharmacosiderite, (Na 0.75 K 0.14 Ba 0.11 ) S1.00 Fe 4 (AsO 4 ) 3 (OH) 3.89 O 0.11 •4H 2 O, is cubic, space group P43m, with a 7.928(9) Å, Z = 1 and R = 0.0654. The Na position is displaced by ~0.2 Å from the Wyckoff 3c site, and is coordinated by four H 2 O groups and eight oxygen atoms of arsenate groups. A new general formula for "excess cation" pharmacosiderite is proposed, involving deprotonation of bridging hydroxide ions.