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Abstract Preferred orientation in polycrystalline materials is one of the most challenging problems for structural analysis. Significant preferred orientation can severely affect the structure‐property analysis of the systems having substantial crystallographic anisotropy. Here, an extremely high degree of preferred orientation has been demonstrated in R‐block hollandite hexagonal PbFexV6−xO11 compounds, which has recently been shown to exhibit an unusual colossal electroresistance response that has a strong dependence on structural modifications. The present results warn against possible errors in understanding the evolution of the crystal structure of these hollandites, which might adversely affect the estimation of the influence of the same on their spectacular physical properties.

Preferred orientation in polycrystalline materials is one of the most challenging problems for structural analysis. Significant preferred orientation can severely affect the structure‐property analysis of the systems having substantial crystallographic anisotropy. Here, an extremely high degree of preferred orientation has been demonstrated in R‐block hollandite hexagonal PbFexV6−xO11 compounds, which has recently been shown to exhibit an unusual colossal electroresistance response that has a strong dependence on structural modifications. The present results warn against possible errors in understanding the evolution of the crystal structure of these hollandites, which might adversely affect the estimation of the influence of the same on their spectacular physical properties. [ABSTRACT FROM AUTHOR]

Ceramics based on hollandites are known to mainly produced by the conventional solid-phase method. However, the use of various sol–gel methods in some cases makes it possible to change the temperature and time its synthesis, morphological characteristics, porosity, and electrophysical properties. In this work, ceramic materials based on a number of cesium titanate hollandite phases were synthesized by combustion of citrate–nitrate mixtures. The structure of the obtained materials was studied by X-ray powder diffraction analysis and scanning electron microscopy. Investigation of the electrophysical properties showed that, in a hydrogen flow, the electrical conductivity of hollandites with aluminum and nickel increases significantly (by 2.5–3 orders of magnitude) throughout the studied temperature range. Thus, ceramics based on these hollandites can be considered promising for creating hydrogen sensors and fuel cells. [ABSTRACT FROM AUTHOR]

Using the pyrolysis of citrate-nitrate compositions, ceramic materials are synthesized in the titanium-rich region of the K2O–TiO2–Al2O3 system. The resulting materials are compositions with different phase contents: K2Ti6O13, hollandite KxAlxTi8– xO16, and titanium oxide with a rutile structure. The results of a study of their electrically conductive properties in a wide temperature range are presented. The highest conductivity (σ = 1.2 × 10–3 S/cm at 750°C) is exhibited by single-phase potassium hexatitanate. The activation energy for this sample in the range 450–750°С is 0.75 eV. [ABSTRACT FROM AUTHOR]

This review provides a broad overview of the structural characteristics, compositional flexibility, and structure–property relationships of hollandite materials. Hollandites have a general formula AxB8O16, x ≤ 2, with 'A' cations located in one-dimensional tunnels formed by a framework of 'B'–O octahedra. With numerous possibilities for chemical and structural modifications, hollandite family provides many opportunities to manipulate its properties for specific applications. First, we review the chemistry, structure–property relationship, and processing techniques for various applications. The primary focus is on the cumulative effects of A- and B-cation interaction, and the resultant parameters including unit cell symmetry, cation order–disorder, electronic and/or magnetic coupling, that dictate the material's applicability. Then, selected applications, such as crystalline hosts for radioactive caesium disposal, electrode material for Li-ion batteries, and ferromagnetic materials, are outlined from a structure–property relationship perspective. Finally, processing strategies in correlation with structural evolution and applications are briefly addressed. [ABSTRACT FROM AUTHOR]

Materials of the hollandite structure in the K2O–MeO(Mе2O3)–TiO2 (Ме = Al, Ni, Mg) system, synthesized by two methods (solid-phase synthesis and pyrolysis of citrate–nitrate compositions), were studied. The pyrolysis, compared to the traditional solid-phase synthesis, yielded the materials with the more developed specific surface and, as a consequence, enhanced performance in sorption of a model dye, Methylene Blue (for K2MgTi7O16, q = 18.75 mg g–1). The K2Al2Ti6O16 sample, also prepared by pyrolysis, showed the highest catalytic performance in oxidation of СО and Н2. The hydrogen oxidation on this catalyst occurred to 95% at 355°С with the performance of ~0.21 × 10–5 mol g–1 s–1, which is two times higher compared to the sample of the same composition prepared by the solid-phase method. The hollandites show promise as sorbents and catalysts for gas treatment. [ABSTRACT FROM AUTHOR]

Hollandites with compositions Ba 1.15−x Cs 2 x Cr 2.3 Ti 5.7 O 16 (0 ≤ x ≤ 1.15) intended for the immobilization of cesium (Cs) from nuclear waste have been prepared, characterized, and analyzed for Cs retention properties. Sol-gel synthesized powders were used for structural characterization using a combination of X-ray, neutron, and electron diffraction techniques. Phase-pure hollandites adopting tetragonal (I4/m) or monoclinic symmetry (I2/m) were observed to form in the compositional range 0 ≤ x ≤ 0.4. Structural models for the compositions, x = 0, 0.15, and 0.25 were developed from Rietveld analysis of powder diffraction data. Refined anisotropic displacement parameters ( β ij ) for the Ba and Cs ions in the hollandite tunnels indicate local disorder of Ba/Cs along the tunnel direction. In addition, weak superlattice reflections were observed in X-ray and electron diffraction patterns that were due to the compositional modulation i.e., ordering of ions and vacancies along tunnel direction. Our overall observations suggest the phase-pure hollandites studied assumed supercell structures with ordered tunnel cations, which in turn have positional disorder in individual supercells. [ABSTRACT FROM AUTHOR]

The synthesis and physical properties of the new distorted-Hollandite PbIr 4 Se 8 are reported. Powder X-ray diffraction and transmission electron microscopy show that the structure consists of edge- and corner-sharing IrSe 6 octahedra, with one-dimensional channels occupied by Pb. The structure contains Se-Se anion-anion bonding, leading to an electron count of Pb 2+ (Ir 3+ ) 4 (Se 2 ) 2- (Se 2− ) 6 , confirmed by bond-valence sums and diamagnetic behavior. Structural and heat capacity measurements demonstrate disorder on the Pb site, due to the combination of lone-pair effects and the large size of the one-dimensional channels. Comparisons are made to known Hollandite and pseudo-Hollandite structures, which demonstrates that the anion-anion bonding in PbIr 4 Se 8 distorts its structure, to accommodate the Ir 3+ state. An electronic structure calculation indicates semiconductor character with a band gap of 0.76(11) eV. [ABSTRACT FROM AUTHOR]

We have studied the crystallization of potassium polytitanates (PPTs) modified in aqueous solutions of salts of various transition metals. The results demonstrate that the modified materials crystallize in the form of solid solutions with the hollandite structure. Ceramic materials obtained using the PPT/Fe, PPT/Ni, and PPT/Co systems have high dielectric permittivity in the low-frequency region (ε ~ 10), whereas PPT/Cu ceramics have lower ε (~10), retaining it to a frequency of 1 MHz, and very low dielectric losses (tanδ ~ 10). [ABSTRACT FROM AUTHOR]

Through salt-flux methods, single crystals of eight novel hollandite-type titanates have been grown with the stoichiometry K x (Ti 8− y M y ) 8 O 16 for x = y ≈1.5 where M 3+ =Sc, Ti, V, Cr, Mn, and Fe, and for x /2= y ≈0.75 where M 2+ =Co and Ni. Single crystal and powder X-ray diffraction reveals phase pure crystals with tetragonal space group I 4/ m . These titanates are microporous oxides with 1-dimensional channels where the non-framework cation, K, resides. In addition to diffraction, spectroscopic methods were used to characterize the chemical composition of both the bulk and surface. X-ray photoelectron spectroscopy affords the oxidation states of the substituted transition metals M on the surface and energy dispersive X-ray spectroscopy the metal ratios, including that of potassium, for an overall accurate description of the chemical composition of both the bulk and surface. Magnetization versus temperature measurements indicate that all phases are Curie paramagnets with effective magnetic moments consistent with the d -electron count given by combined X-ray diffraction and elemental analysis techniques. [ABSTRACT FROM AUTHOR]

Abstract: Diamonds from Juína area, Brazil, are known by their superdeep origin related to recycling of oceanic crust, basalts and pelagic sediments in the lower mantle. Chemical and structural data on mineral inclusions support this hypothesis. We have studied the inclusions of five diamonds from Juininha alluvial deposits, part of the Juína province, and we have found for the first time in terrestrial rocks, a paragenetic assemblage only observed in high-pressure experiments and/or extraterrestrial rocks. The results suggest the presence of a veined-type lower mantle, where an assemblage of hydrous basaltic origin (MgFeAl-perovskite) is crosscut by pockets and/or veins of intermediate to acid compositions, dominantly constituted by SiO2 +(K,Na)-Hollandite. The silicate paragenesis (KNa-Hollandite and SiO2) can be produced by melting of a pelagic metasedimentary source subducted together with the oceanic slab down to the uppermost lower mantle. Such a veined-type lower mantle puts in evidence a derived source modified by melting/dehydration of multicomponent from a subducted slab. [Copyright &y& Elsevier]

Abstract: We report the synthesis and structural characterization of the new Ir holladite, Rb0.17IrO2. Rb0.17IrO2 crystallizes in the tetragonal hollandite structure. In contrast to the previously reported monoclinic Ir hollandite K0.25IrO2, we do not observe a difference in Ir–O bond lengths in Rb0.17IrO2 and thus find Ir in an average oxidation state of + 3.83. We also report and compare the electronic and magnetic properties of Rb0.17IrO2 and K0.25IrO2, finding that they are both metallic and Pauli paramagnets further supporting that the electrons are delocalized in the Ir 5d states. [Copyright &y& Elsevier]

The results of experimental studies of synthesis of the hollandite phase KGaTiO using initial mixtures of different dispersion compositions obtained by two methods, i.e., mechanical dispersion (MD) (followed by solid-phase sintering) and sol-gel method (Pechini method, MP), are reported. The catalytic properties of the obtained materials in the reactions of carbon monoxide (CO) and hydrogen (H) oxidation have been determined. It has been demonstrated that an increase in the catalytic activity in the CO oxidation reaction is observed on the samples obtained using the sol-gel method, in which the hollandite phase content is higher and crystallization is more complete. The samples obtained using the MD method are characterized by a low porosity and activity in comparison with those produced by the Pechini method. [ABSTRACT FROM AUTHOR]

Abstract: The crystal structures, synthesis and physical properties of ruthenium hollandites ALi2Ru6O12 (A=Na, K) with a new pseudo-hexagonal structure type are described. Analogous to tetragonal hollandites, the framework is made of MO6 octahedra in double chains that share corner oxygens with each other to create interstitial tunnels. The tunnels are either hexagonal or triangular in cross-section. Magnetic susceptibilities, low temperature specific heat, and electrical resistivities are reported. The data indicate that these materials are normal, low density of states metals. This new structure type can be extended from A=Group I to A=Group II ions with the synthesis of CaLi2Ru6O12 and SrLi2Ru6O12. [Copyright &y& Elsevier]