Hollandite-type Compounds Research Articles

Heavy ion irradiations on synthetic hollandite-type materials: Ba1.0Cs0.3A2.3Ti5.7O16 (A=Cr, Fe, Al)

The hollandite supergroup of minerals has received considerable attention as a nuclear waste form for immobilization of Cs. The radiation stability of synthetic hollandite-type compounds described generally as Ba1.0Cs0.3A2.3Ti5.7O16 (A=Cr, Fe, Al) were evaluated by heavy ion (Kr) irradiations on polycrystalline single phase materials and multiphase materials incorporating the hollandite phases. Ion irradiation damage effects on these samples were examined using grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM). Single phase compounds possess tetragonal structure with space group I4/m. GIXRD and TEM observations revealed that 600keV Kr irradiation-induced amorphization on single phase hollandites compounds occurred at a fluence between 2.5×1014Kr/cm2 and 5×1014Kr/cm2. The critical amorphization fluence of single phase hollandite compounds obtained by in situ 1MeV Kr ion irradiation was around 3.25×1014Kr/cm2. The hollandite phase exhibited similar amorphization susceptibility under Kr ion irradiation when incorporated into a multiphase system.

Ferrihollandite, BaMn4+

The recent revisitation of the nomenclature of tunnel oxides belonging to the hollandite supergroup led to the definition of a new end-member, ferrihollandite, ideally BaMn 4+ 6 Fe 3+ 2 O 16 . In particular, the type hollandite from Kajlidongri, Jhabua District, Madhya Pradesh, India, proved to be ferrihollandite. A new occurrence of ferrihollandite is here described from the manganese hydrothermal ores from Vagli, Apuan Alps, Tuscany, Italy, on the basis of chemical and X-ray diffraction data. Electron-microprobe data point to the following composition (in wt%): TiO 2 3.06, MnO 2 58.83, Mn 2 O 3 8.64, Fe 2 O 3 9.76, Al 2 O 3 0.79, ZnO 0.03, SrO 2.77, BaO 14.63, Na 2 O 0.09, K 2 O 0.05, sum 98.65. The structural formula, based on 16 oxygen atoms and 8 octahedral cations per formula unit, is (Ba 0.793 Sr 0.222 Na 0.024 K 0.009 ) ∑=1.048 (Mn 4+ 5.624 Ti 0.318 Fe 3+ 1.016 Mn 3+ 0.910 Al 0.129 Zn 0.003 ) ∑=8.000 O 16 . The TG-DSC data show three main endothermic processes, related to the loss of water and the release of oxygen upon heating. The total mass loss is ~ 9.7 wt% in the temperature range between 25 and 1200 °C; the mass loss related to the release of water is ~ 1.5 wt%. Single-crystal X-ray diffraction study gives a monoclinic unit cell, a 10.0001(7), b 5.7465(4), c 9.8076(8) A, β 90.713(2)°, V 563.56(2) A 3 , space group P 2/ n . The crystal structure has been solved and refined on the basis of 1675 observed reflections, with a final R 1 = 0.037. The basic structural features of ferrihollandite are the same as the 2.9 A hollandite-type compounds, but with long-range ordering of the tunnel cations. The occurrence of superstructure reflections could be related to the ordered sequence Ba–H 2 O–Ba–H 2 O within the tunnels.

Correlativity of the nitrogen oxide adsorption mechanism and crystal structure in hollandite-type compounds

Hollandite-type K1.88Ga1.88Sn6.12O16 and K1.58Ga1.58Ti6.42O16 powders obtained by the solid-state reaction method were examined by DRIFT spectroscopy to evaluate their NO adsorption properties. Two key findings were deduced from the refined structure information and evaluation of NO adsorption on a hollandite surface: first, the presence of oxygen in carrier gas contributed significantly to NO adsorption; second, the occupation probability and atomic coordinate of alkaline-metal at the end of the one-dimensional tunnel structure influenced the adsorption volume per unit cell.

Crystal growth and refinement of K 1.88Ga 1.88Sn 8.12O 16 hollandite-type compound

We grew single crystals of K 1.88Ga 1.88Sn 6.12O 16 hollandite-type compound and precisely determined their crystal structure by X-ray diffraction. K 1.88Ga 1.88Sn 6.12O 16 fibrous crystals measuring an average of about 2 mm in length and 60 μm in diameter were obtained by slow-cooling (from 1623 K to 1273 K) of a flux melt consisting of a 40 mol% crystal composition of (K 2O) 1(Ga 2O 3) 1(SnO 2) 1 and a 60 mol% flux composition of (K 2O) 2(MoO 3) 1(B 2O 3). The unit cell of K 1.88Ga 1.88Sn 6.12O 16 ( a = 1.0389(2) nm, c = 0.3132(2) nm, V = 0.3381(2) nm 3) was larger than that of K 1.52Ga 1.52Ti 6.48O 16. The ionic size difference with the replacement of Ti by Sn, 10%, was relaxed by an anisotropic expansion of the octahedra by 2.6% in the (001) plane and 5.8% along the c-axis.

Synthesis and photo-catalytic property of a hollandite-type compound (K 2Ga 2Sn 6O 16)

Hollandites crystallize with one-dimensional tunnel-like framework structures. The tunnels in the hollandite are usually occupied by alkaline or alkaline earth elements and a case in point is K 2Ga 2Sn 6O 16 (KGSO). This compound has been shown to be catalytically active in the decomposition of endocrine-disrupting chemicals, pentachlorophenol (C 6Cl 5OH) and heptachloro-dioxin (C 12Cl 7O 2H), under weak ultraviolet illumination. It was observed that the morphology and size of the prepared materials was a significant factor in determining the extent of catalytic activity. Fine powders of KGSO were prepared using a sol–gel route. However, variations in the hydrolysis step allowed for control of the morphology of the produced, fine powders which in turn produced variations in the catalytic activity. The morphology of these powders was observed to be a key in maximizing the photocatalytic oxidative decomposition of both C 6Cl 5OH and C 12Cl 7O 2H with particles which had a more rounded morphology having the greater activity. KGSO is a promising photocatalyst for decomposition of harmful aromatic compounds and with careful chemical control of its synthesis, the material can be optimized for chemical activity.

Improvement of oxidative decomposition activity on hollandite type photocatalyst against pentachlorophenol

Endocrine-disrupting chemicals such as pentachlorophenol (C6Cl5OH) are hazardous for the generative function of living creatures. Photocatalysts give us an effective route for removing these pollutants. Most of the reports are related to a photocatalyst of titanium dioxide (TiO2) fine particles loaded with metal particles such as Pt or Ag. However, a diversity of photocatalysts is required to deal with the increasing number of identified harmful chemicals which occur under a range of different conditions.Hollandites have one-dimensional tunnel-like framework structure. K2Ga2Sn6O16 (KGSO) is one of hollandite type compounds. In this study, hollandite type KGSO fine powders were fabricated using sol–gel method. The hydrolysis route was carefully observed for preparation of KGSO fine powders with high photocatalytic activity. To prepare the fine powder, the influence of hydrolysis temperature on preparation of fine powder was examined. It was found that the control of aforementioned temperature was very important for preparation of nano-sized KGSO powders with high activity. In addition, the authors controlled particle morphology of KGSO using different preparation conditions. The oxidative decomposition activity against C6Cl5OH on round shape KGSO nano-particles (78%) was much higher than that on the elongated shape KGSO particles (46.2%). It is concluded that the control of morphology and size of KGSO particles maximized the photocatalytic activity on KGSO for oxidative decomposition of C6Cl5OH. In addition, KGSO did not produce organic chlorides as by-products in the photocatalysis. Accordingly, it is expected that KGSO will be one of promising photocatalysts for decomposition of harmful aromatic compound C6Cl5OH under weak UV illumination.

Photoinduced hydrophilicity and photocatalytic decomposition of endocrine-disrupting chemical pentachlorophenol on hollandite

Surface properties and photocatalytic oxidation reactions on the hollandite-type compound K2Ga2Sn6O16 (KGSO) were examined for photoinduced hydrophilicity and oxidative decomposition of an endocrine-disrupting chemical, pentachlorophenol (C6Cl5OH, PCP), under ultraviolet (UV) illumination. The thin films and mesoporous powders of hollandite were used for examination of surface properties and photocatalysis, respectively. The photoinduced surface property was examined by measurement of the contact angle of water, ortho-chlorophenol (o-C6H4ClOH), and toluene on the surface of KGSO. The contact angle of H2O and o-C6H4ClOH decreased to 0° under UV illumination. The toluene showed little change in contact angle under UV irradiation. It is concluded that the surface of KGSO shows photoinduced hydrophilicity for H2O and aromatic compounds with hydroxyl groups (−OH). In addition, KGSO clearly showed a photo-oxidative decomposition of PCP under weak UV illumination at room temperature. The decomposition speed of C6Cl5OH on KGSO was much faster than that on previous reported nano-sized SnO2 photocatalysts. It is expected that photo-oxidative decomposition of aromatic compound will be controlled by a combination of optimum composition of the hollandite phase and control of the morphology of the hollandite particles. This suggests that hollandite would be a promising photocatalyst for decomposition of aromatic compounds in endocrine-disrupting chemicals.

ホランダイト型 K<SUB><I>x</I></SUB>Nb<SUB><I>y</I></SUB>Ti<SUB>8 - <I>y</I></SUB>O<SUB>16</SUB> (<I>x</I> ≤ 1.15, 0 ≤ <I>y</I> ≤ 1) の合成と結晶構造解析

Hollandite-type compounds, KxNbyT8 - yO16 (x ≤ 1.15, 0 ≤ y ≤ 1) were prepared by high temperature reaction under H2 atmosphere. Black single crystals of Holladite-type K1.15Nb0.67Ti7.33O16 were obtained by heating a mixture of K2CO3, TiO2 and Nb2O5 with the molar ratio of 1 : 1 : 1 under H2 atmosphere at 1200°C for 1h. From single crystal X-ray diffraction analysis, the tetragonal lattice parameters of this compound are a = 1.02260(7) and c = 0.2974(1)nm, and the cystal structure refinement led to the final R-factors of R = 0.037 and Rw = 0.041 for 773 unique diffractions. The extent of solubility of niobium for powder samples was found to be y ≤ 1 in KxTi8 - yNbyO16 from solid state reaction under H2 atmosphere at 1200°C and the value of x ranged from 0.8 to 0.9.

Surface property and photocatalytic oxidation on hollandite type compound

Surface property and photocatalytic oxidation on hollandite type compound

Barium hollandite-type compounds BaxFe2xTi8−2xO16 with x=1.143 and 1.333

Experimental X-ray powder diffraction patterns and refined unit cell parameters for two barium hollandite-type compounds, BaxFe2xTi8−2xO16, with x=1.143 and 1.333, are reported here. Compared to the tetragonal parent structure, both compounds exhibit monoclinic distortions that increase with Ba content [Ba1.333Fe2.666Ti5.334O16: a=10.2328(8), b=2.9777(4), c=9.899(1) Å, β=91.04(1)°, V=301.58(5) Å3, Z=1, ρcalc=4.64 g/cc; Ba1.143Fe2.286Ti5.714O16: a=10.1066(6), b=2.9690(3), c=10.064(2) Å, β=90.077(6)°, V=301.98(4) Å3, Z=1, ρcalc=4.48 g/cc]. The X-ray powder patterns for both phases contain a number of broad, weak superlattice peaks attributed to ordering of the Ba2+ ions within the tunnels of the hollandite framework structure. According to the criteria developed by Cheary and Squadrito [Acta Crystallogr. B 45, 205 (1989)], the observed positions of the (0k1)/(1k0) superlattice peaks are consistent with the nominal x-values of both compounds, and the k values calculated from the corresponding d-spacings suggest that the Ba ordering within the tunnels is commensurate for x=1.333 and incommensurate for x=1.143. High-temperature X-ray diffraction data indicate that the x=1.333 compound undergoes a monoclinic→tetragonal phase transition between 310 and 360 °C.

Hollandite-type compounds K 2Me(FeV) 3.5O 16 (Me  Co or Ni) with magnetic ordering

The crystal structure and magnetic properties of two new materials KNi 0.5Fe 3.5V 3.5O 16 and KCo 0.5Fe 3.5V 3.5O 16 are presented. The basic model for their structure analysis was derived from the known structure of hollandite-type compounds. The magnetization of these compounds has been measured from 4.2 to 300 K and in magnetic fields up to 14 T. Both compounds are ferrimagnetically ordered with T n above RT.

Bi 1.7V 8O 16: The First Bi-Hollandite-Type Compound

Bi 1.7V 8O 16 was synthesized by high temperature solid state reaction between Bi 2O 3, V 2O 5, and V 2O 3. The compound crystallizes with tetragonal symmetry, space group I4/ m. The structure refinement using single crystal data confirms that the compound belongs to the hollandite family. It is the first hollandite-type compound containing bismuth in the large tunnels of the V 8O 16 framework. The bismuth atom forms a very flat BiO 4 square pyramid with the skeleton oxygen atoms. Bi 1.7V 8O 16 is oxidized to BiVO 4 and V 2O 5 at 400°C in air, as evidenced by TGA and DSC.

Frequency-independent ionic conductivity of hollandite type compounds

The frequency- and temperature-dependences of a.c. ionic conductivity of one-dimensional superionic conductors K-priderites, K1.6Mg0.8Ti7.2O16(KMTO) and K1.6Al1.6Ti6.4O16(KATO), which were grown by using a flux with high grade purity, were investigated from 100 Hz to 10 MHz. The complex ionic conductivity along a c-axis was analyzed by using an equivalent circuit constructed by considering the frequency-dependent ionic conductivity σmb and the frequency-independent ionic conductivity σdc which was not observed for samples grown by using a flux with normal grade purity. The σdc was thermally activated and the activation energies for KMTO and KATO were 0.26 and 0.30 eV respectively. The lateral conductivity was very small.

Ac ionic conductivity of hollandite type compounds from 100 Hz to 37.0 GHz

The frequency- and temperature-dependences of a.c. ionic conductivity of one-dimensional super-ionic conductors K-priderites with a hollandite type structures were investigated from 100 Hz to 37.0 GHz. Four kinds of K-priderite, K 1.6Mg 0.8Ti 7.2O 16, (K 1.3, Li 0.1) Mg 0.7Ti 7.3O 16, K 1.6Al 1.6Ti 6.4O 16 and (K 1.3, Li 0.2) Al 1.5Ti 6.5O 16, were studied. An equivalent circuit to combine the data of the complex conductivity at low and high frequencies was proposed. The data of complex conductivity at low frequencies can be analyzed in terms of the moving box model proposed by Beyeler et al. The transport of K + ions at low frequencies is characterized by the cooperative motion of the K + ions with various mobilities and is accompanied with the polarization of the K + ions in the channels. The ion transport across intrinsic barriers at or above microwave frequencies is characterized by the frequency-independent ionic conductivity and is interpreted by the configurational model proposed by Beyeler et al. The height of intrinsic barriers is related to the lattice constants of a crystal.

Ionic conductivity of hollandite type compound at 9.54 and 32.8 GHz

The microwave ionic conductivity σ of single-crystals of K-Mg priderite ( a hollandite type compound ) K1·6Mg0·8Ti7·2O16 has been measured using a standing wave method at 9.54 and 32.8 GHz from 83 to 616 K. The results of σT−1T characteristic are divided into two regions, one is the part of higher temperatures I and the other is the part of lower temperatures II. As the ionic conductivity at 32.8 GHz at the region I is interpreted by the configurational model proposed by Bayeler et al., the frequency-independent intrinsic ionic conductivity dominated by the configurational model is predicted to be observed at higher frequencies than 32.8 GHz.

Syntheses of hollandite type Rb 2Cr 8O 16, K 2Cr 2V 6O 16 and K 2V 8O 16

Hollandite-type compounds, Rb 2Cr 8O 16, K 2Cr 2V 6O 16 and K 2V 8O 16, were synthesized under high P-T conditions up to 1200°C and 7GPa. The structural refinement using a single crystal of Rb 2Cr 8O 16 confirms that the structure is similar to that of K 2Cr 8O 16. Magnetic measurements indicate that Rb 2Cr 8O 16 is ferromagnetic below 295K, K 2Cr 2V 6O 16 paramagnetic down to 77K and K 2V 8O 16 has susceptibility anomaly at 175K. These compounds are all semiconductive and show discontinuities in temperature-resistivity curves at points corresponding to magnetic anomalies.