α-PbO2 Structure Research Articles

Structural, dielectric and magnetic properties of MFeNbO6 materials (M=Ti, Zr, Hf)

We study the structural, magnetic and electronic properties of MFeNbO6 materials (where A = Ti, Zr or Hf). TiFeNbO6 crystallises with the disordered rutile structure with the P42/mnm space group. Increasing the ionic radii of the M4+ cation going from Ti4+ (0.605 Å) to Zr4+ (0.72 Å) and Hf4+ (0.710 Å) results in a buckling of the octahedral chains with these materials crystallising with the disordered α-PbO2 structure (space group Pbcn). In contrast, with previous reports for TiFeNbO6 we find no evidence of relaxor ferroelectric behaviour with all three materials instead exhibiting Maxwell-Wagner-like relaxation. Impedance spectroscopy confirms semiconductor behaviour. Magnetically, all three materials can be described by infinite irregular antiferromagnetic S = 5/2 chains with weak ferromagnetism below 10 K. These chains have random directions of propagation between nearest Fe–Fe neighbours consistent with the disordered nature of these materials. We observed a negative imaginary part of the ac susceptibility related to the defect magnetic topology.

Zharchikhite, AlF(OH)2: a novel structure type related to α-PbO2.

The crystal structure of zharchikhite, AlF(OH)2, from the Zharchikhinskoe deposit (Buryatia, Russia) is solved here using single-crystal X-ray diffraction. The mineral is monoclinic, space group P21/c, a = 5.1788 (4), b = 7.8386 (4), c = 5.1624 (4) Å, β = 116.276 (10)°, V = 187.91 (3) Å3 and Z = 4. Zharchikhite demonstrates a novel structure type roughly related to the α-PbO2 structure type and different from other compounds of the Al-F-OH system. The crystal structure of zharchikhite is based on the octahedral pseudoframework built from zigzag chains of edge-sharing AlF2(OH)4 octahedra; adjacent chains are linked via F vertices and the pseudoframework contains wide channels.

Phase stability of entropy stabilized oxides with the α-PbO2 structure

The prediction of new high entropy oxides (HEOs) remains a profound challenge due to their inherent chemical complexity. In this work, we combine experimental and computational methods to search for new HEOs in the tetravalent AO2 family, using exclusively d0 and d10 cations. Our aim is to explain the phase stability of the α-PbO2 structure, which is found for the medium entropy oxide (Ti, Zr, Hf, Sn)O2. Using a pairwise approach to approximate the mixing enthalpy, we confirm that α-PbO2 is the expected lowest energy structure for this material above other candidates including rutile, baddeleyite, and fluorite structures. We also show that no other five-component compound composed of the tetravalent cations considered here is expected to form under solid state synthesis conditions, which we verify experimentally. Ultimately, we conclude that the flexible geometry of the α-PbO2 structure can be used to understand its stability among tetravalent HEOs.

Electronic structure characterization of TiO2-II with the α-PbO2 structure by electron-energy-loss-spectroscopy and comparison with anatase, brookite, and rutile

TiO2-II is a high pressure form of titania with a density about 2% larger than that of rutile. In contrast to the common polymorphs anatase, brookite and rutile its electronic structure and optical properties are poorly characterized. Here we report on a comparative electron-energy-loss-spectroscopy (EELS) study for which high resolution valence-loss and core-loss EELS data were acquired from nanocrystalline (<75 ​nm sized) titania particles with an energy resolution of about 0.2 ​eV. Electronic structure features revealed from titanium L3,2 and oxygen K electron energy loss near-edge structures show a strong similarity of TiO2-II with both rutile and brookite, which is attributed to similarities in the connectivity of octahedral TiO6 units with neighboring ones. From combined valence-loss EELS and UV-VIS diffuse reflectance spectroscopy data the band gap of TiO2-II was determined to be indirect and with a magnitude of ∼3.18 ​eV, which is very similar to anatase (indirect, ∼3.2 ​eV), and distinctly different from rutile (direct, ∼3.05 ​eV) and brookite (direct, ∼3.45 ​eV).

α-PbO2-type niobate as efficient cathode materials for steam and CO2 electrolysis

α-PbO2-type Mn0.5Ti0.5NbO4-based oxides are studied as cathode materials in solid oxide electrolysis cell (SOEC) for the direct electrolysis of steam or CO2 at 800 °C. Comparison between Mn0.5Ti0.5NbO4 and the Fe-doped ones, Fe0.3Mn0.35Ti0.35NbO4 and Fe0.6Mn0.2Ti0.2NbO4, indicates that the Fe3+ doping increases the electric conductivity in air but decreases the stability of the α-PbO2 structure in reducing atmosphere as in Fe0.6Mn0.2Ti0.2NbO4. The electric conductivity in reducing atmosphere is found to be ascribed to Ti4+-O-Ti3+ and Fe3+-O-Fe2+ in Mn0.5Ti0.5NbO4 and Fe doped ones, respectively. The steam or CO2 electrolsysis indicates that 30% Fe3+ substitution for Mn/Ti decreases the polarization resistance at low bias, but 60% Fe doping increases the polarization resistance dramatically than Mn0.5Ti0.5NbO4. The excessive Fe3+ doping is found to induce the phase transformation and delamination on the cathode/electrolyte interface under a cathodic current. The electrolyser with Fe0.3Mn0.35Ti0.35NbO4 cathode on zirconia-based electrolyte imparts a stable current density of 2.32 Acm−2 at −1.6 V if Ar-88% H2O is used as feeding gas. Analysis of the gas product indicates that H2 is produced and the Faradaic efficiency at −1.4 V and −1.6 V is found to be 100% and 93%, respectively.

Synthesis, Characterization and Electrochemical Analysis of FeNbO4 Phase with α-PbO2 Structure

The Nanometer and micrometer sized samples of FeNbO4 were prepared by sol-gel polymeric precursor and solid state reaction methods. Electrochemical lithium insertion was performed into both samples. Structural changes during the lithium insertion were followed by ex-situ XRD. The electrochemical charge discharge curves shows insertion of 0.3 Li is obtained in micrometer sized samples when discharged to 1.0 V and extraction of 0.1 Li is observed during charge. On the other hand, in the nano structured sample, insertion of 3Li/FeNbO4 were observed and during the first discharge, 2.2Li is extracted when charged to 3 V. A reversible capacity of 120 mA h/g is observed after 20 cycles.

Shock synthesis and characterization of titanium dioxide with α-PbO2 structure

The phase transformation behavior of anatase and rutile titanium dioxide with particle sizes of 60 nm and 150 nm under shock compression have been investigated. To increase the shock pressure and reduce the shock temperature, copper powder and a small amount of paraffin were mixed with the TiO2 powder. The shock recovered samples were characterized by x-ray diffraction, Raman spectroscopy, and transmission electron microscope. The results indicate that both anatase and rutile TiO2 can transform to α-PbO2 phase TiO2 through shock-induced phase transition. The transformation rate of α-PbO2 phase TiO2 for anatase TiO2 under shock compression is 100% and pure α-PbO2 phase TiO2 can be obtained, while the transformation rate for rutile TiO2 is over 90%. The influence of the particle size on the yield of α-PbO2 phase TiO2 is not noticeable. The thermal stability of the recovered pure α-PbO2 phase TiO2 was characterized by high temperature x-ray diffraction, thermogravimetric analysis and differential scanning calorimetry. The results show that α-PbO2 phase TiO2 transforms to rutile TiO2 when heated to temperature higher than 560 °C. The mechanisms of the phase transition of TiO2 under shock compression are discussed.

An implementation of artificial neural-network potentials for atomistic materials simulations: Performance for TiO2

Machine learning interpolation of atomic potential energy surfaces enables the nearly automatic construction of highly accurate atomic interaction potentials. Here we discuss the Behler–Parrinello approach that is based on artificial neural networks (ANNs) and detail the implementation of the method in the free and open-source atomic energy network (ænet) package. The construction and application of ANN potentials using ænet is demonstrated at the example of titanium dioxide (TiO2), an industrially relevant and well-studied material. We show that the accuracy of lattice parameters, energies, and bulk moduli predicted by the resulting TiO2 ANN potential is excellent for the reference phases that were used in its construction (rutile, anatase, and brookite) and examine the potential’s capabilities for the prediction of the high-pressure phases columbite (α-PbO2 structure) and baddeleyite (ZrO2 structure).

Novel Alkali Earth Borohydride Sr(BH4)2 and Borohydride-Chloride Sr(BH4)Cl

Two novel alkali earth borohydrides, Sr(BH4)2 and Sr(BH4)Cl, have been synthesized and investigated by in-situ synchrotron radiation powder X-ray diffraction (SR-PXD) and Raman spectroscopy. Strontium borohydride, Sr(BH4)2, was synthesized via a metathesis reaction between LiBH4 and SrCl2 by two complementary methods, i.e., solvent-mediated and mechanochemical synthesis, while Sr(BH4)Cl was obtained from mechanochemical synthesis, i.e., ball milling. Sr(BH4)2 crystallizes in the orthorhombic crystal system, a = 6.97833(9) Å, b = 8.39651(11) Å, and c = 7.55931(10) Å (V = 442.927(10) Å(3)) at RT with space group symmetry Pbcn. The compound crystallizes in α-PbO2 structure type and is built from half-occupied brucite-like layers of slightly distorted [Sr(BH4)6] octahedra stacked in the a-axis direction. Strontium borohydride chloride, Sr(BH4)Cl, is a stoichiometric, ordered compound, which also crystallizes in the orthorhombic crystal system, a = 10.8873(8) Å, b = 4.6035(3) Å, and c = 7.4398(6) Å (V = 372.91(3) Å(3)) at RT, with space group symmetry Pnma and structure type Sr(OH)2. Sr(BH4)Cl dissociates into Sr(BH4)2 and SrCl2 at ~170 °C, while Sr(BH4)2 is found to decompose in multiple steps between 270 and 465 °C with formation of several decomposition products, e.g., SrB6. Furthermore, partly characterized new compounds are also reported here, e.g., a solvate of Sr(BH4)2 and two Li-Sr-BH4 compounds.

Transformation of rutile to TiO2-II in a high pressure hydrothermal environment

The high pressure transformation of rutile to TiO2-II with the α-PbO2 structure is known to be kinetically hindered. In this study we show that a hydrothermal environment at 6 GPa and 650 °C provid ...

High-pressure phase relations in the system TiO2–ZrO2 to 12 GPa: stability of αPbO2-type srilankite solid solutions of (Ti1−x , Zr x )O2 (0 ≤ x ≤ 0.6)

Phase relations in the system TiO2–ZrO2 were examined in the pressure range of 3.5–12 GPa at 1,800 °C, using multianvil apparatus. At 1,800 °C, TiO2 rutile transforms to αPbO2 structure at 10 GPa, and the αPbO2-type solid solution is stable in compositional range between TiO2 and about (Ti0.6, Zr0.4)O2 at 3.5–12 GPa. Combination of the present results with the published data at 0–3 GPa demonstrates that continuous solid solution with the αPbO2-type structure is stable between TiO2 and (Ti1−x, Zrx)O2 (x ≈ 0.6) at 0–12 GPa. This indicates that both the αPbO2-type TiO2 and srilankite Ti2ZrO6 with the same structure belong to the continuous solid solution system though the two phases have been regarded as different minerals. With increasing ZrO2 content, lattice parameters of a- and c-axes of the αPbO2-type solid solution increase, but b-axis is almost constant or slightly decreases. At higher pressure, the αPbO2-type solid solution dissociates into two phases, αPbO2-type phase and tetragonal zirconia. Srilankite with more TiO2-rich composition than Ti2ZrO6 might be found in natural rocks derived from the deep upper mantle.

Structural Evolution of TiO2 Precipitates in Ti-DopedSapphire (α-Al2O3)

A time-dependent phase stability diagram has been developed describing the structural evolution of TiO2 precipitates in Ti-doped sapphire (α-Al2O3). A high-pressure TiO2 phase with the α-PbO2 structure (α-TiO2) forms as a coherent precipitate in Ti-doped sapphire annealed at 1200°–1300°C; extended annealing times at these temperatures result in twinned particles, the twinning minimizing the precipitate–matrix misfit strain energy. Twinned α-TiO2 eventually transforms to the rutile structure, coincident with the loss of coherency with the sapphire matrix. Rutile nucleates directly in sapphire annealed at temperatures at 1350°C and above. Rutile is present mainly as semicoherent and twinned precipitates, the twinning occurring relatively early in the precipitation sequence.

Pressure-Induced Amorphization and Polyamorphism in One-Dimensional Single-Crystal TiO2 Nanomaterials

The structural phase transitions of single-crystal TiO2−B nanoribbons were investigated in situ at high pressure using synchrotron X-ray diffraction and Raman scattering. Our results have shown a pressure-induced amorphization (PIA) occurred in TiO2−B nanoribbons upon compression, resulting in a high-density amorphous (HDA) form related to the baddeleyite structure. Upon decompression, the HDA form transforms to a low-density amorphous (LDA) form, while the samples still maintain their pristine nanoribbon shape. A high-resolution transmission electron microscopy (HRTEM) image reveals that the LDA phase has an α-PbO2 structure with short-range order. We propose a homogeneous nucleation mechanism to explain the pressure-induced amorphous phase transition in the TiO2−B nanoribbons. Our study demonstrates for the first time that PIA and polyamorphism occurred in the one-dimensional (1D) TiO2 nanomaterials and provides a new method for preparing 1D amorphous nanomaterials from crystalline nanomaterials.

Seifertite, a dense orthorhombic polymorph of silica from the Martian meteorites Shergotty and Zagami

Seifertite is a dense orthorhombic polymorph of silica with the scrutinyite (α-PbO2) type structure that was found as lamellae occurring together with dense silica glass lamellae in composite silica grains in the heavily shocked Martian meteorite Shergotty. The mineral is also intergrown in some grains with minor stishovite and a new unnamed monoclinic dense silica polymorph with a ZrO2-type structure. Seifertite has also been found in the Martian shergottite Zagami and is a minor constituent in other Martian shergottites. Chemical analyses of seifertite in Shergotty indicate major SiO2 with minor concentrations of Al2O3 and Na2O. Selected-area electron diffraction (SAED) and X-ray diffraction can be interpreted in terms of an orthorhombic pattern from a scrutinyite (α-PbO2) structure. The cell parameters are a = 4.097(1) A, b = 5.0462(9) A, c = 4.4946(8) A, V = 92.92 A3, Z = 4, and the space group is Pbcn or Pb 2 n . Density is (calc.) = 4.294 g/cm3 (with pure SiO2), 4.309 g/cm3 (with empirical formula). It is inferred that seifertite was formed by shock-induced solid-state transformation of either tridymite or cristobalite on Mars at an estimated minimum equilibrium shock pressure in excess of 35 GPa. The new mineral is named after Friedrich A. Seifert (b. 1941), founding Director of the Bayerisches Geoinstitut, Universitat Bayreuth, Germany, for his seminal contributions to high-pressure geoscience.

Schreyerite, V2Ti3O9: New occurrence and crystal structure

A new occurrence of schreyerite, V2Ti3O9, has recently been found in metamorphic rocks of the Sludyanka complex at the southern shore of Lake Baikal, Russia. In contrast to previously known schreyerite lamellae, which are intergrown with rutile, crystals from the Sludyanka complex occur as isolated single crystals associated with titanite, allowing single-crystal X-ray diffraction experiments. The chemical composition was determined with an electron microprobe giving the composition (V1.785Cr0.157Fe0.036)(Ti2.536V0.468)O9. A peculiarity of this schreyerite sample is the partial substitution of V4+ for Ti4+. The crystal structure was determined by single-crystal X-ray diffraction and was refined in the monoclinic space group C2/c [a = 17.102(2), b = 5.0253(5), c = 7.0579(8) Å, β = 106.636(10)°] to R1 = 2.84%. The structure is in agreement with the qualitative model of Grey et al. (1973) determined on the basis of electron diffraction and X-ray powder data for synthetic (Fe,Cr)2Ti3O9. Reinvestigation of schreyerite from the type locality indicates that this sample has cell dimensions and symmetry corresponding to the Sludyanka sample. The structure of schreyerite may be considered as a 1:1 polysome composed of slabs of berdesinskiite, V2TiO5, and Ti2O4 (a high-pressure phase of TiO2 with α-PbO2 structure).

High Temperature—High Pressure Synthesis and Crystal Structure of the Incommensurately Modulated, α‐PbO2 Related, Compound MnTa2O6.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Characterization of a high-pressure phase of silica from the Martian meteorite Shergotty

Recently, there has been substantial interest in post-stishovite high-pressure polymorphs of SiO2, discovered as extraterrestrial minerals, or synthesized in the laboratory. Previous investigators reported the presence of “α-PbO2-like” and “baddeleyite-like” SiO2 in the Martian meteorite Shergotty, and also the synthesis of an a-PbO2-like phase at pressures of 60-80 GPa in a laser-heated diamond anvil cell. To provide definitive information about the nature of the natural “a-PbO2 phase,” we recovered a small sample from the Shergotty meteorite, obtained powder X-ray diffraction patterns, and performed a Rietveld refinement of the structure. The resulting cell parameters and space group are a = 4.097(1) Å, b = 5.0462(9) Å, c = 4.4946(8) Å, and Pbcn. The structure refinement confirms that this sample does have the α-PbO2 structure

X-ray diffraction study on pressure-induced phase transformationsin nanocrystalline anatase/rutile (TiO2)

An in situ x-ray diffraction study was conducted to study thepressure-induced phase transformation in nanocrystalline anatase/rutile(TiO2) to 35.1 GPa. The nano-anatase phase remains stable to approximately16.4 GPa, and then transforms to an amorphous phase, which is returned uponrelease of pressure. The nano-rutile phase starts to transform to thebaddeleyite (ZrO2) structure at ~8.7 GPa, and the transformation iscomplete at approximately 16.4 GPa. On release of pressure the ZrO2structure transforms to the α-PbO2 structure. The results arecompared to previous work on phase changes in TiO2 with different particlesizes.

Thermoelastic properties of the high-pressure phase of SnO 2 determined by in situ X-ray observations up to 30 GPa and 1400 K

In situ synchrotron X-ray experiments in the system SnO2 were made at pressures of 4–29 GPa and temperatures of 300–1400 K using sintered diamond anvils in a 6–8 type high-pressure apparatus. Orthorhombic phase (α-PbO2 structure) underwent a transition to a cubic phase (Pa3¯ structure) at 18 GPa. This transition was observed at significantly lower pressures in DAC experiments. We obtained the isothermal bulk modulus of cubic phase K0 = 252(28) GPa and its pressure derivative K′=3.5(2.2). The thermal expansion coefficient of cubic phase at 25 GPa up to 1300 K was determined from interpolation of the P-V-T data obtained, and is 1.7(±0.7) × 10−5 K−1 at 25 GPa.

Post-Rutile High-Pressure Phases in TiO2

The crystal structures of rutile (TiO2) and its high-pressure polymorphs have been studied by X-ray powder diffraction in the pressure range up to 60 GPa. At 12 GPa, rutile transforms to a phase with the baddeleyite (ZrO2) structure. Upon decompression, this phase transforms at 7 GPa to another phase with the α-PbO2 structure. At ambient conditions, the α-PbO2-type phase is 2.1 (3)% denser than rutile and the baddeleyite-type phase is 11.3 (9)% denser than rutile. In the pressure range of the rutile-to-baddeleyite transition, the difference in density between the two phases is 9.75 (15)%. The zero-pressure bulk moduli, as determined from the equation of state, are 230 (20), 260 (30) and 290 (20) GPa for rutile, the α-PbO2-type phase and the baddeleyite-type phase, respectively.