Rutile Compounds Research Articles

Germanium dioxide: A new rutile substrate for epitaxial film growth

Rutile compounds have exotic functional properties that can be applied for various electronic applications; however, the limited availability of epitaxial substrates has restricted the study of rutile thin films to a limited range of lattice parameters. Here, rutile GeO2 is demonstrated as a new rutile substrate with lattice parameters of a=4.398Å and c=2.863Å. Rutile GeO2 single crystals up to 4 mm in size are grown by the flux method. X-ray diffraction reveals high crystallinity with a rocking curve having a full width half-maximum of 0.0572°. After mechanical polishing, a surface roughness of less than 0.1 nm was obtained, and reflection high-energy electron diffraction shows a crystalline surface. Finally, epitaxial growth of (110)-oriented TiO2 thin films on GeO2 substrates was demonstrated using molecular beam epitaxy. Templated by rutile GeO2 substrates, our findings open the possibility of stabilizing new rutile thin films and strain states for the tuning of physical properties.

Correlation of bond metallicity measures to electronegativity for binary oxides

AbstractWe have investigated alkali, alkaline‐earth, and rutile binary oxides within density functional theory (DFT) and Bader's atoms‐in‐molecules theory, focusing on properties of bond and ring critical points, and their relations to band gap and Pauling electronegativity. We find linear relations of kinetic energy density, electron density, and the gap divided by kinetic energy density at the bond critical points to the difference of Pauling electronegativities of the cation and oxygen anion. At the ring critical points of rutile compounds, we also find that some bond metallicity measures are linearly related to the difference of electronegativities. This study extends our knowledge about the relations between bond critical points, band gap, and electronegativity, but also shows for the first time a quantitative relation between quantities at the ring critical points and global properties of the compounds.

Band filling dependence of the Curie temperature in CrO2

Rutile CrO2 is an important half-metallic ferromagnetic material, which is also widely used in magnetic recording. In an attempt to find the conditions, which lead to the increase of the Curie temperature (TC), we study theoretically the band-filling dependence of interatomic exchange interactions in the rutile compounds. For these purposes, we use the effective low-energy model for the magnetic t2g bands, derived from the first-principles electronic structure calculations in the Wannier basis, which is solved by means of dynamical mean-field theory. After the solution, we calculate the interatomic exchange interactions, by using the theory of infinitesimal spin rotations, and evaluate TC. We argue that, as far as the Curie temperature is concerned, the band filling realized in CrO2 is far from being the optimal one and much higher TC can be obtained by decreasing the number of t2g electrons (n) via the hole doping. We find that the optimal n is close to 1, which should correspond to the case of VO2, provided that it is crystallized in the rutile structure. This finding was confirmed by using the experimental rutile structure for both CrO2 and VO2 and reflects the general tendency towards ferromagnetism for the narrow-band compounds at the beginning of the band filling. In particular, our results suggest that the strong ferromagnetism can be achieved in the thin films of VO2, whose crystal structure is controlled by the substrate.

Structural instability of the rutile compounds and its relevance to the metal–insulator transition of VO2

The metal–insulator transition (MIT) of VO2 is discussed with particular emphasis on the structural instability of the rutile compounds toward dimerization. Ti substitution experiments reveal that the MIT is robust up to 20% Ti substitutions and occurs even in extremely thin V-rich lamellas in spinodally decomposed TiO2–VO2 composites, indicating that the MIT is insensitive to hole doping and essentially takes on a local character. These observations suggest that either electron correlation in the Mott–Hubbard sense or Peierls (Fermi-surface) instability plays a minor role on the MIT. Through a broad perspective of crystal chemistry on the rutile-related compounds, it is noted that VO2 and another MIT compound NbO2 in the family eventually lie just near the borderline between the two structural groups with the regular rutile structure and the distorted structures characterized by the formation of dimers with direct metal–metal bonding. It is also shown that the two compounds of the rutile form do not follow the general trends in structure observed for the other rutile compounds, giving clear evidence of an inherent structural instability present in the two compounds. The MITs of VO2 and NbO2 are natural consequences of structural transitions between the two groups, as all the d electrons are trapped in the bonding molecular orbitals of dimers at low temperatures. Such dimer crystals are ubiquitously found in early transition metal compounds having chain-like structures, such as MoBr3, NbCl4, Ti4O7, and V4O7, the latter two of which also exhibit MITs probably of the same origin. In a broader sense, the dimer crystal is a kind of “molecular orbital crystals” in which virtual molecules made of transition metal atoms with partially-filled t2g shells, such as dimers, trimers or larger ones, are generated by metal–metal bonding and are embedded into edge- or face-sharing octahedron networks of various kinds. The molecular orbital crystallization opens a natural route to stabilization of unpaired t2g electrons in crystals.

Spinodal Decomposition in the TiO2–VO2 System

Spinodal decomposition is a ubiquitous phenomenon leading to phase separation from a uniform solution. We show that a spinodal decomposition occurs in a unique combination of two rutile compounds of TiO2 and VO2, which are chemically and physically distinguished from each other: TiO2 is a wide-gap insulator with photo catalytic activities and VO2 is assumed to be a strongly correlated electron system which exhibits a dramatic metal-insulator transition at 342 K. The spinodal decomposition takes place below 830 K at a critical composition of 34 mol% Ti, generates a unidirectional composition modulation along the c axis with a wavelength of approximately 6 nm, and finally results in the formation of self-assembled lamella structures made up of Ti-rich and V-rich layers stacked alternately with 30-50 nm wavelengths. A metal-insulator transition is not observed in quenched solid solutions with intermediate compositions but emerges in the thin V-rich layers as the result of phase separation. Interestingly, the metal-insulator transition remains as sharp as in pure VO2 even in such thin layers and takes place at significantly reduced temperatures of 310-340 K, which is probably due to a large misfit strain induced by lattice matching at the coherent interface.

Origin of XAS Resonances for 3D Metals in Rutile Compounds

In this work, we want to clear up the origin of the XAS structures for rutile compounds. To determine the connections between the atomic arrangement and XAS structures, we have calculated the Ti K edge in TiO 2 and compared results when few atoms are removed from the cluster. Multiple Scattering approach from FEFF6 code is used.

Resonant photoemission from TiO 2(110) surfaces: implications on surface bonding and hybridization

Valence band photoemission obtained near the Ti 3p absorption edge has been used to investigate the surface electronic structure of TiO 2(110)-(1 × 1), −(1 × 2) and Na/TiO 2(110). The variation of the energy distribution curves with photon energy suggests that the observed resonant emission should be divided into two parts. A low-energy part is attributed to Ti 3p → 3d initial exitations, while a high-energy part is assigned to Ti 3p → 4s exitations. We find that both the intensity and the shape of the resonance profile as well as the spectral shape of the resonant emission vary substantially with surface conditions, the largest changes being associated with Ti 4s initial excitations. By applying a molecular orbital bonding scheme developed for rutile compounds and by considering the surface-induced perturbations to this, we are able to relate our experimental findings to recent structural models for the (1 × 1) and (1 × 2) surfaces. Particularly for the (1 × 1) surface the existence of bonding Ti 4s-O 2p hybridization observed at E B = 5.6 and E B = 6.7 eV is recognized for the first time. The observed restriction of this resonant emission to the clean (1 × 1) surface is readily associated with the contraction of Ti(5-fold)O bond lengths expected for this surface. For the (1 × 2) surface we observe a strong damping of the 4s-related bonding resonances, a decrease in 3d-bonding emission and an increase of the 3d-“non-bonding” emission. These observations are in accordance with a recent structural model for this surface. Finally, the resonant-emission data demonstrate that the surface Ti present at the (1 × 2) surface should be considered as Ti 4+ states.

ESR determination of TiO 2 and SnO 2 Tammann temperatures

The thermal behaviour of vanadium(IV) paramagnetic ions deposited by impregnation on two rutile compounds (TiO 2 and SnO 2) has been investigated by ESR. Beyond a critical temperature, these probe V(IV) ions migrate from the surface into the bulk of the oxide. The most significant ESR features of the intralattice and extralattice V(IV) species are briefly analysed. For the V/TiO 2 system, the amount of detectable bulk V(IV) ions depends strongly on the annealing temperature; their signal intensity drastically increases above 865 ± 15 K. This sudden change indicates the temperature at which cations and cationic vacancies begin to move inside TiO 2, namely the Tammann temperature of this oxide. This value agrees with the empirical range generally admitted for the Tammann temperature of oxides (between 0.3 T m and 0.5 T m where T m represents the oxide melting temperature, i.e. between 634 and 1057 K in the case of TiO 2). It is also consistent with a previous experimental determination (870 K) obtained by specific surface measurements. A poor resolution hampers a similar analysis for the V/SnO 2 system. However, near 650 K, the ESR spectra are strongly modified and indicate that diffusion mechanisms have begun. A 650 ± 50 K value appears therefore as a reasonable estimate of the SnO 2 Tammann temperature. Such a small value agrees with the low T m of this oxide (1400 K) which leads to a 400–700 K empirical range.

Valence force field treatment of the rutile structure at zero-wave vector

The lattice vibrational frequencies at zero-wave vector of 10 MX 2-type rutile structures (X = O, F) are analyzed using a valence bond force field which includes parameters to accommodate the two types of M-X bonds and the forces associated with the two types of nearest neighbor X…X interactions. The two different M-X force constants are constrained to a single adjustable parameter by applying the relationship K mx = K ′ mx( r mx r ′ mx ) 6 . This simplified force field with three adjustable paramete and one constraining relationship reproduces the Raman active phonon frequencies of all 10 rutile compounds with an average error of ⩽ 5% in almost all cases. The agreement between observation and calculation is not as good for the infrared active phonons—a result which is attributed in part to discrepancies in the measurement and analysis of infrared reflectivity spectra and in part to the absence in the simplified valence bond force field of a term to account for forces along the c-axis of the rutile structure.

Standardization of automobile tire fabric testing

The metal–insulator transition (MIT) of VO2 is discussed with particular emphasis on the structural instability of the rutile compounds toward dimerization. Ti substitution experiments reveal that the MIT is robust up to 20% Ti substitutions and occurs even in extremely thin V-rich lamellas in spinodally decomposed TiO2–VO2 composites, indicating that the MIT is insensitive to hole doping and essentially takes on a local character. These observations suggest that either electron correlation in the Mott–Hubbard sense or Peierls (Fermi-surface) instability plays a minor role on the MIT. Through a broad perspective of crystal chemistry on the rutile-related compounds, it is noted that VO2 and another MIT compound NbO2 in the family eventually lie just near the borderline between the two structural groups with the regular rutile structure and the distorted structures characterized by the formation of dimers with direct metal–metal bonding. It is also shown that the two compounds of the rutile form do not follow the general trends in structure observed for the other rutile compounds, giving clear evidence of an inherent structural instability present in the two compounds. The MITs of VO2 and NbO2 are natural consequences of structural transitions between the two groups, as all the d electrons are trapped in the bonding molecular orbitals of dimers at low temperatures. Such dimer crystals are ubiquitously found in early transition metal compounds having chain-like structures, such as MoBr3, NbCl4, Ti4O7, and V4O7, the latter two of which also exhibit MITs probably of the same origin. In a broader sense, the dimer crystal is a kind of “molecular orbital crystals” in which virtual molecules made of transition metal atoms with partially-filled t2g shells, such as dimers, trimers or larger ones, are generated by metal–metal bonding and are embedded into edge- or face-sharing octahedron networks of various kinds. The molecular orbital crystallization opens a natural route to stabilization of unpaired t2g electrons in crystals.