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NbO2 is Molybdenite structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is two-dimensional and consists of two NbO2 sheets oriented in the (0, 0, 1) direction. Nb4+ is bonded in a 6-coordinate geometry to six equivalent O2- atoms. All Nb–O bond lengths are 2.10 Å. O2- is bonded in a distorted T-shaped geometry to three equivalent Nb4+ atoms.
NbO2 is Rutile structured and crystallizes in the tetragonal P4_2/mnm space group. The structure is three-dimensional. Nb4+ is bonded to six equivalent O2- atoms to form a mixture of edge and corner-sharing NbO6 octahedra. The corner-sharing octahedral tilt angles are 44°. There are two shorter (2.04 Å) and four longer (2.10 Å) Nb–O bond lengths. O2- is bonded in a distorted T-shaped geometry to three equivalent Nb4+ atoms.
NbO2 is zeta iron carbide-like structured and crystallizes in the tetragonal I4_1 space group. The structure is three-dimensional. there are two inequivalent Nb4+ sites. In the first Nb4+ site, Nb4+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing NbO6 octahedra. The corner-sharing octahedra tilt angles range from 32–58°. There are a spread of Nb–O bond distances ranging from 1.95–2.21 Å. In the second Nb4+ site, Nb4+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing NbO6 octahedra. The corner-sharing octahedra tilt angles range from 32–58°. There are a spread of Nb–O bond distances ranging from 1.95–2.21 Å. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded in a 3-coordinate geometry to three Nb4+ atoms. In the second O2- site, O2- is bonded in a 3-coordinate geometry to three Nb4+ atoms. In the third O2- site, O2- is bonded in a distorted trigonal planar geometry to three Nb4+ atoms. In the fourth O2- site, O2- is bonded in a distorted trigonal planar geometry to three Nb4+ atoms.
NbO2 crystallizes in the triclinic P-1 space group. The structure is three-dimensional. there are two inequivalent Nb4+ sites. In the first Nb4+ site, Nb4+ is bonded to six O2- atoms to form a mixture of distorted corner and edge-sharing NbO6 octahedra. The corner-sharing octahedra tilt angles range from 9–29°. There are a spread of Nb–O bond distances ranging from 1.96–2.27 Å. In the second Nb4+ site, Nb4+ is bonded to six O2- atoms to form a mixture of distorted corner and edge-sharing NbO6 octahedra. The corner-sharing octahedra tilt angles range from 9–33°. There are a spread of Nb–O bond distances ranging from 1.89–2.39 Å. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded in a 3-coordinate geometry to three Nb4+ atoms. In the second O2- site, O2- is bonded in a 3-coordinate geometry to three Nb4+ atoms. In the third O2- site, O2- is bonded in a linear geometry to two Nb4+ atoms. In the fourth O2- site, O2- is bonded to four Nb4+ atoms to form a mixture of distorted corner and edge-sharing ONb4 trigonal pyramids.
NbO2 is trigonal omega-like structured and crystallizes in the tetragonal I4_1/amd space group. The structure is three-dimensional. Nb4+ is bonded to six equivalent O2- atoms to form a mixture of corner and edge-sharing NbO6 octahedra. The corner-sharing octahedral tilt angles are 3°. There are four shorter (2.04 Å) and two longer (2.18 Å) Nb–O bond lengths. O2- is bonded in a T-shaped geometry to three equivalent Nb4+ atoms.
NbO2 is Baddeleyite-like structured and crystallizes in the orthorhombic Pbca space group. The structure is three-dimensional. Nb4+ is bonded to seven O2- atoms to form a mixture of distorted corner and edge-sharing NbO7 pentagonal bipyramids. There are a spread of Nb–O bond distances ranging from 2.05–2.22 Å. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted trigonal non-coplanar geometry to three equivalent Nb4+ atoms. In the second O2- site, O2- is bonded to four equivalent Nb4+ atoms to form a mixture of distorted corner and edge-sharing ONb4 tetrahedra.
NbO2 is zeta iron carbide-like structured and crystallizes in the tetragonal I4_1/a space group. The structure is three-dimensional. there are two inequivalent Nb4+ sites. In the first Nb4+ site, Nb4+ is bonded to six O2- atoms to form a mixture of corner and edge-sharing NbO6 octahedra. The corner-sharing octahedra tilt angles range from 32–57°. There are a spread of Nb–O bond distances ranging from 1.95–2.22 Å. In the second Nb4+ site, Nb4+ is bonded to six O2- atoms to form a mixture of corner and edge-sharing NbO6 octahedra. The corner-sharing octahedra tilt angles range from 32–57°. There are a spread of Nb–O bond distances ranging from 1.95–2.18 Å. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded in a 3-coordinate geometry to three Nb4+ atoms. In the second O2- site, O2- is bonded in a distorted trigonal planar geometry to three Nb4+ atoms. In the third O2- site, O2- is bonded in a 3-coordinate geometry to three Nb4+ atoms. In the fourth O2- site, O2- is bonded in a distorted trigonal planar geometry to three Nb4+ atoms.
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The information available on the BaO(SrO)-NbO-NbO2 system with the niobium atom in the lower oxidation degree is very limited. Very few compounds have been found previously in this system. They are BaNbO3, SrxNbO3(0,7=x=1), Ba2Nb2O9, SrNb8O14; and some suggestions on the BaNb8O14 existence have been made also. At the same time Nb-based oxide compounds could be quite interesting in the search of new noncopper high T(sub c) superconductors Researchers studied Ba(Sr) NbxO2x-2 and Ba2(Sr2)-NbxO2x-1 compositions in the phase diagram of BaO(SrO)-NbO-NbO2 system. The synthesis of the materials was carried out in vacuum at the temperatures of 1000 to 1500 C. Barium carbonate and niobium pentoxide were used as initial components. X-ray analysis was carried out.
We have investigated microstructure and microchemistry of precipitates and dislocation loops in high-burnup M5® using (scanning) transmission electron microscopy ((S)TEM) equipped with energy dispersive X-ray spectroscopy (EDS). Two (S)TEM lamellae were made by cryo-FIB from the same cladding sample. The Nb-rich native precipitates were found in the metal, in the suboxide and in the oxide. Upon diffraction analysis, most of the Nb-rich native precipitates in the metal matrix remain as β-Nb phase, while no β-Nb native precipitates were found in the oxide. Nearby the oxide and metal (O/M) interface, the native precipitates in the oxide were already oxidized into t-NbO2 phase. At further distance away from the O/M interface, the Nb-rich native precipitates were gradually oxidized and became amorphous. Besides the native precipitates, Nb-rich irradiation induced precipitates (IIPs) were found in the metal matrix. Using g = <0002> vector for imaging, the length of the IIPs was aligned with dislocation loops. However, no Nb segregation to the dislocation loops themselves was observed. For the first time, we report that IIPs indeed exist in the oxide but only within about 1.5 µm away from O/M interface. However, the oxidation state of the IIPs in the oxide is still unclear. The presence of both native precipitates and IIPs in the oxide may indicate the Nb concentration in the oxide solid solution remain low nearby the O/M interface, which may explain the reduced corrosion kinetics of in-pile M5®. On the other hand, no IIPs were observed in the oxide at further distance and this may indicate that they have eventually dissolved back into the oxide. A few mechanisms related to IIPs stability in the oxide are presented.
Both XPS and ToF-SIMS data and analysis suggest that Nb oxides grow rapidly and overwhelm Nb metal within the first 24 hours following a logarithmic growth. Nb2O5 growth is dominant out of all types of Nb oxide with reaching 80% At conc within 80 days. NbO2 is quickly saturated, maintaining 10% At conc over time with a logarithmic growth. NbO on the opposite follows an exponential decay. Oxide thickness growth also implies sample preparation techniques impact oxide saturation and growth rate.
Superconducting qubits are a leading platform for quantum computation. These circuits are typically made from superconducting materials like aluminum or niobium. However, the amorphous niobium oxide and aluminum oxide on the surface of these circuits introduce considerable RF loss due to the presence of two-level systems (TLS), which limits the maximum coherence times T1 to ~100 μs. Capping the niobium qubits with a tantalum layer leads to a 3- 5x improvement [Bal et al., 2024]. But even in this case, tantalum forms an amorphous surface oxide that introduces loss. In an effort to devise strategies to eliminate the presence of this oxide, we present a comprehensive study on the nature of Ta oxide using x-ray photoemission spectroscopy (XPS) and secondary ion mass spectrometry (ToF-SIMS) as a function of heat treatment. The thin films were annealed in ultra-high vacuum conditions and analyzed in situ to characterize the composition and evolution of the native tantalum oxide layer and oxide-metal interface. Our analysis reveals two critical differences between tantalum and niobium oxides: Nb2O5 completely dissolves at 400°C, while Ta2O5 persists even at 800°C. Additionally, tantalum oxide contains only a single suboxide (TaO), in contrast to niobium's two suboxides (NbO and NbO2). The suboxide of tantalum contributes minimally to the total oxide content and shows a relative increase with temperature. Understanding the oxide’s behavior will open new pathways for optimizing coherence times in tantalum qubits.
Tantalum dioxide (TaO2) is a metastable tantalum compound. Here, we report the epitaxial stabilization of TaO2 on Al2O3 (1-102) (r-plane sapphire) substrates using suboxide molecular-beam epitaxy (MBE) and thermal laser epitaxy (TLE), demonstrating single-oriented, monodomain growth of anisotropically strained thin films. Microstructural investigation is performed using synchrotron X-ray diffraction and scanning transmission electron microscopy. The tetravalent oxidation state of tantalum is confirmed using X-ray absorption and photoemission spectroscopy as well as electron energy-loss spectroscopy. Optical properties are investigated via spectroscopic ellipsometry and reveal a 0.3 eV Mott gap of the tantalum 5d electrons. Density-functional theory and group theoretical arguments are used to evaluate the limited stability of the rutile phase and reveal the potential to unlock a hidden metal-insulator transition concomitant with a structural phase transition to a distorted rutile phase, akin to NbO2. Our work expands the understanding of tantalum oxides and paves the way for their integration into next-generation electronic and photonic devices.
Isothermal oxidation behaviors of NbAl3 alloy synthesized by laser cladding were investigated at temperatures between 800 and 1400 C, and the effect of vanadium microalloying on the oxidation of the laser-clad alloy was examined. The oxidation kinetics of the two alloys were monitored using thermal gravimetric weight gain data, and the bulk and surface chemistries were analyzed using XRD and XPS, respectively. It was found that NbAl3 did not form an exclusive layer of protective Al2O3. The oxidation products at 800 C were found to be a mixture of Nb2O5 and Al2O3. At 1200 C, a mixture of NbAlO4, Nb2O5, and Al2O3 formed; and at 1400 C, a mixture of NbAlO4, Al2O3, NbO2, NbO(2.432), and Nb2O5 formed. The addition of V led to a dramatic increase of the oxidation rate, which may be related to the formation of (Nb, V)2O5 and VO2, which grows in preference to protective Al2O3.
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