Christoph Reimann

Bixbyite-type V2O3--a metastable polymorph of vanadium sesquioxide.

A metastable polymorph of vanadium sesquioxide was prepared by the reaction of vanadium trifluoride with a water-saturated gaseous mixture of 10 vol % hydrogen in argon. The new polymorph crystallizes in the bixbyite-type structure. At temperatures around 823 K a transformation to the well-known corundum-type phase is observed. Quantum-chemical calculations show that the bixbyite-type structure is about 9 kJ/mol less stable than the known corundum-based one. This result, in combination with the absence of imaginary modes in the phonon density of states, supports the classification of the bixbyite-type phase as a metastable V(2)O(3) polymorph. At ~50 K a paramagnetic to canted antiferromagnetic transition is detected.

Synthesis and crystal structure of δ-TaON, a metastable polymorph of tantalum oxide nitride.

δ-TaON was prepared by reaction of gaseous ammonia with an amorphous tantalum oxide precursor. As a representative of the anatase structure (aristotype) it crystallizes in the tetragonal crystal system with lattice parameters a = 391.954(16) pm and c = 1011.32(5) pm. At temperatures between 800 and 850 °C an irreversible phase transformation to baddeleyite-type β-TaON is observed. While quantum-chemical calculations confirm the metastable character of δ-TaON, its transformation to β-TaON is kinetically controlled. The anion distribution of the anatase-type phase was studied theoretically. In agreement with previous studies, it was found that a configuration with maximal N-N distances is most stable. The calculated band edge energies indicate that δ-TaON is a promising photocatalytic material for redox reactions, e.g., water splitting.

Electronic structure and thermodynamics of V2O3 polymorphs

A metastable bixbyite‐type polymorph of vanadium sesquioxide, V2O3, has recently been synthesized, and it transforms to the corundum‐type phase at temperatures around 550 °C. The possibility of a paramagnetic to canted antiferromagnetic or even spin‐glass‐like transition has been discussed. Quantum‐chemical calculations on the density‐functional theory level including explicit electronic correlation confirm the metastability as well as the semiconducting behavior of the material and predict that the bixbyite‐type structure is about 0.1 eV less stable than the well‐known corundum‐type phase. Nonetheless, quasiharmonic phonon calculations manifest that bixbyite‐type vanadium sesquioxide is a dynamically stable compound. Other possible V2O3 polymorphs are shown to be even less suitable candidates for the composition V2O3.

Anosovite-type V3O5: a new binary oxide of vanadium.

The reaction of either V(2)F(6)·4H(2)O or a mixture of 60 wt % VF(2)·4H(2)O and 40 wt % VF(3)·3H(2)O with a water-saturated gaseous mixture of 15-20 vol % hydrogen in argon leads to the formation of a new polymorph of V(3)O(5) crystallizing in the orthorhombic anosovite-type structure. Quantum-chemical calculations show that the anosovite-type structure is about 15 kJ/mol less stable than the corresponding monoclinic Magnéli phase. In addition, there are no imaginary modes in the phonon density of states, supporting the classification of the anosovite-type phase as a metastable V(3)O(5) polymorph. Susceptibility measurements down to 3 K reveal no hint for magnetic ordering.

On the DFT ground state of crystalline bromine and iodine.

We report on an erroneous ground state within common density functional theory (DFT) methods for the solid elements bromine and iodine. Phonon computations at the GGA level for both molecular crystals yield imaginary vibrational modes, erroneously indicating dynamic instability-that fact alone could easily pass as a computational artefact, but these imaginary modes lead to energetically more favorable and dynamically stable structures, made up of infinite monoatomic chains. In contrast, meta-GGA and hybrid functionals yield the correct energetic order for bromine, while for iodine, most global hybrids do not improve the GGA result significantly. The qualitatively correct answer, in both cases, is given by the long-range corrected hybrid LC-ωPBE, the Minnesota functionals M06L and M06, and by periodic Hartree-Fock and MP2 theory. This poor performance of economic DFT functionals should be kept in mind, for example, during global structure optimizations of systems with significant contributions from halogen bonds.

Synthesis and characterization of metastable transition metal oxides and oxide nitrides

Abstract New routes to vanadium sesquioxide and tantalum oxide nitride (γ- and δ-phase) are presented. Phase pure V2O3 with bixbyite-type structure, a metastable polymorph, was obtained from vanadium fluoride hydrates at ~750 K. It crystallizes in the cubic crystal system in space group Ia3¯

A density-functional theory approach to the existence and stability of molybdenum and tungsten sesquioxide polymorphs

Ia\bar 3

Oxide nitrides: From oxides to solids with mobile nitrogen ions

with lattice parameter a=939.30(5) pm. The catalytical properties of the corresponding oxide nitride phases and their oxidation and reduction solid-state kinetics were investigated. The preparation of γ-TaON as a phase pure sample can be realized by ammonolysis of X-ray amorphous tantalum oxide precursors at 1073 K. This metastable tantalum oxide nitride crystallizes in the monoclinic VO2(B)-type structure in space group C2/m. The same precursors can be used to synthesize the δ-modification with an anatase-type structure at 1023 K. It crystallizes in the tetragonal crystal system in space group I41/amd. A maximum yield of 82 m % could be obtained. The fundamental band gaps of the synthesized and of other metastable TaON polymorphs were calculated from first principles using the GW method. The present results are compared to experimental data and to previous calculations at hybrid DFT level.

Adsorption of nitrogen and ammonia at zirconia surfaces

Abstract The sesquioxides of molybdenum and tungsten have been reported as thin films or on surfaces as early as 1971, but the preparation of bulk materials and their crystal structures are still unknown up to the present day. We present a systematic ab initio approach to their possible syntheses and crystal structures applying complementary methods and basis-set types. For both compounds, the corundum structure is the most stable and does not display any imaginary frequencies. Calculations targeted at a high-pressure synthesis starting from the stable oxides and metals predict a reaction pressure of 15 GPa for Mo2O3 and over 60 GPa for W2O3.