Bixbyite-Type V₂O₃—A Metastable Polymorph of Vanadium Sesquioxide

Abstract

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 characterization of metastable transition metal oxides and oxide nitrides

“…The absence of remaining precursor-fluoride was proved by EDX, WDX, and XFA. The new vanadium oxide is metastable (about 9 kJ/mol less stable than the thermodynamically stable polymorph [1] and largely independent from the quantum-mechanical method) and transforms to corundum-type V 2 O 3 at ~823 K in argon atmosphere. An oxidation to V 2 O 5 can be observed in air at ~523 K. At ambient conditions it oxidizes to VO 2 in a period of 3 weeks.…”

“…In a previous publication we presented the synthesis of bixbyite-type V 2 O 3 with a maximum yield of about 82 wt % with corundum-type V 2 O 3 as side phase [1]. In addition, the electronic structure and thermodynamics have already been discussed [26].…”

“…In most cases it is very difficult to obtain phase pure materials important for an unambiguous interpretation of the data from the properties measurements. During the last years we published on synthesis and characterization of four new metastable phases: bixbyite-type V 2 O 3 [1], anosovite-type V 3 O 5 [2], VO 2 (B)-type TaON [3], and anatase-type TaON [4]. Unfortunately, only anosovite-type V 3 O 5 can be synthesized as single phase [2].…”

“…All other samples contained large amounts of side phases leading to severe problems concerning the investigation of physical properties. However, the crystal structures could be determined, also in combination with quantum-chemical methods [1][2][3][4]. Recently, we optimized the preparation techniques and in this contribution we present simple routes to phase pure bixbyitetype V 2 O 3 and VO 2 (B)-type TaON (γ-TaON) [5].…”

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

“…As a first experimental attempt, a conventional synthetic approach inspired by the successful synthesis of the metastable sesquioxides of vanadium [4] was used. Different oxide (crystalline and amorphous) and oxide fluoride precursors were reduced with altered H 2 /Ar flow rates.…”