Large Single Crystals of V_1–xMo_xO₂ from a Two-Step Chemical Vapor Transport Synthesis

Abstract

Single crystals of molybdenum doped VO 2 with the formula V 1−x Mo x O 2 have been successfully synthesized with molybdenum compositions up to x = 0.60. These crystals were obtained using a novel, two-step chemical vapor transport (CVT) synthesis. The first CVT reaction uses a stoichiometric amount of VO 2 , V 2 O 3 , MoO 2 , and MoO 3 with MoCl 3 as a transport agent to produce a largely polycrystalline, single phase V 1−x Mo x O 2 product. This product can then be pulverized and used in a second CVT reaction with TeCl 4 as a transport agent to produce large single crystals. This two-step synthetic method results in larger single crystals with a higher degree of compositional control compared to any single step CVT syntheses attempted, as well as allowing for an increase in dopant range compared to what had previously been achieved for this dopant system. Crystals obtained from this synthetic method are ideal for physical property measurements and diffuse scattering experiments which require large samples to be able to properly mount the samples and obtain useful data.

Geometric Frustration Suppresses Long-Range Structural Distortions in NbxV1–xO2

“…It is apparent that the various lattice parameters found here interpolate well with the prior ones for the determined compositions. Moreover, this same method was used previously to determine the Mo concentration in VO 2 and was shown to be accurate when compared against other, independent characterization methods. Since the (Nb,V)­O 2 and (V,Mo)­O 2 phase diagrams show remarkably similar lattice parameter changes, the compositions calculated via PXRD in this way will be treated as “actual” throughout this study.…”

“…The HZ tended to be above the target stoichiometry, suggesting that Nb and V disproportionate across temperature zones much more than V and Mo do. 24 Indeed, this may be related to the overall difficulties in controlling output stoichiometry for the V−Nb phases.…”

“…Attempts to determine the oxidation state of Mo have yielded ambiguous results, with bulk probes suggesting an effective Mo 4+ and delocalized electronic states, − while surface-sensitive ones such as XPS find that Mo 5+ and Mo 6+ are present . However, given that the same “Mo 5+ /Mo 6+ ” surface oxidation state was found in MoO 2 , which must be considered a bulk Mo 4+ species, it seems likely that the surface oxidation states in V 1– x Mo x O 2 do not represent the bulk values.…”

“…■ EXPERIMENTAL METHODS Crystal Growth. Single crystals of Nb x V 1−x O 2 were grown using a chemical vapor transport technique (CVT) similar to the method recently reported for the (V,Mo)O 2 , 24 though more reducing conditions are required when using Nb instead of Mo. The crystal growth method is divided into two categories, one for Nb concentrations in the range 0 < x < 0.5 and another for higher concentrations of 0.5 < x < 1.…”