Preparation and characterization of powders and crystals of V_n‐xTi_xO_2n‐1 Magneli oxides

Abstract

V n-x Ti x O 2n-1 Magnéli phases have been synthesized under vacuum in powder form (n=4, 0≤x≤0.4) and crystals (n = 4 and 5, x = 0.5 and 1.4, respectively), grown by chemical vapour transport in closed ampoules. TeCl 4 and NH 4 Cl were used as transporting agents. Needle-shaped crystals as long as 200-300 micrometers or 2-3 mm were obtained when in presence of NH 4 Cl or TeCl 4 , respectively. The powder and crystal structures were examined by X-ray diffraction and the transport and magnetic characteristics were measured.. The powders resulted to be single-phase and the relevant composition was assumed to be equal to the nominal one. The overall stoichiometry of compounds, n, was determined from single crystal X-ray diffraction data. The Ti content, x, was deduced from the elementary cell volume, by applying the Végard law. Crystals were mainly untwinned and of good quality. The elementary cell of both, powders and crystals, was triclinic (P-1) and did not change with doping. DC electrical resistivity of the crystals was measured in a four-points (van der Pauw) configuration. DC magnetic susceptibility of the powders was measured in a SQUID magnetometer. The Ti doping was found to progressively smooth and finally to suppress the magnetic transitions occurring in the V 4 O 7 . The metal-insulator transitions observed in V 4 O 7 and V 5 O 9 , at around 235 and 125 K respectively, were not observed in the doped crystals, thus indicating some significant change of the electronic structure of the V oxides.

The Thermal-Oxidation Behavior of Pristine and Doped Magnéli Phase Titanium Oxides

“…Therefore, the preparation of Mn-doped Ti n O 2n−1 via H 2 reduction of TiO 2 are infeasible notwithstanding other preparations such as metallothermic reduction. Rule 2 is observed for other reported dopants of Ti n O 2n−1 : C, 22 N, 19,21 Cu, 17 Y, 26 Zr, 22 Nb, 18,24,26,27 Mo, 26 Ta, 27 and Ce. 20 However, they violate rules 1 (C, N, and Mo) and 3 (C IV , N III , and Cu III low-spin complex, Y III , Zr IV , Ce III , and Ce IV ), or there is insufficient data to evaluate rule 4 (all but Zr IV ).…”

“…Doping is an emerging method for tailoring Ti n O 2n−1 to better suit their application. 9 The inclusion of various dopants is demonstrated to be capable of modifying tribological, 18,19 electrical, [20][21][22] photocatalytic, 23,24 thermoelectric, [25][26][27] and electrocatalytic 21,22,28,29 properties. However, the rationale for selecting dopants is generally sparse apart from a size criterion, which Gardos invokes in selecting Cu.…”

Large Size Single Crystal Growth of Ti4O7 by the Floating-Zone Method

“…Chemical vapor transport (CVT) has been the main method used to grow Ti 4 O 7 single crystals for decades. ,,,− However, there are some shortcomings in Ti 4 O 7 single crystal grown by CVT. First, the single crystals grown by CVT contain inevitably impurities (e.g., TeCl 4 and NH 4 Cl) owing to the transport medium.…”

The Thermal-Oxidation Behavior of Pristine and Doped Magnéli Phase Titanium Oxides

“…10 Doping is an emerging method for tailoring Ti n O 2n-1 to better suit their application. 3 The inclusion of various dopants is demonstrated to be capable of modifying tribological, 11,12 electrical, 13,14 photocatalytic, 15,16 thermoelectric, [17][18][19] and electrocatalytic 14,20 properties. Hayfield conducted a preliminary investigation of doped Ebonex® electrodes, a trademark for Magnéli phase titanium oxide-based products, including their thermal-oxidation behavior in air.…”