Effect of Pressure on Electrical Resistivity and Crystal Structure of Quasi‐Two‐Dimensional Mo₄O₁₁

Abstract

Electrical resistivities p and lattice parameters of quasi-two-dimensional crystals of monoclinic q-Mo,O,, and orthorhombic y-Mo,O,, are measured under hydrostatic pressure up to 1.2 GPa a t room temperature. Both crystals have negative pressure coefficients of e, which can be qualitatively explained by a phonon stiffening effect. In particular, the pressure dependence of p for y-Mo,O,, shows a drastic drop or jump a t Pc = 0.3 GPa, whose magnitude and the pressure coefficients are strongly dependent on the sample resistivity or degree of stoichiometry. It is suggested that the change of a small amount of MOO, tetrahedra into MOO, octahedra induced by excess oxygen atoms is responsible for such structure-sensitive properties.Bei Zimmertemperatur und hydrostatischen Driicken bis zu 1,2 GPa werden die elektrischen Widerstande p und Gitterparameter von quasizweidimensionalen Kristallen von monoklinen qMo40,, und rhombischen y-Mo40,, gemessen. Beide Kristalle haben negative Druckkoeffizienten von p, die qualitativ durch einen Phononenversteifungseffekt erkliirt werden konnen. Insbesondere zeigt die Druckabhangigkeit von e fur y-Mo,O,, einen drastischen Abfall oder Sprung bei P , = = 0,3 GPa, dessen GroDe und die Druckkoeffizienten stark von dem Probenwiderstand oder Grad der Stoichiometrie abhangig sind. Es wird angenommen, daI3 die h d e r u n g eines geringen Anteils von Moo,-Tetraedern in Moo,-Oktaedern, die durch iiberschiissige Sauerstoffatome induziert wird, fiir solche strukturempfindlichen Eigenschaften verantwortlich ist.

Molybdenum Oxides MoOx: Spark-Plasma Synthesis and Thermoelectric Properties at Elevated Temperature

“…At room temperature, for MoO 2.750 , the electrical conductivity reported for single crystals is strongly anisotropic and varies from 0.23 × 10 5 Sm −1 parallel to the bc plane 16 to 5−13 × 10 5 Sm −1 perpendicular to it. 16,25,41,42 A value of σ = 3.12 × 10 5 Sm −1 measured for our polycrystalline samples ranges in between, which is plausible because of approximately isotropic distribution of crystallite directions and increased scattering at grain boundaries.…”

“…The synthesis of the η polymorph is usually performed directly at temperatures below the γ stability range, which excludes the possible necessity of supercooling γ to reach a γ → η transition. ,,,,, Thus, under SPS conditions, the equilibrium may be influenced by the applied electric field, which introduces a driving force additional to the concentration gradient and therefore affects the diffusion paths of mobile charged species, that is, Mo cations and O anions. For comparison, reports on SPS processed hyperstoichiometric UO 2+ x show the uranium oxidation state to gradually increase toward the anode (+) due to directional migration of oxygen defects .…”

“…This supports the statement that none of the MoO x phases has a homogeneity range, 10,20 and challenges on stoichiometric deviations as reason for different resistivity transitions in γ-Mo 4 O 11 crystals under high pressure were reported pre- viously. 25 In comparison to the composition x = 2.750, the full width at half-maximum (FWHM) of many reflections is increased in both, the oxygen deficient sample (21 of 31 reflections) and the oxygen excessive sample (17 of 31 reflections), which indicates disorder whenever a secondary phase is formed (Tables S3−S5, Supporting Information). Thermoelectric Transport Properties.…”

“…Thus, TE properties at elevated temperatures and the respective potential for high-temperature application are mostly unknown. Moreover, research was often constrained to single crystals obtained from solid-state reactions ,− and chemical vapor transport (CVT) ,− or to diverse nanostructures . During spark-plasma synthesis (SPS), sample powder is encapsulated in an electrical conducting die.…”

Developments in Measuring and Calculating Chemical Vapor Transport Phenomena Demonstrated on Cr, Mo, W, and Their Compounds

“…In the past iodine − and TeCl 4 ,,− were used for the synthesis of Mo 4 O 11 crystals by chemical transport reactions. No systematic examinations of the transport behavior in these systems were carried out.…”