Increasing the Phase-Transition Temperatures in Spin-Frustrated Multiferroic MnWO₄ by Mo Doping

Ceramic samples of MnW_1–<i>x</i>Mo_<i>x</i>O₄ (<i>x</i> ≤ 0.3) solid solution were prepared by a solid-state route with the goal of increasing the magnitude of the spin-exchange couplings among the Mn²⁺ ions in the spin spiral multiferroic MnWO₄. Samples were characterized by X-ray diffraction, optical spectroscopy, magnetization, and dielectric permittivity measurements. It was observed that the Néel temperature <i>T</i>_N, the spin spiral ordering temperature <i>T</i>_M2, and the ferroelectric phase-transition temperature <i>T</i>_FE2 of MnWO₄ increased upon the nonmagnetic substitution of Mo⁶⁺ for W⁶⁺. Like pure MnWO₄, the ferroelectric critical temperature <i>T</i>_FE2(<i>x</i>) coincides with the magnetic ordering temperature <i>T</i>_M2(<i>x</i>). A density functional analysis of the spin-exchange interactions for a hypothetical MnMoO₄ that is isostructural with MnWO₄ suggests that Mo substitution increases the strength of the spin-exchange couplings among Mn²⁺ in the vicinity of a Mo⁶⁺ ion. Our study shows that the Mo-doped MnW_1–<i>x</i>Mo_<i>x</i>O₄ (<i>x</i> ≤ 0.3) compounds are spin-frustrated materials that have higher magnetic and ferroelectric phase-transition temperatures than does pure MnWO₄

A One-Pot Approach to Mesoporous Metal Oxide Ultrathin Film Electrodes Bearing One Metal Nanoparticle per Pore with Enhanced Electrocatalytic Properties

The controlled incorporation of single metal nanoparticles within the pores of mesostructured conducting metal oxide ultrathin films is demonstrated, taking advantage of the controlled metal precursor loading capacities of PS-<i>b</i>-P4VP inverse micellar templates. The presented one-pot approach denoted as Evaporation-Induced Hydrophobic Nanoreactor Templating (EIHNT) unusually involves the nanostructuration of the metal oxide via the hydrophobic shell of the micellar template, while the concomitant nanostructuration of the metal is achieved via its confinement in the hydrophilic micellar core. This approach is applied to tin-rich ITO and gold, to yield unique mesoporous tin-rich ITO ultrathin film electrodes remarkably loaded with one size-controlled gold nanoparticle per pore. Interestingly, the resulting tin-rich ITO-supported gold nanoparticles exhibit improved catalytic activity and durability in electrocatalytic CO oxidation compared to similarly sized gold nanoparticles supported on conventional ITO coatings