Insights into BaTi_1–<i>y</i>Zr_<i>y</i>O₃ (0 ≤ <i>y</i> ≤ 1) Synthesis under Supercritical Fluid Conditions

The production of BaTi_1–<i>y</i>Zr_<i>y</i>O₃ (0 ≤ <i>y</i> ≤ 1, BTZ) nanocrystals is known to be challenging due the low reactivity of zirconium precursors. Here we have successfully studied the impact of zirconium on the BTZ particle formation in sub- and supercritical fluid conditions along the entire solid solution. <i>In situ</i> synchrotron wide angle X-ray scattering (WAXS) analyses were conducted in batch at 150 and 400 °C to follow, in real time, the BTZ crystallite synthesis. This approach revealed the complexity behind the nucleation and growth mechanisms of ABO₃ nanocrystals, especially toward high zirconium content (more than 50 atomic %). This type of substitution induces, among other things, microstrain within the structure. Moreover, for the cases of BaTi_0.4Zr_0.6O₃ and BaTi_0.2Zr_0.8O₃, the experiments showed the apparition of two crystallite size populations. In the BaTi_0.4Zr_0.6O₃ case, at 400 °C, these two size populations merged into a single one after at least 8 min; in contrast to what was observed for the case of BaTi_0.2Zr_0.8O₃. This is a manifestation of how with increasing the zirconium content the particles become more refractory and in these cases the temperature is not high enough to enable their ripening. It is important to note that this behavior was not observed for particles produced at 400 °C using a flow synthesis method, with a residence time of only 50 s. There, the particles presented a single size population close to the one obtained after 8 min in batch. This thus suggests that for batch syntheses a longer time is required to achieve a similar product quality to the one obtained with a flow process

Incorporation of Jahn–Teller Cu²⁺ Ions into Magnetoelectric Multiferroic MnWO₄: Structural, Magnetic, and Dielectric Permittivity Properties of Mn_1–<i>x</i>Cu_<i>x</i>WO₄ (<i>x</i> ≤ 0.25)

Polycrystalline samples of Mn_1–<i>x</i>Cu_<i>x</i>WO₄ (<i>x</i> ≤ 0.5) have been prepared by a solid-state synthesis as well as from a citrate synthesis at moderate temperature (850 °C). The goal is to study changes in the structural, magnetic, and dielectric properties of magnetoelectric type-II multiferroic MnWO₄ caused by replacing Jahn–Teller-inactive Mn²⁺ (d⁵, <i>S</i> = 5/2) ions with Jahn–Teller-active Cu²⁺ (d⁹, <i>S</i> = 1/2) ions. Combination of techniques including scanning electron microscopy, powder X-ray and neutron diffraction, and Raman spectroscopy demonstrates that the polycrystalline samples with low copper content 0 ≤ <i>x</i> ≤ 0.25 are solid solution that forms in the monoclinic <i>P</i>2/c space group. Rietveld analyses indicate that Cu atoms substitutes for Mn atoms at the Mn crystallographic site of the MnWO₄ structure and suggest random distributions of Jahn–Teller-distorted CuO₆ octahedra in the solid solution. Magnetic susceptibility reveals that only 5% of Cu substitution suppresses the nonpolar collinear AF1 antiferromagnetic structure observed in pure MnWO₄. Type-II multiferroicity survives a weak Cu substitution rate (<i>x</i> < 0.15). Multiferroic transition temperature and Néel temperature increase as the amount of Cu increases. New trends in some of the magnetic properties and in dielectric behaviors are observed for <i>x</i> = 0.20 and 0.25. Careful analysis of the magnetic susceptibility reveals that the incorporation of Cu into MnWO₄ strengthens the overall antiferromagnetic interaction and reduces the magnetic frustration

Persistent Type-II Multiferroicity in Nanostructured MnWO₄ Ceramics

Persistent Type-II Multiferroicity in Nanostructured MnWO₄ Ceramic

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