Michaël Josse

Effect of Nonmagnetic Substituents Mg and Zn on the Phase Competition in the Multiferroic Antiferromagnet MnWO4

The effects of substituting nonmagnetic Mg2+ and Zn2+ ions for the Mn2+ (S = 5/2) ions on the structural, magnetic, and dielectric properties of the multiferroic frustrated antiferromagnet MnWO4 were investigated. Polycrystalline samples of Mn1−xMgxWO4 and Mn1−xZnxWO4 (0 ≤ x ≤ 0.3) solid solutions were prepared via a solid-state route and characterized via X-ray and neutron diffraction, magnetization, and dielectric permittivity measurements. Mg and Zn substitutions result in very similar effects. The Neel temperature (TN), the AF3-to-AF2 magnetic phase-transition temperature (T2), and the critical ferroelectric temperature (Tc = T2) of MnWO4 are reduced upon nonmagnetic doping. At the lowest temperature (T = 1.5 K), the incommensurate magnetic structure for x(Mg) = 0.15 and x(Zn) = 0.15 corresponds to either a sinusoidal spin arrangement or an elliptical spin-spiral phase similar to the polar AF2 structure observed in MnWO4. These findings were discussed by considering the effects of the Mg and Zn substi...

Influence of Ta5+ content on the crystallographic structure and electrical properties of [001]PC-oriented (Li,Na,K)(Nb,Ta)O3 single crystals

A series of centimeter-sized lead-free piezoelectric Li+- and Ta5+-modified (Na,K)NbO3 single crystals with an ABO3 perovskite structure was successfully grown by the top-seeded solution growth method. Effective segregation of the elements was considered in order to further develop the growth of Li+- and Ta5+-modified (Na,K)NbO3 single crystals. The crystallographic structure and electrical behaviour along the [001]PC orientation were studied. X-ray diffraction, Raman spectroscopy and dielectric studies reveal that the increase in Ta5+ content reduces the orthorhombic–tetragonal phase transition. Bipolar and unipolar strain curves were investigated at 2 kV mm−1. The highest bipolar and unipolar strains associated with a large-signal piezoelectric constant of 368 pm V−1 were achieved for Li0.02Na0.626K0.354Nb0.808Ta0.192O3. The asymmetric bipolar strain curves were observed and related to the existence of internal bias fields induced by defect dipoles, which were created during the crystal growth process.

Coexistence of ferroelectric and relaxor states in Ba 2 Pr x Nd 1-x FeNb 4 O 15

We have investigated the dielectric response of Ba₂Pr_xNd_1-xFeNb₄O₁₅ ceramics (x = 0, 0.2, 0.5, 0.6, 0.8, 1) in the frequency range from 20 Hz to 1 GHz. The obtained results confirmed the continuous transformation from the ferroelectric behavior of Ba₂NdFeNb₄O₁₅ to the pure relaxor response of Ba₂PrFeNb₄O₁₅ with increasing x. For intermediate x values, coexisting ferroelectric transition and relaxor dielectric signatures were observed, corresponding to two different phenomena in the framework of these materials. Increasing the amount of Pr decreases the ferroelectric phase transition temperatures in these ceramics; a large cooling-heating hysteresis exceeding 50K was also observed.

Phase transitions and magnetic structures in MnW1-x Mo x O4 compounds (x ⩽ 0.2).

Temperature-dependent specific heat, magnetization and neutron diffraction data have been collected in zero magnetic field for polycrystalline samples of MnW1-x Mo x O4 (x  ⩽  0.2) solid solution whose end-member MnWO4 exhibits a magnetoelectric multiferroic phase (AF2 phase) between T 1  ≈  8 K and T 2  =  12.5 K. In MnW1-x Mo x O4, diamagnetic W(6+) are replaced with diamagnetic Mo(6+) cations and magnetic couplings among Mn(2+) (3d (5), S  =  5/2) ions are modified due the doping-induced tuning of the orbital hybridization between Mn 3d and O 2p states. It was observed that magnetic phase transition temperatures which are associated with the second-order AF3-to-paramagnetic (T N) and AF2-to-AF3 (T 2) transitions in pure MnWO4 slightly increase with the Mo content x. Magnetic specific heat data also indicate that the first-order AF1-to-AF2 phase transition at T 1 survives a weak doping x  ⩽  0.05. This latter phase transition becomes invisible above the base temperature 2 K for higher level of doping x  ⩾  0.10. Neutron powder diffraction datasets collected above 1.5 K for a sample of MnW0.8Mo0.2O4 were analyzed using the Rietveld method. The magnetic structure below  ≈  14 K is a helical incommensurate spin order with a temperature-independent propagation vector k  =  (-0.217(6), 0.5, 0.466(4)). This cycloidal magnetic structure is similar to the polar AF2 structure observed in MnWO4. The AF1 up-up-down-down collinear spin arrangement observed in MnWO4 is absent in our MnW0.8Mo0.2O4 sample.

Growth and Characterization of Lead-free Piezoelectric Single Crystals

Lead-free piezoelectric materials attract more and more attention owing to the environmental toxicity of lead-containing materials. In this work, we review our first attempts of single crystal grown by the top-seeded solution growth method of BaTiO3 substituted with zirconium and calcium (BCTZ) and (K0.5Na0.5)NbO3 substituted with lithium, tantalum, and antimony (KNLSTN). The growth methodology is optimized in order to reach the best compositions where enhanced properties are expected. Chemical analysis and electrical characterizations are presented for both kinds of crystals. The compositionally-dependent electrical performance is investigated for a better understanding of the relationship between the composition and electrical properties. A cross-over from relaxor to ferroelectric state in BCTZ solid solution is evidenced similar to the one reported in ceramics. In KNLSTN single crystals, we observed a substantial evolution of the orthorhombic-to-tetragonal phase transition under minute composition changes.

Investigation of Dielectric Relaxation Processes in Ba2NdFeNb4-xTaxO15 Ceramics

Dielectric response of Ba2NdFeNb4-xTaxO15 (x = 0.3, 0.6 and 2) solid solutions with Tetragonal Tungsten Bronze structure has been investigated by means of broadband dielectric spectroscopy in the frequency range from 20 Hz to 37 GHz. Pure Ba2NdFeNb4O15 compound displays ferroelectric behavior. Substitution of Nb5+ by Ta5+ considerably alters dielectric response of this system. Ferroelectric state was observed only for x = 0.3 compound with slightly lowered ferroelectric phase transition temperature. Further increase of x causes formation of relaxor state in these solid solutions and considerably lowers corresponding dielectric permittivity dispersion temperature region.

Dielectric Study of Unexpected Transitions in Multiferroic Mn1-x(Mg,Zn)xWO4 Ceramics

The influence of magnetic field on the dielectric properties of several Mn1-x(Mg,Zn)xWO4 multiferroic ceramics is investigated, and confirms an additional phase transition below the ferroelectric Curie temperature previously detected in this system in zero magnetic field. In highly doped samples an original plateau of the capacitance is observed between T2 and this additional transition. Pyroelectric measurements in zero magnetic field for a Mn0.85Mg0.15WO4 ceramic confirm the existence of this unexpected low temperature phase transition. The magnetic or structural origin of this transition is discussed.

Cool-SPS: an opportunity for low temperature sintering of thermodynamically fragile materials

Sintering has been achieved by Spark Plasma Sintering at low temperatures ( 90% of theoretical density) fragile materials, and also highlights the extra stability afforded by this approch for certain materials which have been sintered above their normal decomposition temperature. Using multiple examples, this work illustrates the potential of “Cool-SPS” for fast and efficient sintering of fragile materials which are impossible to sinter by conventional sintering techniques.

Crystal-chemistry design of new single-phase multiferroics

Most single-phase multiferroics are of perovskite (or narrowly related) crystal structure. N. Hill pointed out the significant drawback of this kind of materials for multiferroism: as ferromagnetism (requiring ’’d’’ configuration) and ferroelectricity (favoured by ’’d’’ configuration) rely on the same cationic sublattice, this one should contain both ’’d’’ and ’’d’’ cations. This condition is hardly fulfillable and most of the multiferroic materials currently studied are derived frommagnetic materials and do not contain ’’ferroelectrically active’’ cations such as Ti, Nb, Ta ... Our crystal-chemistry driven approach is to elaborate potentially multiferroic materials from ferroelectrics in which wemay substitute magnetic cations without significant alteration of the ’’ferroelectric framework’’. Ferroelectrics possessing several cationic sublattices are the most likely candidates, and in the wide range of ferroelectric materials studied at the ICMCB, Tetragonal Tungsten Bronzes (TTB) appear to be the ideal ones. Wewill present the structural, dielectric andmagnetic properties of substituted TTB niobates, the multiferroic properties of which will be discussed in terms of crystal-chemistry. A survey of the ’’core-shell multiferroic composites’’ activities at the ICMCB will be also given.