Synthesis, sintering by Cool-SPS and characterization of A2Cu(CO3)2 (A = K, Na): evidence for multiferroic and magnetoelectric cupricarbonates.

We report here the synthesis and densification of two magnetic materials in the system A2Cu(CO3)2 (A = Na, K). Based on literature data, the synthesis route has been modified to offer the possibility of gram-scale production with high powder purity. The cool spark plasma sintering (Cool-SPS) method has been used to obtain highly dense samples (>95% of theoretical density). Not only did the sintering method prove its efficiency for giving dense ceramics, but it also extended the stability domain of both compounds to higher temperatures. The case of Na2Cu(CO3)2 is highly instructive of the possibilities offered by the Cool-SPS technique, since the pure phase can be obtained during the sintering process, while it is impossible through conventional heat treatments in a furnace. Obtaining dense ceramics allowed the exploration of dielectric properties, leading to the observation of a step-like anomaly in K2Cu(CO3)2 similar to some multiferroic systems, while Na2Cu(CO3)2 exhibits an interesting magnetocapacitance evolution, probably linked to a magnetoelectric coupling.

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.

Incorporation of Jahn-Teller Cu(2+) Ions into Magnetoelectric Multiferroic MnWO4: Structural, Magnetic, and Dielectric Permittivity Properties of Mn1-xCuxWO4 (x ≤ 0.25).

Polycrystalline samples of Mn1-xCuxWO4 (x ≤ 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 Mn(2+) (d(5), S = 5/2) ions with Jahn-Teller-active Cu(2+) (d(9), S = 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 ≤ x ≤ 0.25 are solid solution that forms in the monoclinic P2/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 (x < 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 x = 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.

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.

Coexistence of ferroelectric and relaxor states in Ba2PrxNd1-xFeNb4O15.

We have investigated the dielectric response of Ba(2)Pr(x)Nd(1-x)FeNb(4)O(15) 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 Ba(2)PrFeNb(4)O(15) 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.

Cation ordering in the double tungstate LiFe(WO4)2.

Single crystals of lithium iron tungstate, LiFe(WO(4))(2), were obtained using a high-temperature solution growth method. The analysis was conducted using the monoclinic space group C2/c, with ß = 90.597 (2)°, giving R1 = 0.0177. The Li and Fe atoms lie on twofold axes. The structure can also be refined using the orthorhombic space group Cmcm, giving slightly higher residuals. The experimental value of ß and the residuals mitigate in favour of the monoclinic description of the structure. Calculated bond-valence sums for the present results are closer to expected values than those obtained using the results of a previously reported analysis of this structure.

Flexible relaxor materials: Ba(2)Pr(x)Nd(1-x)FeNb(4)O(15) tetragonal tungsten bronze solid solution.

Relaxors are very interesting materials but most of the time they are restricted to perovskite materials and thus their flexibility is limited. We have previously shown that tetragonal tungsten bronze (TTB) niobate Ba(2)PrFeNb(4)O(15) was a relaxor below 170 K and that Ba(2)NdFeNb(4)O(15) displays a ferroelectric behavior with a T(C) = 323 K. On scanning the whole solid solution Ba(2)Pr(x)Nd(1-x)FeNb(4)O(15) (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1), we demonstrate here a continuous crossover between these end member behaviors with a coexistence of ferroelectricity and relaxor in the intermediate range. This tunability is ascribed to the peculiar structure of the TTB networks which is more open than the classical perovskites. This allows for the coexistence of long range and short range orders and thus opens up the range of relaxor materials.

Anti-KSbF6 structure of CaTbF6 and CdTbF6: a confirmation of the singular crystal chemistry of Tb4+ in fluorides.

The crystal structures of two new tetravalent terbium fluorides, CaTbF6 and CdTbF6, have been determined from X-ray and neutron powder diffraction data. The title compounds exhibit an anti-KSbF6 structure, the three-dimensional framework of which is built of [TbF6]2- chains of edge-sharing dodecahedra further linked, by sharing corners, to isolated [MF6]4- octahedra (M=Ca, Cd). The mechanism of the anionic sublattice rearrangement when going from KSbF6 to CaTbF6 is described and related to a simple cubic fluoride-ion packing. Comparison with the crystal structures of beta-BaTbF6 and other representatives of the M(II)M('IV)F6 family allows the singular crystal-chemical properties of some fluoroterbates to be emphasized.