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In this report, the high-temperature transport properties of (Dy1−xCax)(Zr0.2Hf0.2Sn0.2Ti0.2Ge0.2)O7 pyrochlore oxides with x = 0, 0.05, and 0.1 are studied in dry and humid air. The phase composition and crystal structure were determined by using x-ray and neutron diffraction. The addition of calcium to the structure caused an increase in the concentration of oxygen vacancies, indicating an ionic charge compensation mechanism. Electrical studies allowed us to determine the total electrical conductivity as a function of the synthesis atmosphere and pH2O. The electrical conductivity turned out to be at the level of ∼10−3 S/cm at 800 °C, and only a slight effect of the presence of protonic defects in the structure on the total electrical conductivity was observed. In general, the samples had a low electrical conductivity with a relatively high activation energy of conduction. [ABSTRACT FROM AUTHOR]

This article reports on the exploration of the chemical versatility of entropy‐stabilized pyrochlores (A2B2O7) with large chemical disorder introduced on the B site, as well as the study of isovalent and aliovalent substitution on the B site and of Ca2+‐doping on the A site. Among all the 20 new compounds obtained in this study, 8 are entropy‐stabilized: RE2(TiZrHfGeSn)2 (with RE = Gd–Ho and Y), Pr2(TiZrHfScNb)2O7, and RE2(TiGeSnAlNb)2O7 (with RE = Er and Y). Moreover, some physical properties (magnetic, electrical, thermal, and optical properties) of these high‐entropy pyrochlores have been screened, and some of them could be potentially interesting for applications. For example, we observed low thermal diffusivity values (between 0.4 and 0.7 m2 s−1) making them interesting for thermal barrier coating or tunable optical bandgaps in the visible region that could make them appealing as photocatalysts or in optical applications. Furthermore, single‐phased compounds were obtained for almost all the rare earths (REs) on the A site. Therefore, when five nonmagnetic cations are used for the B site, it enables to study the magnetic properties and, more specifically, the influence of the local chemical and structural disorder on the exotic ground states observed in monocationic RE pyrochlores. [ABSTRACT FROM AUTHOR]

This paper aims at showing the complementarity between time-of-flight and triple-axis neutron scattering experiments, on the basis of two topical examples in the field of geometrical magnetic frustration. Rare earth pyrochlore magnets R2Ti2O7 (R is a rare earth) play a prominent role in this field, as they form model systems showing a rich variety of ground states, depending on the balance between dipolar, exchange interactions and crystal field. We first review the case of the XY antiferromagnet Er2 Ti2 O7. Here a transition towards a Néel state is observed, possibly induced by an order-by-disorder mechanism. Effective exchange parameters can be extracted from S(Q,ω). We then examine the case of the spin liquid Tb2 Ti2 O7. Recent experiments reveal a complex ground state characterized by “pinch points” and supporting a low energy excitation. These studies demonstrate the existence of a coupling between crystal field transitions and a transverse acoustic phonon mode.

In the orthorhombic manganites o-RMnO3, where R is a heavy rare earth (R = Gd-Yb), the Mn3+ sublattice is known to undergo two magnetic transitions. The low temperature phase has an antiferromagnetic structure (collinear or elliptical), which has been well characterized by neutron diffraction in most of these compounds. The intermediate phase, occurring in a narrow temperature range (a few K), is documented for R = Gd–Ho as a collinear modulated structure, incommensurate with the lattice spacings. We report here on a 57Fe Mössbauer study of 2% 57Fe doped o-YbMnO3, where the spin only Fe3+ ion plays the role of a magnetic probe. From the analysis of the shape of the magnetic hyperfine Mössbauer spectra, we show that the magnetic structure of the intermediate phase in o-YbMnO3 (38.0 K < T < 41.5 K) is also modulated and incommensurate. [ABSTRACT FROM AUTHOR]

This article reviews recent achievements on the crystal growth of a new series of pyrochlore oxides-lanthanide zirconates, which are frustrated magnets with exotic magnetic properties. Oxides of the type A2B2O7 (where A = Rare Earth, B = Ti, Mo) have been successfully synthesised in single crystal form using the floating zone method. The main difficulty of employing this technique for the growth of rare earth zirconium oxides A2Zr2O7 arises from the high melting point of these materials. This drawback has been recently overcome by the use of a high power Xenon arc lamp furnace for the growth of single crystals of Pr2Zr2O7. Subsequently, large, high quality single crystals of several members of the zirconate family of pyrochlore oxides A2Zr2O7 (with A = La→Gd) have been grown by the floating zone technique. In this work, the authors give an overview of the crystal growth of lanthanide zirconates. The optimum conditions used for the floating zone growth of A2Zr2O7 crystals are reported. The characterisation of the crystal boules and their crystal quality is also presented. [ABSTRACT FROM AUTHOR]

The preparation, structural, microstructural, dielectric, and low temperature magnetic properties of Mg x Ni1− x Fe2 O4 ( x = 0, 0.17, 0.34, 0.50, 0.66, 0.83, 1) ferrites synthesized by using a self-combustion sol-gel method is presented. Good insulating properties were found for all the compositions at high frequencies (kHz and MHz range), which might drive the present ceramics as interesting for RF/microwaves applications. By increasing the Mg2+ concentration, the total resistivity strongly increases (from ~106 Ωm for the Ni ferrite to 109 Ωm for the Mg ferrite), corresponding to conductivities in the range 10−9-10−6 S/m at f = 1 Hz. Typical ferrimagnetic character with a small coercivity and saturation magnetization in the range (30-50) Am2/kg, which slightly decreases with increasing the Mg content, were found. On the basis of the combined results from the infrared spectroscopy and XRD analysis, it was shown that the magnetic properties depend on the Mg2+ ions distribution on the octahedral and tetrahedral sites and the experimental saturation magnetization allowed to compute the cation distribution for the Mg x Ni1− x Fe2 O4 ferrites. [ABSTRACT FROM AUTHOR]

Examples of materials where an "order by disorder" mechanism is at play to select a particular ground state are scarce. It has recently been proposed, however, that the antiferromagnetic XY pyrochlore Er2Ti2O7 reveals a most convincing case of this mechanism, with the observation of a spin gap at zone centers having recently been interpreted as a corroboration of this physics. Here we argue, however, that the anisotropy generated by the interaction-induced admixing between the crystal-field ground and excited levels provides for an alternative mechanism. It especially predicts the opening of a spin gap of about 15 μeV, which is of the same order of magnitude as the one observed experimentally. We report high-resolution inelastic neutron scattering data which can be well understood within this scenario. [ABSTRACT FROM AUTHOR]

The dynamical magnetic correlations in Tb2Ti2O7 have been investigated using polarized inelastic neutron scattering. Dispersive excitations are observed, emerging from pinch points in reciprocal space and characterized by an anisotropic spectral weight. Anomalies in the crystal field and phonon excitation spectrum at Brillouin zone centers are also reported. These findings suggest that Coulomb phases, although they present a disordered ground state with dipolar correlations, allow the propagation of collective excitations. They also point out a strong spin-lattice coupling, which likely drives effective interactions between the 4f quadrupolar moments. [ABSTRACT FROM AUTHOR]

We have studied the low-energy spin dynamics between 4.6 and 0.07 K in a Tb2Ti2O7 single-crystal sample by means of inelastic neutron scattering experiments. The spectra consist in a dual response, with a static and an inelastic contribution, showing striking Q dependencies. We propose an interpretation involving an anisotropic exchange interaction in combination with a breaking of the threefold symmetry at the rare earth site. Simulations of the g-dependent scattering in the random phase approximation account well for the inelastic response. [ABSTRACT FROM AUTHOR]

Zero-field muon spin rotation and magnetization measurements were performed in La2Cu1-xMxO4, for 0 ⩽ x ⩽ 0.12, where Cu2+ is replaced either by M = Zn2+ or by M = Mg2+ spinless impurity. It is shown that while the doping dependence of the sublattice magnetization [M(x)] is nearly the same for both compounds, the Néel temperature [TN(x)] decreases unambiguously more rapidly in the Zn-doped compound. This difference, not taken into account within a simple dilution model, is associated with the frustration induced by the Zn2+ impurity onto the Cu2+ antiferromagnetic lattice. In fact, from TN(x) and M(x) the spin stiffness is derived and found to be reduced by Zn doping more significantly than expected within a dilution model. The effect of the structural modifications induced by doping on the exchange coupling is also discussed. [ABSTRACT FROM AUTHOR]

The dynamical magnetic correlations in Tb2Ti2O7 have been investigated using polarized inelastic neutron scattering. Dispersive excitations are observed, emerging from pinch points in reciprocal space and characterized by an anisotropic spectral weight. Anomalies in the crystal field and phonon excitation spectrum at Brillouin zone centers are also reported. These findings suggest that Coulomb phases, although they present a disordered ground state with dipolar correlations, allow the propagation of collective excitations. They also point out a strong spin-lattice coupling, which likely drives effective interactions between the 4f quadrupolar moments.