[en] Magnetization and susceptibility studies on single crystals of the pyrochlore Ho₂Ti₂O₇ are reported. Magnetization isotherms are shown to be qualitatively similar to that predicted by the nearest-neighbor spin-ice model. Below the lock-in temperature, T^*≅1.97 K, magnetization is consistent with the locking of spins along [111] directions in a specific two-spins-in, two-spins-out arrangement. Below T^* the magnetization for B||[111] displays a two step behavior signaling the breaking of the ice rules

[en] The pyrochlore-type matrices containing up to 10% wt. of U, Np, Pu and Am oxides were produced by self-propagating high-temperature synthesis (SHS) proposed as a method of preparation of the ceramic mineral-like matrices for immobilization of actinides. It was shown that basically the matrices consist of a phase of yttrium titanate having pyrochlore structure with actinides incorporated isomorphically. Stability of these matrices relative to hydrothermal leaching of the actinides at 90 C was studied: the leaching rate does not exceed 1 * 10^-7 g/(cm² day). (authors)

[en] Transition-metal spinels (general formula AB₂X₄) have been, for many years, the subject of intense experimental and theoretical activity. Structurally, the most interesting feature of these systems is the fact that the B cation occupies the nodes of a pyrochlore lattice, which is known to be geometrically frustrated. Therefore, one can explore how the natural tendency of the transition metals to order in the charge, magnetic and orbital sectors is affected by geometrical frustration. Recently, orbital ordering has become a topical subject in a variety of both non-frustrated systems, such as manganites and other perovskites, and in the spinels. In this paper, I review the recent experimental activity on the subject of orbital ordering in transition-metal spinels and relate this to models of orbital ordering that are being developed by theoreticians

[en] The traditional BCS superconductors AOs_2O_6 (A=K, Rb, and Cs) were investigated to find the relationship between their structures and superconducting transition temperatures. The T_c decreases with increasing the unit cell parameter of AOs_2O_6. This is in contrast to the case of conventional BCS superconductivity in a single bond model, where T_c may increase with increasing the the unit cell parameter since the DOS at Fermi level increases as the unit cell parameter increases. Instead, the T_c of a β-pyrochlore oxide is proportional to the lattice softness of the compound

[en] Pyrochlore-rich and zirconolite-rich ceramics have been developed for the immobilization of excess weapons Pu. The ceramics are composed of a mixture of pyrochlore, zirconolite, brannerite and rutile. Impurity ions are present in most Pu-waste streams. Most of these impurities can be incorporated into the phases present in the ceramic; however some, such as B and Si, can promote the formation of additional phases. In this work, the impurity ions were classed into sets with supposedly the same valency (2+, 3+, 4+, 5+ and 6+). One set containing Np and Th and another set containing the glass formers (Al, Si, B, Na and K) were also made. These sets of elements were then added to a 'baseline' ceramics of nominal sintered composition, 95 wt.% pyrochlore (Ca_0.89Gd_0.22(Pu or Ce)_0.22U_0.44Hf_0.23Ti₂O₇) and 5 wt.% HF-doped rutile, (Ti_0.9Hf_0.1O₂). A sufficient amount of each of these sets of impurity ions was added so that the primary phases of the baseline ceramic were saturated with them and secondary phases formed. Both Pu/U-doped Ce/U-doped samples were made. The impurity elements were added as nitrates to an alkoxide-route precursor, which was calcined and then ball milled. Pellets were pressed from the powder and sintered in Ar, air or 3.7% H₂ in Ar at about 1350 degrees Celsius for 4 hours. The obtained results are summarized in this work. As a matter of fact, most of the ions can be accommodated in the ceramic, but the partitioning across the different phases in the ceramic is not even. The groups will preferentially move to certain phases or, if sufficient amounts are present, result in the formation of new phases. The conclusion is that all ions of similar size and valency behave in a similar manner unless there are volatility problems

[en] Niobium is usually considered to be one of the least mobile elements during alteration of rocks in geological processes and is thought to only be mobile at elevated temperatures and/or pressures. Since niobium is assumed to be unaffected by weathering it can be used in geochemical-evolution calculations to estimate the loss of material from the weathering of rocks. There are roughly a hundred niobium-bearing minerals, including those in the pyrochlore group, which comprises the main ores for Nb and Ta. Furthermore, pyrochlores are identified as potential hosts for nuclear waste because of their resistance to weathering. Here we show that niobium can be highly mobile even at near-surface geochemical conditions as a soluble and unreactive polyoxometalate ion. Discovery of the first natural hexaniobate minerals, all formed at near-surface conditions, provides evidence that niobium is being actively leached out of nearby primary minerals and easily transported as polyoxometalate ions. In addition, we show how Nb and Ta can be fractionated during low-temperature processes. (author)

[en] It has recently been suggested that long-range magnetic dipolar interactions are responsible for spin ice behavior in the Ising pyrochlore magnets Dy₂Ti ₂O₇ and Ho₂Ti ₂O₇ . We report here numerical results on the low temperature properties of the dipolar spin ice model, obtained via a new loop algorithm which greatly improves the dynamics at low temperature. We recover the previously reported missing entropy in this model, and find a first order transition to a long-range ordered phase with zero total magnetization at very low temperature. We discuss the relevance of these results to Dy₂Ti ₂O₇ and Ho₂Ti ₂O₇

[en] Neutron scattering measurements have been performed on polycrystalline Tb₂Sn₂O₇ at temperatures above and below that of the phase transition, T_N = 0.87 K, to investigate further the spin dynamics in the magnetically ordered state. In particular, new neutron spin echo results are presented showing a dependence on Q in the dynamics. We show evidence of the coexistence of static ferromagnetism and dynamically fluctuating spins down to 30 mK and we make a comparison of this partially ordered system to the spin liquid Tb₂Sn₂O₇.

[en] Neutron powder diraction and inelastic measurements were performed examining the 5d pyrochlore Y₂Ir₂O₇. Temperature dependent measurements were performed between 3.4 K and 290 K, spanning the magnetic transition at 155 K. No sign of any structural or disorder induced phase transition were observed over the entire temperature range. In addition, no sign of magnetic long-range order was observed to within the sensitivity of the instrumentation. These measurements do not rule out long range magnetic order, but the neutron powder diraction structural renements do put an upper bound for the ordered iridium moment of ∼0.2 (micro)_B/Ir (for a magnetic structure with wave vector Q ≠ 0) or 0.5 (micro)_B/Ir (for Q = 0).

[en] It is shown in a new class of pyrochlore compound YMn_2Zn_2_0_-_xIn_x (with x = 2.36) that the spin fluctuation rate (v) is proportional to temperature (v ∝ T) below T_0 ∼10 K where heavy-fermion(HF)-like behavior is observed. Such a linear T dependence, commonly found in Y(Sc)Mn_2 and LiV_2O_4, is expected for intersecting quasi-1D Hubbard chains, suggesting that the quasi-1D character of the t_2_g band associated with the pyrochlore lattice plays an important role in the emergence of the HF-like state, where T_0 may correspond to intra-chain antiferromagnetic interaction energy between Mn spins