The observation of canonical spin-glass behavior in the pyrochlore oxide Y 2 Mo 2 O 7 has been a subject of considerable interest as the original structural studies were interpreted in terms of a well-ordered crystallographic model. It is widely held that the stabilization of the spin-glass state requires some level of positional disorder along with frustration. Recent reports from local probe measurements, extended x-ray-absorption fine structure ͑EXAFS͒ and 89 Y NMR, have been interpreted in terms of disorder involving the Mo-Mo distances ͑EXAFS͒ and multiple Y sites ͑NMR͒. This work reports results from temperature-dependent ͑15-300 K͒ neutron diffraction ͑ND͒ and neutron pair distribution function studies which can provide from the same data set information on both the average and local structures. The principal findings are that: ͑1͒ there is no crystallographic phase transition over the temperature region studied within the resolution of the ND data; ͑2͒ the diffraction data are well fitted using a fully ordered model but with large and anisotropic displacement parameters for three of the four atomic sites; ͑3͒ the pairwise real-space correlation function G͑r͒ shows clear evidence that the principal source of disorder is associated with the Y-O1 atom pairs rather than the Mo-Mo pairs, in disagreement with the interpretation of the EXAFS results; ͑4͒ fits to the G͑r͒ improve significantly when anisotropic displacements for all sites are included; ͑5͒ inclusion of a split-site position parameter for O1 improves, slightly, both the G͑r͒ fits and the Rietveld fits to the ND data; and ͑6͒ for all models the fits become worse as the temperature decreases and as the fitting range decreases. These results are qualitatively consistent with the 89 Y NMR observations and perhaps recent muon-spin-relaxation studies. The issue of static versus dynamic disorder is not resolved, definitively. An estimate of the distribution of exchange constants due to the disorder is made using spin-dimer analysis and compared with the Saunders-Chalker model for the generation of spin-glass behavior from "weak" disorder on geometrically frustrated lattices.

A novel Y 3(1Àx) Er 3x Ga 5 O 12 nanocrystalline garnet has been synthesized by a sol-gel technique and a complete structural, morphological, vibrational, and optical characterization has been carried out in order to correlate the local structure of the Er 3+ ions with their optical properties. The synthesized nanocrystals are found in a single-phase garnet structure with an average grain size of around 60 nm. The good crystalline quality of the garnet structure is confirmed by FTIR and Raman measurements, since the phonon modes of the nano-garnet are similar to those found in the single crystal garnet. Under blue laser excitation, intense green and red visible and 1.5 mm infrared luminescences are observed, whose relative intensities are very sensitive to the Er 3+ concentration. The dynamics of these emissions under pulsed laser excitations are analyzed in the framework of different energy transfer interactions. Intense visible upconverted luminescence can be clearly observed by the naked eye for all synthesized Er 3+ -doped Y 3 Ga 5 O 12 nano-garnets under a cw 790 nm laser excitation. The power dependency and the dynamics of the upconverted luminescence confirm the existence of different two-photon upconversion processes for the green and red emissions that strongly depend on the Er 3+ concentration, showing the potential of these nano-garnets as excellent candidates for developing new optical devices. A IntroductionNowadays, rare earth (RE 3+ )-doped nanocrystals attract great attention due to their size, shape, and phase-dependent structural and luminescence properties, which make them suitable for fundamental and technological applications.1,2 On the other hand, the favorable physical and chemical properties of the oxide garnet crystals, such as high transparency from the UV to the mid-IR, high thermal conductivity, hardness, good chemical stability, and relatively low-energy phonons, make them one of the most important families of host matrices for the RE 3+ ions with interesting luminescence properties already used in lasers and phosphors. ions without charge compensation. 8,9From the point of view of the potential optical applications of the RE 3+ ions, one of the most interesting phenomena is their capacity to convert the infrared absorbed radiation into visible emitting light, known as energy upconversion.10 The particular selection of one or various RE 3+ ions and their concentrations allows controlling the upconverted visible luminescence to match a specific coordinate of colour, or even the generation of white light as a combination of red, green and blue (RGB) emissions. Thus there is an increasing demand for upconversion materials with important applications in upconversion lasers, due to the availability of powerful near-infrared commercial laser diodes, IR detection by conversion to visible light, where detectors are more efficient, and biological fluorescence labels and imaging or 3-D displays. 1,2,[11][12][13][14][15][16] When the RE 3+ ions are incorporated into the nanocrystals their upcon...

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