Hydrothermal Crystal Growth of Rare Earth Tin Cubic Pyrochlores, RE₂Sn₂O₇ (RE = La-Lu): Site Ordered, Low Defect Single Crystals

A hydrothermal route to single crystals of rare earth stannates RE₂Sn₂O₇ (RE = La-Lu) in the cubic pyrochlore structure is reported. Growth reactions were performed in aqueous fluids at 700 °C and 200 MPa with CsF mineralizers in concentrations ranging from 0 to 30 M, with 20 M CsF providing the most consistent results. Single crystals of the entire range of lanthanides were grown and characterized by single crystal X-ray diffraction and found to be isostructural in the Fd3 m space group. The unit cell sizes range from 10.7106(16) Å for La₂Sn₂O₇ to 10.3005(9) Å for Lu₂Sn₂O₇. Both the unit cell size and RE-O distances are found to be essentially linear with respect to the ionic radius of the rare earth ion. The high quality diffraction data strongly suggests that there is very little site disorder or lattice defects in the sample. Of particular interest is the synthesis and single crystal growth of Ce₂Sn₂O₇, which represents one of the few f¹ pyrochlore samples. Specific heat measurements were obtained down to 50 mK on both Yb₂Sn₂O₇ and Ce₂Sn₂O₇, with the Yb material displaying a single sharp peak at 138 mK suggesting that the sample does not contain any site disorder or "lattice stuffing". The Ce analog shows only a broad featureless transition below 100 mK, with this lack of long-range magnetic order consistent with quantum spin liquid behavior.