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 Fd3̅m 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.
Large single crystals of Yb 2 Ge 2 O 7 in the cubic Fd3̅ m space group, are synthesized and characterized from a high-temperature hydrothermal method (650°C/200 MPa in 1 M KF). The cubic phase displays spin frustration and possibly nonclassical quantum-spin behavior at low temperature. This is the first report of single crystals of this important phase of size and quality suitable for single-crystal neutron diffraction.
Ising-like spin-1/2 magnetic materials are of interest for their ready connection to theory, particularly in the context of quantum critical behavior. In this work we report detailed studies of the magnetic properties of a member of the rare earth pyrosilicate family, D-Er 2 Si 2 O 7 , which is known to display a highly anisotropic Ising-like g-tensor and effective spin-1/2 magnetic moments. We used powder neutron diffraction, powder inelastic neutron spectroscopy (INS), and single crystal AC susceptibility to characterize its magnetic properties. Neutron diffraction enabled us to determine the magnetic structure below the known transition temperature (T N = 1.9 K) in zero field, confirming that the magnetic state is a four-sublattice antiferromagnetic structure with two non-collinear Ising axes, as was previously hypothesized. Our powder INS data revealed a gapped excitation at zero field, consistent with anisotropic (possibly Ising) exchange. An applied field of 1 T produces a mode softening, which is consistent with a field-induced second order phase transition. To assess the relevance of D-Er 2 Si 2 O 7 to the transverse field Ising model, we performed AC susceptibility measurements on a single crystal with the magnetic field oriented in the direction transverse to the Ising axes. This revealed a transition at 2.65 T at 0.1 K, a field significantly higher than the mode-softening field observed by powder INS, showing that the field-induced phase transitions are highly field-direction dependent as expected. These measurements suggest that D-Er 2 Si 2 O 7 may be a candidate for further exploration related to the transverse field Ising model.
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