Structural, optical-vibration and magnetic properties of tetragonal lanthanide pyrogermanates obtained by molten-salt synthesis

Abstract

Abstract Lanthanide pyrogermanates of general formula Ln2Ge2O7 (Ln = Tb, Dy, Ho, Er) were synthesized by molten-salt synthesis. Single-phase, impurity-free materials were obtained, which belong to the tetragonal P41212 space group. Highly aggregated powders with good crystallinity were visualized by transmission electron microscopy techniques. Optical-vibration properties of the pyrogermanates were examined by Raman scattering. Lattice vibrations related to lanthanide ions presented upshifts due to decreasing lattice volumes (from Tb to Er), while Raman modes related to Ge-O vibrations remain unchanged or with little shift. Dy2Ge2O7 clearly shows a peak in the dc magnetic susceptibility around 2.5 K under a relatively low applied field (paramagnetic to antiferromagnetic transition), while long-range magnetic ordering for the Tb-, Ho- and Er-containing ceramics could be observed down to 2 K. Curie-Weiss curves showed a higher Weiss temperature compared to literature in the case of Tb2Ge2O7, suggesting a weak long-range order at low temperature, in the case of our sample. For the Er2Ge2O7 compound, the presence of small Curie-Weiss temperatures reinforces the idea that antiferromagnetic and ferromagnetic dipolar exchange interactions between nearest neighbors compete at low temperature in our powders. All these features are coherent with a magnetic frustrated complex ground state such as the ones observed in the spin ices. Isothermal magnetization curves almost overlapped for the Dy, Ho and Tb compounds while it shows a sensitive non-standard behavior at low field in the case of the Er2Ge2O7.