The pyrochlore Ho2Ti2O7: Synthesis, crystal growth, and stoichiometry

We have investigated the effect of synthesis and growth conditions on the magnetic, structural, and compositional properties of the pyrochlore oxide holmium titanate and demonstrate a method for growing high quality stoichiometric single crystals. A series of polycrystalline samples with various contents of Ti (−0.08 ≤ x ≤ 0.08, and nominal compositions of Ho2Ti2+xO7) were synthesized at different temperatures, and characterized using powder X-ray diffraction. The results show that synthesizing powders at a higher temperature of 1500 °C yields single phase compounds. Ti deficient powders showed an increase of lattice constant due to stuffing (Ho into Ti positions), while Ti rich powders showed a decrease in lattice constant due to anti-stuffing (Ti into Ho positions). A post-reaction annealing in O2 was found to be necessary to accomplish the anti-stuffing process. Use of the conventional floating zone (FZ) technique introduced Ti deficiency, stuffing, and oxygen vacancies in the grown crystal. Growth of high structural quality and stoichiometric single crystals of Ho2Ti2O7 by the traveling solvent floating zone (TSFZ) method is reported. AC susceptibility measurements revealed that the stoichiometric crystal shows a higher spin ice freezing temperature, indicating that crystal quality and stoichiometry play a key role on low temperature spin ice properties of this compound.