In situ neutron diffraction study of the formation of Ho₂Ge₂O₇ pyrochlore at high temperature and pressure

The formation of the spin-ice pyrochlore Ho₂Ge₂O₇ by two different high temperature, high pressure routes has been explored using in situ neutron diffraction. The first route involves the solid-state reaction of Ho₂O₃ and GeO₂, and formation of the pyrochlore phase is observed at 994(27) °C and 3.81(2) GPa, which are significantly milder conditions than those previously reported. The second route involves the hydrothermal synthesis of the tetragonal Ho₂Ge₂O₇ pyrogermanate from Ho(NO₃)₃·5H₂O and GeO₂ and its subsequent transformation to the pyrochlore phase, which is observed at 683(23) °C and 3.89(3) GPa. The lowering of the formation temperature of high pressure phases by employment of a precursor of appropriate stoichiometry may also have applications in the wider field of solid-state chemistry.