New high-pressure form of the negative thermal expansion materials zirconium molybdate and hafnium molybdate

High-pressure X-ray diffraction experiments on cubic ZrMo2O8 have shown tha t the material undergoes a first-order phase transition involving an 11% vo lume decrease between 0.7 and 2.0 GPa under quasi-hydrostatic conditions. T he transition is reversible upon decompression, but shows considerable hyst eresis. Similar behavior was observed for cubic HfMo2O8. Under nonhydrostat ic conditions, the cubic materials start to amorphize above 0.3 GPa and can be converted to the monoclinic polymorphs when heated under pressure. This is the first time that monoclinic HfMo2O8 was observed.