Structural changes of (K,Gd)2Ta2O7 pyrochlore at high pressure

The structure of K-bearing tantalate pyrochlore (K2-xGdx)Ta2O6+x(x∼0.4) was studied at high pressures using in situ X-ray diffraction and Raman scattering methods. Experimental results indicated that (K2-xGdx)Ta2O6+x(x∼0.4) retains the pyrochlore structure up to 40 GPa, but partial amorphization occurred at pressures above 23 GPa. The amorphous phase was also confirmed in the quenched sample by means of transmission electron microscopy. The tantalate pyrochlore lattice is more stable than pyrochlore compounds in other systems, such as rare earth titanates, zirconates and stannates. The structural stability of pyrochlore tantalate may be mainly related to the size ratio of cations on the 16d and 16c sites in the lattice. - Graphical Abstract: In the pyrochlore tantalate, K and Gd co-occupy the 16d site and Ta is on the 16c site and forming octahedra with nearby anions. The pyrochlore tantalate is quite resistant to high pressure even when the structure is partially amorphized. Highlights: → In situ XRD measurements revealed that (KGd)2Ta2O7 pyrochlore is much stable than other pyrochlore oxides at high pressures. → Pyrochlore tantalate can be partially amorphized at high pressures. → The enhanced phase stability of tantalate pyrochlore is due to the large size difference of cations.