Observation of all-in type tetrahedral displacements in nonmagnetic pyrochlore niobates

We observed all-in type Nb tetrahedral displacement in nonmagnetic pyrochlore niobates A2Nb2O7(A=Nd0.5Ca0.5 and Y0.5Ca0.5) through the analysis of the neutron pair distribution function and the extended x-ray absorption function spectroscopy. The all-in type Nb tetrahedral displacement, which has the character of a charge singlet state, is driven by the formation of the bonding orbital. The diffuse scattering in the x-ray diffraction, which has the resonant component in the Nb L3 edge, indicates that the all-in type Nb tetrahedral displacement has the periodicity with its short-range correlation.