  Density Functional Theory calculations have been performed to obtain lattice
parameters, elastic constants, and electronic properties of ideal pyrochlores
with the composition A$_2$B$_2$O$_7$ (where A=La,Y and B=Ti,Sn,Hf, Zr). Some
thermal properties are also inferred from the elastic properties. A decrease of
the sound velocity (and thus, of the Debye temperature) with the atomic mass of
the B ion is observed. Static and dynamical atomic charges are obtained to
quantify the degree of covalency/ionicity. A large anomalous contribution to
the dynamical charge is observed for Hf, Zr, and specially for Ti. It is
attributed to the hybridization between occupied $2p$ states of oxygen and
unoccupied d states of the B cation. The analysis based on Mulliken population
and deformation charge integrated in the Voronoi polyhedra indicates that the
ionicity of these pyrochlores increases in the order Sn--Ti--Hf--Zr. The charge
deformation contour plots support this assignment.
