First Principles Studies of the Phonon, Polarization, Dielectric and Piezoelectric Responses of Pyrochlore Cd₂Nb₂O₇

Abstract

First-principles calculations were used to investigate the lattice dynamics, spontaneous polarization, dielectric and piezoelectric responses of the Cd₂Nb₂O₇ compound. Starting from the highest cubic phase, we obtained tetragonal I4₁md and orthogonal Ima2 phase by freezing in the unstable polar phonon of the structure. By further freezing in unstable phonon of Ima2 phase, we get its ground state monoclinic Cc symmetry. Structural properties of the calculated results are in good agreement with the experimental. The spontaneous polarization of I4₁md, Ima2 and Cc are 4.91, 17.82, 56.95 μC/cm², respectively. We explore the lattice contributions coming from individual atoms and individual phonons. For the Cc structure, large magnitudes of dielectric and piezoelectric response are come from ϵ₂₂ and e₂₆, respectively. They are mainly due to the low frequency A″ phonons with ω = 47.8 and 71.5 cm⁻¹. Detailed analysis shows that, the structure of Nb-O octahedra provides extraordinary large contribution.

Keywords:

• Cd₂Nb₂O₇
• lattice contributions
• spontaneous polarization
• dielectric
• piezoelectric

Funding

This work was supported by the National Science Foundation of China (Grant No. 51162019 and 51462019).