Structure and stability of Cd2Nb2O7 and Cd2Ta2O7 explored by ab initio calculations

Structural instabilities of Cd2Nb2O7 have been explored by all-electron ab initio calculations in the framework of density-functional theory. The calculated phonon-dispersion curve of the cubic high-temperature phase displays several modes with imaginary frequencies. A distortion according to a T-1u mode at the Gamma point leads to the energetically most favorable configuration. The resulting structure has been optimized in space-group symmetries Ima2 and I1a1 (Cc). The stability of these structures suggests a sequence of phase transitions Fd (3) over barm-Ima2-Cc. The dominant structural changes occur as a result of the ferroelectric transition to orthorhombic symmetry, whereas the distortions associated with the subsequent transition to monoclinic symmetry are relatively small. With respect to the cubic phase, Nb is displaced most strongly. In both low-temperature phases, two crystallographically distinct Nb and Cd positions exist. The Nb displacements give rise to an ordered arrangement of short and long Nb-O bonds. In the Ima2 phase, chains of alternating long and short bonds propagate along [001] and along < 111 >, while corresponding chains parallel to [100] still consist of symmetric Nb1-O bonds. The latter bonds distort only with the transition to the Cc phase. In contrast to Cd2Nb2O7, the structure of Cd2Ta2O7 returns to cubic symmetry when subjected to similar distortions.