Frustrated Octahedral Tilting Distortion in the Incommensurately Modulated Li_(3x)Nd_(2/3_x)TiO3 Perovskites

摘要

Perovskite-structured titanates with layered A-site ordering form remarkably complex superstructures. Using transmission electron microscopy, synchrotron X-ray and neutron powder diffraction, and ab initio structure relaxation, we present the structural solution of the incommensurately modulated Li_(3x)Nd_(2/3-x)TiO3 perovskites (x - 0.05, superspace group Pmmm(α1,1/2,0)000(1/2,β2 0)000, a = 3.831048(5) A,b = 3.827977(4) A, c = 7.724356(8) A, q1 = 0.4513l(8)a~* + 1/2b~*, q2 = 1/2a~* + 0.41923(4)b~*). In contrast to earlier conjectures on the nanoscale compositional phase separation in these materials, all peculiarities of the superstructure can be understood in terms of displacive modulations related to an intricate octahedral tilting pattern. It involves fragmenting the pattern of the out-of-phase tilted TiO6 octahedra around the a- and b-axes into antiphase domains, superimposed on the pattern of domains with either pronounced or suppressed in-phase tilt component around the c-axis. The octahedral tilting competes with the second order Jahn—Teller distortion of the TiO6 octahedra. This competition is considered as the primary driving force for the modulated structure. The A cations are suspected to play a role in this modulation affecting it mainly through the tolerance factor and the size variance. The reported crystal structure calls for a revision of the structure models proposed for the family of layered A-site ordered perovskites exhibiting a similar type of modulated structure.