Structural Distortions and Uniaxial Negative Thermal Expansion in the Polar Dion–Jacobson Oxide RbNdTa ₂ O ₇

We provide deeper insight into the crystal structures, sequential structural phase transitions (<i>I2cm</i> → <i>Cmce</i> → <i>I</i>4/<i>mcm</i> → <i>P</i>4/<i>mmm</i>), thermal expansion, and electronic properties of the <i>n</i> = 2 Dion-Jacobson polar oxide RbNdTa₂O₇, through X-ray powder diffraction, neutron powder diffraction, Raman studies, and density functional theory calculations. We observed a uniaxial negative thermal expansion (NTE) across the first-order transition, <i>I2cm</i> → <i>Cmce</i>, where the unit cell contracts along the <i>c-</i>axis, which is driven by a contraction of the NdTa₂O₆ layer. This NTE occurs within the temperature range of the first-order phase transition and contrasts with the corkscrew mechanism typically observed in Ruddlesden-Popper phases. In RbNdTa₂O₇, the <i>I2cm</i> (hybrid improper ferroelectric) → <i>Cmce</i> (antipolar) transition involves crucial changes in the bond lengths of Nd and Ta polyhedra, coupled with polar to antipolar displacement of the Nd ions, leading to a net contraction in the NdTa₂O₆ layer along the <i>c-</i>axis, while preserving the overall octahedral tilting magnitude. This transition highlights the intricate interplay between the Nd and Ta coordination and the associated TaO₆ distortions. Temperature-dependent Raman spectra analysis further confirms the first-order structural transition and associated NTE, providing evidence for increased bond stiffness across this transition. Additionally, using neutron powder diffraction, we have determined that the transition <i>I</i>4/<i>mcm</i> → <i>P</i>4/<i>mmm</i> occurs at approximately 1150 K. Finally, we have calculated from DFT + <i>U</i>, the partial density of states, the energy bandgaps, and effective masses of the charge carriers of the polar ground structure.