# Structural Distortions and Uniaxial Negative Thermal Expansion in the Polar Dion–Jacobson Oxide RbNdTa2O7

## TL;DR

This paper studies the structural and thermal properties of RbNdTa2O7, revealing a unique contraction mechanism during a phase transition.

## Contribution

The paper identifies a uniaxial negative thermal expansion mechanism in RbNdTa2O7 distinct from typical corkscrew mechanisms in similar materials.

## Key findings

- A uniaxial negative thermal expansion occurs during the I2cm → Cmce phase transition in RbNdTa2O7.
- The transition involves polar to antipolar displacement of Nd ions and contraction of the NdTa2O6 layer.
- Neutron diffraction reveals a phase transition at approximately 1150 K from I4/mcm to P4/mmm.

## Abstract

We provide deeper insight into the crystal structures,
sequential
structural phase transitions (I2cm → Cmce → I4/mcm → P4/mmm), thermal expansion, and electronic
properties of the n = 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, I2cm → Cmce, where the unit cell contracts along the c-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 I2cm (hybrid improper
ferroelectric) → Cmce (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 c-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 I4/mcm → P4/mmm occurs at approximately 1150 K.
Finally, we have calculated from DFT + U, the partial
density of states, the energy bandgaps, and effective masses of the
charge carriers of the polar ground structure.