Inverse Melting of Polar Order in Chemically Substituted BaTiO3

TL;DR

This study reveals an unusual inverse melting phenomenon in a chemically disordered ferroelectric oxide, where polar order decreases at low temperatures due to the influence of quenched disorder and competing interactions.

Contribution

It provides direct atomic-scale visualization of inverse melting in a doped ferroelectric, linking the phenomenon to random field effects from chemical disorder.

Findings

Inverse melting of polar order observed in BaTi1-xZrxO3

Disordered state re-emerges at low temperatures due to quenched disorder

Visualization shows competition between thermal fluctuations and random fields

Abstract

In many condensed matter systems, long range order emerges at low temperatures as thermal fluctuations subside. In the presence of competing interactions or quenched disorder, however, some systems can show unusual configurations that become more disordered at low temperature, a rare phenomenon known as "inverse melting". Here, we discover an inverse melting of the polar order in a ferroelectric oxide with quenched chemical disorder (BaTi1-xZrxO3) through direct atomicscale visualization using in situ scanning transmission electron microscopy. In contrast to the clean BaTiO3 parent system in which long range order tracks lower temperatures, we observe in the doped system BaTi1-xZrxO3 that thermally driven fluctuations at high temperature give way to a more ordered state and then to a re-entrant disordered configuration at even lower temperature. Such an inverse melting of the polar order is likely linked to the random field generated by Zr dopants, which modulates the energy landscape arising from the competition between thermal fluctuations and random field pinning potential. These visualizations highlight a rich landscape of order and disorder in materials with quenched disorder, which may be key to understanding their advanced functionalities.