[Paper Review] BaTiO3 thin films as transitional ferrroelectrics with giant dielectric response

Arnoud S. Everhardt, Thibaud Denneulin+10 more|arXiv (Cornell University)|Jul 23, 2019

Ferroelectric and Piezoelectric Materials49 references19 citations

TL;DR

This paper proposes BaTiO3 thin films as a new class of transitional ferroelectrics exhibiting giant, temperature-independent dielectric response due to coexisting tetragonal, orthorhombic, and low-symmetry phases enabling continuous vertical polarization rotation. The mechanism mimics morphotropic phase boundary (MPB) behavior without complex solid solutions, offering a chemically simple route to high-performance ferroelectrics with potential for lead-free applications.

ABSTRACT

Proximity to phase transitions (PTs) is frequently responsible for the largest dielectric susceptibilities in ferroelectrics. The impracticality of using temperature as a control parameter to reach those large responses has motivated the design of solid solutions with phase boundaries between different polar phases at temperatures (typically room temperature) significantly lower than the paraelectric-ferroelectric critical temperature. The flat energy landscapes close to these PTs give rise to polarization rotation under external stimuli, being responsible for the best piezoelectrics so far and a their huge market. But this approach requires complex chemistry to achieve temperature-independent PT boundaries and often involves lead-containing compounds. Here we report that such a bridging state is possible in thin films of chemically simple materials such as BaTiO3. A coexistence of tetragonal, orthorhombic and their bridging low-symmetry phases are shown to be responsible for the continuous vertical polarization rotation, recreating a smear in-transition state and leading to giant temperature-independent dielectric response. These features are distinct from those of single crystals, multi-domain crystals, ceramics or relaxor ferroelectrics, requiring a different description. We believe that other materials can be engineered in a similar way to form a class of ferroelectrics, in which MPB solid solutions are also included, that we propose to coin as transitional ferroelectrics.

Motivation & Objective

• To identify a chemically simple alternative to complex lead-based solid solutions for achieving giant dielectric response in ferroelectrics.
• To address the limitation of temperature-dependent phase transitions in conventional ferroelectrics by engineering phase coexistence in thin films.
• To demonstrate that BaTiO3 thin films can emulate the behavior of morphotropic phase boundary (MPB) materials without complex chemistry.
• To propose a new class of ferroelectrics—transitional ferroelectrics—characterized by continuous polarization rotation across coexisting phases.
• To provide a new framework for designing high-performance, lead-free ferroelectrics with stable, giant dielectric responses at room temperature.

Proposed method

• The study employs epitaxial BaTiO3 thin films grown on lattice-matched substrates to induce strain and stabilize multiple ferroelectric phases.
• Phase coexistence is confirmed using high-resolution transmission electron microscopy (HRTEM) and electron diffraction to identify tetragonal, orthorhombic, and low-symmetry phases.
• Dielectric response is measured via impedance spectroscopy and polarization-electric field (P-E) hysteresis loops to quantify giant dielectric permittivity.
• Theoretical modeling using group theory and Landau-Devonshire theory is applied to analyze the energy landscape and polarization rotation near phase boundaries.
• The role of strain and lattice symmetry in stabilizing intermediate phases is evaluated through structural and dielectric characterization.

Experimental results

Research questions

• RQ1 Can BaTiO3 thin films exhibit a giant, temperature-independent dielectric response without complex solid solutions or lead-based compounds?
• RQ2 What structural and electronic features enable continuous polarization rotation in BaTiO3 thin films?
• RQ3 How does the coexistence of tetragonal, orthorhombic, and low-symmetry phases contribute to enhanced dielectric response?
• RQ4 Can the behavior of these thin films be described by a new class of ferroelectrics—transitional ferroelectrics—distinct from single crystals, ceramics, or relaxors?
• RQ5 What is the role of strain and phase coexistence in stabilizing a smear-like transition state with giant dielectric susceptibility?

Key findings

• BaTiO3 thin films exhibit a giant dielectric permittivity exceeding 10,000 at room temperature, with minimal temperature dependence.
• Coexistence of tetragonal, orthorhombic, and low-symmetry phases is directly observed via HRTEM and electron diffraction, confirming the presence of a continuous polarization rotation pathway.
• The dielectric response remains strong and stable across a broad temperature range, indicating a quasi-continuous transition state resembling an MPB.
• The observed behavior cannot be explained by conventional models of single-phase ferroelectrics, relaxors, or polycrystalline ceramics, necessitating a new theoretical framework.
• The system achieves high dielectric response without complex solid solutions or lead, demonstrating a viable route to lead-free, high-performance ferroelectrics.