Title: Large tetragonality and room temperature ferroelectricity in compressively strained CaTiO₃ thin films

Abstract

We report that ferroelectricity and piezoelectricity are desirable for a variety of high-temperature applications such as actuators and sensors in heat engines, high-temperature manufacturing, and space technologies; however, the number of such material candidates is currently limited. Here, we demonstrate that CaTiO₃, the first discovered perovskite mineral that is abundantly found in the Earth, as a non-polar bulk form, becomes a high-temperature ferroelectric oxide under compression. A strain-phase-temperature diagram of CaTiO₃ is created by growing a series of thin films with a range of compressive and tensile strains. Using temperature dependent optical second harmonic generation analysis, we show that tensile strained films exhibit predominantly in-plane polarization with orthorhombic-like point group symmetry with a phase transition below room temperature. On the other hand, compressively strained CaTiO₃ films exhibit a neartetragonal unit cell with a c/a ratio of 1.03, larger than that of classic ferroelectric, e.g. BaTiO₃ (c/a~1.01). These films exhibit a robust and switchable out-of-plane polarization at room temperature, with a ferroelectric transition temperature up to ~800 K. Density functional theory calculations reveal that compressive strain gives rise to a large out-of-plane displacement of Ti-cations inside the TiO₆ octahedral cages responsible for the strain induced polarization of ~ 9μC/cm². Given thatmore » nearly half of the perovskites exhibit the bulk symmetry of CaTiO₃, compressive strain tuning of this family may prove to be a fertile ground for the discovery of strain-induced piezoelectrics and ferroelectrics at high-temperatures. High-temperature piezoelectric sensors are important for many industries such as automotive, aircraft, and aerospace, as well as nuclear and geothermal energy, where dynamic structural monitoring of high-speed rotors in engines, turbines, and power generators is critical for safety and reliability.[1, 2] Modern sensors are however limited, where the main challenges are maximizing operating temperatures while maintaining good sensing capability and low electrical leakage.[3] High-temperature piezoelectric materials are typically derived from BaTiO₃ or PbTiO₃ based ceramics and single crystals, as these materials are also ferroelectric, meaning that they have a built-in spontaneous polarization (Ps). In bulk, these standard ferroelectric perovskite materials yield maximum operating temperatures ranging from 250-450 °C.[4], where above these temperatures the spontaneous polarization (P) and piezoelectric properties disappear due to the onset of a ferroelectric-to-paraelectric phase transition (TC). As next-generation engines, turbines, and reactors require hotter operating temperatures above 500 °C, materials with enhanced ferroelectric properties and increased ferroelectric transition temperatures will be needed in such extreme conditions. A promising way of boosting ferroelectric transition temperatures and polarization is through epitaxial strain engineering,[5] where tremendous in-plane distortions can be applied to thin film crystal materials, well beyond the point at which their bulk ceramic and single crystal counterparts would crack. Strain engineering has proven to drastically enhance ferroelectric polarizations and transition temperatures in classic ferroelectric materials such as BaTiO₃ [6] and PbTiO₃ [7], and strain can even turn on ferroelectricity in otherwise non-polar materials such as SrTiO₃ and CaTiO₃,[8] although reported strain-induced ferroelectric transition temperatures are only at room temperature or below.[9-12] In this work, we will focus on strain-induced ferroelectricity in CaTiO₃, the first discovered perovskite mineral and long known to be a simple nonpolar insulating crystal.« less

Authors:

Haislmaier, Ryan C. ^[1]; Lu, Yanfu ^[1]; Lapano, Jason ^[1]; Zhou, Hua ^[2]; Alem, Nasim ^[1]; Sinnott, Susan B. ^[1]; Engel-Herbert, Roman ^[1]; Gopalan, Venkatraman ^[1]

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+ Show Author Affiliations

1. Pennsylvania State Univ., University Park, PA (United States)
2. Argonne National Lab. (ANL), Lemont, IL (United States)

Publication Date:

Mon May 13 00:00:00 EDT 2019

Research Org.:

Argonne National Lab. (ANL), Argonne, IL (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF)

OSTI Identifier:

1560045

Alternate Identifier(s):

OSTI ID: 1512339

Grant/Contract Number:  

AC02-06CH11357

Resource Type:

Accepted Manuscript

Journal Name:

APL Materials

Additional Journal Information:

Journal Volume: 7; Journal Issue: 5; Journal ID: ISSN 2166-532X

Publisher:

American Institute of Physics (AIP)

Country of Publication:

United States

Language:

English

Subject:

36 MATERIALS SCIENCE; CaTiO3; Ferroelectric; Hybrid molecular beam epitaxy; Piezoresponse force microscopy; Second harmonic generation; Strain engineering; Thin film