Broadband critical dynamics in disordered lead-based perovskites**Article In Memoriam of Professor Roger A Cowley.

C Stock,M Songvilay,B Roessli,Guangyong Xu,P M Gehring

doi:10.1088/1361-648x/ab86ee

Abstract

Materials based on the cubic perovskite unit cell continue to provide the basis for technologically important materials with two notable recent examples being lead-based relaxor piezoelectrics and lead-based organic–inorganic halide photovoltaics. These materials carry considerable disorder, arising from site substitution in relaxors and molecular vibrations in the organic–inorganics, yet much of our understanding of these systems derives from the initial classic work of Prof. Roger A Cowley, who applied both theory and neutron scattering methods while at Chalk River Laboratories to the study of lattice vibrations in SrTiO3. Neutron scattering continues to play a vital role in characterizing lattice vibrations in perovskites owing to the simple cross section and the wide range of energy resolutions achievable with current neutron instrumentation. We discuss the dynamics that drive the phase transitions in the relaxors and organic–inorganic lead-halides in terms of neutron scattering and compare them to those in phase transitions associated with a ‘central peak’ and also a soft mode. We review some of the past experimental work on these materials and present new data from high-resolution time-of-flight backscattering spectroscopy taken on organic–inorganic perovskites. We will show that the structural transitions in disordered lead-based perovskites are driven by a broad frequency band of excitations.

Highlights

The perovskite crystal structure forms the backbone of many materials that have inspired and challenged theories in magnetism, superconductivity, and structural phase transitions
The dynamics governing this antiferrodistortive transition are outlined in gure 2, which shows a soft mode that varies with temperature as expected according to Landau theory and condenses into a new Bragg peak at the structural transition
One possibility is that the molecular motions re ect the critical dynamics that soften near the structural transition in the organic–inorganic perovskites

Summary

Introduction

The perovskite crystal structure forms the backbone of many materials that have inspired and challenged theories in magnetism, superconductivity, and structural phase transitions. The lead-based relaxor ferroelectrics have generated great interest within the materials science and condensed matter physics communities owing to their exceptional dielectric properties and ultra-high piezoelectric responses [3, 4] Many of these properties are enhanced in single crystal form compared to ceramics. We will show using high resolution backscattering results that the low-energy dynamics critical to the structural transitions in MAPbCl3 do not stem from a single soft mode phonon, as in SrTiO3, but rather from a broad continuum of low-energy frequencies These compounds present new theoretical and experimental challenges to efforts to understand how the dynamics and structural properties are connected on a microscopic level. We will discuss recent ideas that relate the low-energy lattice vibrations to the exceptional semiconducting properties in these materials

SrTiO3—the soft optical phonon and the central peak

Lead-based relaxors—the soft mode and ‘quasielastic’ scattering

Organic–inorganic photovoltaic perovskites and critical dynamics

Discussion and conclusions