BaBiO₃ is a well-ordered crystal and would be expected to conduct heat well. However, BaBiO₃ is a poor thermal conductor, much like an amorphous material such as glass. Using the powerful SXD instrument at ISIS the team could confirm the high quality of their BaBiO₃ samples, which helped to determine the mechanism underpinning the low thermal conductivity. The study was the first experiment conducted on SXD after its upgrade.
The team consisted of researchers from the Institute of Physics at the University of São Paulo, Brazil, and the Federal University of Minas Gerais, working with ISIS scientists Gøran Nilsen and Matthias Gutmann, and researchers from the Max Planck Institute, Germany.
This publication is the first to come out of the Brazilian use of ISIS funded through the International Science Partnerships Fund (ISPF) programme. ISPF supports international research partnerships in strategic priority areas. The research was led by Professor Valentina Martelli.
The following article was originally published by the Institute of Physics, University of São Paulo, Brazil:
Researchers from Institute of Physics, University of São Paulo (IFUSP) and the Federal University of Minas Gerais (UFMG) discovered that barium bismuth crystal (BaBiO₃) has thermal conductivity as low as that of glass, despite its ordered structure. The research, published in the journal Advanced Science, sheds light on novel mechanisms that could revolutionize the use of functional oxides in thermal insulation technologies and thermoelectric devices.
Materials do not always behave as we expect. A good example of this is barium bismuthate, with the chemical formula BaBiO3. Despite being a well-ordered crystal (with different structural phases), it conducts heat as poorly as a glass, which are amorphous materials. This recent discovery has just been published in the journal Advanced Science.
Barium bismuthate has been known of since the 1960s, but it had not been explored from this angle, as previous research has focused on applications such as superconductivity, photocatalysis, and others. The work utilised the expertise in thermal transport of the group led by researcher Valentina Martelli (IFUSP) to investigate the material from a new perspective, in the context of the doctoral research project of the student Alexandre Henriques (IFUSP). The theoretical elements of the research were led by researcher Walber Hugo Brito (UFMG), who used state-of-the-art computational techniques to calculate the thermal conductivity of the material.
In crystals, heat conduction is governed primarily by two types of heat carriers: electrons and phonons. The latter can be considered to be quantizations of the vibrations of the crystal lattice. While electrons play a very important role in the transport of heat in metallic materials, phonons play a central role in insulators and semiconductors, as in the case of BaBiO3. The efficiency with which these carriers transfer thermal energy depends on several factors, such as the crystal structure, the presence of defects, and the scattering mechanisms. As an example, we can mention diamond, which is a poor electrical conductor, but a great thermal conductor. Glass, on the other hand, which has a disordered structure, hinders the movement of heat carriers, resulting in low thermal conductivity.
What is striking about BaBiO3 is that, despite having its atoms organized in a crystalline pattern, it has an extremely low thermal conductivity, comparable to that of glassy (amorphous) materials. This indicates that there is something unusual going on in its internal structure. According to the researchers, the material has a combination of unusual mechanisms, such as dynamic instabilities, which make it difficult to transport the phonons: they are spread out too much and are transmitted only by a very short path within the sample, as if the material were glass.
To reach this conclusion, the scientists combined thermal conductivity experiments with computer simulations. Of particular note is the fact that it is a Brazilian theoretical-experimental collaboration, with the use of sophisticated experimental methods and significant theoretical effort, involving advanced calculations and high computational power.
The experiments were conducted at IFUSP, combining two experimental methods: the first called the stationary method, based on a stable heat flow through the material, and the second, called 3ω, which is an advanced methodology based on the use of an oscillatory thermal excitation. The combination of methodologies allowed the team to measure heat conduction in a considerable range of temperatures: from -271°C to 122°C.
The simulations, carried out on Brazilian supercomputers such as Santos Dumont (LNCC), Ada Lovelace (CENAPAD-SP) and Coaraci (Unicamp), made it possible to study the behaviour of phonons in BaBiO3 in detail, with the support of tools such as Density Functional Theory (DFT) and machine learning.
The discovery contributes to the search for new functional materials with characteristics that can be controlled for various applications. Functional oxide materials, in particular, are compounds that have physical or chemical properties that are still poorly understood, and with great potential in this context.
From a fundamental point of view, BaBiO3 is an oxide with emergent physical properties that have not yet been fully explained. In addition to having a poorly understood insulating phase, the compound has a superconducting phase (which conducts electricity without resistive loss) when doped with Pb/K; this phase is characterized by a critical temperature of approximately 32K and classified as unconventional. Additionally, BaBiO3 has been considered a candidate for topological electronic phases.
In the sense of technological applications, the discovery of a behaviour of BaBiO3 similar to glass is very attractive because it indicates that this material can be used, for example, as a thermal insulator in devices that require low thermal dissipation or as a platform (after appropriate electronic doping) for the development of new thermoelectric devices.
These unusual thermal properties of BaBiO3 place it as a candidate for the design of functional heterostructures, which arise from the combination of two or more different materials, usually in thin and alternating layers, which have optimised or novel properties compared to the separate materials.
The study is part of a theoretical-experimental collaboration coordinated by Professor Valentina Martelli (IFUSP). The experiments were carried out by her and doctoral student Alexandre Henriques (IFUSP) and postdoctoral student Mariana Lima (IFUSP). The computer simulations and theoretical calculations run by Professor Walber Brito (DF-UFMG). The group also included researchers Dr. Nilsen and Dr. Gutmann from the ISIS Neutron and Muon Source - UK, and Dr. Steffen Wirth from the Max Planck Institute in Germany. We are grateful for the financial support of FAPESP and the Serrapilheira Institute. V. Martelli acknowledges dupport from J. Larrea Jiménez for experiments in the Laboratory for Quantum Matter under Extreme Conditions.
Read the full paper at: Anomalous Glassy Thermal Conductivity in a Perovskite Bismuthate Induced by Structural Dynamic Instability - Henriques - Advanced Science - Wiley Online Library
