First Principles Study On Thermal Transport And Thermal Expansion Properties Of Perovskite-type Materials

Perovskite materials,as important functional materials,exhibit a rich array of physical and chemical properties,such as ferroelectricity,optoelectronic properties,piezoelectric effects,magnetism,and high dielectric constants,making them highly valuable in numerous key domains.Based on their chemical composition,perovskite materials can be broadly categorized into oxide perovskites,halide perovskites,and perovskite-related materials.Among these,halide perovskites,as novel optoelectronic semiconductor materials,offer advantages such as tunable bandgaps,high quantum efficiencies,strong photoluminescence,and extended carrier lifetimes.They are also easy to fabricate and cost-effective,presenting vast application prospects in fields such as photovoltaics,lighting/displays,and thermoelectrics.At present,the photoelectric conversion efficiency of halide perovskite solar cells（PSCs）has rapidly increased from3.8%to 25.7%.However,due to the intrinsic low thermal stability of perovskite,the service life of its devices cannot be compared with that of traditional silicon-based solar cells,which is one of the obstacles to the commercialization of perovskite devices.Therefore,many optimization strategies have been proposed to improve device stability,such as the development of polymer/glass closed packaging process,which can effectively inhibit the decomposition of perovskite solar cells to improve stability.Although these optimization processes have effectively improved the stability of perovskite devices,the significant thermal expansion characteristics of halide perovskite due to weak ion bonding still pose a serious challenge to the long-term stability of the devices.In addition,understanding and regulating other thermal properties of perovskite materials（such as thermal transport properties）is also important for optimizing device performance,and helps to develop thermal management techniques and strategies for perovskite materials,and promote the development of related optoelectronic devices.Therefore,the study of thermal properties and regulation rules of perovskite materials has important scientific value,and is one of the key scientific problems in this field.In this study,based on first-principles lattice dynamics and combined with Boltzmann transport theory,we explore the physical mechanisms underlying the thermal properties of perovskite-type materials and conduct the following research:1.The thermal expansion properties and chemical trends of inorganic halide perovskite are revealed,and an effective descriptor for thermal expansion coefficient is proposed.We conducted a comprehensive and in-depth investigation of the thermal expansion properties of halide perovskites at 300 K using first-principles calculations.By comparing them to oxides as references,we found that the large thermal expansion in halide perovskites is primarily attributed to the low bulk modulus and volume heat capacity of their soft lattice.When oxides and halides share the same structural type,composition-dependent anharmonicity was identified as the most significant factor determining the magnitude of thermal expansion.We discovered that weakening the B-X bond strength promotes octahedral anharmonicity,leading to a reduction in the thermal expansion of halide perovskites.Subsequently,we proposed an effective descriptor for the thermal expansion coefficient of halide perovskites,with a Pearson correlation coefficient close to-80%.Our findings provide insights into the potential mechanisms and chemical trends of thermal expansion in halide perovskites.2.Alloying strategy is proposed to reduce the lattice thermal conductivity of Ruddlesden-Popper（RP）phase halide perovskite,and the anisotropy of the perovskite can be controlled in a wide range after alloying.We have employed first-principles lattice dynamics calculations combined with the Boltzmann transport theory to investigate the thermal transport properties of two-dimensional Ruddlesden-Popper（RP）phase halide perovskites through alloying at the B and X sites.We have achieved a significant modulation of lattice thermal conductivity,ranging from the lowest value（_c=0.05 W/m K@Cs₄Ag Bi I₈）to the highest value(_a/b=0.95 W/m K@Cs₄Na Bi Cl₄I₄),with an anisotropy ratio ranging from 1.22 to 4.13.In comparison to pure RP phase halide perovskites and three-dimensional halide perovskite alloys,the two-dimensional halide perovskites introduce more phonon branches and enhance phonon-phonon coupling through alloying,resulting in effective phonon scattering and reduced thermal conductivity.Further analysis of the phonon transport modes in these structures reveals that lower phonon velocities and shorter phonon lifetimes are the main contributors to the low thermal conductivity.This study provides in-depth insights into the intriguing phonon transport phenomena in these compounds and offers strong theoretical guidance.3.The physical source of the thermal transport properties of perovskite-related Re O3 structures is revealed,and the thermal conductivity can be regulated by changing the chemical composition.We have systematically investigated the thermal transport properties of six perovskite-related Re O₃-type structures related to perovskites using first-principles lattice dynamics calculations combined with the Boltzmann transport equation.We obtained these materials with not only low lattice thermal conductivity but also achieved a wide range of values,ranging from the lowest（Cd Zr F₆@4.15 W/m K）to the highest（Fe Zr F₆@16.3 W/m K）lattice thermal conductivity.This indicates that lattice thermal conductivity can be significantly controlled by varying the chemical composition,making them suitable for different application requirements.Furthermore,we observed a certain regularity between the anharmonicity of the structures and their lattice thermal conductivity,where larger anharmonicity corresponds to lower lattice thermal conductivity.Additionally,we conducted an in-depth analysis of the phonon transport properties in these materials and identified the phonon velocity and phonon lifetime as the main factors contributing to the significant variation in lattice thermal conductivity.This study provides theoretical insights into the unique thermal transport characteristics of perovskite-related Re O₃-type structures.4.The thermal expansion behavior of perovskite-related Re O3-type structures was predicted by machine learning,and two materials with negative expansion characteristics were obtained.We construct a set of descriptors consisting of connectivity and element information to describe the thermal expansion properties of perovskite-related Re O₃-type structures.Based on the descriptors we developed,combined with the machine learning algorithm,we can effectively learn the thermal expansion behavior of the collected data set.The model we trained achieved a prediction accuracy of up to 92%,highlighting the effectiveness of descriptor sets in accurately predicting thermal expansion behavior.In addition,based on the trained machine learning model,we successfully predicted two perovskite structurally related Re O₃-type materials（VF₃ and Zr F₃）that are kinetically stable and have negative expansion properties.This study provides a strong support for the development and application of the thermal expansion of perovskite materials.