Thermal expansion is a fundamental physical property of solids, originating from anharmonic phonon effects, which profoundly influences various important properties of solids, such as phase transition processes and bandgaps. For conventional semiconductors bonded by covalent or ionic bonds, their degree of anisotropic thermal expansion is generally weak, with the relative difference in linear expansion coefficients along two crystallographic directions typically being less than 1. Achieving highly anisotropic thermal strain in solids and clarifying the underlying mechanism remain a fundamental challenge.
Figure. Phase evolution and lattice dynamics.(a, b) Temperature-dependent linear thermal expansion coefficients for (a) phase Ia and (b) phase Iβ crystals, demonstrating their cascaded phase evolution across the temperature range of 120–560 K. Key lattice planes belonging to intermediate metastable phases are numerically labeled. Shaded blue and red regions indicate periods of pronounced lattice contraction and expansion, respectively. (c) Schematic illustration of the cascaded structural evolution. (d) Comparison of anisotropic thermal expansion between the studied hierarchical crystals and typical conventional semiconductors. The relative difference in the linear expansion coefficient (a) for the hierarchical crystals is calculated between their most expansive (e.g., (006)/(103)) and least expansive (e.g., (110)/(10
)) lattice planes at representative temperatures.
The research group of Professor Jiyang Fan from the School of Physics at Southeast University, combining experiments and quantum-mechanics-based theoretical calculations, has reported a rare thermal expansion anisotropy in a class of hierarchical hybrid semiconductors: in these materials, giant positive thermal expansion and significant negative thermal expansion coexist simultaneously along different crystallographic axes, resulting in an anisotropic thermal expansion metric as high as 5.6. The study reveals that this extreme anisotropy originates from strong anharmonic phonon dynamics with crystallographic-orientation dependence, driven by symmetry-breaking local microscopic interactions dominated by hydrogen bonds and van der Waals forces, as well as global electronic flat bands that promote spatially selective localization of excited-state electrons. Unlike purely covalent or purely ionic interactions, these complex microscopic forces significantly amplify the thermal anisotropy in the hybrid semiconductor framework. Over a broad temperature range from 120 to 560 K, the accumulated anisotropic strain induces a breathing-mode lattice response and triggers cascading structural phase transitions. These results unveil the microscopic mechanism of giant thermal anisotropy in hierarchical hybrid solids, and also provide insights for the design of thermal strain materials in high-precision microelectromechanical systems, tunable optoelectronic devices, and strain-adaptive functional devices.
Recently, the above results were published in Applied Physics Reviews, a journal of the American Institute of Physics, under the title "Exceptionally anisotropic thermal expansion in flat-band zero-dimensional hybrid perovskites". The authors of the paper include doctoral students Huaxin Wu, Wenjie Liu, and Qin Ling from the School of Physics at Southeast University, along with Professor Jiyang Fan from the Key Laboratory of Quantum Materials and Devices of Ministry of Education and School of Physics at Southeast University serving as the corresponding author. This work was supported by the National Natural Science Foundation of China.
Link of this article: https://doi.org/10.1063/5.0323338
