Spatial Modulation and Thermal-Induced Spin Phase Transition on the Negative Thermal Expansion of ScF₃ with Metal Dopants

Abstract

Negative thermal expansion (NTE) is an intriguing physical property of solids, which is often related to the lattice, phonons, charges, and spin. However, most of the studies are mainly focused on the phonons, and the spin-related mechanism in open framework compounds remains unclear owing to the lack of more experimental pieces of evidence. Here, based on the first-principles calculations combined with the quasi-harmonic approximation, we carried out studies of the NTE properties of ScF3 with metal dopants to reveal the roles in NTE of both the spatial effects and spin states. For most of the dopants, the NTE coefficient decreases with the decrease of ionic radius, obeying a linear scaling relationship. Interestingly, for magnetic impurities with a spin phase transition from a high spin state to a low spin state, we show that they will exhibit a Jahn–Teller distortion behavior and enhance the longitudinal vibration mode. In turn, the longitudinal vibration mode has a large positive Grüneisen parameter, which can offset the NTE obviously. This work not only proposes a new insight into the NTE mechanism about impurities in ScF3 but also provides an approach to tailor the NTE properties for fluorides with magnetic impurities as well as oxides.

Exploring negative thermal expansion materials with bulk framework structures and their relevant scaling relationships through multi-step machine learning

“…Specifically, they are completed by the Vienna Ab initio Simulation Package and the PHONOPY code, 135 and the detailed description can be found in ref. 136–140. In all, the flowchart of the ML model in our prediction is shown in Fig.…”

“…149,150 Since the DFT calculations cannot take into account the contribution of entropic elasticity to NTE, there may be some deviations in calculating CNTE using both the static DFT calculations and ab initio molecular dynamics simulation with the machine learning interatomic potentials (MLIPs), compared to the experimental results. Nevertheless, our previous DFT calculations of CNTE for some NTE materials with framework structures (such as fluorides, [151][152][153][154] oxides, [155][156][157] cyanides 158 and PBAs [159][160][161] still have achieved comparable results with experiments, and it is shown that using the DFT calculations as a roughly qualitative criterion to judge whether a material has NTE characteristics should be reasonable and unquestionable. Due to the fact that the first-principles calculation of the CNTE will require a lot of computer time, especially for oxide systems with a large number of atoms in the primitive cells and complex crystal structures, here, we select a few compounds with simple structures and small number of atoms in the primitive cell for calculation.…”