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

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

High-Dielectric-Permittivity Layered Nitride CaTiN₂

High-Dielectric-Permittivity Layered Nitride CaTiN<sub>2</sub

Structure, Magnetism, and Tunable Negative Thermal Expansion in (Hf,Nb)Fe₂ Alloys

Structure, Magnetism, and Tunable Negative Thermal Expansion in (Hf,Nb)Fe₂ Alloy

Twin Crystal Induced near Zero Thermal Expansion in SnO₂ Nanowires

Knowledge of controllable thermal expansion is a fundamental issue in the field of materials science and engineering. Direct blocking of the thermal expansions in positive thermal expansion materials is a challenging but fascinating task. Here we report a near zero thermal expansion (ZTE) of SnO₂ achieved from twin crystal nanowires, which is highly correlated to the twin boundaries. Local structural evolutions followed by pair distribution function revealed a remarkable thermal local distortion along the twin boundary. Lattice dynamics investigated by Raman scattering evidenced the hardening of phonon frequency induced by the twin crystal compressing, giving rise to the ZTE of SnO₂ nanowires. Further DFT calculation of Grüneisen parameters confirms the key role of compressive stress on ZTE. Our results provide an insight into the thermal expansion behavior regarding to twin crystal boundaries, which could be beneficial to the applications

Structure and Phase Transformation in the Giant Magnetostriction Laves-Phase SmFe₂

As one class of the most important intermetallic compounds, the binary Laves-phase is well-known for its abundant magnetic properties. Samarium–iron alloy system SmFe₂ is a prototypical Laves compound that shows strong negative magnetostriction but relatively weak magnetocrystalline anisotropy. SmFe₂ has been identified as a cubic <i>Fd</i>3̅<i>m</i> structure at room temperature; however, the cubic symmetry, in principle, does not match the spontaneous magnetization along the [111]_cubic direction. Here we studied the crystal structure of SmFe₂ by high-resolution synchrotron X-ray powder diffraction, X-ray total scattering, and selected-area electron diffraction methods. SmFe₂ is found to adopt a centrosymmetric trigonal <i>R</i>3̅<i>m</i> structure at room temperature, which transforms to an orthorhombic <i>Imma</i> structure at 200 K. This transition is in agreement with the changes of easy magnetization direction from [111]_cubic to [110]_cubic direction and is further evidenced by the inflection of thermal expansion behavior, the sharp decline of the magnetic susceptibility in the field-cooling–zero field-cooling curve, and the anomaly in the specific heat capacity measurement. The revised structure and phase transformation of SmFe₂ could be useful to understand the magnetostriction and related physical properties of other RM₂-type pseudocubic Laves-phase intermetallic compounds