Due to the high structural flexibility and controllable thermal expansion, cubic double ReO 3 -type negative thermal expansion (NTE) fluorides provide a solution for solving the prominent phenomenon of thermal expansion mismatch between materials. However, the expensive raw materials and complex synthesis steps limit its practical application. In this work, we have designed a more advantageous method for the synthesis of NTE material CaZrF 6 , and it is expected to be generalized to the synthesis of other double ReO 3 -fluorides. Intriguingly, a new orthorhombic phase CaZrF 6 has been synthesized via this method in a lower temperature. Unlike the strong isotropic NTE of the cubic phase CaZrF 6 , the orthorhombic phase shows the strong anisotropic positive thermal expansion (PTE). The combined analysis of temperature-dependent X-ray diffraction (XRD), Raman spectra, and first-principles calculations shows that the low frequency phonon vibration mode with negative Grüneisen parameter in cubic CaZrF 6 are strongly correlated with the transverse thermal vibration of F atoms and dominates the NTE of the material.
3D anisotropic functional properties (such as magnetic, electrical, thermal, and optical properties, etc.) in a single material are not only beneficial to the multipurpose of a material, but also helpful to enrich the regulatory dimensionality of functional materials. Herein, a colossal 3D electrical anisotropy of layered MAB-phase MoAlB single crystal is introduced and dissected. Using high-temperature metal-solution method, high-quality MoAlB single crystals are obtained and a surprisingly strong out-of-plane ( σ a / σ b = 1.43 × 10 5 , at 2 K) and in-plane ( σ a / σ c = 12.12, at 2 K) electrical anisotropies are first observed. After a series of experimental and theoretical investigations, it is demonstrated that the 3D anisotropic crystal structure and chemical bond of MoAlB result in its 3D anisotropic phonon vibration and electronic structure, influence the corresponding electron-electron as well as electron-phonon interactions, and finally give rise to its colossal 3D anisotropy of electrical conductivity. This work experimentally and theoretically proves MoAlB single crystal possessing the 3D anisotropies of crystal structure, chemical bond, phonon vibration, electronic structure, and electrical transport, but also provides a promising platform for the future design of functionalized electronic devices as well as synthesis of new and large-sized in-plane anisotropic 2D material (MoBene).
The realization of terahertz (THz) detectors using organic-inorganic hybrid perovskite (OHP) materials presents a challenge because of the high real conductivities and broad THz absorption spectra required. The control of phonon vibration modes in OHPs depends on the control of defect states such as the defect-incorporated perovskite structure and the δ/α -mixed interfacial structure observed in MAPbI 3 and δ/α -mixed FAPbI 3 , respectively. Difficulties with defect control in particular materials may amplify relative risk factors and invite strong resistance to commercialization. As an alternative to OHPs, the all-inorganic perovskite material CsPbI 3 (lacking the hydrophobic organic cations that generate structural defects) constitutes a good candidate for THz detectors. We investigated the phonon vibration modes of thin films of the all-inorganic perovskite γ -CsPbI 3 containing no atomic or chemical defect states. We observed the three expected major phonon vibration modes at 0.9, 1.5, and 1.8 THz that originate from the transverse I–Pb–I frame, the Cs–I–Cs optical vibration, and the longitudinal I–Pb–I frame, respectively, finding good agreement with theoretical simulations. Significantly, the real conductivity of these all-inorganic perovskite thin films ranged from approximately 10–40 S/cm across a broad frequency range of 0.5–3.0 THz, demonstrating the material's considerable potential for application as a THz-band detector.
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