Spin–Phonon Interactions and Anharmonic Lattice Dynamics in Fe3GeTe2

Abstract Raman scattering is performed on Fe3GeTe2 (FGT) at temperatures from 8 to 300 K and under pressures from the ambient pressure to 9.43 GPa. Temperature‐dependent and pressure‐dependent Raman spectra are reported. The results reveal respective anomalous softening and moderate stiffening of the two Raman active modes as a result of the increase of pressure. The anomalous softening suggests anharmonic phonon dynamics and strong spin–phonon coupling. Pressure‐dependent density functional theory and phonon calculations are conducted and used to study the magnetic properties of FGT and assign the observed Raman modes, E2g2$E_{2{ m{g}}}^2$ and A1g1$A_{1{ m{g}}}^1$. The calculations proved the strong spin–phonon coupling for the E2g2$E_{2{ m{g}}}^2$ mode. In addition, a synergistic interplay of pressure‐induced reduction of spin exchange interactions and spin–orbit coupling effect accounts for the softening of the E2g2$E_{2{ m{g}}}^2$ mode as pressure increases

Thermal and lattice dynamical properties of Na8Si46 clathrate

The experimental heat capacity of Na8Si46 has been determined from 35\u2013300 K, and its lattice parameters have been measured over the range 100\u2013330 K. The experimental heat capacity and the thermal-expansion coefficient are compared with theoretical lattice-dynamical calculations for Na8Si46. The latter accurately reproduce the experimental thermal expansion and also give the first reliable assignment of the vibrational spectrum of this material, as judged by comparison of the calculated and experimental heat capacities. In addition, the theoretical results allow a calculation of the Gr\ufcneisen parameter of Na8Si46, which shows enhanced anharmonicity at low temperatures.NRC publication: Ye

K₃Fe(CN)₆ under External Pressure: Dimerization of CN^– Coupled with Electron Transfer to Fe(III)

The addition polymerization of charged monomers like CC^2– and CN^– is scarcely seen at ambient conditions but can progress under external pressure with their conductivity significantly enhanced, which expands the research field of polymer science to inorganic salts. The reaction pressures of transition metal cyanides like Prussian blue and K₃Fe­(CN)₆ are much lower than that of alkali cyanides. To figure out the effect of the transition metal on the reaction, the crystal structure and electronic structure of K₃Fe­(CN)₆ under external pressure are investigated by <i>in situ</i> neutron diffraction, <i>in situ</i> X-ray absorption fine structure (XAFS), and neutron pair distribution functions (PDF) up to ∼15 GPa. The cyanide anions react following a sequence of approaching–bonding–stabilizing. The Fe­(III) brings the cyanides closer which makes the bonding progress at a low pressure (2–4 GPa). At ∼8 GPa, an electron transfers from the CN to Fe­(III), reduces the charge density on cyanide ions, and stabilizes the reaction product of cyanide. From this study we can conclude that bringing the monomers closer and reducing their charge density are two effective routes to decrease the reaction pressure, which is important for designing novel pressure induced conductor and excellent electrode materials

The mechanism behind SnO metallization under high pressure

SnO is known to undergo metallization at ∼ 5 GPa while retaining its tetragonal symmetry. However, the mechanism of this metallization remains speculative. We present a combined experimental and computational study including pressure-dependent infrared spectroscopy, resistivity, and neutron powder diffraction measurements. We show that, while the excess charge mobility increases with pressure, the lattice distortion, in terms of the z-position of Sn, is reduced. Both processes follow a similar trend that consists of two stages, a moderate increment up to ∼ 3 GPa followed by a rapid increase at higher pressure. This behavior is discussed in terms of polaron delocalization. The pressure-induced delocalization is dictated by the electron–phonon coupling and related local anisotropic lattice distortion at the polaron site. We show that these polaronic states are stable at 0 GPa with a binding energy of ∼ 0.35 eV. Upon increasing the pressure, the polaron binding energy is reduced with the electron–phonon coupling strength of Γ and M modes, enabling the electrical phase transition to occur at ∼ 3.8 GPa. Further compression increases the total electron–phonon coupling strength up to a maximum at 10 GPa, which is a strong evidence of dome-shaped superconductivity transition with Tc = 1.67 K

Polymerization of Acetonitrile via a Hydrogen Transfer Reaction from CH 3 to CN under Extreme Conditions

Acetonitrile (CH$_{3}$CN) is the simplest and one of the most stable nitriles. Reactions usually occur on the C≡N triple bond, while the C−H bond is very inert and can only be activated by a very strong base or a metal catalyst. It is demonstrated that C−H bonds can be activated by the cyano group under high pressure, but at room temperature. The hydrogen atom transfers from the CH3 to CN along the CH⋅⋅⋅N hydrogen bond, which produces an amino group and initiates polymerization to form a dimer, 1D chain, and 2D nanoribbon with mixed sp2 and sp3 bonded carbon. Finally, it transforms into a graphitic polymer by eliminating ammonia. This study shows that applying pressure can induce a distinctive reaction which is guided by the structure of the molecular crystal. It highlights the fact that very inert C−H can be activated by high pressure, even at room temperature and without a catalyst

Pressure induced polymerization of acetylide anions in CaC2 and 107 fold enhancement of electrical conductivity

Transformation between different types of carbon–carbon bonding in carbides often results in a dramatic change of physical and chemical properties. Under external pressure, unsaturated carbon atoms form new covalent bonds regardless of the electrostatic repulsion. It was predicted that calcium acetylide (also known as calcium carbide, CaC2) polymerizes to form calcium polyacetylide, calcium polyacenide and calcium graphenide under high pressure. In this work, the phase transitions of CaC2 under external pressure were systematically investigated, and the amorphous phase was studied in detail for the first time. Polycarbide anions like C66− are identified with gas chromatography-mass spectrometry and several other techniques, which evidences the pressure induced polymerization of the acetylide anions and suggests the existence of the polyacenide fragment. Additionally, the process of polymerization is accompanied with a 107 fold enhancement of the electrical conductivity. The polymerization of acetylide anions demonstrates that high pressure compression is a viable route to synthesize novel metal polycarbides and materials with extended carbon networks, while shedding light on the synthesis of more complicated metal organics