New Insights into the Negative Thermal Expansion: Direct Experimental Evidence for the “Guitar-String” Effect in Cubic ScF₃

Abstract

The understanding of the negative thermal expansion (NTE) mechanism remains challenging but critical for the development of NTE materials. This study sheds light on NTE of ScF3, one of the most outstanding materials with NTE. The local dynamics of ScF3 has been investigated by a combined analysis of synchrotron-based X-ray total scattering, extended X-ray absorption fine structure, and neutron powder diffraction. Very interestingly, we observe that (i) the Sc-F nearest-neighbor distance strongly expands with increasing temperature, while the Sc-Sc next-nearest-neighbor distance contracts, (ii) the thermal ellipsoids of relative vibrations between Sc-F nearest-neighbors are highly elongated in the direction perpendicular to the Sc-F bond, indicating that the Sc-F bond is much softer to bend than to stretch, and (iii) there is mainly dynamically transverse motion of fluorine atoms, rather than static shifts. These results are direct experimental evidence for the NTE mechanism, in which the rigid unit is not necessary for the occurrence of NTE, and the key role is played by the transverse thermal vibrations of fluorine atoms through the "guitar-string" effect.

Two-dimensional nanoscale correlations in the strong negative thermal expansion material ScF3

“…Recent classical molecular dynamics simulations and general arguments have suggested strict rigidity of molecular units is not possible or necessary for NTE 15,[17][18][19] , fully consistent with the finite correlation length implied by our experiments and divergent kinetic energy density in the thermodynamic limit of the CLM. Here we propose a mechanism of strain relief and kinetic energy lowering consistent with experiments, the quantized nature of the vibrational spectrum, and hierarchy of stiffness present in the bond patterns.…”

“…Best agreement is met when ω p 1.6 meV, which is similar in magnitude to the observed branch energy at low temperature (1.2-1.4 meV) and somewhat lower than the observed values at ambient temperature (3.4-3.6 meV). We remark that existing computational work gives mode energies ranging from 3-6.5 meV 11,17,18 , and that inclusion of other low-energy modes off of the high-symmetry cut is expected to raise this effective energy scale somewhat.…”

“…Building on this idea, some researchers find an adequate description of NTE in terms of rigid molecular units which draw in the lattice when thermally activated, with still finer points of debate on which senses of motion are appropriate for which particular system [13][14][15] , or across which momentum manifolds rigid external modes exist 16 . Others have suggested that molecular rigidity is neither necessary nor favorable as an NTE mechanism 15,[17][18][19] . Separately from any consideration of NTE, studies of the perovskite space group landscape show that the cubic open perovskites in the P m3m structure like ScF 3 and ReO 3 can undergo coordinated tilt patterns 20 of rigid octahedra.…”

Atypically small temperature-dependence of the direct band gap in the metastable semiconductor copper nitrideCu3N

“…Below 50 K, negative thermal expansion is predicted, but this is not supported or ruled out by the experimental results; at higher temperatures, the DFT expansion coefficient matches well to the experimental Debye model. This contrasts with the ReO 3 -structured ScF 3 , where strong anharmonicity leads to large variations in predicted and observed thermal expansion [69,70]. In other semiconductors, overestimated DFT-and QHA-predicted thermal expansion has been attributed to higher-order anharmonic effects [71].…”

Facile synthesis of scandium fluoride oriented single-crystalline rods and urchin-like structures by a gas–solution interface technique

“…20 Importantly, scandium fluoride belongs to a very specific and narrow group of materials demonstrating negative thermal expansion (NTE) in the broad temperature range from 10 K to 1100 K. [21][22][23] It is known that the magnitude of NTE is reduced with decreasing crystal size. 24 Crystal morphology is also known to affect the optical, mechanical, electrochemical, catalytic and other properties of materials.…”