Scandium trifluoride maintains a cubic ReO(3) type structure down to at least 10 K, although the pressure at which its cubic to rhombohedral phase transition occurs drops from >0.5 GPa at ∼300 K to 0.1-0.2 GPa at 50 K. At low temperatures it shows strong negative thermal expansion (NTE) (60-110 K, α(l) ≈ -14 ppm K(-1)). On heating, its coefficient of thermal expansion (CTE) smoothly increases, leading to a room temperature CTE that is similar to that of ZrW(2)O(8) and positive thermal expansion above ∼1100 K. While the cubic ReO(3) structure type is often used as a simple illustration of how negative thermal expansion can arise from the thermally induced rocking of rigid structural units, ScF(3) is the first material with this structure to provide a clear experimental illustration of this mechanism for NTE.
Scandium fluoride displays isotropic negative thermal expansion (NTE) from at least 10 to 1100 K and retains a cubic ReO3-type structure over this range; the NTE is most pronounced at low temperatures. Control of thermal expansion was explored by forming Sc1–xYxF3 (x≤0.25), which were characterized with synchrotron powder diffraction at ambient pressure from 100 to 800 K. The behavior of the solid solutions under pressure (≤0.276 GPa) was also examined while heating from 298 to 523 K. Insertion of the relatively large Y3+ ion into ScF3 results in a cubic-to-rhombohedral phase transition upon cooling from ambient temperature to 100 K, even at low substitution levels (5%). The coefficient of thermal expansion (CTE) of the solid solutions in the rhombohedral phase is strongly dependent on both composition and temperature; however, above 400 K, where all samples are cubic, the CTE appears to be largely independent of composition. The isothermal bulk modulus and CTE of ScF3, but not those of the solid solutions, are independent of temperature and pressure, respectively. Yttrium substitution lowers the bulk modulus, even at temperatures where the samples are cubic. Finally, the solid solutions stiffen upon heating.
Materials with the cubic ReO3-type structure are, in principle, excellent candidates for negative thermal expansion (NTE). However, many such materials, including TaO2F, do not display NTE. It is proposed that local distortions away from the ideal structure, associated with the need to accommodate the different bonding requirements of the disordered O/F, contribute to the occurrence of near zero thermal expansion rather than NTE. The local structure of TaO2F is poorly described by an ideal cubic ReO3-type model with O and F randomly distributed over the available anion sites. A supercell model featuring −Ta–O–Ta–O–Ta–F– chains along ⟨1 0 0⟩, with different Ta–O and Ta–F distances and O/F off-axis displacements, gives much better agreement with pair distribution functions (PDFs) derived from total X-ray scattering data for small separations (<8 Å). Analyses of PDFs derived from variable temperature measurements (80 to 487 K), over different length scales, indicate an average linear expansion coefficient of close to zero with similar contributions from the geometrically distinct Ta–OTa and Ta–FTa links in TaO2F.
Pair distribution function (PDF) methods have great potential for the study of diverse high‐pressure phenomena. However, the measurement of high‐quality, high‐resolution X‐ray PDF data (to Qmax > 20 Å−1) remains a technical challenge. An optimized approach to measuring high‐pressure total scattering data for samples contained within a diamond anvil cell (DAC) is presented here. This method takes into account the coupled influences of instrument parameters (photon energy, detector type and positioning, beam size/shape, focusing), pressure‐cell parameters (target pressure range, DAC type, diamonds, pressure‐transmitting media, backing plates, pressure calibration) and data reduction on the resulting PDF. The efficacy of our approach is demonstrated by the high‐quality, high‐pressure PDFs obtained for representative materials spanning strongly and weakly scattering systems, and crystalline and amorphous samples. These are the highest‐resolution high‐pressure PDFs reported to date and include those for α‐alumina (to Qmax = 20 Å−1), BaTiO3 (to Qmax = 30 Å−1) and pressure‐amorphized zeolite (to Qmax = 20 Å−1).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.