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Anomalous thermal expansion with infrared spectroscopy.
Applied Optics
|February 2, 2010
Summary
Anomalous thermal expansion in CuCl and CuBr is explained by lattice vibrations. Changes in bonding fractions during re-expansion lead to increased interatomic distances, contrary to typical contraction.
Area of Science:
- Solid-state physics
- Materials science
- Spectroscopy
Background:
- Anomalous thermal expansion is a phenomenon observed in some materials where they expand upon heating over a certain temperature range.
- Understanding the underlying mechanisms is crucial for material design and application.
- Lattice dynamics and bonding characteristics play significant roles in thermal expansion behavior.
Purpose of the Study:
- To investigate the anomalous thermal expansion in copper chloride (CuCl) and copper bromide (CuBr) using an analytical approach.
- To correlate the observed anomalous expansion with the anharmonic behavior of lattice vibrations.
- To explore the changes in bonding characteristics during the re-expansion phase.
Main Methods:
- Utilized analytical methods to study the anharmonic behavior of lattice vibrations.
- Analyzed complete infrared (IR) spectroscopic data available for the low-temperature region of CuCl and CuBr.
- Examined the relationship between anomalous thermal expansion, anharmonic factor, and temperature.
Main Results:
- A mirror-like proportionality was observed between anomalous thermal expansion and the change in the anharmonic factor with temperature for both CuCl and CuBr.
- Drastic changes in IR energy absorption occurred during the re-expansion temperature region.
- These changes suggest a significant increase in the ionic fraction of binding and a decrease in the covalent fraction.
Conclusions:
- The study attributes the anomalous thermal expansion and re-expansion phenomenon in CuCl and CuBr to significant shifts in bonding character.
- The increased ionic radii sum compared to covalent radii sum during re-expansion leads to an enlargement of interatomic distances.
- Anharmonic lattice vibrations are identified as the fundamental driver for these observed thermal expansion anomalies.
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