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Infrared phonon anomaly in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>BaFe</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mtext>As</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>
The detailed optical properties of ${\text{BaFe}}_{2}{\text{As}}_{2}$ have been determined over a wide frequency range above and below the structural and magnetic transition at ${T}_{N}\ensuremath{\simeq}138\text{ }\text{K}$. A prominent in-plane infrared-active mode is observed at $253\text{ }{\text{cm}}^{\ensuremath{-}1}$ (31.4 meV) at 295 K. The frequency of this vibration shifts discontinuously at ${T}_{N}$; for …