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Ordered low-temperature structure in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">K</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">C</mml:mi></mml:mrow><mml:mrow><mml:mn>60</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>detected by infrared spectroscopy
Infrared spectra of a ${\mathrm{K}}_{4}{\mathrm{C}}_{60}$ single-phase thin film have been measured between room temperature and 20 K. At low temperatures, the two high-frequency ${T}_{1u}$ modes appear as triplets, indicating a static ${D}_{2h}$ crystal-field stabilized Jahn-Teller distortion of the ${\mathrm{C}}_{60}{}^{4\ensuremath{-}}$ anions. The ${T}_{1u}(4)$ mode changes into a known doublet above 250 …