Experimental and theoretical study of infrared active phonons in heavily doped K <i> _x p </i> -terphenyl

We present an experimental and theoretical study of the infrared-active phonon modes of heavily potassium (K) doped <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mi>p</a:mi> </a:math> -terphenyl, a material of great current interest due to reports of high-temperature superconductivity. First, we show that the phonon features in the fingerprint region of the midinfrared domain can be well reproduced through the density functional theory (DFT) quantum-chemical simulations. Compelling evidence for a distorted organic molecular framework is obtained here from the emergence of up to three absorption bands that are predicted to be infrared (IR)-inactive under idealized <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:msub> <b:mi>D</b:mi> <b:mrow> <b:mn>2</b:mn> <b:mi>h</b:mi> </b:mrow> </b:msub> </b:math> planar symmetry and a systematic softening of specific vibrational modes. Computational models of twisted conformers exhibiting reduced <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:msub> <c:mi>D</c:mi> <c:mn>2</c:mn> </c:msub> </c:math> or <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:msub> <d:mi>C</d:mi> <d:mrow> <d:mn>2</d:mn> <d:mi>h</d:mi> </d:mrow> </d:msub> </d:math> symmetry are shown to successfully reproduce the activation of these silent modes and the redshifting shown by others. Second, we present an analysis of the lineshape of the experimental IR-phonon modes in a wide temperature range from 6 to 300 K. We show that several of the IR phonons have a strongly asymmetric shape that is indicative of a Fano-like interaction with the doped charge carriers. Notably, we find that the phonon asymmetries, and the corresponding Fano parameters, undergo anomalous changes around 100 K that are reminiscent of a charge localization transition around 90 K that has been previously reported from a corresponding study of the low-energy electronic response.