Constraint on the potassium content for the superconductivity of potassium-intercalated phenanthrene

Raman-scattering measurements were performed on K(x)phenanthrene (0 ⩽ x ⩽ 6.0) at room temperature. Three phases (x = 3.0, 3.5, and 4.0) are identified based on the obtained Raman spectra. Only the K3phenanthrene phase is found to exhibit the superconducting transition at 5 K. The C-C stretching modes are observed to broaden and become disordered in K(x)phenanthrene with x = 2.0, 2.5, 6.0, indicating some molecular disorder in the metal intercalation process. This disorder is expected to influence the nonmetallic nature of these materials. The absence of metallic character in these nonsuperconducting phases is found from the calculated electronic structures based on the local density approximation.