Synthesis, Crystal Structure, Local Structure, and Magnetic Properties of Polycrystalline and Single-Crystalline Ce[sub.2]Pt[sub.6]Al[sub.15]

Asymmetry, such as non-centrosymmetry in the crystal or chiral structure and local symmetry breaking, plays an important role in the discovery of new phenomena. The honeycomb structure is an example of an asymmetric structure. Ce[sub.2]Pt[sub.6]Al[sub.15] is a candidate for a frustrated system with honeycomb Ce-layers, which have been reported to show near the quantum critical point. However, the ground state of Ce[sub.2]Pt[sub.6]Al[sub.15] depends on the sample, and analysis of the crystal structure is difficult due to the presence of stacking disorder. We synthesized polycrystalline Ce[sub.2]Pt[sub.6]Al[sub.15] using arc melting method (AM-Ce[sub.2]Pt[sub.6]Al[sub.15]) and single-crystalline Ce[sub.2]Pt[sub.6]Al[sub.15] using flux method (F-Ce[sub.2]Pt[sub.6]Al[sub.15]). The prepared samples were characterized by electron probe micro-analysis (EPMA), single and powder X-ray diffraction methods, measured magnetic properties and X-ray absorption spectroscopy (XAS). The composition ratio of AM-Ce[sub.2]Pt[sub.6]Al[sub.15] was stoichiometric, although it contained a small amount (i.e., a few percent) of the impurity Ce[sub.2]Pt[sub.9]Al[sub.16]. Meanwhile, the composition ratio of F-Ce[sub.2]Pt[sub.6]Al[sub.15] deviated from stoichiometry. The X-ray absorption fine structure (XAFS) spectrum of AM-Ce[sub.2]Pt[sub.6]Al[sub.15] at the Ce L[sub.3]-edge was similar to that of CeF[sub.3], which possesses the Ce[sup.3+] configuration, indicating that the valence of Ce in Ce[sub.2]Pt[sub.6]Al[sub.15] is trivalent; this result is consistent with that for the magnetic susceptibility. To determine the precise structure, we analyzed the extended X-ray absorption fine structure (EXAFS) spectra of Ce L[sub.3]- and Pt L[sub.3]-edges for Ce[sub.2]Pt[sub.6]Al[sub.15], and found that the EXAFS spectra of Ce[sub.2]Pt[sub.6]Al[sub.15] can be explained not as a hexagonal Sc[sub.0.6]Fe[sub.2]Si[sub.4.9]-type structure but, instead, as an orthorhombic structure with honeycomb structure.