Anisotropic plasmons due to carrier electrons in Cs-doped hexagonal WO₃ studied by momentum transfer resolved electron energy-loss spectroscopy

Cs-doped hexagonal WO₃ (CWO) is used as a solar heat-shielding material for windows, in which plasma oscillation due to carrier electrons (carrier plasmon) plays an important role for near infrared scattering. Despite the hexagonal crystal structure of CWO, the anisotropic properties of the carrier plasmons have not been investigated. This study reports the momentum transfer resolved electron energy-loss spectroscopic measurements of CWO to investigate the anisotropic properties of carrier plasmons. The experimental results clarified that the two plasma oscillation modes at 1.2 and 1.8 eV have different excitation properties in CWO. One plasma oscillation at 1.2 eV was excited for q along the ab plane with a large damping effect, which indicated that electron excitations occur for the q//ab plane. Another mode at 1.8 eV was an oscillation excited for q along the c-axis with a small damping effect, i.e., a long plasmon relaxation time. These two modes can be interpreted by the anisotropic energy dispersion of the electronic states around the Fermi level of CWO. Such anisotropic properties of the carrier plasmons led to an accurate understanding of the heat-shielding mechanism.