Optical Properties of Single Crystals of NbSe2 and Nb1.04Se2

Niobium-selenium compounds in the composition range NbSe2–Nb1.14Se2 can form layered crystals which are readily cleaved in a direction perpendicular to the c axis. For compositions in the range NbSe2–Nb1.05Se2, two structures, a high-temperature ζ phase and a lower-temperature ε phase may be formed. Single crystals of ε-NbSe2, ε-Nb1.04Se2, ζ-NbSe2, and ζ-Nb1.04Se2 were grown by the vapor transport technique and the optical absorption spectra of thin-cleaved specimens in the visible and near infrared regions were studied. An infrared absorption spectrum to λ = 15 μ was also obtained for ε-NbSe2. Accurate values of the absorption coefficient α were measured at 10 nm intervals in the spectral region 400–620 nm. For all crystal types, α was found to have peak value ∼4.5×105 cm−1 at λ = 460 nm, decreasing steadily to ∼1.3× 105 cm−1 at λ = 620 nm. The values of α for ε-NbSe2, along with bulk reflectivities R0 measured on basal plane cleavage surfaces of opaque sections of ε-NbSe2, have been used to calculate the optical constants n and k. The most notable feature of the visible and near infrared studies is the similarity of the gross features of the absorption spectra of all four crystal types. This indicates that the absorption is governed principally by a mechanism operating largely within the Se–Nb–Se ``layer molecules'' and relatively independent of structure or the presence of interstitial excess metal. An absorption minimum centered about λ = 1.2 μ has been attributed to plasma oscillation of the conduction electrons, and calculations based on this assumption have yielded a value m*≤5.9 m for the effective mass of these electrons.