Optical properties of UBe13, CeBe13, and ThBe13.

Abstract

We report the results of optical reflectivity measurements on M${\mathrm{Be}}_{13}$(M=U,Ce,Th) from 0.1 to 30 eV at T=300 K. The dielectric function \ensuremath{\epsilon}(\ensuremath{\omega})=${\ensuremath{\epsilon}}_{1}$+i${\ensuremath{\epsilon}}_{2}$(\ensuremath{\omega}) is determined from a Kramers-Kronig analysis of the data. The intraband and interband contributions to \ensuremath{\epsilon}(\ensuremath{\omega}) were separated via a low-energy Drude analysis. The following free-carrier plasma frequencies were obtained: ${\ensuremath{\omega}}_{p}$(${\mathrm{UBe}}_{13}$)=5.9 eV, ${\ensuremath{\omega}}_{p}$(${\mathrm{CeBe}}_{13}$)=5.3 eV, and ${\ensuremath{\omega}}_{p}$(${\mathrm{ThBe}}_{13}$)=4.4 eV. Estimated values for the optical mass of the free carriers fall in the range 1${m}_{\mathrm{opt}2}$ for the three beryllides, consistent with the presence of a wide band at ${E}_{F}$. If a value of ${m}_{\mathrm{opt}=1}$ is chosen, the values of ${\ensuremath{\omega}}_{p}$ are consistent with 3.4 (${\mathrm{UBe}}_{13}$), 2.8 (${\mathrm{CeBe}}_{13}$), and 2.0 (${\mathrm{ThBe}}_{13}$) free carriers per M${\mathrm{Be}}_{13}$ formula unit. Prominent structure in ${\ensuremath{\epsilon}}_{2,\mathrm{b}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}}$ is observed in all three compounds at \ensuremath{\approxeq}2 and \ensuremath{\approxeq}13 eV and are tentatively assigned to Be(2s\ensuremath{\rightarrow}2p) transitions. M-atom-dependent structure in the spectra is also observed and identified with the d and f states near ${E}_{F}$. The results for ${\mathrm{UBe}}_{13}$ are discussed in terms of recent energy-band calculations, but a true quantitatitve comparison of the data and theory will have to await calculation of the dielectric functions from the band-structure results.