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We determine the frequency dependence of four independent CdWO$_4$ Cartesiandielectric function tensor elements by generalized spectroscopic ellipsometrywithin mid-infrared and far-infrared spectral regions. Single crystal surfacescut under different angles from a bulk crystal, (010) and (001), areinvestigated. From the spectral dependencies of the dielectric function tensorand its inverse we determine all long wavelength active transverse andlongitudinal optic phonon modes with $A_u$ and $B_u$ symmetry as well as theireigenvectors within the monoclinic lattice. We thereby demonstrate that suchinformation can be obtained completely without physical model line shapeanalysis in materials with monoclinic symmetry. We then augment the effect oflattice anharmonicity onto our recently described dielectric function tensormodel approach for materials with monoclinic and triclinic crystal symmetries[Phys. Rev. B, 125209 (2016)], and we obtain excellent match between allmeasured and modeled dielectric function tensor elements. All phonon modefrequency and broadening parameters are determined in our model approach. Wealso perform density functional theory phonon mode calculations, and we compareour results obtained from theory, from direct dielectric function tensoranalysis, and from model lineshape analysis, and we find excellent agreementbetween all approaches. We also discuss and present static and abovereststrahlen spectral range dielectric constants. Our data for CdWO$_4$ are inexcellent agreement with a recently proposed generalization of theLyddane-Sachs-Teller relation for materials with low crystal symmetry [Phys.Rev. Lett. 117, 215502 (2016)].

Anisotropy, phonon modes, and lattice anharmonicity from dielectric function tensor analysis of monoclinic cadmium tungstate