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We measure the reflectivity of pure and doped polycrystalline La2 -"Sr"Cu04 in the gap region and up to 40000 cm ' (=5 eV). For the doped sample we analyze the reflectivity in terms of a superposition of a metallic a-b plane component and a nearly insulating e-axis component with ordinary phonons. This approach removes several unphysical results obtained assuming a homogeneous surface and a11ows agreement between the measured infrared and dc properties, which has not been previously obtained. This approach further indicates that our infrared gap signature is dominated by the c-axis contribution and insensitive to the gap in the a-b plane. We estimate the c-axis gap to be 23, =2kgT"which is small compared to both the isotropic Bardeen-CooperSchrieffer prediction and tunneling measurements.Recently, high-temperature superconductivity has been discovered in metallic oxides in the layered K2NiF4 structure, ' prompting a great deal of research on the properties of these and related compounds. Infrared (ir) measurements are useful both for the study of the superconducting energy gap and the normal-state excitations and modes, ' which may be relevant to the superconductivity. In anisotropic systems such as these one would like to study polarized reAectivity from single crystals, since both the normal-state and superconducting properties are strongly dependent on orientation. " At the present time, high quality single crystals which are large enough for the ir measurements are not available and considerable effort has been expended in the study of polycrystalline samples. A number of controversies exist regarding both the gap measurement and the interpretation of the normalstate reflectivity data ' in these samples.In the present work we measure the reflectivity of pure and doped polycrystalline La2 Sr"Cu04 over a wide frequency range. In the individual crystallites the conductivity is believed to be much larger in the a bplane thanalong the c axis. Reflectivity measurements on single crystals of the isostructural compound LazNi04 exhibit a metallic reflectivity for the incident electric field in the a bplane, and a -nearly insulating response for E parallel to the c axis. ' Based on these observations we model the reAectivity as a composite spectrum consisting of independent contributions from different crystallite orientations, one metallic contribution from the a bplane, and on-e nearly insulating c-axis contribution. This approach enables us to understand the primary features of the reflectivity in terms of ordinary phonon and plasmon contributions, and to obtain for the first time agreement between the measured ir and the dc properties (conductivity and carrier density). We also find that our infrared measurement of the superconducting energy gap is dominated by the contribution from the low reflectivity c axis and is insensitive to the a-b plane gap. Our modeling indicates that this out-of-plane gap is about 25.=2k' T,. This value is substantially less than the gap measured by tunneling' (25 = 4ktt T, ), suggesting...

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