Vol. 6, No. 3/March 1989/J. Opt. Soc. Am. B 385 Diffuse reflectance and its relationship to oxygen content in YBa 2 Cu 3 07- 6 and Bi2.l(CaXSr1-x)n+1CunO2n+4+6 (ln = 1, 2) A. Bittar, M. P. Staines, and R. G. Buckley Physics and Engineering Laboratory, Department of Scientific and Industrial Research, P.O. Box 31313, Lower Hutt, New Zealand Received August 9, 1988; accepted November 14, 1988 We report on variations of the diffuse reflectance of the YBa 2 Cu 3 07-5 and Bi 2 .l(Ca.Srl-)n+1Cun2n+4+b (n = 1, 2) superconducting ceramics with oxygen content 0 < 6 < 0.8. We model the diffuse refectance of the ceramics and showthat the observed near-infrared edge may be approximated by a single-carrier free-electron Drude model. For the bismuth-based compounds, the infrared Drude edge and structures at -2.0 and 3.8 eV show little variation with . For the YBa 2 Cu 3 07-_, the Drude edge and the interband features vary strongly with . The interband peaks at 1.7 and 4.1 eV are both attributed to transitions involving the oxygen-loading copper planes. INTRODUCTION Studies of the optical properties of the layered perovskite superconductors are made difficult by the porous, polycrys- talline nature of these ceramic materials. 1 - 5 Sizeable single- crystal specimens are difficult to prepare and, in the case of YBa 2 CU 3 0 7 -6 (YBCO), suffer from the additional problems of inhomogeneous 0 loading and twinning. Diffuse reflec- tance circumvents problems associated with surface condi- tions in measurements on ceramic samples; however, this technique is not generally favored in the spectroscopy of solids because of the difficulty in relating the measurements to the intrinsic dielectric properties of the bulk material. Nevertheless we have applied this technique to ceramic su- perconductors in the energy range 0.3-6.0 eV and have found good correspondence between our infrared results and spec- ular Drude reflectance and are confident that spectral fea- tures in the visible and ultraviolet are likewise well repre- sented in the diffuse reflectance spectra that we have mea- sured. Here we report the results of optical experiments on YBCO and on two members of the Bi 2 .1(CaxSri-x)n+lCun- 0 2n+4+5 homologous series. 6 In each case the 0 stoichiom- etry has been varied by quenching or vacuum annealing. We find that the near-infrared reflectance is well described by the Drude model in all our samples. In YBCO we con- firm interband features at 1.7, 2.6, 4.1, and 4.6 eV, as ob- served by others, 5 and deduce their variations with 0 stoi- chiometry; in addition we observe that the 4.1-eV peak is sensitive to ordering in the O-loading Cu(1) plane. In the Bi-based superconductors, O-related variations in spectral features are small and are found only above 4.0 eV, possibly linking them to 0 sorption in the Bi-O planes. SAMPLE CHARACTERIZATION AND OPTICAL MEASUREMENTS Both the YBCOand the n = 1 and n = 2 Bi-based supercon- ductors were prepared by mixing stoichiometric amounts of the oxides and carbonates. The n = 2 member (Bi 2 .,CaSr 2 - Cu 2 08) was reacted and then sintered at 8650Cin air; the n = 1 member (Bi 2 .,CaSrCuO 6 ) was reacted and sintered at 8100C in air. YBCOwas prepared in the usual way at 9200C and, after regrinding, was sintered at 9400Cand slowcooled in 0. All samples were shown to be single phase by x-ray diffraction and were characterized by measuring their super- conducting transition temperatures. For n = 2, T, = 91 K; for n = 1, T, = 78 K; and for YBCO T, = 90 K. The 0 stoichiometry of the samples was varied by quench- ing and annealing treaments. The n = 1 and n = 2 Bi samples were annealed in air at temperatures between 400 and 8000C and then quenched in liquid N 2 . The accompa- nying small but systematic changes in T, from -70 to 91 K are readily understood in terms of 0 loading. 7 The YBCO samples were either quenched in liquid N 2 after air anneals or annealed in vacuum and then slow cooled to room tem- perature immediately before the optical meaurements were made. 0 stoichiometries were measured by gravimetric methods. The room-temperature diffuse reflectance of the thick ceramic samples was measured with integrating sphere at- tachments to our high-accuracy spectrophotometer 8 and a Bomen DA3 Fourier-transform infrared spectrometer. Over the 0.3-6.0-eV range covered, the spectra were refer- enced to a white diffuser and corrected for systematic errors by using standard photometric techniques. 9 In Fig. 1 wepresent specular and diffuse reflectance spec- tra for two n = 2 Bi samples. Clearly a considerable amount of light is lost in the specular reflectance measurement, a situation that is hardly improved by use of a Au-coated sample as reference; furthermore, although the specular re- flectance of the two samples differs markedly in the near infrared, their diffuse reflectance spectra are almost identi- cal. This remarkable reproducibility of the reflectance val- ues and structural features is directly attributable to the quasi-total collection of the light reflected or backscattered by the samples. In general the spectral diffuse reflectance, R,,(w), of opti- cally thick samples measured with this technique depends on the intrinsic absorption coefficient K(w) of the ceramics as well as their scattering coefficients S(w). Because of the particle size (-1-10 ,um), an effective medium analysis is not applicable in our wavelength range, and a Kramers-Kronig 0740-3224/89/030385-04$2.00 © 1989 Optical Society of America Bittar et al.