Optical Band Gaps of Selected Ternary Sulfide Minerals
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American Mineralogist, Volume 83, pages 865±871, 1998 Optical band gaps of selected ternary sul®de minerals STEVEN I. BOLDISH*AND WILLIAM B. WHITE² Intercollege Materials Research Laboratory, Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A. ABSTRACT Optical band gaps for a set of 23 ternary and quaternary sul®de minerals have been measured by diffuse re¯ectance spectroscopy. Comparison of band gaps measured by dif- fuse re¯ectance with band gaps determined by single-crystal methods for 12 binary sul®des demonstrates that the diffuse re¯ectance measurement produces results accurate to within 0.1 eV. Unlike the band gaps of binary sul®des that plot linearly with various measures of bond energy, the ternary and quaternary sul®des cluster within ranges determined by the chemical composition. The band gaps for the ternary and quaternary sul®des tend to lie between the band gaps of the component binary sul®des. INTRODUCTION position. For the binary compounds, reagent grade chem- Many sul®de minerals are classed as semiconductors icals were used and the crystal structures were checked (see e.g., Shuey 1975). They have substantial electrical by X-ray diffraction. conductivity, high refractive indices, and often a metallic Some compounds of interest, which were not available luster. A fundamental property of semiconductor com- as natural minerals, were synthesized. The appropriate bi- pounds is the band gap, the energy separation between nary sul®des were mixed in stoichiometric proportions, the ®lled valence band constructed mainly from sulfur p- ground together under acetone, pressed into pellets, and orbitals, and the empty conduction band constructed ®red in sealed evacuated silica tubes. Details are given in mainly from empty cation orbitals. The bonding arrange- Table 2. ment for sul®de minerals, particularly ternary sul®de minerals, is often quite complicated (see e.g., Vaughan Diffuse re¯ectance spectroscopy and Craig 1978). Samples were ground to a ®ne powder under acetone Optical excitation of electrons across the band gap is and packed into shallow aluminum sample plaques. The strongly allowed, producing an abrupt increase in absorp- spectra were measured with a Beckman DK-2A spectro- tivity at the wavelength corresponding to the gap energy. photometer equipped with an integrating sphere diffuse This feature in the optical spectrum is known as the op- re¯ectance attachment. The sphere was coated with a Ko- tical absorption edge. For those structures with band gaps dak BaSO4 optical paint. A sample plaque coated with in the range of 0.5 to 3 eV, the optical absorption edge the optical paint was also used as the reference. A PbS can be easily measured by conventional optical spectros- solid state detector was used from 2700 nm to 500 nm. copy. The complete band structure includes states span- A photomultiplier detector was used at wavelengths ning 30 to 50 eV (Vaughan 1985). The optical absorption shorter than 500 nm. Only the diffuse component of the edge gives only the lowest energy of this group of states. re¯ected light was collected. The present paper presents optical absorption edges for Diffuse re¯ectance data are often converted to absorp- a selection of ternary sul®de minerals, mainly sulfosalts, tivities by the Kubelka-Munk function (Wendlandt and and discusses the estimated band gaps in relation to the Hecht 1966). chemical bonding in these structures. 2 (1 2 R`) } EXPERIMENTAL METHODS F(R ) 55 (1) ` 2 Rs Sources of minerals ` The minerals used in this investigation were obtained where R` 5 (I/I0)diffuse, the diffuse re¯ectivity from an from various museum and commercial sources (Table 1). in®nitely thick layer of powder (about 2 mm for typical Identi®cation and phase purity of all minerals were sul®des), a5absorptivity in units of cm21, and s 5 scat- checked by X-ray diffraction. In a few cases, electron tering factor largely independent of wavelength for par- microprobe analyses were made to determine phase com- ticle sizes larger than the wavelength of light. However, it was found that a plot of log(I0/Idiffuse) against wavelength * Present address: Advanced Process Technology, Inc., 911 E. gave an acceptably accurate representation of the spec- Arapaho Rd., Suite 105E, Richardson, Texas 75081, U.S.A. trum. This quantity is termed ``absorbance'' in the ®gures ² E-mail: [email protected] and is de®ned as 0003±004X/98/0708±0865$05.00 865 866 BOLDISH AND WHITE: OPTICAL BAND GAPS OF SULFIDES TABLE 1. Sources of minerals Mineral Composition Source Locality Boulangerite 5PbS´2Sb2S3 SWMS Zacatecas, Mexico Bournonite 2PbS´Cu2S´Sb2S3 MU Mineral King Mine, British Columbia Cylindrite Pb3Sn4Sb2S14 MI Poopo, Bolivia Enargite Cu3AsS4 MI Zacatecas, Mexico Franckeite Pb5Sn3Sb2S14 MI Itos Mine, Oruro, Bolivia FuÈloÈppite 3PbS´4Sb2S3 MU Baia Mare, Romania Gratonite 9PbS´2As2S3 W Cerre de Pasco, Peru Livingstonite HgSb4S8 W Unknown Lorandite Tl2S´As2S3 W Allchar, Macedonia Meneghinite Cu2S´26PbS´7Sb2S3 W Barrie Twp., Ontario Miargyrite AgSbS2 MU Santa Fe Mine, Red Mt. Randsburg mining district Owyheeite 5PbS´Ag2S´3Sb2S3 MU Near Gabbs, Nye Co., Nevada Plagionite 5PbS´4Sb2S3 USNM Wolfsberg, Harz, Germany Proustite 3Ag2S´As2S3 W Marienberg, Saxony, Germany Pyrargyrite 3Ag2S´Sb2S3 W Highland Bell Mine, Beaverdell, British Columbia Semseyite 9PbS´4Sb2S3 W Herja, Romania Stephanite Ag5SbS4 W Freiberg, Saxony, Germany Zinckenite 6PbS´7Sb2S3 W Stevens Co., Washington Note: MI 5 Penn State Mineral Museum; MU 5 Minerals Unlimited, Ridgecrest, CA; SWMS 5 Southwest Mineral Supplies; USNM 5 National Museum of Natural History, Washington, DC; W 5 Wards, Rochester, NY. diffuse reflectance of BaSO Extraction of the optical absorption edge abs 5 log4 (2) []diffuse reflectance of sample An ideal semiconductor or insulator, with no defects or A basic assumption in diffuse re¯ectance spectroscopy impurities, has an intrinsic absorptivity in the range of is that light is transmitted through the powder particles a 1022 to 1024/cm at wavelengths between the Urbach tail number of times combined with multiple re¯ections as of the electronic edge at the short wavelength limit and the light moves from grain to grain into the powder layer the phonon edge at the long wavelength limit. The pho- and back out again. When the powder is highly absorb- non edge is determined by the multiphonon tail of the ing, surface re¯ectivity of the grains becomes a dominant intrinsic vibrational modes. The phonon edge occurs at effect and the observed spectrum becomes more like a roughly twice the highest wavenumber longitudinal pho- specular re¯ectance spectrum than an absorption spec- non (White 1990). For sul®des the phonon edge occurs trum. For this reason, diffuse re¯ectance measurements in the range of 1000±1500 cm21, well beyond the range at very high absorbances are not true measures of the of the present measurements. The diffuse re¯ectance at absorbance. This effect is responsible for the slight bend- the long wavelength end of the measured spectra is dom- ing over of some of the spectra near the top of the ab- inated by re¯ection and scattering due to the high refrac- sorbance scale. tive index of the sul®de compounds. In a few of the spectra, the ``absorbance'' drops below At the absorption edge, the absorptivity rises rapidly to zero in the low absorbance region. This is an artifact due values in the range of 105 to 106/cm due to the strongly to spectral regions in which the re¯ectance of the sample allowed transition moments of the interband transitions. Measurement of the entire interband absorption spectrum is greater than the re¯ectance of the BaSO4 reference. The small dips near 1400 and 1900 nm are due to tiny by direct transmission requires ®lms with thicknesses of amounts of water adsorbed on the reference material. 10 to 100 nm. Alternatively the spectra can be measured by specular re¯ectance followed by deconvolution of the re¯ectance curves to separate the real and imaginary parts of the refractive index. TABLE 2. Conditions for synthesis Diffuse re¯ectance spectroscopy can be used to deter- Mineral Composition mine the optical absorption edge but not much more of ÐAg2S´Sb2S3 Heated for 45 h at 450 8C and cooled the interband spectrum although Wood and Strens (1979) (cubic) slowly for 2 h to room temperature. attempted to use diffuse re¯ectance measurements to the Dufrenoysite 2PbS´As2S3 Heated at 620 8C for 24 h, then quenched in water. Annealed for 4 edge of the vacuum ultraviolet at 200 nm. The ideal dif- days at 200 8C. fuse re¯ectance spectrum (for example, the spectra of Jordanite 27PbS´7As S Heated for 60 h at 450 8C and cooled 2 3 wurtzite and greenockite in Fig. 1) consists of a nearly for 2 h to room temperature. ¯at, low absorbance region at long wavelengths that Proustite 3Ag2S´As2S3 Heated for 30 h at 450 8C and cooled slowly. abruptly transforms to a steeply rising absorption edge at Pyrargyrite 3Ag S´Sb S Heated for 48 h at 450 8C. 2 2 3 shorter wavelengths. Smithite AgAsS2 Heated at 595 8C for 14 h and quenched. The resulting material The most direct way of extracting the optical band gap was powdered and annealed for 14 is to simply determine the wavelength at which the ex- h at 380 8C. trapolations of the base line and the absorption edge BOLDISH AND WHITE: OPTICAL BAND GAPS OF SULFIDES 867 TABLE 3. Comparison of measured band gaps with literature values Compound Eg (ex) Eg (McL) Eg (lit) Method Reference Ag2S 0.92 Ð Ð Diffuse Re¯ectance This investigation As2S3 Ð Ð 2.37 Transmission Evans and Young (1967) CdS (h) 2.30 2.43 2.452 Electrore¯ectance Cardona et al.