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Optical absorption edge in rare earth sesquisulfides
Materials Research Bulletin 41 (2006) 448–454 www.elsevier.com/locate/matresbu
Optical absorption edge in rare earth sesquisulfides Cheryl M. Forster 1, William B. White * Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA Received 21 July 2005; accepted 29 July 2005 Available online 19 August 2005
Abstract Optical absorption edge spectra have been measured by diffuse reflectance spectroscopy on a complete suite of rare earth sesquisulfide compounds of varying structure type. Optical band gaps were extracted from these data. The observed band gaps tend to vary with crystal structure type but many compounds with the same structure have similar band gaps independently of the particular rare earth ion. The ordered a-structure has a substantially lower band gap than the defect g-structure for all compounds examined except for La2S3. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Chalcogenides; A. Optical materials; D. Optical properties
1. Introduction The sesquisulfides of the trivalent rare earths are a large family of high band gap semiconductors with potential application as optical window materials [1]. Infrared window materials must have to be transparent within the atmospheric window regions of 1–5 and/or 8–14 mm. To evaluate the rare earth sulfides as window materials, it is necessary to measure the transparent region by determining the onset of phonon and electron absorption regions that border it. The crystal structures of the rare earth sesquisulfides are well known and are summarized in Table 1. The Greek letter notation is that of Flahout and his colleagues [2] and is widely, although not universally, used. The feature of interest in the present paper is the optical band gap. Some compounds have * Corresponding author. Tel.: +1 814 865 1152; fax: +1 814 865 2326. E-mail address: [email protected] (W.B. White). 1 Present address: Department of Materials Science, University of Utah, Salt Lake City, UT, USA. 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.07.035
C.M. Forster, W.B. White / Materials Research Bulletin 41 (2006) 448–454
449
Table 1 Summary of rare earth sesquisulfide structures Notation a b g d e a b c
Space group
Pnma I41/acd ¯ I 43d P21/m ¯ R3c Structure determination by Prewitt and Sleight [4]. Notation of Sleight and Prewitt [5]. Structure determination by White et al. [6].
Ln3+ coordination
Alternate notation a
b
Gd2S3-type ; A-type B-type b Th3P4-type; C-typeb Ho2S3-typec; D-typeb Corundum-type; E-type b
8–7 8–7 8–8 7–6 6
complicated optical absorption spectra due to f ! f transitions. In addition, most have a sharp optical absorption edge in the visible or near-infrared region that permits a reasonable estimate of the optical band gap by direct spectroscopic measurement. Some absorption edges, measured from diffuse reflectance spectra of powdered samples, were reported previously [3]. Since the first report, it has been possible to perform better syntheses of the materials and to prepare compounds in structure types that were not previously available. One objective of the present paper is simply to extend the data base and report optical band gaps on the new compounds and also to obtain improved data on several compounds that are now available in very fine-grained form. The second objective is to elucidate the relationship between the optical band gap and the crystal structure. The gap energy shows systematic variations across the rare earth series, which merits interpretation.
2. Experimental methods The compounds used for the spectroscopic measurements were prepared by a two-step process. The first step was to prepare fine-grained oxides by spray pyrolysis [7]. The second step was to sulfidize the oxides with minimum grain growth. The initial starting materials were rare earth nitrates. These were dissolved in water and pumped into the spray pyrolysis furnace at 900 8C as previously described [8]. The generic reaction is LnðNO3 Þ3 nH2 O ! Ln2 O3 þ nH2 O þ nitrogen oxides The end product of this step was fully reacted, near-spherical particles with particle sizes in the range of 2–5 mm. The highly reactive oxide particles were placed in a graphite boat and the temperature ramped up to values in the range of 1000–1200 8C. The exact temperature for any particular experiment was the temperature needed to produce the desired polymorph of the rare earth sesquisulfides. Heat-up and cooldown were done under an argon atmosphere. When the furnace reached the final reaction temperature, the gas flow was switched to H2S. The final products were phase-pure rare earth sesquisulfides as determined by X-ray diffraction. The particle sizes were similar to those of the parent oxides but many particles were connected by narrow necks, indicating that some sintering had taken place in the H2S atmosphere. The particle size was taken as 3–5 mm. The previous measurements [3] were made on powders prepared by direct high temperature reaction in a H2S atmosphere which produced particle sizes in the range of 30–40 mm. Measurements were also
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made on a limited set of commercial (Cerac Inc.) rare earth sulfide powders. These also had particle sizes in the 30–40 mm range. Before measurement they were heated in flowing H2S to strip out any residual oxygen. All spectra were measured by diffuse reflectance on plaques of packed powder. Spectra were obtained on a Varian Cary 2300 UV-vis-near IR spectrophotometer fitted with an integrating sphere coated with Kodak BaSO4 white optical paint. Microcrystalline MgO was used as a reference material. The spectral range was 2000–340 nm. It was found that absorbance could be adequately represented by log (reflectance of reference)/(reflectance of sample) and that a Kubelka–Munck function was not needed.
3. Results 3.1. Color The rare earth sesquisulfides are strongly colored materials with color varying systematically with structure type. The colors were quantified according to the Munsell color system [9] (Table 2). The notation is given as Munsell hue (H) which is the perceived color, the Munsell value (V) which is the lightness based on a scale of 0 for ideal black and 10 for ideal white, and chroma (C) for the colors’ Table 2 Munsell color for rare earth sesquisulfides Phase
Visual color
Munsell color
Alpha-structure phases La2S3 Pr2S3 Nd2S3 Sm2S3 Gd2S3 Dy2S3
Yellow Maroon Brown Maroon Maroon Orange
7.5 Y 7/4 10 R 2.5/2 5 R 3/4 7.5 R 3/6 2.5 YR 4/6 2.5 YR 4/8
Beta-structure phases La2S3 Pr2S3 Nd2S3
Yellow Lime green Lime green
10 Y 9/8 2.5 GY 8/8 2.5 GY 6/8
Gamma-structure phases La2S3 Pr2S3 Nd2S3 Sm2S3 Gd2S3 Dy2S3
Yellow Lime green Lime green Gold Brown Yellow
10 Y 9/6 2.5 GY 6/6 2.5 GR 5/6 10 YR 5/8 10 YR 4/4 2.5 Y 7/6
Delta-structure phases Y2S3 Ho2S3 Er2S3
Yellow Tan Yellow
5 Y 7/4 2.5 Y 8/10 2.5 Y 8/8
Epsilon-structure phases Yb2S3
Yellow
2.5 Y 7/10
C.M. Forster, W.B. White / Materials Research Bulletin 41 (2006) 448–454
451
Fig. 1. Diffuse reflectance spectrum of a-La2S3.
departure from neutral gray on a scale of 0 for gray materials to 20 for color saturated materials. The notation is written H V/C. The data in Table 2 were obtained by visual matching of plaques of the rare earth sesquisulfide samples with matte-finish Munsell color plaques. 3.2. Spectra Spectra typical of those observed are shown in Figs. 1–3. There is a transparent region at longer wavelengths and an abrupt absorption edge followed by a region of very intense absorption at shorter
Fig. 2. Diffuse reflectance spectrum of a-Sm2S3.
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Fig. 3. Diffuse reflectance spectrum of d-Ho2S3.
wavelengths. For many of the rare earth sesquisulfides, the f ! f transitions of the rare earth ions are superimposed as a sequence of sharp, weak lines. The examples given show La2S3 which has only an absorption edge, Sm2S3 in which the f ! f transitions are separated from the absorption edge, and Ho2S3 in which the f ! f transitions are superimposed on the absorption edge. Because the data presented are obtained from small particle-size materials, the diffuse reflectance spectra should be a better representation of the absorption spectra than earlier measurements on coarser-grained materials. Not all absorption edges are equally sharp. For some, of which Sm2S3 is an example, the edge knees over abruptly and the absorption edge rises rapidly, to give a very precise value for the onset of absorption. In the spectra of other compounds, such as La2S3, the absorption edge bends over gradually over a substantial wavelength range. This behavior is generally attributed to the presence of exciton or defect states directly below the bottom of the conduction band. 3.3. Band gaps The optical absorption edge was determined by drawing straight lines through the base line in the transparent region and through the absorption edge. The wavelength where these lines cross was taken as the absorption edge. Conversion to energy units gives the optical band gap (Table 3). In a different study [10], band gaps determined from diffuse reflectance spectra for a number of binary sulfides were compared with band gaps determined by single crystal reflectance and electrical measurements. In general, the diffuse reflectance band gaps were within 0.1 eV or less of the single crystal measurements. The discrepancy, if any, was systematically on the low side. These and other data suggest that a comparable accuracy should be assigned to the data in Table 3. Three sets of data are given in Table 3. First are the data measured on the fine-grained powders obtained from sulfidizing spray pyrolysis oxides. Second are the data measured on the H2S-treated but otherwise unmodified commercial sulfides. Third, for comparison, are the partial data obtained from the previous investigation [3]. It can be seen that the band gaps determined from the commercial compounds
C.M. Forster, W.B. White / Materials Research Bulletin 41 (2006) 448–454
453
Table 3 Optical band gaps for rare earth sesquisulfides (in eV) Compound
Phase
EDS-powders
Commercial
Previous report [3]
La2S3 Pr2S3 Nd2S3 Sm2S3 Gd2S3 Tb2S3 Dy2S3 La2S3 Pr2S3 Nd2S3 La2S3 Pr2S3 Nd2S3 Sm2S3 Gd2S3 Dy2S3 Ho2S3 Y2S3 Er2S3 Yb2S3
a a a a a a a b b b g g g g g g d d d e
2.73 1.64 1.93 1.69 1.67 1.68 1.85 2.76 2.48 2.43 2.70 2.28 2.53 2.18 1.56 2.53 2.53 2.53 2.48 2.39
2.64 1.67 1.77 1.69 1.74 1.70 1.70 – – – – – – – – – 2.00 2.53 2.59 –
2.70 1.68 1.68 1.71 1.73 1.70 1.77 – – – – – – – – – 2.58 2.58 2.58 2.38
are very similar to those obtained in the previous investigation. The band gaps measured on the finegrained powders are systematically somewhat lower and are believed to be more accurate.
4. Discussion and conclusions The band gaps measured on fine-grained powders are plotted against the radii of the rare earth ions in Fig. 4. The influence of crystal structure is clear in that, with the exception of La2S3, the gaps for the same
Fig. 4. Optical band gap as a function of rare earth ionic radius. Radii are 8-coordination Shannon–Prewitt radii [11].
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rare earth sulfide vary considerably depending on the polymorph. All three polymorphs of La2S3 have essentially the same band gap. Curiously, the defect g-structure has a higher band gap than the more ordered a-structure. The rare earth sesquisulfides with the b, g, and d-structures nearly all have band gaps in the range of 2.5 eVor above. Compounds with the a-structure have much lower band gaps, in the range of 1.7 eV. An unexplained exception is Gd2S3 for which the g-structure polymorph has the lowest observed band gap.
Acknowledgement Research supported by the Office of Naval Research under Contract Nr. N00014-85-K-0219.
References [1] W.B. White, Proc. S.P.I.E. 1326 (1990) 80. [2] J. Flahout, in: K.A. Gschneidner, L. Eyring (Eds.), Handbook on the Physics and Chemistry of the Rare Earths, vol. 4, North Holland, Amsterdam, 1979, p. 1. [3] O. Schevciw, W.B. White, Mater. Res. Bull. 18 (1983) 1059. [4] C.T. Prewitt, A.W. Sleight, Inorg. Chem. 7 (1967) 1090. [5] A.W. Sleight, C.T. Prewitt, Inorg. Chem. 7 (1968) 2282. [6] J.G. White, P.N. Yocum, S. Lerner, Inorg. Chem. 6 (1967) 1872. [7] G.L. Messing, S.-C. Zhang, G.V. Jayanthi, J. Am. Ceram. Soc. 76 (1993) 2707. [8] C.M. Vaughn-Forster, W.B. White, J. Am. Ceram. Soc. 80 (1997) 273. [9] Munsell Book of Color, Kollmorgen Corp., Baltimore, MD, 1976. [10] S.I. Boldish, W.B. White, Am. Miner. 83 (1998) 865. [11] R.D. Shannon, C.T. Prewitt, Acta Cryst. B 25 (1969) 925.
Optical absorption edge in rare earth sesquisulfides Cheryl M. Forster 1, William B. White * Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA Received 21 July 2005; accepted 29 July 2005 Available online 19 August 2005
Abstract Optical absorption edge spectra have been measured by diffuse reflectance spectroscopy on a complete suite of rare earth sesquisulfide compounds of varying structure type. Optical band gaps were extracted from these data. The observed band gaps tend to vary with crystal structure type but many compounds with the same structure have similar band gaps independently of the particular rare earth ion. The ordered a-structure has a substantially lower band gap than the defect g-structure for all compounds examined except for La2S3. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Chalcogenides; A. Optical materials; D. Optical properties
1. Introduction The sesquisulfides of the trivalent rare earths are a large family of high band gap semiconductors with potential application as optical window materials [1]. Infrared window materials must have to be transparent within the atmospheric window regions of 1–5 and/or 8–14 mm. To evaluate the rare earth sulfides as window materials, it is necessary to measure the transparent region by determining the onset of phonon and electron absorption regions that border it. The crystal structures of the rare earth sesquisulfides are well known and are summarized in Table 1. The Greek letter notation is that of Flahout and his colleagues [2] and is widely, although not universally, used. The feature of interest in the present paper is the optical band gap. Some compounds have * Corresponding author. Tel.: +1 814 865 1152; fax: +1 814 865 2326. E-mail address: [email protected] (W.B. White). 1 Present address: Department of Materials Science, University of Utah, Salt Lake City, UT, USA. 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.07.035
C.M. Forster, W.B. White / Materials Research Bulletin 41 (2006) 448–454
449
Table 1 Summary of rare earth sesquisulfide structures Notation a b g d e a b c
Space group
Pnma I41/acd ¯ I 43d P21/m ¯ R3c Structure determination by Prewitt and Sleight [4]. Notation of Sleight and Prewitt [5]. Structure determination by White et al. [6].
Ln3+ coordination
Alternate notation a
b
Gd2S3-type ; A-type B-type b Th3P4-type; C-typeb Ho2S3-typec; D-typeb Corundum-type; E-type b
8–7 8–7 8–8 7–6 6
complicated optical absorption spectra due to f ! f transitions. In addition, most have a sharp optical absorption edge in the visible or near-infrared region that permits a reasonable estimate of the optical band gap by direct spectroscopic measurement. Some absorption edges, measured from diffuse reflectance spectra of powdered samples, were reported previously [3]. Since the first report, it has been possible to perform better syntheses of the materials and to prepare compounds in structure types that were not previously available. One objective of the present paper is simply to extend the data base and report optical band gaps on the new compounds and also to obtain improved data on several compounds that are now available in very fine-grained form. The second objective is to elucidate the relationship between the optical band gap and the crystal structure. The gap energy shows systematic variations across the rare earth series, which merits interpretation.
2. Experimental methods The compounds used for the spectroscopic measurements were prepared by a two-step process. The first step was to prepare fine-grained oxides by spray pyrolysis [7]. The second step was to sulfidize the oxides with minimum grain growth. The initial starting materials were rare earth nitrates. These were dissolved in water and pumped into the spray pyrolysis furnace at 900 8C as previously described [8]. The generic reaction is LnðNO3 Þ3 nH2 O ! Ln2 O3 þ nH2 O þ nitrogen oxides The end product of this step was fully reacted, near-spherical particles with particle sizes in the range of 2–5 mm. The highly reactive oxide particles were placed in a graphite boat and the temperature ramped up to values in the range of 1000–1200 8C. The exact temperature for any particular experiment was the temperature needed to produce the desired polymorph of the rare earth sesquisulfides. Heat-up and cooldown were done under an argon atmosphere. When the furnace reached the final reaction temperature, the gas flow was switched to H2S. The final products were phase-pure rare earth sesquisulfides as determined by X-ray diffraction. The particle sizes were similar to those of the parent oxides but many particles were connected by narrow necks, indicating that some sintering had taken place in the H2S atmosphere. The particle size was taken as 3–5 mm. The previous measurements [3] were made on powders prepared by direct high temperature reaction in a H2S atmosphere which produced particle sizes in the range of 30–40 mm. Measurements were also
450
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made on a limited set of commercial (Cerac Inc.) rare earth sulfide powders. These also had particle sizes in the 30–40 mm range. Before measurement they were heated in flowing H2S to strip out any residual oxygen. All spectra were measured by diffuse reflectance on plaques of packed powder. Spectra were obtained on a Varian Cary 2300 UV-vis-near IR spectrophotometer fitted with an integrating sphere coated with Kodak BaSO4 white optical paint. Microcrystalline MgO was used as a reference material. The spectral range was 2000–340 nm. It was found that absorbance could be adequately represented by log (reflectance of reference)/(reflectance of sample) and that a Kubelka–Munck function was not needed.
3. Results 3.1. Color The rare earth sesquisulfides are strongly colored materials with color varying systematically with structure type. The colors were quantified according to the Munsell color system [9] (Table 2). The notation is given as Munsell hue (H) which is the perceived color, the Munsell value (V) which is the lightness based on a scale of 0 for ideal black and 10 for ideal white, and chroma (C) for the colors’ Table 2 Munsell color for rare earth sesquisulfides Phase
Visual color
Munsell color
Alpha-structure phases La2S3 Pr2S3 Nd2S3 Sm2S3 Gd2S3 Dy2S3
Yellow Maroon Brown Maroon Maroon Orange
7.5 Y 7/4 10 R 2.5/2 5 R 3/4 7.5 R 3/6 2.5 YR 4/6 2.5 YR 4/8
Beta-structure phases La2S3 Pr2S3 Nd2S3
Yellow Lime green Lime green
10 Y 9/8 2.5 GY 8/8 2.5 GY 6/8
Gamma-structure phases La2S3 Pr2S3 Nd2S3 Sm2S3 Gd2S3 Dy2S3
Yellow Lime green Lime green Gold Brown Yellow
10 Y 9/6 2.5 GY 6/6 2.5 GR 5/6 10 YR 5/8 10 YR 4/4 2.5 Y 7/6
Delta-structure phases Y2S3 Ho2S3 Er2S3
Yellow Tan Yellow
5 Y 7/4 2.5 Y 8/10 2.5 Y 8/8
Epsilon-structure phases Yb2S3
Yellow
2.5 Y 7/10
C.M. Forster, W.B. White / Materials Research Bulletin 41 (2006) 448–454
451
Fig. 1. Diffuse reflectance spectrum of a-La2S3.
departure from neutral gray on a scale of 0 for gray materials to 20 for color saturated materials. The notation is written H V/C. The data in Table 2 were obtained by visual matching of plaques of the rare earth sesquisulfide samples with matte-finish Munsell color plaques. 3.2. Spectra Spectra typical of those observed are shown in Figs. 1–3. There is a transparent region at longer wavelengths and an abrupt absorption edge followed by a region of very intense absorption at shorter
Fig. 2. Diffuse reflectance spectrum of a-Sm2S3.
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Fig. 3. Diffuse reflectance spectrum of d-Ho2S3.
wavelengths. For many of the rare earth sesquisulfides, the f ! f transitions of the rare earth ions are superimposed as a sequence of sharp, weak lines. The examples given show La2S3 which has only an absorption edge, Sm2S3 in which the f ! f transitions are separated from the absorption edge, and Ho2S3 in which the f ! f transitions are superimposed on the absorption edge. Because the data presented are obtained from small particle-size materials, the diffuse reflectance spectra should be a better representation of the absorption spectra than earlier measurements on coarser-grained materials. Not all absorption edges are equally sharp. For some, of which Sm2S3 is an example, the edge knees over abruptly and the absorption edge rises rapidly, to give a very precise value for the onset of absorption. In the spectra of other compounds, such as La2S3, the absorption edge bends over gradually over a substantial wavelength range. This behavior is generally attributed to the presence of exciton or defect states directly below the bottom of the conduction band. 3.3. Band gaps The optical absorption edge was determined by drawing straight lines through the base line in the transparent region and through the absorption edge. The wavelength where these lines cross was taken as the absorption edge. Conversion to energy units gives the optical band gap (Table 3). In a different study [10], band gaps determined from diffuse reflectance spectra for a number of binary sulfides were compared with band gaps determined by single crystal reflectance and electrical measurements. In general, the diffuse reflectance band gaps were within 0.1 eV or less of the single crystal measurements. The discrepancy, if any, was systematically on the low side. These and other data suggest that a comparable accuracy should be assigned to the data in Table 3. Three sets of data are given in Table 3. First are the data measured on the fine-grained powders obtained from sulfidizing spray pyrolysis oxides. Second are the data measured on the H2S-treated but otherwise unmodified commercial sulfides. Third, for comparison, are the partial data obtained from the previous investigation [3]. It can be seen that the band gaps determined from the commercial compounds
C.M. Forster, W.B. White / Materials Research Bulletin 41 (2006) 448–454
453
Table 3 Optical band gaps for rare earth sesquisulfides (in eV) Compound
Phase
EDS-powders
Commercial
Previous report [3]
La2S3 Pr2S3 Nd2S3 Sm2S3 Gd2S3 Tb2S3 Dy2S3 La2S3 Pr2S3 Nd2S3 La2S3 Pr2S3 Nd2S3 Sm2S3 Gd2S3 Dy2S3 Ho2S3 Y2S3 Er2S3 Yb2S3
a a a a a a a b b b g g g g g g d d d e
2.73 1.64 1.93 1.69 1.67 1.68 1.85 2.76 2.48 2.43 2.70 2.28 2.53 2.18 1.56 2.53 2.53 2.53 2.48 2.39
2.64 1.67 1.77 1.69 1.74 1.70 1.70 – – – – – – – – – 2.00 2.53 2.59 –
2.70 1.68 1.68 1.71 1.73 1.70 1.77 – – – – – – – – – 2.58 2.58 2.58 2.38
are very similar to those obtained in the previous investigation. The band gaps measured on the finegrained powders are systematically somewhat lower and are believed to be more accurate.
4. Discussion and conclusions The band gaps measured on fine-grained powders are plotted against the radii of the rare earth ions in Fig. 4. The influence of crystal structure is clear in that, with the exception of La2S3, the gaps for the same
Fig. 4. Optical band gap as a function of rare earth ionic radius. Radii are 8-coordination Shannon–Prewitt radii [11].
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rare earth sulfide vary considerably depending on the polymorph. All three polymorphs of La2S3 have essentially the same band gap. Curiously, the defect g-structure has a higher band gap than the more ordered a-structure. The rare earth sesquisulfides with the b, g, and d-structures nearly all have band gaps in the range of 2.5 eVor above. Compounds with the a-structure have much lower band gaps, in the range of 1.7 eV. An unexplained exception is Gd2S3 for which the g-structure polymorph has the lowest observed band gap.
Acknowledgement Research supported by the Office of Naval Research under Contract Nr. N00014-85-K-0219.
References [1] W.B. White, Proc. S.P.I.E. 1326 (1990) 80. [2] J. Flahout, in: K.A. Gschneidner, L. Eyring (Eds.), Handbook on the Physics and Chemistry of the Rare Earths, vol. 4, North Holland, Amsterdam, 1979, p. 1. [3] O. Schevciw, W.B. White, Mater. Res. Bull. 18 (1983) 1059. [4] C.T. Prewitt, A.W. Sleight, Inorg. Chem. 7 (1967) 1090. [5] A.W. Sleight, C.T. Prewitt, Inorg. Chem. 7 (1968) 2282. [6] J.G. White, P.N. Yocum, S. Lerner, Inorg. Chem. 6 (1967) 1872. [7] G.L. Messing, S.-C. Zhang, G.V. Jayanthi, J. Am. Ceram. Soc. 76 (1993) 2707. [8] C.M. Vaughn-Forster, W.B. White, J. Am. Ceram. Soc. 80 (1997) 273. [9] Munsell Book of Color, Kollmorgen Corp., Baltimore, MD, 1976. [10] S.I. Boldish, W.B. White, Am. Miner. 83 (1998) 865. [11] R.D. Shannon, C.T. Prewitt, Acta Cryst. B 25 (1969) 925.