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Infrared optical properties of calcium lanthanum sulfide
Materials Letters 64 (2010) 334–336
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Infrared optical properties of calcium lanthanum sulfide Celia I. Merzbacher 1, Daniel L. Chess 2, William B. White ⁎ Materials Research Institute, Materials Research Lab. Bldg., The Pennsylvania State University, University Park, PA 16802, USA
a r t i c l e
i n f o
Article history: Received 31 October 2009 Accepted 3 November 2009 Available online 10 November 2009 Keywords: IR windows CaLa2S4 IR spectra
a b s t r a c t The fundamental infrared optical properties of CaLa2S4 were determined by specular reflectance measurements from theoretically dense, optically transparent, polycrystalline ceramics. Kramers–Kronig transformation of the reflectance spectra permitted the extraction of refractive index, and transverse and longitudinal optic modes. Five transverse modes predicted by factor group analysis were observed with the intense high wavenumber mode occurring at 142 and 205 cm− 1. The other three modes are weak and occur at 37, 59, and 86 cm− 1. The two high wavenumber intense modes have identifiable longitudinal modes at l51 and 318 cm− 1. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Calcium lanthanum sulfide, CaLa2S4, has been developed as a farinfrared optical ceramic with a bandpass in the 8–14 μm atmospheric window somewhat wider than that of zinc sulfide [1,2]. The material is also harder and more refractory than zinc sulfide although its thermal expansion coefficient is substantially larger [3]. Extensive research on the ceramic processing of CaLa2S4 has produced specimens of good optical quality, transparent in the visible spectrum and with few impurity features in the infrared [4–7]. The objective of the present paper is to present the fundamental infrared optical properties of this material. This is accomplished by measuring the mid- and far-infrared specular reflectance spectra of theoretically dense CaLa2S4 ceramics. The optical properties are obtained by deconvoluting the reflectance spectra. Good spectra can be obtained from dense ceramics, especially if the ceramic grains are optically isotropic which is true for CaLa2S4 [8]. 2. Preparation of specimens and spectroscopic measurements CaLa2S4 was synthesized by the spray pyrolysis method using solutions of calcium and lanthanum nitrates as starting materials. Spraying the nitrate solutions into a hot-wall furnace at 800 °C produced a stoichiometric oxide precursor with a particle size of 1–2 μm. This material was converted to sulfide by firing in an atmosphere of a 50–50 mixture of H2S and H2 at 1000 °C for 48 h. The fine-grained sulfide powder was cold-pressed into 2 cm diameter disks, and sintered at ⁎ Corresponding author. Tel.: +1 814 865 1152. E-mail address: [email protected] (W.B. White). 1 Present address: Semiconductor Research Corp., 1101 Slater Rd., Suite 120, Durham, NC 27703, USA. 2 Present address: Environmental Sciences Division, Thomas J. Watson Research Center, IBM Corp., Yorktown Heights, New York, USA. 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.11.007
1500 °C in an H2S atmosphere to produce a hard ceramic with on the order of 90% of theoretical density. Hot isostatic pressing at 1400 °C and 20 MPa pressure for 1.5 h produced a theoretically dense ceramic that was optically transparent. Great care was required to avoid impurities that would produce absorption features in the mid-infrared, the important IR window region. Three specimens were prepared. Due to variations in the processing parameters, one was somewhat darkened, although transparent, one was intermediate, and the third was a lighter yellow-orange color. These were simply labeled A, B, and C and served to determine the effect, if any, of processing variables on the IR optical properties. Specular reflectance spectra were obtained using an IBM Instruments Fourier transform infrared spectrometer equipped with a 15° off-axis specular reflectance attachment. A front surface aluminum mirror was used as a reference. Spectra were measured over the range of 30 to 3900 cm− 1. Optical parameters were obtained by deconvolution of the reflectance spectra by a Kramers–Kronig transform [9]. The ideal K–K transform requires integration over a wavenumber range of zero to infinity, thus some method is needed to allow for the finite wavenumber range of real spectrometers. There are some discrepancies among the various software packages and approaches to the calculation [10]. The K–K program used was written in this laboratory and used the wing correct approach of Andermann et al. [11] and gives results that compare favorably with deconvolutions obtained by classical oscillator fits. The full range of the reflectance measurements were used for the K–K analysis.
3. Results and discussion The reflectance spectra of the three specimens were all similar. The spectrum of specimen A is shown in Fig. 1. Because the features of interest all lie in the low wavenumber range, the printed spectra are
C.I. Merzbacher et al. / Materials Letters 64 (2010) 334–336
335
Table 1 Vibrational mode wavenumbers for CaLa2S4 (cm− 1).
A B C
TO-1
LO-1
TO-2
LO-2
TO-3
TO-4
TO-5
204 202 209
319 318 318
143 143 144
153 157 151
87 85 86
62 58 58
35 36 41
Table 2 Factor group classification of normal modes of CaLa2S4. Td
Fig. 1. Far IR reflectance spectrum of specimen A.
A1 A2 E T1 T2
Acoustic modes
Vibrational modes
Selection rules
(x,y,z)
1 2 3 5 5
Raman, αii Inactive Raman, αii Inactive IR; Raman, αij
shown only to 600 cm− 1. The reflectance is 23% at 10 μm and rises slowly to 24% at the upper end of the measured spectral range. The K–K transform gives the real part, n, and the imaginary part, k, of the refractive index as a function of wavenumber. The transverse vibrational modes appear as maxima in the imaginary part of the permittivity, ε″ = 2nk (Fig. 2A). The longitudinal modes appear as maxima in the function Im(1/ε) (Fig. 2B). The three specimens gave slightly different values for the mode wavenumbers (Table 1) but are within the expected error of both reflectance measurements and the calculations which used a 5 cm− 1 spacing between data points. The refractive index at 10 μm is 2.85. Table 2 gives a factor group classification for the zone-center ̅ phonons of the Th3P4 structure, space group I4 3d. Five modes are predicted to be infrared-active and that is what is observed. The results obtained for CaLa2S4 may be compared with an earlier study of SrNd2S4 [12]. The SrNd2S4 ceramic was prepared by sintering a disk in flowing H2S. It had a grain size of 20–50 μm and retained considerable porosity so that the specimen was not transparent. IR reflectance could be measured only down to 100 cm− 1 so that the low wavenumber modes listed in Table 1 were not observed. The two reflectance bands were deconvoluted by a classical oscillator fit and the best fit was obtained using oscillators at 132, 186, and 200 cm− 1. The two high wavenumber modes were not apparent in the reflectance spectrum. In contrast, the present reflectance spectra (Fig. 1) show two maxima but the K–K transform generates only one peak in the ε″ spectrum. For CaLa2S4, as contrasted with SrNd2S4 these modes could be too closely spaced to be resolved in the K–K transform. If this should be the case, then one of the weak low wavenumber modes must be a 2-phonon peak. 4. Conclusions Good quality spectra in the far infrared allow calculation of the fundamental phonon energies and other IR optical properties of the IR window material, CaLa2S4. Acknowledgement This work was supported by the Office of Naval Research under contract no. N00014-85-K-0129. References Fig. 2. (A) Transverse phonon peaks for CaLa2S4 derived by calculating ε″ from the reflectance spectrum. (B) Longitudinal phonon peaks for CaLa2S4 derived by calculating Im (1/ε) from the reflectance spectrum.
[1] [2] [3] [4]
White WB, Chess D, Chess CA, Biggers JV. SPIE Trans 1981;297:38. White WB. Refractory sulfides as IR window materials. Proc SPIE 1990;1326:80. Schevciw O, White WB. Mater Res Bull 1983;18:1059. Chess DL, Chess CA, Biggers JV, White WB. J Amer Ceram Soc 1983;66:18.
336
C.I. Merzbacher et al. / Materials Letters 64 (2010) 334–336
[5] Saunders KJ, Wong TY, Hartnett TM, Tustison RW, Gentilman RL. Proc SPIE 1986;683:72. [6] Savage JA, Lewis KL, Kinsman BE, Wilson AR, Riddle R. Proc SPIE 1986;683:78. [7] Harris DC, Hills ME, Gentilman RL, Saunders KJ, Wong TY. Adv Ceram Mater 1987;2:74.
[8] [9] [10] [11] [12]
Bliss M, Walden BL, White WB. J Amer Ceram Soc 1990;73:1078. Turrell G. Infrared and Raman spectra of crystals. London: Academic Press; 1972. Lichvár P, Liška M, Galusek D. Ceram-Silikáty 2002;46:25. Andermann G, Caron A, Dows DA. J Opt Soc Amer 1965;55:1210. Provenzano PL, Boldish SI, White WB. Mater Res Bull 1977;12:939.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Infrared optical properties of calcium lanthanum sulfide Celia I. Merzbacher 1, Daniel L. Chess 2, William B. White ⁎ Materials Research Institute, Materials Research Lab. Bldg., The Pennsylvania State University, University Park, PA 16802, USA
a r t i c l e
i n f o
Article history: Received 31 October 2009 Accepted 3 November 2009 Available online 10 November 2009 Keywords: IR windows CaLa2S4 IR spectra
a b s t r a c t The fundamental infrared optical properties of CaLa2S4 were determined by specular reflectance measurements from theoretically dense, optically transparent, polycrystalline ceramics. Kramers–Kronig transformation of the reflectance spectra permitted the extraction of refractive index, and transverse and longitudinal optic modes. Five transverse modes predicted by factor group analysis were observed with the intense high wavenumber mode occurring at 142 and 205 cm− 1. The other three modes are weak and occur at 37, 59, and 86 cm− 1. The two high wavenumber intense modes have identifiable longitudinal modes at l51 and 318 cm− 1. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Calcium lanthanum sulfide, CaLa2S4, has been developed as a farinfrared optical ceramic with a bandpass in the 8–14 μm atmospheric window somewhat wider than that of zinc sulfide [1,2]. The material is also harder and more refractory than zinc sulfide although its thermal expansion coefficient is substantially larger [3]. Extensive research on the ceramic processing of CaLa2S4 has produced specimens of good optical quality, transparent in the visible spectrum and with few impurity features in the infrared [4–7]. The objective of the present paper is to present the fundamental infrared optical properties of this material. This is accomplished by measuring the mid- and far-infrared specular reflectance spectra of theoretically dense CaLa2S4 ceramics. The optical properties are obtained by deconvoluting the reflectance spectra. Good spectra can be obtained from dense ceramics, especially if the ceramic grains are optically isotropic which is true for CaLa2S4 [8]. 2. Preparation of specimens and spectroscopic measurements CaLa2S4 was synthesized by the spray pyrolysis method using solutions of calcium and lanthanum nitrates as starting materials. Spraying the nitrate solutions into a hot-wall furnace at 800 °C produced a stoichiometric oxide precursor with a particle size of 1–2 μm. This material was converted to sulfide by firing in an atmosphere of a 50–50 mixture of H2S and H2 at 1000 °C for 48 h. The fine-grained sulfide powder was cold-pressed into 2 cm diameter disks, and sintered at ⁎ Corresponding author. Tel.: +1 814 865 1152. E-mail address: [email protected] (W.B. White). 1 Present address: Semiconductor Research Corp., 1101 Slater Rd., Suite 120, Durham, NC 27703, USA. 2 Present address: Environmental Sciences Division, Thomas J. Watson Research Center, IBM Corp., Yorktown Heights, New York, USA. 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.11.007
1500 °C in an H2S atmosphere to produce a hard ceramic with on the order of 90% of theoretical density. Hot isostatic pressing at 1400 °C and 20 MPa pressure for 1.5 h produced a theoretically dense ceramic that was optically transparent. Great care was required to avoid impurities that would produce absorption features in the mid-infrared, the important IR window region. Three specimens were prepared. Due to variations in the processing parameters, one was somewhat darkened, although transparent, one was intermediate, and the third was a lighter yellow-orange color. These were simply labeled A, B, and C and served to determine the effect, if any, of processing variables on the IR optical properties. Specular reflectance spectra were obtained using an IBM Instruments Fourier transform infrared spectrometer equipped with a 15° off-axis specular reflectance attachment. A front surface aluminum mirror was used as a reference. Spectra were measured over the range of 30 to 3900 cm− 1. Optical parameters were obtained by deconvolution of the reflectance spectra by a Kramers–Kronig transform [9]. The ideal K–K transform requires integration over a wavenumber range of zero to infinity, thus some method is needed to allow for the finite wavenumber range of real spectrometers. There are some discrepancies among the various software packages and approaches to the calculation [10]. The K–K program used was written in this laboratory and used the wing correct approach of Andermann et al. [11] and gives results that compare favorably with deconvolutions obtained by classical oscillator fits. The full range of the reflectance measurements were used for the K–K analysis.
3. Results and discussion The reflectance spectra of the three specimens were all similar. The spectrum of specimen A is shown in Fig. 1. Because the features of interest all lie in the low wavenumber range, the printed spectra are
C.I. Merzbacher et al. / Materials Letters 64 (2010) 334–336
335
Table 1 Vibrational mode wavenumbers for CaLa2S4 (cm− 1).
A B C
TO-1
LO-1
TO-2
LO-2
TO-3
TO-4
TO-5
204 202 209
319 318 318
143 143 144
153 157 151
87 85 86
62 58 58
35 36 41
Table 2 Factor group classification of normal modes of CaLa2S4. Td
Fig. 1. Far IR reflectance spectrum of specimen A.
A1 A2 E T1 T2
Acoustic modes
Vibrational modes
Selection rules
(x,y,z)
1 2 3 5 5
Raman, αii Inactive Raman, αii Inactive IR; Raman, αij
shown only to 600 cm− 1. The reflectance is 23% at 10 μm and rises slowly to 24% at the upper end of the measured spectral range. The K–K transform gives the real part, n, and the imaginary part, k, of the refractive index as a function of wavenumber. The transverse vibrational modes appear as maxima in the imaginary part of the permittivity, ε″ = 2nk (Fig. 2A). The longitudinal modes appear as maxima in the function Im(1/ε) (Fig. 2B). The three specimens gave slightly different values for the mode wavenumbers (Table 1) but are within the expected error of both reflectance measurements and the calculations which used a 5 cm− 1 spacing between data points. The refractive index at 10 μm is 2.85. Table 2 gives a factor group classification for the zone-center ̅ phonons of the Th3P4 structure, space group I4 3d. Five modes are predicted to be infrared-active and that is what is observed. The results obtained for CaLa2S4 may be compared with an earlier study of SrNd2S4 [12]. The SrNd2S4 ceramic was prepared by sintering a disk in flowing H2S. It had a grain size of 20–50 μm and retained considerable porosity so that the specimen was not transparent. IR reflectance could be measured only down to 100 cm− 1 so that the low wavenumber modes listed in Table 1 were not observed. The two reflectance bands were deconvoluted by a classical oscillator fit and the best fit was obtained using oscillators at 132, 186, and 200 cm− 1. The two high wavenumber modes were not apparent in the reflectance spectrum. In contrast, the present reflectance spectra (Fig. 1) show two maxima but the K–K transform generates only one peak in the ε″ spectrum. For CaLa2S4, as contrasted with SrNd2S4 these modes could be too closely spaced to be resolved in the K–K transform. If this should be the case, then one of the weak low wavenumber modes must be a 2-phonon peak. 4. Conclusions Good quality spectra in the far infrared allow calculation of the fundamental phonon energies and other IR optical properties of the IR window material, CaLa2S4. Acknowledgement This work was supported by the Office of Naval Research under contract no. N00014-85-K-0129. References Fig. 2. (A) Transverse phonon peaks for CaLa2S4 derived by calculating ε″ from the reflectance spectrum. (B) Longitudinal phonon peaks for CaLa2S4 derived by calculating Im (1/ε) from the reflectance spectrum.
[1] [2] [3] [4]
White WB, Chess D, Chess CA, Biggers JV. SPIE Trans 1981;297:38. White WB. Refractory sulfides as IR window materials. Proc SPIE 1990;1326:80. Schevciw O, White WB. Mater Res Bull 1983;18:1059. Chess DL, Chess CA, Biggers JV, White WB. J Amer Ceram Soc 1983;66:18.
336
C.I. Merzbacher et al. / Materials Letters 64 (2010) 334–336
[5] Saunders KJ, Wong TY, Hartnett TM, Tustison RW, Gentilman RL. Proc SPIE 1986;683:72. [6] Savage JA, Lewis KL, Kinsman BE, Wilson AR, Riddle R. Proc SPIE 1986;683:78. [7] Harris DC, Hills ME, Gentilman RL, Saunders KJ, Wong TY. Adv Ceram Mater 1987;2:74.
[8] [9] [10] [11] [12]
Bliss M, Walden BL, White WB. J Amer Ceram Soc 1990;73:1078. Turrell G. Infrared and Raman spectra of crystals. London: Academic Press; 1972. Lichvár P, Liška M, Galusek D. Ceram-Silikáty 2002;46:25. Andermann G, Caron A, Dows DA. J Opt Soc Amer 1965;55:1210. Provenzano PL, Boldish SI, White WB. Mater Res Bull 1977;12:939.