Contains research objectives and reports on two research projects.Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U.S. Air Force) under Contract DA 36-039-AMC-03200(E)U.S. Air Force (ESD Contract AF19(628)-6066)Sloan Fund for Basic Research (M.I.T. Grant

Fertel, Jeanne H.

McNelly, T. F.

Perry, C. H.

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VIII. OPTICAL AND INFRARED SPECTROSCOPY*Academic and Research StaffProfessor C. H. PerryGraduate StudentsJeanne H. FertelD. J. MuehlnerJ. F. ParrishN. TornbergRESEARCH OBJECTIVES AND SUMMARY OF RESEARCHThe activities of this group have been concentrated on the study of theerties of solids determined by both infrared and Raman spectroscopy.being extended to the study of low-frequency electronic and magneticsolids at liquid-helium temperatures.optical prop-This work istransitions in1. Interferometric Spectroscopyl' 2The far infrared Michelson interferometer has been operating successfully over thewavelength range 25-1000 k on a reasonably routine basis. A step-drive sampling inter-val is being considered and the frequency range has now been extended almost to650 cm - 1 A 212 bit analog-to-digital converter and a new lock-in amplifier areexpected to yield more accurate intensity measurements so that full use can be made ofthe low-temperature bolometer.References1. C. H. Perry, R. Geick, and E. F. Young, Appl. Opt. 5, 1171 (1966).2. C. H. Perry, "Solid-State Spectroscopy in the Far Infrared," Optical Society ofAmerica Annual Meeting, San Francisco, California, October 19-21, 1966(Paper FC3).2. Lattice Vibrationsa. Infrared Spectra of SolidsThe far infrared instrumentation has been described in previous reports and publi-1,2 3 4 5cations. Materials investigated have included BN; CdSe; Mg 2 Sn; NH 4 C1 andNH4 Br; 6 KTaO3 ; M PdX4 , PdL 4X2 and trans and cis isomers PdLX with M= NH++ + floik 9 (K , Rb or Cs ; L = NH3; X = Cl-, Br or I-; some ABF 3 fluoride perovskites (with+ + 2+ 2+ 2+ 2+ 2+ 2+ .2+A = K or Rb ; B = Mg , Ni Co , Mn+, Zn , and Mg x  i x) and someThis work was supported in part by the joint services Electronics ProgramsU.S. Army, U.S. Navy, and U.S. Air Force) under Contract DA 36-039-AMC-03200(E),the U.S. Air Force (ESD Contract AF19(628)-6066), and the Sloan Fund for BasicResearch (M.I.T. Grant). C!: $ 14)QPR No. 84ir~B I ~. : ' I~ ~,P~~Y(VIII. OPTICAL AND INFRARED SPECTROSCOPY)tektites.10 A temperature-dependent study of the dielectric dispersion of some alkalihalides and related ionic crystals continues and will include some mixed crystals ofsimple alkali halides. Some mixed semiconductors, ferroelectrics, antiferromagneticmaterials and other organic solids will be investigated. We also propose to studycondensed-phase gases.b. Raman Spectra of SolidsThe temperature-dependent Raman spectrum of NH 4 C1, ND 4 C1, NH 4 Br, (Na K(Nax 1-x)TaO 3 , KTaO3 , and SrTiO3 (see Sec. VIII-B) have been studied, and this work con-tinues so that it can be complementary to the far infrared measurements. A Spectra-Physics 125 He/Ne laser is to be incorporated with the present spectrometer, andconsiderable improvement in signal-to-noise ratio and sample handling is to be expected.A new improved gas-flow Dewar system is being constructed for accurate low-temperature measurements in the region of various phase transitions.C. H. PerryReferences1. C. H. Perry, Japan. J. Phys., Vol. 4, Suppl. 1, p. 564, 1965.2. C. H. Perry, R. Geick and E. F. Young, Appl. Opt. 5, 1171 (1966).3. R. Geick, C. H. Perry, and G. Rupprecht, Phys. Rev. 146, 543 (1966).4. R. Geick, C. H. Perry, and S. S. Mitra, J. Appl. Phys. 37, 1994 (1966).5. R. Geick, W. Hakel, and C. H. Perry, Phys. Rev. 148, 824 (1966).6. J. F. Reintjes, Jr. and C. H. Perry, Symposium on Molecular Structure and Spec-troscopy, Ohio State University, Columbus, Ohio, September 1966 (Paper U10);Quarterly Progress Report No. 82, Research Laboratory of Electronics, M. I. T.,July 15, 1966, pp. 48-53.7. C. H. Perry, EUCHEM FAR INFRA-RED Conference, Culham, England, Septem-ber 12-16, 1966; T. F. McNelly and C. H. Perry, Quarterly Progress Report No. 83,Research Laboratory of Electronics, M.I.T., October 15, 1966, pp. 26-29; (thispaper will appear in The Physical Review).8. C. H. Perry, D. P. Athans, E. F. Young, J. R. Durig, and B. R. Mitchell, QuarterlyProgress Report No. 81, Research Laboratory of Electronics, M.I.T., April 15,1966, pp. 27-32; (this paper will appear in Spectrochimica Acta).9. E. F. Young, Ph.D. Thesis, Department of Physics, M.I.T., August 1966; E. F. Youngand C. H. Perry, Symposium on Molecular Structure and Spectroscopy, Ohio StateUniversity, Columbus, Ohio, September 6-10, 1966 (Paper U9);Quarterly ProgressReport No. 83, Research Laboratory of Electronics, M. I. T., October 15, 1966,pp. 17-25.10. C. H. Perry and J. D. Wrigley, Jr., Quarterly Progress Report No. 82, ResearchLaboratory of Electronics, M.I.T., July 15, 1966, pp. 45-48; (this paper will appearin Applied Optics).11. J. R. Reintjes, Jr., S.B. Thesis, M.I.T., May 1966; J. F. Reintjes and C. H. Perry,Symposium on Molecular Structure and Spectroscopy, Ohio State University, Colum-bus, September 6-10, 1966 (Paper U10);Quarterly Progress Report No. 82, ResearchLaboratory of Electronics, M.I.T., July 15, 1966, pp. 48-53.12. T. F. McNelly, S.B. Thesis, Department of Physics, M.I.T., May 1966.13. T. F. McNelly and C. H. Perry, Symposium on Molecular Structure and Spectroscopy,Ohio State University, Columbus, Ohio, September 6-10 (Paper Ull).QPR No. 84(VIII. OPTICAL AND INFRARED SPECTROSCOPY)A. WORK COMPLETEDA thesis, entitled "Normal Vibrations of Cubic Fluoride Perovskites" was submittedby E. F. Young to the Department of Physics, M.I.T., August 1966, in partial fulfillmentof the requirements for the degree of Doctor of Philosophy.C. H. PerryB. TEMPERATURE DEPENDENCE OF THE RAMAN SPECTRUM OF KTaO3AND SrTiO31. IntroductionThe Raman spectrum of single crystals KTaO3 and SrTiO3 have been observed from-1 -130-1200 cm frequency shift from the 22, 938 cm mercury "e" line over the temper-ature range 4-4400 K. The room-temperature Raman spectrum of SrTiO 3 has been pre-viously observed by Narayanan and Vedam, and their results are in qualitativeagreement with our measurements. Both SrTiO3 and KTaO3 are paraelectric cubiccrystals at room temperature and apart from a slight structural change in SrTiO3 at110 0 K no observation of the onset of ferroelectricity has been observed above a fewdegrees Kelvin. These crystals exhibit a Curie-Weiss law behavior,3 however, andrecent theories of ferroelectricity in perovskite crystals5 have been confirmed experi-mentally for both of these crystals.6-8The temperature-dependent Raman spectra observed in this work has beeninterpreted mainly as second-order spectra in both cases. The known cubic sym-metry implies three first-order infrared active modes and one infrared and Ramaninactive mode, together with the translational modes, none of which are first-order Raman active. Narayanam and Vedaml have erroneously interpreted theirresults as a first-order spectrum for SrTiO3.2. ExperimentThe Raman spectrum was obtained on several different samples by using aCary 81 spectrophotometer. A gas-flow technique, which has been described pre-viously,8 was used to obtain sample temperatures at 4-440 0 K. The spectra atvarious temperatures for KTaO3 are shown in Fig. VIII-1, and for SrTiO3 inFig. VIII-2.3. DiscussionThe temperature-dependent spectra for KTaO3 are surprisingly strong and9complex. No correlation with the known infrared bands could be seen, how-ever, and the spectra have been interpreted as primarly second order. Table VIII-1QPR No. 841200Av WAVE NUMBER (CM"1)Fig. VIII-1. Spectra for KTaO3 at various temperatures.3 Av- 8cm305 K438K 223K175'K123 K MA Kz_WAVE NUMBERS (CM- 1)Fig. VIII-2. Spectra for SrTiO3 at various temperatures.QPR No. 84II I - - I I(VIII. OPTICAL AND INFRARED SPECTROSCOPY)shows the frequency dependence of the various peaks as a function of tempera--1 -1ture. Of particular interest are the bands at -125 cm and 460 cm . Theintensity of the latter band decreases as the temperature is lowered, which indi-cates the presence of a difference band, whereas the intensity of the formerband increases with a decrease in temperature, thereby showing a combinationprocess. This effect is true for a number of other bands, although not to sucha marked extent.From Table VIII-1 it can be seen that most of the frequencies slowly increasewith increasing temperature. This behavior is somewhat unusual but may be theinfluence of the "soft" ferroelectric mode which markedly lowers its frequencyas the temperature is lowered toward the "Curie" temperature at 3K. 8 All ofthe bands have been interpreted in terms of combinations of phonons at the edgeof the Brillouin zone where both energy and momentum considerations can besatisfied and the density of states is high. The assignment of the multi-phononpeaks at 295 0 K are given in Table VIII-2, together with the averaged energies-1(in cm- ) at the edge of the Brillouin zone.Table VIII-1. Temperature dependence of the second-order Raman spectra-1of KTaO3 (in cm- ).90 0 K 123 0 K 295 0 K 333 0 K 348 0K 413 0K 473 0KA -- 100± 8sh 100± 8sh 100± 8 100± 8B 105 110± 5 118± 5 120± 8 122± 8 127±10 132+ 10C -- 160 ± 15sh 160 ± 15sh 160 ± 15sh 160± 15shD 230+ 10sh 230 10sh 230+ 10sh 230+ 10sh 230± 10sh 230 ± 10sh 233± 10E 253± 5 256± 5 265± 5 268± 5 270± 5 277± 5 286± 5F 370 10sh 370 ± 10sh 370± 10sh 370± 10sh 370± 10shG 458± 458± 5 460± 5 460± 5 461± 5 463± 5 465± 5H -- - - 530±10sh 530±10I 580± 5 582± 5 587± 5 592± 5 592± 8 596± 10 598± 10K 685± 15 686± 15 688±15 693± 15 695± 15 695±20 695±20L 750 15 750±15 760±15 760±15 765±20 765±20 770±20M 1100±50 1100±50 1100±50 1100±50sh = shoulder.QPR No. 84Table VIII-2. Assignment to the multi-phonon peaks at 2950K for KTaO3 .Observed (cm - l )  Calculated (cm-1) AssignmentA 100 100 TO Z-LAB 120 120 TA +LA140 O4-TO 2C 160 170 LOZ-LA170 LO 1-TO2225 TO -LAD 230 1225 TO2+TA290 LO +TAE 270 2275 TO +LAF 375 360 T0 3-TO 2G 460 465 T0 3 -LAH 530 530 LO +TO 2585 TO3+TAI 590 585 04+LO2595 LO +LOJ 650 640 0 4 +TO1650 LOl+T0 1K 690 690 LOl+ 0 4705 LO3+TA740 TO3+TO 2L 760 2755 LO3+LA1020 LO3+TO 3M 1100 3+LO1210 LO3+LO3 L1H at 473 0K only.J observed in IR only.K and L observed in IR and Raman.Average energies (in cm -l ) at the edgeTA 35 TO 1 310 TO2 190 TO 3 540 04LA 90 LO1 360 LO 2 255 LO3 670.of the Brillouin zone.330.Table VIII-3. Raman bands observed in SrTiO3.40K 78 0 K 98 0 K 123 0 K 175 0 K 223 0 K 305 0 K 438 0KA 45+ 5 60±5 65± 5 65± 5 75± 5 80± 5 85± 5B 63+ 8 75± 8C 85 ± 8D 235+ 5 235± 5 240± 5 240± 8 240± 8 250+10 250±15E 280± 5 285+ 8 290+10 295±10 300± 15 310±15 320±20F 315+10 - 325±20 - 350±15 360±15 360±15G - 400±20 400±20 375±20 380±20 - -H 445 ± 5I 615± 5 620± 5 620± 8 625± 10 620±10 620± 10 630+10J 670±10 675±10 675±10 680±10 675±10 675±10 680±10K 720±10 720±10 720+10 715±10 715±10 720±10 715±10L 1035±20 1035±20 1030±20QPR No. 84(VIII. OPTICAL AND INFRARED SPECTROSCOPY)Cowley1 0 has discussed thoroughly the lattice dynamics and phase transitions inSrTiO 3 , based on both experimental and theoretical studies. The frequency versuswave-vector dispersion curves for some of the normal modes were measured byneutron spectroscopy. 10The Raman spectra of SrTiO3 as a function of temperature is shown in Fig. VIII-2and is drastically different from that of KTaO3.-1The spectra consist of three main bands centered around 85, 320, and 640 cmThe two broad bands show considerable structure and these smaller peaks arequite temperature-dependent. The highest frequency band appears to consist mainlyof summation processes, whilst the central peak, from its marked temperaturedependence, appears to consist of both sum and difference multi-phonon processes.The frequency-versus-temperature dependence is shown in Table VIII-3 and theassignments are given in Table VIII-4. In most cases, the interpretation has beenmade from the neutron dispersion curves given by Cowley.10 Low-frequency bandsobserved in the spectra below the phase transition are not fully understood, how--1ever. The strongly temperature-dependent line (A) which occurs at 85 cm at-1room temperature and which appears to lower to ~40 cm-1 at 4 0 K could be the first-order ferroelectric "soft" mode (TO 1 ) at the zone center, although the frequencydependence with temperature is not quite the same as that observed in the infraredand neutron measurements. 6 ' 7 The other bands (B and C), shown at 78 0 K and 4 0Kclose to the Rayleigh line, are probably due to additional first-order lattice modesappearing with the transition of the crystal to the tetragonal phase, or they may becaused by the crystal splitting up into small domains as proposed by Rimai andde Mars. Z  On the other hand, it is possible to attribute these bands to differenceprocesses but it appears to be unlikely that they would still be present at liquid-helium temperatures.The temperature dependence of the spectrum between 250 cm-1 and 410 cm-1can be explained by assuming that difference multi-phonon processes occur in thisregion. Possible combinations are the following.Observed Assignment Calculated (cm- l)Broad Band TO3-TO Z  330-1250-400 cm LO -TO 2  360TO 3 -LA 365LOZ-LA 395TO3-TO1 405TO3-TA 410QPR No. 84Table VIII-4. Assignment to the multi-phonon peaks at 305*K for SrTiO3.Observed (cm ) Calculated (cm-1) AssignmentD 250 240 TO +TAE 310 285 LA +TA290 LA +T0 1315 TOI+TO 2 (in zone)320 TA +TO 2325 TO +TO ZF 360 365 TO +LAG 3 8 0 a 385 LO +TA390 LO1 +TO 1H 4 4 5 b 430 LA +LO 1450 TA + 04455 TO + 0 4LA + 04I 620 620 04+ 04J 675 670 TO +TA675 TO3+TO 1680 LO 2 +TA685 LO +TO 1695 LO +TAK 720 725 LA +LO 2L 1030 1025 LO3+LO 111050 LO + 0 4a980K-2230K.b 4 oK.Table VIII-4a. Average energies (in cm-) at the edge of the Brillouin zone.Table VIII-4a. Average energies (in cm ) at the edge of the Brillouin zone.TA -120TO 1 - 125TO2 - 2000 4 - 330TO 3 - 530LA -165LO - 265104 - 290LO2 - 560LO 3 - 760QPR No. 84(VIII. OPTICAL AND INFRARED SPECTROSCOPY)In conclusion, it appears that these multi-phonon Raman spectra are extremely dif-ficult to interpret with any certainty. Cowley's results for SrTiO3 can be assigned, butthere is a multiple choice in most cases. The results of the energies of the dispersioncurves at the edge of the Brillouin zone for KTaO3 are quite tentative. Consequently,one must await the complete neutron spectrum as a function of wave vector so thatworth-while dynamical calculations can be made of the phonon dispersion curves. Wewould like to thank Professor A. Smakula and Dr. A. Linz of the Materials Center, M.I.T.for the samples.C. H. Perry, Jeanne H. Fertel, T. F. McNelly[Mr. T. F. McNelly is now in the Physics Department, Cornell University, Ithaca,New York.]References1. P. S. Narayanan and K. Vedam, Physik 163, 158 (1961).2. L. Rimai and G. A. de Mars, Phys. Rev. 123, 702 (1962).3. R. O. Bell and G. Rupprecht, Phys. Rev. 129, 90 (1963).4. G. Rupprecht and R. O. Bell, Phys. Rev. 135, A748 (1964).5. W. Cochran, Advances in Physics, Vol. 9, p. 387, 1960.6. A. S. Barker and M. Tinkham, Phys. Rev. 125, 1527 (1962).7. R. A. Cowley, Phys. Rev. Letters 9, 159 (1962).8. C. H. Perry and T. McNelly, Quarterly Progress Report No. 82, Research Labo-ratory of Electronics, M.I.T., October 15, 1966, pp. 26-30.9. D. B. Hall and C. H. Perry, Quarterly Progress Report No. 78, Research Laboratoryof Electronics, M.I.T., pp. 51-61; Phys. Rev. Letters 15, 600 (1965).10. R. A. Cowley, Phys. Rev. 134, A981 (1964).QPR No. 84

Optical and Infrared Spectroscopy

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Optical and Infrared Spectroscopy

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