Contains report on one research project.Joint Services Electronics Programs (U.S. Army, U.S. Navy, and U.S. Air Force) under Contract DA 28-043-AMC-02536(E)U.S. Air Force (ESD Contract AFl9(628)-6066)M. I. T. Sloan Fund for Basic Researc

Parrish, J. F.

Perry, C. H.

DSpace@MIT

IV. OPTICAL AND INFRARED SPECTROSCOPYAcademic and Research StaffProf. C. H. PerryGraduate StudentsT. G. Davis D. J. Muehlner J. F. ParrishJeanne H. Fertel N. E. TornbergA. OPTICAL PHONONS IN CdS Se MIXED CRYSTALSx l-x1. IntroductionThe infrared transmittance and reflectance of pure crystals of CdS and CdSe have1-4been reported several years ago. More recently, the infrared reflectance of thehexagonal mixed crystals of CdS/CdSe have been reported. 5 ' 6 The reflection spectraof CdS Se for several values of x have been remeasured and are reported here.x 1-xThe Raman spectra of these crystals have been measured at 300 0 K and 80 0 K in col-laboration with Brafman, Chang and Mitra at the University of Rhode Island and arereported for the first time. The transmission spectra of several more of the mixedcrystals initially discussed in a previous report 7 have been measured at 300 0 K, 80 0 Kand 100 K.It is evident that in the reststrahlen spectra of CdSxSel- x the infrared active phononsoccur in two main bands with frequencies near those of pure CdS and pure CdSe, whichis characteristic of a system in which short-range forces influence the lattice dynam-ics. 5 In this respect the present system neither behaves as ionic mixed crystals norentirely as covalent mixed crystals. In mixed systems composed primarily of ioniccrystals,8 for example, NixCol x , KC1 xBr -x' NaxRb -xI, CaxSrl-xF , etc., on accountof the presence of long-range Coulomb interactions only a single reststrahlen bandoccurs, and the long-wavelength longitudinal optic (LO) and transverse optic (TO) modesshift continuously and linearly with concentration. On the other hand, in a mixed sys-tem composed of covalent crystals like Ge :Si the triply degenerate k - 0 optic-modefrequency of Ge shifts slightly downward with increasing Si content, rather than shiftingupward continuously from the heavier to the lighter components' frequency.The present investigation, which includes Raman measurements, reveals thebehavior of both k - 0 LO and TO phonons as functions of x. At presentThis work was supported principally by the Joint Services Electronics Programs(U.S. Army, U.S. Navy, and U.S. Air Force) under Contract DA 28-043-AMC-02536(E),and in part by the U.S. Air Force (ESD Contract AFl9(628)-6066), and the M. I. T. SloanFund for Basic Research.QPR No. 87(IV. OPTICAL AND INFRARED SPECTROSCOPY)CdS Sex f-xthus far.constitutes by far the most complete and interesting system studied2. Measurements and ResultsThe CdSxSel x system appears to form good mixed crystals with hexagonal unit cells7similar to the wurtzite CdSe and CdS structures. Seven mixed crystals were investi-gated in the concentration range 0.07 < x < 0.71. The samples were quantitatively ana-lyzed and found to be stoichiometric with a standard deviation of one mole per cent orless. Power x-ray diffraction measurements revealed unique lattice constants "a" and"c" throughout the concentration range that was studied and varied linearly with x,thereby indicating that the CdS/CdSe system does form a 'good' mixed crystal with a hex-agonal lattice. 7Since for both CdS and CdSe the anisotropy splitting of the transverse optic phononsis limited3, 4 to only 6 or 7 cm- , and that of the LO phonons hardly 1 cm , all mea-surements were made on unoriented crystals.The reflection spectra were measured with the R.I.I.C. F-S-520 Fourier Spectrom-eter. The spectra were extracted from the quantized interferograms by taking a discretephase-corrected Fourier transform of the data with the IBM 7094 computer at the M.I.T.Computation Center. A typical reflection spectrum is shown in Fig. IV-l.Table IV-1. Raman band positions and their assignments for CdS 79Se.21.Raman Band Assignmentposition (cm-1)41 E 2 (F)87 Multiphonon137 ?182 E 1 , A 1 (r) TO(CdSe)196* LO(CdSe) (F)213 Multiphonon242 E 1 , Al(r) TO(CdS)256 E 2 (r)298 LO(CdS) (r)335 Multiphonon360 Multiphonon413 ?390 2LO(CdSe)493 LO(CdSe) + LO(CdS)595 2LO(CdS)Relatively strong bandsQPR No. 8780 -604020- oooo40 80 120 160 200 240 280 320 360 400WAVE NUMBERS (CM- 1 )Fig. IV-1. Reststrahlen spectrum of CdS. 5 6 8 Se 4 3 2 . Solid line:measured curve. Circles: two-resonance damped-1 -1oscillator fit. (E =5.7, v1 =175 cm , 2 = 248 cm ,pl =O.Z, p2=0.12Z, yl=0.045, y2=0.045).6400 6450 X(A)Fig. IV-2. A portion of the Raman spectrum for 3 concentrations:x= 0.57; x= 0.79; x= 0.93. [Brafman, Chang and Mitra,private communication.]QPR No. 87 27i CP II L_ I ~c;(IV. OPTICAL AND INFRARED SPECTROSCOPY)The Raman spectra observed by Brafman, Chang, and Mitra were excited with a He-Nelaser (6328 A) in the perpendicular scattering mode and were analyzed by means of a Spex1400 double monochromator. Measurements were also made with the Cary 81 Raman Spec-trophotometer in the Spectroscopy Laboratory, M.I.T., in the back-scattering (1800 ) con-figuration, with identical band positions but with occasionally altered relative intensities.The Raman spectrum at each concentration contained many bands. An example is shownin Table IV-1, which lists the Raman bands observed with CdS .79Se0.21. An interestingportion of the Raman spectrum for three intermediate x-values is shown in Fig. IV-Z.The far infrared transmission spectra of several unoriented slices of the mixedcrystal samples were measured at several temperatures by means of Perkin-Elmerspectrophotometers (Model 301 and Model 521) and also by an R.I.I.C. FS-520 FourierCdS/CdSe2(TO, LO) S 05/952TOSe 29/71Z TO +L0Z Se+ OSe 57/430%79/21S91/090% 2LOSo%02TOS TOS+LOSTO CdS ZONE CENTER 2(TO,LO)Se0%200 300 400 500 600WAVE NUMBERS - CM- 1Fig. IV-3. Room temperature infrared transmission spectrum(00 -1 -(200 cm -650 cm ) of CdS Se for 5 values of x.x 1-xspectrophotometer. Figure IV-3 shows the room temperature spectra of 5 of the samplesfrom 200 cm to 650 cm . Since the frequencies change very little with temperature,in this report we shall discuss primarily the room temperature data.3. DiscussionInformation about the long-wavelength TO modes of the mixed crystals is containedin the reflection spectra. Kramers-Kronig analyses of the room-temperature spectraQPR No. 87(IV. OPTICAL AND INFRARED SPECTROSCOPY)were performed to obtain the real (E') and imaginary (E") parts of the complex dielec-tric constant and other optical constants. A typical result is shown in Fig. IV-4. In all32272217Zz0U 12ULUoLin_//E~790 120 150 180 210 240 270 300 330WAVE NUMBERS CMIFig. IV-4. Kramers-Kronig analysis of the reflectionspectrum of Fig. IV-1, giving the real andimaginary parts of the dielectric constant.cases, only two distinct peaks with occasional minor structures were noted in the curvesfor extinction coefficient, conductivity or E" vs frequency. The peak frequency of theconductivity curves are assigned to TO(CdS) and TO(CdSe), the assignment depending ontheir positions. Verlur and Barker,6 on the other hand, have used a model of latticedynamics, which utilizes 7 distinct TO phonons at k ~ 0. A closer inspection of their valuesreveals that for each mixed crystal studied, 2 of the 7 modes considered stand out instrength over the rest. Moreover, a two-resonance damped oscillator can be made tofit our room-temperature reststrahlen spectra, as shown in Fig. IV-1.In the absence of direct measurement, the k ~ 0 LO mode frequency for a singleQPR No. 87(IV. OPTICAL AND INFRARED SPECTROSCOPY)oscillator may be inferred either from the Lyddane-Sachs-Teller (LST) relation or fromDrude's rule (the frequency at which E' = 0 or n = k). For multiple oscillators, althougha generalized LST relation involving all of the modes holds good,10 it does not, however,yield the LO mode frequencies individually. Often Drude's rule has been invoked for themultiresonance oscillators also,11 although without any theoretical justification. Further-more, in some cases, for example, when one of the oscillators is much weaker than theothers (for low and high values of x in the present case, the n and k curves may notcross and therefore the application of the Drude's rule is impractical. Chang and Mitral2have shown that it is possible to obtain the LO frequencies individually as functions of3002802602- 240UUZ 220200 -180 - 41600.0 0.2 0.4 0.6 0.8 1.0CdSe CdSX, MOLE FRACTION OF CdSFig. IV-5. k ~ 0 optical phonons of CdS Se as functions of x.x 1-x1: LO(CdS); 2: TO(CdS); 3: LO(CdSe); 4: TO(CdSe);O - Raman (80°K); A - Raman (3000 K); E- Kramers-Kronig analysis, using present reflectivity data;l--Kramers-Kronig analysis, using data of Verlur andBarker 6 ; x - Two-resonance damped oscillator fit ofthe present reflectivity data.QPR No. 87(IV. OPTICAL AND INFRARED SPECTROSCOPY)oscillator parameters. They have also shown that Drude's rule holds only under cer-tain conditions, viz., the LO frequencies are obtainable only if E' = E" = 0 occurs at thesame frequency. The LO mode frequencies for a few of the mixed crystals have beendetermined in this fashion. Thus, for each TO frequency we obtain one LO frequency.Experimental determination of the LO modes is possible by Raman scattering measure-ments. The two strongest bands in the Raman spectra of each crystal were identifiedwith the LO phonons at k - 0. Their positions showed excellent agreement with thoseobtained from the modified Drude's rule. The two LO and the two TO modes for eachmixed crystal also followed the generalized LST relation 1 0 within the limits of experi-mental accuracy.The one-phonon optic frequencies are plotted as functions of the mole fraction x of100 r-WAVE NUMBER(CM 1)"-,./\ I• \I Z-joV aU-I< 012t f100 200 300WAVE NUMBER (CM- I)- ROOM TEMPERATURE---- LIQUID NITROGENTEMPERATURE1.15mm CdSeL-o0>o-00_j _400 500 600Fig. IV-6. Local mode spectrum of S impurity in CdSe. (a) Reflec-tion spectrum. (b) Absorption spectrum at two tempera-tures showing the local mode fundamental. (c) Firstovertone of the local mode.QPR No. 87_II I I -- _~ I ~I -(IV. OPTICAL AND INFRARED SPECTROSCOPY)CdS in Fig. IV-5. It may be noted that the LO frequency of CdS monotonically decreases,while the TO frequency increases with increasing concentration of CdSe. These two linesintersect at a point in the region of 100% CdSe. This triply degenerate phonon at 268 cm-1may be considered the quasi-localized mode of S in CdSe. The local mode frequencymay be approximately evaluated 6 by using the Dawber and Elliott theory. 13 A value of-1285 cm-1 is obtained for the local-mode frequency, a value not too different, consid-ering the approximations involved, from the value predicted from the behavior of the14mixed crystals. Perry, Geick, and Young have noted the presence of this local-1mode at 268 cm in the reflection spectrum of CdSe with a nominal amount of S impu-6rity, which is shown in Fig. IV-6. Verlur and Barker's spectrum of CdS 0 1 5 Se 9 8 5confirms this observation. The local mode spectrum is best demonstrated by the trans-mission spectrum of a CdSe single crystal with S impurity as presented in Fig. IV-6.The local mode and its first overtone show the characteristic absorption and tempera-ture dependence.The behavior of the long-wavelength optic modes of the heavier component may besummarized as follows. The LO of CdSe decreases and its TO increases with increasingconcentration of CdS. The point of intersection occurring in the region of 100% CdS maylikewise be regarded as a resonant gap mode of Se in CdS. This triply degenerate-1phonon occurs at 188 cm- . An approximate estimate of the gap mode may be made byapplying Jaswal'sl5 model on NaI to the present case. Using the phonon assignment ofMarshall and Mitra 2 for CdS, we have obtained a value of 191 cm -l for the gap-modefrequency.A sum rule pertaining to the optic phonons of mixed zincblende crystals was discov-16ered in connection with the system ZnSxSe x . We find that the sum rule also holdsgood for the present system. It has been found that the sum of one-phonon optic-modefrequencies at any concentration, properly weighted for degeneracy (1 for an LO, 2 fora TO, and 3 for a local or a gap mode) is a constant. The 'average' one-phonon fre-quency of the mixed system, which occurs at 225 ± 2 cm - 1 , is to a very good approxi-mation halfway between the quasi-local mode frequency of S in CdSe (268 cm - 1 ) and the-iresonant gap mode of Se in CdS (188 cm- ). Another consequence of this sum rule isthat in the mixed-crystal system ABxC1-x(mB<mC) the sum of the local mode of B inAC and the gap mode of C in AB can be predicted from the k ~ 0 optic phonons of thepure crystals AB and AC alone:Local mode + gap mode = (TO+LO + (TO LO . . (1)AB AC-1In the case of CdSxSel-x, the left-hand side of Eq. 1 is 456 cm -1, whereas theright-hand side is 446 cm - l1 . The sum rule is apparently obeyed rigorously by theZnSxSel-x system.16 The relatively less exact agreement noted in the present caseQPR No. 87(IV. OPTICAL AND INFRARED SPECTROSCOPY)may be attributed (i) to the exclusion of some of the other allowed k - 0 optic phononsfor the wurtzite structure, and (ii) to the fact that, although CdSxSelx is hexagonal forall values of x, the expected anisotropy splitting has been neglected here.The variation of the long-wavelength optic phonon frequencies with concentration, aspresented in Fig. IV-5, is unique, in the sense that it is different from either that of theionic 7 or the covalent 8 crystals. Yet, in some respects, it reflects the pertinent fea-tures of either type of solid. The lines (1) and (4) in Fig. IV-2 are in some ways remi-niscent of ionic crystals,8 whereas lines (2) and (3) are similar to those found forcovalent crystals. The II-VI compounds may thus be regarded in many ways as behavingas ionic crystals (for example, the separation of the LO and TO branches at k - 0), yetin some other respects they are like covalent crystals (for example, the short-rangeforces that give rise to characteristic frequencies for both the components in the mixedcrystal, Ge:Si).Besides the TO and LO modes discussed here, the wurtzite structure has two addi-17 -1tional Raman-active optical modes of the E 2 type. In pure CdS these occur'7 at 44 cm-1and 252 cm . In some of the S-rich mixed crystals these modes have been noted. Butsince these were not followed through the entire concentration range, they could not beincluded in the present discussion. Tell, Damen, and Porto,17 in their study of theRaman spectrum of CdS, have reported 7 multiphonon bands. These have been assignedto various phonon combinations by Mitra 1 8 and by Nusimovici and Birman. 19 Most ofthese bands are also observed in the mixed crystals and may be similarly assigned. Inboth zincblende and wurtzite structures the second harmonic of the Brillouin zone centerLO modes are allowed in the Raman spectrum. What is noteworthy here is that in theRaman spectrum of CdSx Se we not only find bands corresponding to 2 LO (CdS) and2 LO (CdSe) but also a combination band of the type LO (CdS) + LO (CdSe). The observedRaman bands and their assignments for CdS. 79Se have been presented in Table IV-1as an example.The strongest two-phonon infrared-active bands above the fundamental lattice vibra-tions have been given in Fig. IV-3 for a number of x-values. In hexagonal crystals ofpure CdS or CdSe, the strongest two-phonon absorption bands are due only to combina-tion of phonons from critical points at the end of the Brillouin zone. Two-phonon tran-sitions involving the zone-center critical points are excluded by group theory and occuras cutoffs in the absorption spectrum. Although a classical Brillouin zone is not welldefined for these mixed crystals, the two-phonon absorption spectrum still behaves asif it were due only to critical points at the edge of a pseudo Brillouin zone. No overtonesof the observed zone center frequencies have been seen. Moreover, the absorption spec-trum, like the reflection spectrum, appears to be due to nearly independent contribu-tions from both the CdS and the CdSe in each mixed crystal.Figure IV-7 is a plot, as a function of the mole fraction of CdS in the mixed crystalQPR No. 87(IV. OPTICAL AND INFRARED SPECTROSCOPY)300-- Ls system, of an assignment of the frequen-cies contributing to the two-phonon absorp-tion spectra. The assignment shown ismost consistent with the over-all experi-mental data. It shows only one clearly200 - LOSE defined LA mode and that the zone-boundaryE TE optical frequencies and the zone-center opti-cal frequencies associated with a givenLA substance approach each other as the con-centration of their contributor becomes1oo very small. Near zero concentration theyapproach the local mode of that substance-- TA in the host lattice. The TA frequenciesare also shown on a single straight line,but there were insufficient data to locate0 I I I I the TA mode for all samples.0.0 0.5 I.0CdSe MOLE FRACTIONCdS We are grateful to Dr. D. C. Reynoldsof Wright-Patterson Air Force Base, Ohio,Fig. IV-7. for supplying some of the samples andBrillouin zone boundary of to Dr. S. S. Mitra and his co-workersCdS Se as functions of x.x -x at the University of Rhode Island for theircollaboration in this work. We are alsoindebted to the Space Optics Branch, NASA (Electronics Research Center) Cambridge,for their keen interest in this work, for supporting Mr. J. F. Parrish as asummer-employee, and for making available to us the Perkin-Elmer Model 301spectrophotometer.J. F. Parrish, C. H. PerryReferences1. A. Mitsuishi, H. Yoshina and S. Fujta, J. Phys. Soc. (Japan) 13, 1235 (1958).2. R. Marshall and S. S. Mitra, Phys. Rev. 134, A1019 (1964).3. M. Balkanski, J. M. Besson, and R. Le Toullec, Proc. International Conference onthe Physics of Semiconductors, Paris, 1965, pp. 1091-1099.4. R. Geick, C. H. Perry, and S. S. Mitra, J. Appl. Phys. 37, 1994 (1966).5. M. Balkanski, R. Beserman, and J. M. Besson, Solid State Commun. 4, 201 (1966).6. H. N. Verlur and A. S. Barker, Jr., Phys. Rev. 155, 750 (1967).7. J. F. Parrish and C. H. Perry, Quarterly Progress Report No. 85, Research Labo-ratory of Electronics, M.I.T., April 15, 1967, pp. 42-47.QPR No. 87(IV. OPTICAL AND INFRARED SPECTROSCOPY)8. P. J. Gielisse, J. N. Plendl, L. C. Mansur, R. Marshall, S. S. Mitra, R. Mykalajewycz,and A. Smakula, J. Appl. Phys. 36, 2446 (1965); A. Mitsuishi in "U.S.-Japan Cooper-ative Seminar on Far Infrared Spectroscopy," Columbus, Ohio, September 1965 (unpub-lished); Jeanne H. Fertel and C. H. Perry, a paper presented at the Symposium onMolecular Structure and Spectroscopy, Ohio State University, Columbus, Ohio, Sep-tember 6-9, 1967; R. M. Fuller, C. M. Randall, and D. J. Montgomery, Bull. Am.Phys. Soc. 9, 644 (1964); R. K. Chang, B. Lacina, and P. S. Pershan, Phys. Rev.Letters 17, 755 (1966).9. D. W. Feldman, M. Ashkin, and J. H. Parker, Jr., Phys. Rev. Letters 17, 1209(1966).10. W. Cochran, Z. Krist. 112, 465 (1959).11. D. R. Renneke and D. W. Lynch, Phys. Rev. 138, A530 (1965).12. I. F. Chang and S. S. Mitra (private communication, 1967).13. P. G. Dawber and R. J. Elliott, Proc. Roy. Soc. (London) A273, 222 (1963); Proc.Phys. Soc. (London) 81, 459 (1963).14. C. H. Perry, R. Geick, and E. F. Young, Appl. Opt. 5, 1171 (1966).15. S. S. Jaswal, Phys. Rev. 137, 302 (1965).16. S. S. Mitra (private communication, 1967).17. B. Tell, T. C. Daman, and S. P. S. Porto, Phys. Rev. 144, 771 (1966).18. S. S. Mitra, J. Phys. Soc. (Japan) 21 (Supplement), 61 (1966).19. M. A. Nusimovici and J. L. Birman, Phys. Rev. 156, 925 (1967).QPR No. 87I

Optical and Infrared Spectroscopy

Contains reports on two research projects.Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U. S. Air Force)under Contract DA 28-043-AMC-02536(E)U. S. Air Force (ESD Contract AF19(628)-6066

Mase, J. Z.

VI. OPTICAL AND INFRARED SPECTROSCOPY*Academic and Research StaffProf. C. H. PerryGraduate StudentsT. G. Davis D. J. Muehlner J. F. ParrishJeanne H. Fertel N. E. TornbergUndergraduate StudentsE. M. ImmermanJ. Z. MaseA. WORK COMPLETEDThe theses listed here were submitted to the Department of Physics, M. I. T., May1967, in partial fulfillment of the requirements for the degrees indicated.E. M. Immerman, "Analysis of KC1-KBr Mixed Crystals Using Infrared Spectros-copy," S. B. Thesis.J. Z. Mase, "Infrared and Raman Spectra of Several Ionic and Molecular Crystals," cS. B. Thesis.J. F. Parrish, "Lattice Vibrational Spectra of Cd/Sx:Sel- x Mixed Crystals,"S.M. Thesis.C. H. PerryB. FURTHER INFRARED AND RAMAN MEASUREMENTS OF THEAMMONIUM HALIDES1. IntroductionThe reflection spectra of the ammonium halides have been previously reported byReintjes and Perry. 1,2 Kramers-Kronig analyses and classical oscillator fits to thereflectivity have provided the transverse and the longitudinal optical frequencies. TheRaman spectra of NH 4 C1 crystal and powder, ND 4 C1 powder, and NH 4 Br powder werealso measured by Reintjes.1,2 The investigation of the transverse optical phonon fre-quency by thin-film measurements has been completed on NH 4 C1, ND 4 C1, NH 4 Br, andND 4 Br, at 300'K and 1000K. The Raman spectrum of NH 4 Br single crystal has beenmeasured as a function of temperature and was found to be substantially in agreementwith that reported by Krishnan,3 but differs from the powder data. 1 The transmission,This work was supported principally by the Joint Services Electronics Programs(U. S. Army, U.S. Navy, and U. S. Air Force)under Contract DA 28-043-AMC-02536(E),and in part by the U. S. Air Force (ESD Contract AF19(628)-6066).QPR No. 86NH CL \- RT--- NT III, /I I I I I I I I i40 60 80 100 120 140 160 180 200 220WAVE NUMBER (cm l )NH4Br80e 6040S20- RT--- NT40 60 80 100 120 140 160 180 200 220WAVE NUMBER (cmr')N D4 CL-h -- RT--- NTI I I I I I I  I I40 60 80 100 120 140 160 180 200 220WAVE NUMBER (cm')NDO4Br4I /- ROOM TEMPERATURE /--- NITROGEN TEMPERATURE /I /\JiI I  I I I I I I l I I I I i40 60 80 100 120 140 160 180 200 220WAWE NUMBER (cm 1')Fig. VI- 1. Transmission spectra of thin film of NH4 C1, ND4 C1, NH 4 Br, andND 4 Br, at room temperature (RT) and liquid nitrogen temperature(NT) from 40-220 cm - 1 .(NT) from 40-220 em.QPR No. 8660u 40v0< 20i-i l l l l l r l l i l, l l l iI I I I I I l I(VI. OPTICAL AND INFRARED SPECTROSCOPY)-Iof the single crystals NH4Br has also been measured from 10-105 cm- to check theinfared activity of bands seen in the Raman spectrum and to investigate the possibilitiesof impurity-induced absorption.2. Experimental ResultsThe transmission of thin films of NH 4 Cl, ND 4 C1, NH 4 Br, ND 4 Br, at 300 0 K and1000K are shown in Fig. VI-1. The measurements were made on the Michelson inter--lferometer in the range 40-220 cm-1Reagent grade granular NH 4C1 and NH4Br were used to prepare the thin film. Thedeuterated compounds of ND 4C1 and ND 4Br were made by repeated exchange.The thin films were evaporated either on 0. 04-mm polythylene or 0. 5-mm wedgedcrystal quartz in an Edwards Model 6E4 vacuum coating unit. Various film thicknesswere observed at both temperatures, and the transmission minimum was essentiallyindependent of film thickness or substrate. The quartz substrate had the advantage thatat low temperatures equilibrium was reached more rapidly.In addition to the evaporated films, the ammonium halide powders containedin a polythylene matrix were investigated, but only broad absorption bands werefound with this technique and the procedure was abandoned in favor of evaporatedfilms.-1The transmission of a single crystal of NH 4 Br was measured between 15-105 cm-1at ~77 0 K and ~0. 4*K. The results are shown in Fig. VI-2. The reflectivity of the30 - NH4 Br CRYSTAL25 - A 40K--- 770K-2 0 -15 25 35 45 55 65 75 85 95 105WAVE NUMBER (cm-)Fig. VI-2. Transmission spectra for a single crystal ofNH 4 Br (4 mm thick) at 77 0 K and 4 0 K from-l15- 105 cmNH 4 Br crystal was also run at 40 K to insure that it was below the "78°K phase transi-tion." No observable difference in the reflection spectrum previously reported byReintjes and Perry was observed, except for a very slight increase in reflectivityQPR No. 86(VI. OPTICAL AND INFRARED SPECTROSCOPY)at the reststrahlen peak.The Raman spectra were recorded on the Cary 81 spectrophotometer in the Spec-troscopy Laboratory, M. I. T. The measurements of Reintjesl' 2 for the powders wererepeated at various temperatures and extended to 4°K for the NH 4 Br powder. No sig-nificant differences were observed, except for a frequency shift of the bands and77 ° K80 K169 c147cmINSTRUMENT LINES60 100 140AV (cm - )107 K196 K2300K2390K308°K180 220Fig. VI-3. Raman spectra of NH4Br pow-der at various temperatures.60 80 100 120 140 160 180 200LZv (cm- 1)Fig. VI-4. Raman spectra of NH 4 Br sin-gle crystal at various tem-peratures.some intensity changes. The results for NH 4 Br are shown in Fig. VI-3. Measurementsof NH4Br crystals were, however, siginifcantly different from the powder measure-ments, and the spectra recorded at 133°K, 77°K, and 4oK are shown in Fig. VI-4.2. Discussiona. Thin Films of Ammonium HalidesThe transmission spectra of thin films of NH 4 C1, ND 4 C1, NH4 Br, and ND 4 Br are-1shown in Fig. VI-1. The effective resolution is 5 cm -l , as indicated by Av.NH4 C1. In the spectrum of NH 4 C1 evaporated on quartz, the single absorption at-1 -l172 + 1 cm (RT) shifted to 181 ± 1 cm (NT) and became sharper and more intense,as did the reflectivity measured by Reintjes.l The frequencies are only in fairQPR No. 86 44(VI. OPTICAL AND INFRARED SPECTROSCOPY)-1 -1 -1agreement with the wt of 175 cm (RT) and 188 cm (NT) and a Raman line at 188 cm1(NT). In slightly better agreement are classical oscillator frequencies of 173 (RT) and184 cm-1 (NT). The neutron scattering spectra of Woods et al.5 show a peak at 185 cm-at 900K. No error estimate is given; from the spectrum it seems to be on the same6 -iorder as Palevsky's, making it z±5 cm. More favorably, Krishnan 7 has reported-1a very intense Raman line of frequency 183 cm-1 at 85°K.ND .Cl The spectrum of ND 4 C1 evaporated on polythylene likewise exhibited aA.. -1 -1single absorption. The band at 166 ±2 cm (RT)moved to 177±1 cm (NT), deepened,and became sharper. The nitrogen temperature measurement agrees with Reintjes1,-1Raman measurement of 177 cm-1 at 770K.NH 4 Br. The spectrum of NH 4 Br evaporated on polyethylene or quartz showed onlyone absorption at both 300 0 K (RT) and 77 0 K (NT). At room temperature, the maximum-1absorption occurs at 147 ± 1 cm- . This value compares well with the transverse opti-cal frequency of 146. 8 cm-1 obtained by Reintjes1, Z from a Kramers-Kronig analysisof the crystal reflectivity. A classical oscillator fit of the reflectivity has provided afrequency of 144 cm -l for w 1, the primary oscillator. 8 Also in agreement is a value of148 ± 6 cm measured by Palevsky at 293 K, using the inelastic scattering of neutrons.When the sample cooled to 77 ° K, the absorption shifted to a higher frequency, andbecame deeper and sharper. Its frequency of 160 ± 2 cm-1 agrees with Reintjes'l'8values of 159 cm-1 for wt and 155 cm-1 for w 1. The half-height width of the absorptionnarrowed approximately 38 per cent on cooling from 300'K to 77 0 K,while that ofReintjes' reflectivity narrowed 36 per cent. Mikke and Kroh 9 report a frequency of-1~165 cm-1 for a so-called optical peak from their neutron scattering measurements at120 0 K. The curve was less well defined, however, than Palevsky's room temperaturemeasurement, so the error for the 165 cm-1 frequency must be larger than the ±6 cm-of Palevsky. Wagner and Hornigl 0 infer a limiting lattice frequency of approximately-1160 cm for phase IV (below 78 K) NH 4 Br from combination bands in their near infrared-1spectra of thin films. The observed value of 160 ± 2 cm thus agrees well with thereported values, and is identified with the transverse optical lattice frequency, Oto*To insure that the NH 4 Br was indeed in the ordered cubic phase (below 78 0 K), ameasurement was made with liquid helium in the cryostat. The frequency observedwas 159±2 cm - l , which is within the experimental error of the measurements at 77°K.ND 4 Br. The spectra of the deuterated ammonium bromide films are of the same-1type as those of NH 4 Br. The single absorption at 138 ± 1 cm (RT) shifted to-1147 ± 2 cm , and became sharper and more intense on cooling to nitrogen temperature.These frequencies are also identified with Oto, the transverse optical frequency.-1For the run made at 77 0 K, oto of ND 4 Br is ~13 cm less than that of NH 4 Br at thesame temperature. This frequency shift is completly accounted for by the increase inmass of the ammonium ion. Using w ~ 1/N fM, and the values of wto at 77K, we obtainQPR No. 86(VI. OPTICAL AND INFRARED SPECTROSCOPY)M 11/2ND 4 Br NH 4 BrND4B r = 0. 919 and M N = 0.923.NH4Br MND 4Brb. NH 4 Br ReflectivityNo appreciable change was observed in the reflectivity as the crystal was cooledfrom 77 0 K to 4°K, thereby confirming that the nitrogen temperature measurements ofReintjes1,'2 were indeed those of the crystal below the 78 0 K transition.c. NH 4 Br Crystal TransmissionAt ~77 0 K, the large NH 4 Br crystal was essentially black to the far infrared. At~4 0 K, the crystal opened up, but showed only general absorption, nothing sharp. It washoped that infrared evidence would be found for two low-frequency Raman lines reportedby Krishnan 3 and observed in our measurements. Impurity resonance modes may alsobe expected to absorb in this region, but nothing was found. The spectra are shown inFig. VI-2.d. Raman SpectraThe Raman spectra of ND 4 Br powder are shown in Fig. VI-5. No lines were present-1at room temperature; only the arc lines at 147 and 169 cm were observed. The tran-sition temperature from phase III (tetragonal) to phase IV (CsCl) was determined to bebetween 150 0 K and 160'K, which compares favorably with the previously reported11value. A line appears just below the transition temperature near the arc line, andbecomes sharper, more intense, and shifts to higher frequencies as the temperatureis lowered. For comparison, the infrared and Raman frequencies are listed inTable VI-2. The Raman line is assigned as the transverse optical lattice vibration fre-quency, and its value of 149 cm-1 agrees well with the infrared frequency of 147 cm-1at 77 0 K obtained from the transmission of the thin film.NH 4 Br. The Raman spectra of the large NH 4 Br crystal are shown in Fig. VI-4.The spectra at the 3 temperatures are quite similar and show the same lines, with asharpening of the lines as the temperature was lowered. The two strong lines at 138and 185 cm-1 agree with Krishnan's 3 Raman frequencies of 136 and 182 cm-1 observedat 85 0 K.Because of the tetragonal structure in this phase, these lines may be the split-1fundamental, wto' but only a single band at 151 cm 1 was found in the reflectivity at200K. 9 Since unpolarized light was used in the reflectivity measurement, this mayexplain why a single peak was observed at a frequency between the two Raman lines. Ifthese lines are identified as a split wto there should be a change back to a single lineas the sample is cooled below the 78°K transition to the cubic phase. This is, in fact,QPR No. 86(VI. OPTICAL AND INFRARED SPECTROSCOPY)what was observed by Reintjes 1,2 as a sample of NH 4 Br powder was cooled to 77 0 K; a-1single line at 162 cm was observed and identified as the cubic phase wto. There isgood agreement between this value and-1ND4 Br POWDER the infrared frequency of 160 cm-1 mea-sured with the thin film at 770K.When the NH 4 Br crystal was cooledto 130*K, to 770K, and then to 4°K, no77K change was observed in the Raman spec-trum, except for an increase in the inten-Cr 90'Ksity of the bands. There is a possible_ 0explanation for this unexpected result.130/K Reintjes1,2 noticed a lingering of thez 1500 K tetragonal phase for approximately 20160 0K minutes after cooling NH 4 Br powder to<--210K 77°K. Perhaps, what was observed atnitrogen and helium temperatures was147 cm- I  69cm the Raman spectrum of the supercooledARC LINES tetragonal phase. At such low temper-80 100 120 140 160 180 200 220 atures the energy available for theAv WAVE NUMBER (cm - )phase change is undoubtly small, whichexplains the slow transition. FurtherFig. VI-5. Raman spectra of ND4Br pow-evidence of the tetragonal structure isder at various temperatures.found in Fig. VI-3, the Raman spectraof NH 4 Br powder. In the spectrum at107'K the powder is indeed in the tetragonal phase, and the frequencies are listed as138 and 185 cm-1 by Reintjes. '12 These frequencies agree perfectly with the onesobserved in the crystal at 133 0 K and 4°K.-1More difficult to explain are the low-frequency Raman lines at 72 and 79 cmKrishnan's 3 spectrum at 85 0 K shows an intense line at 69 cm - 1 and a weaker one at77 cm -l 1. He identifies these as part of the lattice spectrum, but gives no furtherassignment. Three things are strange about these lines. First, they were not observedin the spectra of the powder (Fig. VI-3), although they may be hidden in the relativelybroad Rayleigh line. Second, they are not infrared active, as is shown by the infraredtransmission of the single crystal of NH 4 Br (Fig. VI-2). Finally, they were not observedin the neutron spectrum, 6 which is not limited by the usual infrared and Raman selec-tion rules. Two explanations of the lines are possible. There might be an impurity inthe crystal that was not in the powder sample. Krishnan3 states that his crystal was apale yellow; the one used here was a pale yellow-green. Stephensonl3 has advised thatthe color arises from a copper impurity. Since both were similarilty colored, the twoQPR No. 86(VI. OPTICAL AND INFRARED SPECTROSCOPY)low-frequency lines may be attributed to a copper impurity. The other explanation isthat they are the frequencies of some lattice fundamental other than the transverse opti-cal mode, possibly librations. The splitting into two lines is again due to the persistenceof the tetragonal structure below the 78 0 K transition. Infrared inactivity explains whythey were not observed in the transmission spectrum. It is likely that the broadRayleigh line obscured them in the Raman spectra of the powder. A spectrum of thecrystal definitely in the low-temperature ordered phase would probably decide the ques-tion. The tetragonal splitting would be removed, and impurity lines would probablystay unchanged. Any change would thus support the lattice vibration hypothesis.The one remaining feature of the NH 4 Br Raman spectrum to be explained is the line-1at 148 cm observed at 4°K with the powder, as shown in Fig. VI-3. All other mea-surements in Fig. VI-3 were made by Reintjes, 1, and have been explained by him.Again, there are two possiblities for the 4oK spectrum. The sample could have beenin a supercooled tetragonal phase, with the 138 cm - l line of the tetragonal phase shiftedto 148 cm- . The absence of the 185 cm-1 tetragonal line makes this a remote possi-bility, however. The other explanation is that the powder actually was in the low-temperature cubic phase and the intense 162 cm-1 line at 77°K shifted to ~170 cm-1 at4 0 K. The 148 cm line had previously been buried in the arc line at 147 cm, anddid not become intense enough to be observed until helium temperature. Neither expla-nation is very satisfactory, however, in view of the lack of change in both the reflectivity-1Table VI- 1. Infrared and Raman frequencies of NH 4 C1 and ND 4 C1 in cm-1Thin Film Reflectivity Reflectivity Raman OthersTransmission Kramers-Kronig DispersionNH 4 C1 172 ± 1 1751 1 = 1738(300°K) W2 = 225821 8 1 2 185 ± 65NH 4 C1 181 ± 1 188 1 = 1848 188 1, 1856(77 K) crystal neutron8 1 2 72 = 246 1882 1837powder RamanND 4 C1 166 ± 2(3 00 0 K)ND 4 C1 177 ± 1 1771,2(77 K)QPR No. 86(VI. OPTICAL AND INFRARED SPECTROSCOPY)-1Table VI-2. Infrared and Raman frequencies of NH 4 Br and ND 4 Br in cm-1 .Thin Film Reflectivity Reflectivity Raman OthersTransmission Kramers-Kronig Dispersion148 ± 66neutron8 1,2=15 1 135 powderS 18018281381,185 powder7279138 crystal185NH 4 Br(300 K)NH 4 Br(2000K)NH 4 Br(~1 100K)NH 4 Br(77°K)NH 4 Br(4 0 K)ND 4 Br(3000K)ND 4 Br(77 K)1, 2powdercrystal165 ± 6neutron160 inferred' 06977136 Raman182148 powder7279138 crystal185149and thin-film transmission from nitrogen to helium temperature. The spectra of boththe crystal and powder should be rechecked at 4 ° K, with care to insure that the sampleshave changed phase at 78°K before the final cooling to 4oK.All of the observed and reported infrared and Raman frequencies of the ammoniumhalides have been listed for comparison in Tables VI-1 and VI-2. Table VI-3 showsfurther work by Reintjes and Perry,8 with a classical oscillator model used to fit thereflectivity spectrum at various temperatures, and shows the frequencies, oscillatorQPR No. 86147 ± 1 146. 81 1c2 =2144818481541W2 =16272791381851591160 ± 2159 ± 2:l = 15582 = 192Z138 ± 1147 ± 2(VI. OPTICAL AND INFRARED SPECTROSCOPY)Table VI-3. Parameters used in the classical oscillator model to fit theinfrared reflectivity of NH 4 CI and NH 4 Br single crystals at-1various temperatures. (All in cm .)2 2s s1 +2E(W) = E + 2 2 2 20 W - W - i Yl y - °2 - i-YNH 4 C1 NH 4 Br300 0 K 85 0 K 300 0 K 200 0 K 85 0Ks1 339. O0 341. 0 295. 0 285. 0 280. 0S1 173.0 184.0 144.0 151. 0 155. 0Y1 18. 0 4. 0 17. 0 9. 0 2. 5s2  87. 0 52. 0 52. 0 55. 0 48.0W2 225. 0 246. 0 184. 0 182. 0 192. 0Y2 45. 0 32. 0 30. 0 28. 0 38.0strengths, and damping constants that were obtained.The good agreement between the transverse optical frequencies and the Raman fre-quencies at 770 K indicates that the Raman lines are due to these lattice vibrations. Thatthese lines are found in the Raman spectrum, at all, may indicate a deviation fromideal cubic symmetry in the low-temperature ordered phase. 1,2In no samples were the longitudinal frequencies, as calculated from the reflectivity,observed in the Raman spectra. Perhaps, the splitting of lines in NH4 Br in the tetrag-onal phase could be observed in the infrared if polarized radiation were used, and thecrystal orientation varied.J. Z. Mase, C. H. PerryReferences1. J. F. Reintjes, Jr., S. B. Thesis, Department of Physics, M. I. T. , June 1966.2. J. F. Reintjes, Jr. and C. H. Perry, Quarterly Progress Report No. 82, ResearchLaboratory of Electronics, M. I. T., July 15, 1966, pp. 48-53.3. R. S. Krishnan. Proc. Ind. Acad. Sci. A27, 321 (1948).4. C. H. Perry, R. Geick and E. F. Young, Appl. Opt. 5, 1171 (1966).5. A. D. B. Woods, B. N. Brockhouse, M. Sakamoto and R. N. Sinclair, "InelasticScattering of Neutrons in Solids and Liquids," I. A. E. A., Vienna, p. 487, 1961.6. H. Palevsky, J. Phys. Soc. Japan 17, Suppl. B-II, 367 (1962).7. R. S. Krishnan, Proc. Ind. Acad. Sci. AZ6, 432 (1947).QPR No. 861111QPR No. 86(VI. OPTICAL AND INFRARED SPECTROSCOPY)8. J. F. Reintjes and C. H. Perry, Symposium on Molecular Structures and Spectros-copy, Ohio State University, Columbus, Ohio, Sept. 6-10, 1966, Paper U10.9. K. Mikke and A. Kroh, "Inelastic Scattering of Neutrons in Solids and Liquids,"I.A.E.A., Vienna, p. 237, 1961.0. E. L. Wagner and D. F. Hornig, J. Chem. Phys. 18, 305 (1950).1. H. A. Levi and S. W. Peterson, J. Am. Chem. Soc. 75, p. 1536 (1953).2. E. L. Wagner and D. F. Hornig, J. Chem. Phys. 18, 296 (1950).3. C. C. Stephenson, Private Communication, 1967.ii~ N i;*.Ot Ir11*C

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