Infrared reflection spectra of KClO4 single crystals

Journal of Molecular Structure, 147 (1986) 47-56 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

INFRARED REFLECTION

SPECTRA OF KC104 SINGLE CRYSTALS

H. D. LUTZ* and G. WASCHENBACH Labomtorium

fiir Anorganische

Chemie,

Universitiit Siegen, D-5900 Siegen (F.R.G.)

S. HAUSSUHL Znstitut fiir Kristallogmphie,

Universitiit KBln, D-5000 Kdn

(F.R.G.)

(Received 17 March 1986)

ABSTRACT The polarized IR reflection spectra of potassium perchlorate single crystals were recorded and analysed for the frequencies of the transversal and longitudinal optical zone centre modes using classical oscillator-fit method and Kramers-Kronig analysis. For comparing the TO and LO mode frequencies obtained with the corresponding Raman data, frequencies (w,) unaffected by the long range electrostatic fields were calculated. These frequencies are similar to those obtained from conventional IR absorption spectra of Nujol mulls or KBr discs. From the TO/LO splittings of both the internal and external vibrations Scott and Szigeti dynamical effective ionic charges were determined and discussed in terms of ionicity and bond polarity. INTRODUCTION

Potassium perchlorate, KC104, is of considerable interest from the point of view of both phase transition to a high-temperature plastic phase and vibrational theory. The perchlorate ions are in a crystal field of low symmetry (C,) and, hence, large splitting of the degenerated vibrational modes is expected. Furthermore strong TO/LO splitting of both the internal and external vibrations should be present, which must be considered when comparing IR and Raman data. Several Raman single crystal studies and the IR spectra of polycrystalline samples have already been reported [l--5]. Polarized reflection spectra of single crystals for calculating the TO and LO phonon mode frequencies, however, are not available in the literature. The present work was undertaken to make a complete assignment of the IR-active modes of low-temperature KC104 and to compare the results with the single crystal Raman data reported in the literature [3, 41. Additionally dynamical effective charges of the atoms involved should be derived from the TO/LO splittings of both the internal and external modes. *Lattice vibration spectra, Part XLII. Part XLI: G. Waschenbach and H. D. Lutz, Spectrochim. Acta, in press. 0022-2860/86/$03.50

0 1986 Elsevier Science Publishers B.V.

48

At ambient temperature the perchlorates under investigation crystallize in the orthorhombic space group Prima-D :g with four formula units in the unit cell (baryte-type). Group theory treatment of the external vibrations (d = 0) yields [3, 51 l-‘r’ = 4A, -t- 2B1, + 4Bzp + 2B3, + 2A, + 3B1, + Bzu + 3B3, + translational modes rR = A, + 2B1, + BZg + 2Bj3, + B1, + 2B2, + B3u + librational modes. The correlation of the internal modes of ClO, ions is as follows Td (free ion)

C, (site group)

Dzh (unit cell group)

41: v,(v)(RS)

OW W) (IR) (RS) UR) UW B3u:

EXPERIMENTAL

AND CALCULATION

VI,

V2,

2V3,

2V4

UR)

PROCEDURES

Large single crystals of KC104 were grown from aqueous solutions by controlled evaporation at ca. 340 K. The relatively high growth temperature is necessary in order to achieve a sufficient concentration of KC104 in the solution. After several growth generations, in which the single crystals of the preceding steps were employed as seed crystals, individuals with large areas of optical quality and dimensions of ca. 30 X 30 X 12 mm could be obtained. The growth velocity was kept below 0.1 mm day-‘. At higher growth rates the crystals formed milky areas due to inclusion of mother liquid. The crystals possess the typical morphology of baryte. The observed forms, ordered by their morphological rant (setting by Groth [6] ) are: {110}, (OOl), {OlO}, (1021, {Oil}, {loo}, (111). For the IR studies, the crystals were cut to give plates about 30 X 30 X 5 mm in size. The surfaces were (010) and (001) crystal planes, respectively. The measurements were performed at near normal incidence on a PerkinElmer model 325 grating spectrophotometer and Bruker IFS-114 Fourier transform interferometer in the range 40-600 and 200-1500 cm-‘, respectively. An aluminium mirror was taken as reference. Polarized IR radiation

49

was obtained using gold wire grid type polarizers with polyethylene and AgBr substrate for the low and the high frequency region, respectively. The polarized IR reflection spectra were converted into the dielectric dispersion relations by means of both classical oscillator model calculations and Kramers-Kronig analyses. Details are ‘given elsewhere [ 7, 81. The transversal and longitudinal optical phonon frequencies, i.e. aTO and oLo, were determined from the peak positions of the imaginary parts of the dielectric constant E” and the inverse dielectric constant -1m( l/Z), respectively. Dynamical effective ionic charges were calculated from the TO/LO splittings of the observed phonon modes using the following equation 47r*c~(w~o - wio) = (4ne24/V)~(nk(e$/e)/mk)

(1)

k

with V = volume of the Unit cell, nk, e&/e, and mk, stoichiometric coefficients, effective charges (Scott charge [ 9]), and atomic masses of the ions involved, respectively. Details are given elsewhere [lo] . RESULTS

The polarized IR reflection spectra of orthorhombic KC104 single crystal plates are shown in Figs. 1-3 including the dielectric dispersion relations obtained by oscillator fit calculations. The oscillator strengths of the breathing mode v 1 and the bending mode u2 (and of the two-phonon modes [ 3-51) are too small for determining oscillator parameters. The oscillator parameters and the frequencies of the TO and LO phonon modes of both the external and internal vibrations are given in Table 1. For comparing the obtained IR data with the results of Raman single crystal studies [3, 41, we calculated hypothetical mode frequencies, o 0, unaffected by the long range electrostatic field using the equation w; = (2o$o + oEo)/3 in the rigid ion approximation in Table 1.

(2) [ll].

The frequencies obtained are included

DISCUSSION

Internal vibrations of the ClO, in orthorhombic KClO, The true TO frequencies of the various unit cell group modes differ considerably from the frequencies of the corresponding peaks in the conventional IR absorption spectra, at least in the case of the asymmetric Cl0 stretching vibration v3 (see Table 1). This finding is caused by both the TO/LO splitting of these bands and the influence of the directional dispersion of the phonon modes in anisotropic compounds [7]. The peak frequencies of the absorption spectra, however, are very similar to the corrected mode

50

frequencies w. (see above). It seems therefore that the error produced by interpreting conventional IR spectra is not excessive. The correlation splittings of the internal vibrations v3 and v4 are small except those due to vibrational interactions of neighbouring ClO: ions which are interchanged by the glide plane n. In particular, mode splittings due to the mutual exclusion principle, i.e. interactions of ClO; interchanged by the center of inversion, are very small. This is revealed by comparing the corrected mode frequencies u o with the corresponding Raman bands, as shown for u3 and u4 in Table 2. Thus, for example, the v3 unit cell modes of species &g and BzU, both in-phase vibrations with regard to the glide plane n and species A” (with respect to the site group C,), possess nearly the same frequencies, i.e. 1086 and 1088 cm-‘, respectively, whereas the unit cell mode of species B1, i.e. the other site group component of species A”, which is antiphase to the glide plane, is shifted by about 35 cm-’ to higher wavenumbers (1120 cm-‘).

200

400

800

800

1000

1200

1400

cm-’

Fig. 1. Polarized IR reflection spectrum (dashed line, oscillator-fit calculation) and dispersion functions of the optical and dielectric constants (dashed lines, n, -Zm(l/;), respectively) of a (010) face of orthorhombic KClO, with bit C(B,,).

51

4”

I

eoow YOO-

I

. (IO0

j 0: II

-

I! II

-

;j

- 200

z

ULOO.2

. .oo

oo-

3800. x)clo2.00.

. 100

_.’

_________I.

400

(100

800

1000

400

. 300

.

100

‘.________________ow

/L 200

100

- so0

:’1: _______

(‘/2)

--too 1200

1400 cm-’

Fig. 2. Polarized IR reflection 3 IIb(B,,)(see Fig. 1).

spectrum of a (001)

face of orthorhombic

KClO, with

Altogether the interpretation of the v3 modes of the Cl04 ions is as follows. The splitting of the two A’ site group components is about 25 cm-‘, as shown from the Raman modes of species A, and B2s (see Table 2). This splitting, however, is not observed in the IR reflection spectra (species B1, and B&. In the case of species B1, the two unit cell modes contribute to a similar extent to the Reststrahlen band observed, shown by an average frequency compared to the two unit cell modes of species A,, in species BJu the high-wavenumbered site group component obviously possesses larger oscillator strength. The A” component of v3 is found at lower wavenumbers than the A’ components. The unit cell group splitting is as discussed above. The same behaviour, but with smaller site group and unit cell group splittings, is foundfor v4. The assignment of the observed bands corresponds to that given for the Raman active modes in previous reports [3, 41. Similar results, but higher site group splittings, are obtained for the isostructural compound BaS04 evaluating the IR and Raman single crystal data reported by Dawson et al. [12].

Fig. 3. Polarized IR reflection @! IIa(&) (see Fig. 1).

External

spectrum of a (001)

vibrations of orthorhombic

face of orthorhombic

KClO, with

KCLO,,

From the group of 11 theoretically-allowed IR-active external modes of KC104 all but one are observed in the reflection spectra (see Figs. l-3). The results obtained, however, are not in good agreement with the sparse literature data [ 1, 41. Correct assignment of the observed bands with regard to librations and translatory modes is not possible. Additionally, more or less mixing among these vibrations may occur [4]. The interpretation given in Table 1 must therefore be considered to be only tentative. Clear distinctions of librational and translator-y modes, however, would be necessary for calculating dynamical effective charges discussed below. Effective

charges of orthorhombic

KC104

Dynamical effective charges of the potassium and perchlorate ion in orthorhombic KC104 were calculated from the TO/LO splittings of the

53 TABLE 1 Oscillator parameters, TO and LO phonon frequencies of orthorhombic

KClO, (cm:‘)

477P

Y

aTo

WL0

W0

w abs

0.33 0.07 1.07 0.01 0.24 0.68

16 2 16 3 6 6

1084 628 133 124 106 87

1150 635 158 126 110 92

1106 630 141 124 107 89

1115 630 137 108 89

; R T’ T’

B *u

0.31 0.07 1.11 0.33 2.32

6 3 14 10 8

1064 625 129 115 70

1133 633 163 118 84

1088 629 141 116 75

1089 628 137 116 74

“3 VI T’ R R

B all

0.28 0.08 1.45 0.50 1.01

11 2 15 7 35

1118 634 136 72 65

1182 644 171 79 69

1140 637 149 74 66

1144 638 137 74 66

“3 v4 T’ T’ T’

B 1u

v3

w ~0 and w ~0 from oscillator fit calculation, deviation of data obtained from KramerKronig analyses <2 cm-‘, wO, frequencies corrected for long-range electrostatic fields using eqn. (2); wahs from IR absorption spectra of polycrystalline samples (KBr pellets and Nujol mulls, respectively).

TABLE 2 Site group and unit cell group splitting of the asymmetric CIO, stretching and bending vibration vJ and v., of orthorhombic KClO, Site groupa

Unit cell groupb

aMean values of the Raman-active unit ceil group modes belonging to the given species. bRaman data from refs. [3, 41, IR, w,, (see Table 1).

54

external phonons. Because separation of librational and translatory modes is not possible, as discussed above, all external modes were used although strictly speaking only the TO/LO splittings of the translatory modes are a measure of the effective charges of the K’ and CIO; ion. The obtained effective charge e&/e, the so-called Scott charge [9], which is equal to flwith 5’ the so-called normalized splitting [ 131, is given in Table 3. For studying ionicity trends in solids, however, the so-called Szigeti charges e$/e, which take into account that the polarizability of the ions contributes to the TO/LO splitting [ 111, are more convenient e.$e = 3eg/e/(e,

+ 2) and eg/e = e?“e&/e

(3)(4)

with the high-frequency dielectric constant represented as E, and the transversal (Born) charge as ef/e. Details are given elsewhere [lo] . The Szigeti charge of the potassium ion in orthorhombic KC104 resembles those of other ionic alkaline metal salts and reveals some anisotropy (see Table 3), as found for other non-cubic compounds. Thus the charge connected with the B,, modes, for which the direction of the oscillating dipole moment of the phonons is parallel to the c axis, is somewhat smaller than for the other directions. To our knowledge the effective charges of the different atoms in polyatomic entities have not been calculated from TO/LO splittings of internal vibrations. These simplified calculation procedures, however, may be allowable for discussing relative trends of polarity in polyatomic ions. We therefore determined Scott charges of the chlorine and oxygen atoms of the Cl04 ions from the TO/LO splittings of the antisymmetric stretching modes, which mostly reveal the Cl0 bond polarity, using the formalism discussed above and the following equation incorporating the electroneutrality condition TABLE

3

Effective

ionic charges

KClO,

value KClO, BaSO, a-LiIO,

and high frequency

dielectric

constants

orthorhombic

e&k4

e&M

(B,,)

0.75 0.88 0.86 0.83

1.23 1.34 1.34 1.30

&h4

e&k 2.16a 2.2 2.03 3.66d

(B,,) (B,,)

orthorhombic

0.83 l.lb o.50c

0.79 0.93 0.91 0.88

2.7 2.3 2.4 2.5

es*&0

-0.75 -0.8 -0.84 -1.39d

e&/e, eg/e, ez/e, Scott, transverse, and Szigeti charges of the metal ions (= M) and the a2.20, 2.12 and 2.15 forspeciesB,,, B*u,andB,,, Cl, S, I (= X) and 0 atoms, respectively. respectively. bEstimated, data for S and 0 calculated using this value and the frequencies given in ref. 12. CFrom ref. 14, data for I and 0 calculated using this value and the frequencies of the asymmetric stretching mode given in ref. 14. dCharges given in ref. 14.

55

e&/e,

+ 4e&/eo = - e&/e,

(5)

The data obtained, which do not reveal anisotropy, together with those calculated for BaS04 and cu-LiIOJusing the frequencies reported in refs. 12 and 14, respectively, are given in Table 3. The nearly equal effective charges obtained for Clvn, Svl, and Iv seem to be reasonable notwithstanding the different oxidation states of these atoms in the anions under discussion. The obviously more polar S-O bond, compared with the Cl-O bond, also shown from the larger TO/LO splitting of the asymmetric stretching mode vg, -80 (BaS04) instead of 66 cm-’ (KC104) on average, is in agreement with the greater differences of the electronegativities. The so-called static charge of iodine in a-LiIOJ calculated in ref. 14 from oscillator strengths and eigenvectors of the vibrational modes, however, differs greatly from the value obtained by the procedure discussed above. CONCLUSION

The transversal and longitudinal optical zone centre modes of KCIO, o.-rh. can be determined and assigned from the polarized IR reflection spectra of single crystals. For comparison with Raman data the TO and LO mode frequencies obtained must be corrected for frequency shifts due to longrange electrostatic fields. The corrected mode frequencies are similar to the band frequencies of conventional IR absorption spectra. Dynamical effective ionic charges derived from TO/LO splittings of both the external and internal modes can be used to discuss relative trends of the ionicity and bond polarity. ACKNOWLEDGEMENTS

The authors would like to thank the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for financial support. REFERENCES 1 E. K. Galanov and I. A. Brodskii, Fiz. Tverd. Tela (Leningrad), 10 (1968) 3392. 2 S. Seetharaman, H. L. Bhat, and P. S. Narayanan, Indian J.Phys. Part B, 58 (1984) 294. 3 H. D. Lutz, R. A. Becker, W. Eckers, B. G. HSlscher, and H. J. Berthold, Spectrochim. Acta, Part A 39 (1983) 7, and references cited therein. 4 N. Toupry, H. Poulet, M. LePostollec, R.M. Pick, and M. Yvinec, J. Raman Spectrosc., 14 (1983) 166. 5 B. Hajek, 0. Smrckova and P. Zaruba, Collect. Czech. Chem. Commun., 49 (1984) 1756. 6 P. Groth, Chemische Krystallographie II. Wilhelm Engelmann Verlag, Leipzig, 1908, p. 167 ff. 7 H. D. Lutz and G. Wiischenbach, Phys. Chem. Minerals, 12 (1985) 155. 8 H. D. Lutz, G. Kliche and H. Haeuseler, Z. Naturforsch. Teil A, 36 (1981) 184. 9 J. F. Scott, Phys. Rev. Sect. B, 4 (1971) 1360. 10 H. D. Lutz, G. Waschenbach, G. Kliche, and H. Haeuseler, J. Solid State Chem., 48 (1983) 196.

56 11 P. Grosse, Freie Elektronen in Festkiirpem, Springer Verlag, Berlin, 1979, p. 212 ff. 12 P. Dawson, M. M. Hargreave, and G. R. Wilkinson, Spectrochim. Acta, Part A, 33 (1977) 83. 13 R. M. Martin, Solid State Commun., 8 (1970) 799. 14 F. Cerdeira, V. Lemos, F. E. A. Melo, and M. Cardona, Phys. Status Solidi B, 122 (1984) 53.