OH---O Raman bands in KDP melts

Solid State Communications, Vol. 49, No. 1, pp. 4 7 - 5 0 , 1984. Printed in Great Britain.

0038-1098/84 $3.00 + .00 Pergamon Press Ltd.

O-H---O RAMAN BANDS IN KDP MELTS Jong-Jean Kim and Byoung-Koo Choi, Physics Department, Korea Advanced Institute of Science and Technology, P.O. Box 150 Chongyangni, Seoul, Korea

(Received 12 April 1983 by M. Balkansld) The room temperature and the low temperature (10 K) O-H---O Raman bands were observed in KDP melts, and the large difference in the low temperature spectra between melt and crystal is attributed to the proton correlations associated with the ferroelectric phase transition. It is also suggested that KDP melts of varying degrees of dehydration may be a promising system to examine the current theories of O-H---O, a strong hydrogen bond. HYDROGEN BONDS ARE HIGHLY susceptible to local varieties and perturbations, and great complicacy arises in the vibrational hydrogen spectra of the hydrogen bonded complexes in solids [1-3], solutions [2, 3], and gases [4]. Hydrogens in KH2PO4(KDP) family crystals are well known to form the strong hydrogen bonds [1] of O-H---O, and the complicated vibrational spectra of O-H---O exhibit very interesting precursor aspects of both the low temperature [5] and the high temperature [6] phase transitions. It is well known [7, 8] that the four hydrogen bonds in the primitive unit cell of the KDP crystal are correlated to form the collective tunnelling modes of A2, B2 and E symmetries in the paraelectric phase, compatible with the D2a lattice symmetry, which allows coupling with the lattice vibrational phonons of the respective symmetries. Indeed the B2 soft-mode is well known to be a coupled mode between the proton tunnelling mode and the B2 phonon mode carrying the c-axis lattice polarization [8]. The high frequency vibrational Raman bands of O-H---O with peaks at around 2700, 2400 and 1800 cm -1 have been observed only in A1 and Bt mode spectra [5, 6], which implies that the O - H vibrational modes may not couple with the collective tunnelling modes but possibly with the single particle tunnelling. The three band structure of the O-H---O Raman band in KDP crystals has not been clearly explained about its origin even though various explanations such as proton tunnelling [9] and Fermi resonance [1, 3, 5, 10] have been proposed. It is rather surprising that the strong coupling theory [2, 4, 11, 12] has not been considered in the previous interpretations [5, 6] of the O-H---O Raman spectra in KDP crystals. The strong coupling theory involving the anharmonic interactions between the stretching vibration of H atom (O-~---O) and the stretching vibration of H bond (~-~---~) may also

explain as well the gross features of the O-H---O band [2, 4, 11, 12]. In this communication we want to report on our Raman spectra of O-H---O in both KDP crystal and KDP melt, which may help to confirm the possible interpretations of the O-H---O Raman bands. The melt sample was obtained by melting of KDP powder stuffed into the pyrex tubing heated in the open air by use of the alcohol burner. The melting temperature was kept as close as possible to the melting point 252°C until complete melting after which the crystalclear melt was vacuum sealed and cooled down slowly to the room temperature. The Ar-Kr mixed gas laser (spectra physics 165) was operated at 4880,~ (or 5145 ,~) with power set at 300 mW. Polarization rotator and Echelon filter were used to select the incident polarizations and to reject spurious plasma lines and fluorescence background. Laser beam was incident on the sample from below, and 90 ° scattered light was collected by the 0.5 m double grating monochrometer (Spex-1302) with all the slit widths set at 200tam (spectral bandpass of ~ 7 cm -1 at 4880,~). Scattered polarizations were selected by the polarization analyzer, and a polarization scrambler was inserted in front of the monochrometer to reduce the grating dependence on the polarization. The photon counter system with the cooled P.M. tube was used to record the spectral output from the monochrometer. The low temperature (10 K) Raman spectra was obtained by using the closed cycle helium refrigerator system (Air Products - Displex). Figure l(a) shows the well known O-H---O bands between 1500 and 3000 cm -1 as observed in the paraelectric a(bb)c Raman spectra of KDP single crystals, which goes changed into the spectra of Fig. l(b) as the crystal temperature falls below the ferroelectric transition temperature Tc. With a crystal cut with faces normal to the (a + b ) direction, i.e., (1 10) we can obtain 47

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O-H---O RAMAN BANDS IN KDP MELTS

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Vol. 49, No. 1

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Fig. 1. Polarized Raman spectra of KDP single crystal. (a) a(bb)c Raman spectra at room temperture, where the O-H---O bands are observed above 1500 cm -1. (b) x'(y'y')z Raman spectra at 10K, where a ~ x' (= x + y) and b ~ y' (= x -- y) transformations are brought about with the phase transition from the (abc) tetragonal to the (xyz) orthorhombic structure. A 1 and A2 modes of C2o symmetry group are selected in this polarized spectra. the B1 mode Raman spectra [5], which is shown in Fig.

2(a). If we compare the spectra between Fig. l(a) and Fig. 2(a), we can see that the O-H---O bands remain more or less the same while the other bands including lattice modes are drastically changed. Also between Fig. l(b) and Fig. 2(b) of the low temperature ferroelectric phase we can observe that the O-H---O bands are very similar to each other but the other bands below 1500 cm -1 are quite different from each other. We can thus see that the vibrational O-H---O bands do not have any apparent coupling with the lattice part spectrum in both the paraelectric and the ferroelectric phases. The same three-band structure of the O-H---O band is observed in the KDP melt sample as shown in Fig. 3. This sample of melt may be a KDP glass, considering that the Raman spectra exhibits the low frequency broad band at ~ 4 0 cm -I characteristic of the glass-like structure [ 13]. Furthermore, the melt spectra shows considerable intensities at the characteristic frequencies v s ( P - O - P ) and v , ( O - - P - O - ) of (KPOs)n glass [ 14], and dehydration during the melting process would have certainly reduced the O-H---O concentration. Hence we may well characterize the melt sample of KDP as a diluted low concentration system of O-H---O bonds, which are linked between various phosphates of

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Fig. 2. Polarized Raman spectra of KDP single crystal. (a) b'(a'b')a' Raman spectra at room temperature which should select only BI modes ofD2a. Crystals are cut normal to a' = a + b and b' = a -- b. (x ripple grows with polarization mismatch). (b)y(xy)x Raman spectra at 10 K for orthorhombic crystal axes of x, y and z. Note the phase transition, from tetragonal to orthorhombic, brings about the transformations of a' ~ x and b' ~ y. randomly distributed glass-like structure. In passing, the O-H---O bonds observed in this KDP melt were absent in the (KPO3)n glass which was produced as a transparent lump by melting KDP powder directly at 1000°C for over 5 h in a quartz tubing and splashquenching between two copper blocks of room temperature. The details of the dehydration effects on the KDP Raman spectra will be described elsewhere in a full paper [ 15]. It is now clear that the correlation between hydrogens of O-H---O bonds, so crucial in the collective tunnelling modes, is not prerequisite for the three-band structure of the O-H---O vibrational Raman band. It is essentially from independent singles of O-H---O as already observed frequently in many other solutions [3]. Now we examine the low temperature spectra depicted in Fig. l(b), Fig. 2(b), and Fig. 3(b). We observe here quite a distinctive low temperature effect between crystal and melt in the O-H---O bands: in single crystals the O-H---O bands show more complex structures involving new additional bands, while in melt the same three-band structure of room temperature remains unchanged except for some narrowing of the bands and frequency shifts as common with the system of randonly oriented strong H-bonded complexes [ 1]. And the complex low temperature structure of the O-H---O bands in single crystal KDP should be attributed most to the proton

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O-H---O RAMAN BANDS IN KDP MELTS

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Fig. 3. (a) Raman spectra of KDP melt at room temperature. Note the ~ 4 0 cm -~ band characteristic of the glass morphology and the O-H---O bands characteristic of the hydrogen bonding. (b) Raman spectra of KDP melt at 10K. correlations, the essential difference between the crystal and the glass of KDP. This conclusion is more appealing if we consider that the O-H---O band shape hardly depends on the lattice ionic part of the spectrum in both paraelectric [compare Fig. l(a) with Fig. 2(a)] and ferroelectric [compare Fig. l(b) with Fig. 2(b)] phases, and also no degeneracy splitting is concerned here because the O-H---O bands correspond to the A1 and B1 nondegenerate modes. The long range proton ordering in the single crystal KDP below the ferroelectric transition temperature means that the protons are now highly correlated, and we have four O-H---O bonds in the primitive unit cell, which can no longer be considered as independent singles. As the independent atoms each interacting with a common radiation field become correlated to give a super-radiance [ 16], the independent protons each interacting with the common field of the ferroelectric polarization below Tc must be long-range correlated. Indeed, the spectral sequence of change of the O-H---O bands below Te was reminiscent of the spontaneous polarization dependence on temperature: the spectral change continued to be observable until ~ 90 K below which any change of the O-H---O Raman band was hardly noticeable. Finally we want to comment on the origin of the three-band structure of the O-H---O bands observed in both paraelectric crystals and melts of KDP. Regarding that the OH---O stretching vibration belongs to AI and B 1 modes of D ~ structure [17] and the

O-H---O bands are observed in the AI and B1 mode specspectra, it seems that the most important coupling may be the anharmonic coupling between the O-H---O and the O-H---~ stretching modes of the strong coupling theory. Further supporting this view, the 1800 cm -~ band (C band) appears much weaker in the melt sample, which may be considered as agreeing with the prediction of the strong coupling theory that the zero-phonon band (possibly C band) is very sensitive to the coupling strength [2]. And the coupling strength is expected to be weaker for longer and nonlinear O-H---O bonds in the KDP melts having randomly oriented H-bonds. However, it still remains to wait for more experiments including the deuteration effects to clarify whether the other two bands observed at ~2700 cm -1 (A) and ~2400 cm-1 (B) correspond to the two vertical FranckCondon transitions to the Born-Oppenheimer states of the strong coupling theory [2, 11] or the Fermi resonance interactions [ 1,3, 10] between the O - H stretching mode and an overtone of the bending mode 2Toll or 2~OHREFERENCES 1.

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A. Novak, in Structure and Bonding, (Edited by J.D. Dunitz) Vol. 18, pp. 177-216. SpringerVerlag, Berlin (1974); D. Hadzi & S. Bratos, in The Hydrogen Bond, (Edited by P. Schuster, G. Zundel & C. Sandorfy) Vol. 2, Chap. 12, pp. 565-611. North-Holland, Amsterdam (1976). G.L. Hofacker, Y. Marechal & M.A. Ratner, in The Hydrogen Bond, (Edited by P. Schuster et al.) Vol. 2, Chap. 6, pp. 295-357. North-Holland, Amsterdam (1976). S. Bratos, J. Lascombe & A. Novak, in Molecular Interactions, (Edited by H. Ratajczak & W.J. Orville-Thomas) Vol. 1, Chap. 10, pp. 301-346. John Wiley (1980). Y. Marechal, in Molecular Interactions, (Edited by H. Ratajczak & W.J. Orville-Thomas) Vol. 1, Chap. 8, pp. 231-272. John Wiley (1980). E.V. Chisler, V. Davydov & I.T. Savatinova, Soy. Phys.-Solid State 13, 1635 (1972); 13, 1339 (1971); V. Davydov & E.V. Chisler, Soy. Phys.Solid State 23, 1978 (1982). C.Y. She & C.L. Pan, Solid State Commun. 17, 529 (1975); P.V. Huong & B. Hilczer, J. Chem. Phys. 72, 4412 (1980); S. Rumble, F. Ninio & S.S. Ti, Solid State Commun. 42, 767 (1982). I.P. Kaminow, Phys. Rev. 138, A1539 (1965). R. Blinc & B. Zeks, Soft Modes in Ferroelectrics and Annferroelectrics, Chap. 5. North-Holland, Amsterdam (1974). R. Blinc & D. Hadzi, J. Mol. Phys. 1, 391 (1958); M.C. Lawrence & G.N. Robertson, J. Phys. C: Solid State Phys. 13, L1053 (1980). D. Hadzi, PureAppl. Chem. 11,435 (1965); S.E. Odinkov & A.V. Iogansen, Spectrochim. Acta 28A, 2343 (1972); M.F. Claydon & N. Sheppard, Chem. Commun. D23, 1431 (1969).

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O-H---O RAMAN BANDS IN KDP MELTS S.F. Fisher, G.L. Hofacker & M.A. Ratner, J. Chem. Phys. 52, 1934 (1970). S. Bratos, J. Chem. Phys. 63, 3499 (1975). R.J. Nemanich, Phys. Rev. B16, 1655 (1977). G.J. Exarhos, P.J. Miller & W.M. Risen Jr., J.

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