T. Runka a, M. Kozielski a, M. Drozdowski a & L. Szczepańska b a Institute of Physics, Poznan University of

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1 This article was downloaded by: [Politechnika Poznanska] On: 04 October 2013, t: 04:02 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Ferroelectrics Publication details, including instructions for authors and subscription information: Raman scattering study of the ferroelectric phase transition in GPI and DGPI single crystals T. Runka a, M. Kozielski a, M. Drozdowski a & L. Szczepańska b a Institute of Physics, Poznan University of Technology, , Poznań, Piotrowo 3, Poland b Institute of Molecular Physics, Polish cademy of Sciences, , Poznań, Smoluchowskiego 17, Poland Published online: 26 Oct To cite this article: T. Runka, M. Kozielski, M. Drozdowski & L. Szczepańska (2000) Raman scattering study of the ferroelectric phase transition in GPI and DGPI single crystals, Ferroelectrics, 239:1, , DOI: / To link to this article: PLESE SCROLL DOWN FOR RTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. ny opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and

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3 Ferroelerfrics, 2000, Vol. 239, pp Reprints available directly from the publisher Photocopying permitted by license only OP (Overseas Publishers ssociation) N.V. Published by license under the Gordon and Breach Science Publishers imprint. Printed in Malaysia Raman Scattering Study of the Ferroelectric Phase Transition in GPI and DGPI Single Crystals T. RUNKa, M. KOZIELSKIa, M. DROZDOWSKIa and L. SZCZEPNSK~ ahstitute of Physics, Poznan University of Technology, Poznai Piotrowo 3, Poland; and blnstitute of Molecular Physics, Polish cademy of Sciences, Poznai Srnoluchowskiego 17, Poland (Received July 12, 1999) The Raman spectra of GPI and DGPI crystals were investigated in the cm-' internal vibrations range and in the temperature range K and K, respectively. Para-ferroelectric phase transition at 224 K for GPI and 322 K for DGPI was examined. The results obtained from Raman investigations confirm the order-disorder nature of a ferroelectric phase transition of second order type. One type of glycinium cations is observed in the paraelectric phase but two types in ferroelectric phase. The dynamical disorder of protons is coupled to the motions of glycinium cations. Keywords: Raman scattering; ferroelectric phase transition; molecular crystals INTRODUCTION In the last years glycinium phosphite crystals NH2CH*COOH*H3P03 and its deuterated analogue NDzCH~COOD*D~PO~ (abbreviated as GPI and DGPI, respectively) have been the object of numerous investigations by various methods. They pointed out the existence of the continuous ferroelectric phase transition order-disorder type at T, = 224 and 322 K for GPI and DGPI, respectively '1v2*31, large anomaly of the dielectric [995]/125

4 T. RUNK et al. permittivity was found at the Curie point along the b-axis. The spontaneous polarkation (Pl) appears along the b-axis below 224 K for GPI 16', however the pyroelectric signal appears below 322 K for DGPI 13]. It seems that P. is due to the ordering of glycinium cations, which forces the protons into order in the interphosphite hydrogen bonds. The temperature dependence of P, is characteristic of a transition close to tncntical point 14'. Very large isotopic effect (- 98 K) on T, indicates the essential role of the strong interphosphite hydrogen bonds in the paraelectric-ferroelectnc phase transition mechanism of GPI I3I. The transition temperature in the both crystals has recently been reported to decrease with increasing pressure ",81. GPI and DGPI belong to the monoclinic system with P2da space group in the paraelectric phase and P21 in the ferroelectric phase. t room temperature (paraelectric phase) the unit cell dimensions are: a = nm, b = nm, c = nm, p = " and Z = 4 for GPI 151. The structure consists of hydrogen-bonded PO3 tetrahedra layers alternate with glycine molecule layers parallel to the bc-plane Is]. There are two type of centrosymmetric hydrogen bonds ( nm and nm). However, glycine molecules in the organic layers are not mutually interconnected. The hydrogen bonds between carboxylic groups and phosphite anions ( nm) appear to be longer than in the case of interphosphite hydrogen bonds 121. In this paper we report the temperature study of Raman scattering in GPI and DGPI crystals. EXPERIMENTL Single crystals of GPI and DGPI used in Raman experiment were grown from saturated aqueous solution of stoichiometric ratio of glycine and phosphorous acid. The deuterated crystals were obtained by sixfold crystallization from D20. The samples of sizes about (3x4~5) mm3 and (2x3~4) mm3 were cut from single crystals and polished to the optical quality. GPI crystal was placed into the static continuous flow liquid helium cryostat making possible the measurements in the temperature range K (stability f 0.1 K). In DGPI case the sample was examined in the thermostat equipped with a special temperature control system that ensured thermal stability of k 0.5 K in the temperature range K. The Raman spectra of GPI and DGPI were recorded using a(b,b)c* and a(c*,c*)b scattering geometry by classic light scattering

5 RMN SCTTERING STUDY [997]/127 system with the spectral slit-width 2 cm-i. The Raman spectra in the range cm-' were measured. Lattice vibrations were not observed due to strong and broad Rayleigh scattering. The argon ion laser (80 mw) operating at 488 nm was used to excite the spectra. The scattered radiation was analysed using a PC-controlled double grating monochromator equipped with EMI photomultiplier and standard photon-counting system. Fitting procedure was employed to obtain the wavenumbers (Raman shift) for the spectral bands recorded in the experiment. RESULTS ND DISCUSSION The investigated spectra cover the range of glycinium cations (NH3'CH2COOH) and anions vibrations (HzP03') for GPI and (ND3'CH2COOD) and (D2P0Y) in DGPI case. The temperature dynamics of these groups for both phases was examined using Raman scattering experiment. FIGURE I. The polarized Raman spectra of GPI and DGPI obtained at room temperature for a(c*,c*)b scattering geometry. The polarized Raman spectra obtained for GPI and DGPI crystals in the spectral range cm" at room temperature are presented in

6 128/[998] T. RUNK et al. figure 1. s it can be seen from figure 1, there are some differences in GPI and DGPI Raman spectra. The bands in the spectral range cm-' arise from the deformation and bending (in-plane and out-of-plane) vibrations of the P-D bonds in DPO3 tetrahedron. The observed bands in the spectral range cm-' arise from the stretching vibrations of -ND3' groups. The asymmetric band of bending vibrations at 652 cm-' of COOH groups shifts to 615 cm-' in deuterated crystal. New band appearing at 1747 cm-' arises from the stretching vibrations of P-D bond. It coincides with the band at 1748 cm-' (arising from the overtone of the stretching C-C bond vibration in GPI). The estimated from Raman experiment frequency ratio of the stretching vibrations (P-H and P-D) VP.H/VP.D = 1.38 is in good agreement with theoretical calculations (- 1.39). weak band for DGPI at 2409 cm'' arising from stretching vibrations of P-H bond is also observed. It can be caused by the partial deuterization of the P-H bond. Temperature dependence of the both, stretching vibrations of the P-0 bonds and bending (in-plane and out-of-plane) vibrations of the P-H bond for GPI are presented in figure 2. On approaching the phase transition a splitting of some glycinium bands and internal vibrations of the phosphite ions is observed. 175K Wavenumbers [cm.'] rn5k 21 SK 230K 250K FIGURE 2. Temperature dependence of the stretching vibrations of the P4 bonds and bending (in-plane and out-of-plane) vibrations of the P-H bond for GPI. Splitting of the bands in the spectral range cm-' for the deformation vibrations of the phosphite ions at 433 cm-' for GPI and DGPI is presented in figures 3a and 3b, respectively.

7 RMN SCTTERING STUDY [999]/ , f I j 430. f 42O w1i90 (8)..' am zb~ 2io 2io zio 2io do Temperature [I(I I,,,,,,,, 290 3W : Temperature [Kj '0 FIGURE 3. Temperature dependence of the Raman shift of deformation vibrations of HP03 tetrahedrons for GPI - (a) and DGPI - (b). Small splitting for bending vibrations of COOH at 652 cm-' was also observed for GPI. Farther splitting of the bands appears for the P-0 stretching vibrations and P-H bending vibrations at 933 cm-' and cm-'. Temperature dependence of the Raman shift and FWHM of the band at 933 cm-' in GPI are presented in figure Temperature [K] FIGURE 4. Temperature dependence of the Raman shill and FWHM of the band at 933 cm-' in GPI crystal. New band at 738 cm-' appears in the region of the deformation vibrations of the P-D bond for the deuterated crystal in the ferroelectric phase. Temperature dependence of the Raman bands in the spectral range cm'' is presented in figure 5.

8 T. RUNK et al. Temperature [K] FIGURE 5. Temperature dependence of the Raman bands in the spectral range cm.' for DGPI. The Raman band in the spectral range cm-' (the region of the stretching vibrations of P-0 bonds) in GPI shows two components at 933 and 958 cm-' with an intensity (19dI933) and area (9)$933) ratio different for both phases. Temperature evolution of the intensity ratio I and area ratio are presented in figure 6. The other bands observed in Raman experiment do not show substantial changes in the vicinity of the phase transition. 4. =/, 1... I.., - I w ZM) Temperature [K] 0 c. 0 U. ' m Temperature [Kl io FIGURE 6. Temperature evolution of the intensity ratio (I = 1958/1933) and area ratio ( = 958/93J) for the stretching vibrations of P-O bonds in GPI.

9 RMN SCTTERING STUPY [1001]/13 1 CONCLUSIONS Only one type of glycinium cations is observed in the paraelectric phase, however splitting of the glycinium and phosphite bands in the ferroelectric phase may indicate the existence of two type of glycinium cations. The changes of stretching and deformation bands of phosphite anion can be related either to the strong coupling between the stretching and deformation vibrations of HPO, tetrahedrons and hydrogen bonds (stretching and bending vibrations) or to the disorder of the protons in the centrosymmetric hydrogen bonds. Moreover, ordering of the protons in the centrosymmetric interphosphite hydrogen bonds is observed. It seems to be obvious that proton vibrations along the hydrogen bonds (c-axis) produce a change of dipole moment along the b-axis. Thus it might be concluded that the dynamics of the organic sublattice of GPI and DGPI can play a key role in the long-range order. The Raman study confirmed the order-disorder nature of the ferroelectric phase transition in both GPI and DGPI crystals. cknowledgments This work was supported in part by the Committee for Scientific Research of Poland under grant No. 2 P03B and the Research Project BW of the Poznan University of Technology. References [I] S. Dacko, 2. Czapla, J. Baran, M. Drozd, Phys.Lett. 223,217 (1996). [2] J. Baran, G. Bator, R. Jakubas, M. Sledi, J. Phys.: Condens. Mutter 8, (1996). [3] J. Baran, M. Sledi, R. Jakubas, G. Bator, Phys. Rev. B55, 169 (1997). [4] R. Tchukvinskyi, Z. Czapla, R. Sobiestianskas,. Brilingas, J.Grigas, 3. Baran, cta Phys. Polon. 92, 1191 (1997). 151 M-T. verbuch-pouchot, cta Cryst. C 49,815 (1993). [6] R. Tchukvinskyi, R. Cach, Z. Czapla, S. Dacko, phys. stut. sol. (a) 165,309 (1998). [7] N. Yasuda, T. Sakurai, Z. Czapla, J. Phys.: Condens. Mutter 9, 347 (1997). [8] N. Yasuda,. Kaneda, Z. Czapla, J. Php: Condens. Mutter 9,447 (1997).