Gunasekaran, S

Hablilburrahman, A

Shanmugham, M

IACS Institutional Repository

Indian J .  Phys. 67B (3). 251 -  257 (1993)Infrared and Raman spectroscopic investigations on ferroelectric lead hydrogen phosphate crystalsA Habecburrahman\ M Shanmugham*^ and S Gunasekaran*+ Department of l^iysics, 'I’hc New College, Madras-6 (X) 014. India + f Departrneni ol Physics. Presidency College, Madras 6 CK) 005, India ♦ Depanmenl of Physics, Pachaiyappa's College, Madras-6 (X) 030. IndiaReceived 29 October 1992. accepted20 April 1993Abstract : U ad hydrogen phosphate (LlIP) or Lead monciile single crystals are grown by gel mcihtxl Infrared and laser Raman spectra of fcrroclccinc PbHP0 4  have been measured at room temperature in the frequency regions 40(K) — 200 cm > and 4000 — 1 0  cm * respectively- Vibrational band assignments have been made, which arc consisicni with ihc ptiini group symmetry of the lerroelectric phase The assignments made lor this IcTToclectric crystals arc classihcd into three pans vi/. (j) lattice modes, (n) K) 4 internal modes and (ui) High (requency Il-n'odcsK eyw ords ; Infran:d spectra,Raman spectra, vibralumal band assignments PACS No s . ;  33.20. Pa, 33 20 Fb1. In tro d u c tio nFrom the structural stand point PbHP0 4  (LHP) crystal is most conlormable to the qua.si onc- dimcnsional hydrogen bonded ferroelectric. Carvalho and Salimas LI] applied their quasi one- dimensional Ising model to the ferroelectric properties of PbHP0 4 . This crystal has the same crystal space group Pe or C s at temperature above and below the transition temperature (T  ^=310K). A neutron diffraction study o f LHP ai room temperature repi^ried by Ncgran ct al [2] yielded particularly single crystal structure with monoclinic symmetry with two formula units per cell. As shown in Figure 1 , the PO 4  groups are linked into chains along c-axis by short O -H - 0  bonds; but these chains are not crojjs-linked to one another as in potassium  dihydrogen orthophosphate (KDP). The O - H - 0  hydrogen bonds play an important role in tlie transition from ferroelectric to paraelectric mechanism at the T .^The infrared and Raman spectra of powdered samples were first reported by Blinc ei d[3] and proposed a pseudo spin tunnelling model for the phase transition o f LHP in analogy to the similar theory for KDP in interpreting their results, A com plete vibrational band assignments studies for the powdered LHP crystals comparing both infrared and laser Raman spectra at room temperature have been reported in the present work. The O - H - 0  hydrogen bonds in PbHP0 4  are found to be similar to those in PbHAs0 4  KH2 PO4  and KH2 ASO4 . LHP252 A Haheeburrahman, M Shanmugham and S Gunasekarancrysuil is monoclinic at room temperature [2]. The PO« groups are linked by O - H - 0  bonds to form a system  o f linear chains along C-axis. The 0 - H - - 0  distances are 2 .52  A [2]. Hydrogen  positions are 0.22 A from the pseudo-centre o f sym m effy in O -H -O  bonds, so  that 0 -H bond length is 1.04 A. The hydrogen bonds should be thus c lose to those in KH 2 PO4 .F ig u re  I . l>ie crystal slruciurc of l^bllPO^ projected on the a c planeIn PbHPO^ however a sysiem  of linear O-H -O chains is deali w iih , rather than a layer like hydrogen bond network in which only half o f the oxygen s are hydrogen bonded as in KH 2 PO4  141 PbHP0 4  IS not isomon:)hous witli orthorhombic BaHP 0 4  crystals and triclinic Ca>IP0 4  crysLals. Preliminary results from a structural study have shown that the ferroelectric phase o f  dcuterated isom orph L D P has the sam e sp ace group as LH P (N egran  ct al  [2]). The space group o f  die para-elcciric phase o f  the L DP is taken to be P 2 /CIC 2 hl(Negran v.t al  [2]), as was tliought previously to be the case o f LHP. H ow ever, in view  o f the recent work on LHP (Lockw(X)d el al 15], N clm es et al [6 J) this assumption must be treated with caution.2 . E x p er im en ta lBased on the principle that good quality crystals can be grown in the gel m edium , the gel technique o f grow ing PbHP 0 4  has been adopted in the present work. T he B D H  Analar chemicals and double distilled water have been used here. Growing o f  larger LHP crystals is very difficult because o f low  solubility o f this compound. In the present m ethod, one o f  the reactants, orihophosphoric acid (0 .5M -L  5M ) was incorporated with the gelling  mixture o f  density varying between 1 0 2 -1 .0 9  gm-cm~^ for the pH range 2-10 , fo llow ed  by the slow  diflusion ol another reactant supernatant lead nitrate solution (0.5M -2M ). The crystals grown in higher density and higher pH value gels arc found to be opaque. G ood transparent crystals arc w ell grown in the lower density gels 1.03-1.04 gm -cm ^ with low  pH values 2-3.Infrared and Raman spectroscopic investigations etc. 253The Fourier transform infrared and far infrared spectra of this crystal have been measured respectively on Perkin Elmer-1760 and 983 spectrometers in the regions 4000-500 dir* and 1000-200 cm"*. The laser Raman spectra have been observed in the region 4000-10 cm* ;^ on DILORZ24 Raman spectrometer using 514.53 nm radiation from an Argon ion laser (100 mW) for excitation. The spectra are presented in Figures 2 to 4 and the observed frequencies with thier assignments are summarized in Table 1. The frequencies for all sharp bands are expected to be accurate to ±1 cm"*. All specua are recorded at 303° K.I.OOMpiFigure 2. Fourier iransfoim infrared specinun of lead hydrogen phosphate.3. Results and discussionVibrational band assignments:Figure 1 shows the ciystal structure of the ferroelectric phase of lead hydrogen phosphate in a projection of the unit cell on the ac plane. The H atoms disordered over two sites related by a centre of inversion at the mid point of the 0-H--0 bonds. The PO4 groups are linked by the hydrogen bonds to form chains alcxig the C-axis. These chains^  ^ re well-separated from each other b^the lead atoms in the a-axis. As far as lead hydrogen phosphate [7] we may consider ihc PO^  to be molecular entities so that the number of modes may be subdivided into PO4254 A flabeeburrahman, M  Shanmugham and S Gunasekaraninternal m odes, external or lattice modes and high frequency H-modcs. The internal modes should occur at higher frequencies than the lattice modes. The high frequency modes o fKiyurc 3. I^ar-mfrared spcclrum of lead hydrogen phosphate in the range of 1000 - 200 cmFigure 4. Laser Raman spcclnim of lead hydrogen phosphate in ihc range of 4000 - 10 ctr\~Infrared (uidRanim spectroscopic investigations etc. 2551protons involved in the O -H -0 bonds are expected to appear at frequencies beyond 1200 cm'^. We now discuss the results obtained in the different spectral regions; the lattice phonon region, the PO4 internal vibration region and the high fipequency H mode region.T ib le  1. Vibnttionil ipectra and band auignm cnu of lead hydiogen phosphale.RamanFrequencyInleniintyinfrared Frequency o n  Iniemitybeaciiptio77 (MS) 106 (VS) 137 (S)152 (M) 249 (VW)425 (VW) 573 (VW) 732 (VW) 946 (M) 1018 (W) 1354 (VW)252 (W) 431 (W) 572 (MS) 745 0 ^  956 (VS) 1004 (VS) 1348 (S)V0 ....0  (H P O ^ Ubradan modea and polar lattice modeV2(P04)V4(P04)6 (OH) vi(P04)V3(P04)6 (OH)1739 (M S)'2362 (M ) v (O H ) oombinationi2735 '( M )  . and ovcitonca* vs - V o y  Itrang: $ - S trong : M > medium:MS - medium A long; W - Weak; VW - Veiy weak3.1 Lattice vibrations:Lattice vibrations which involve the motions of lead atoms and the PO4 complexes as a whole appear in the far infrared region. Further, a soft mode is expected below 110 cm"* [8]. The peak around 250 cm'* may represent probably the 0 -0  hydrogen bond stretching vibration involving liberation motion of the HPO4 group. Such 0 -0  stretching mode could occur as well around 250 cm"  ^ in CaHP04 and around 155 cm“* to 145 cm“* BaHP04 [9]. In the present work, these bands appear in the characteristic region.3.2 PO4 Internal vibrations:In this region, we expect to find the internal vibrations of the PO4 tetrahedra and the out or in plane 0-H deformation vibrations. If there would be a short symmetric O -H-0 bond we should as well find the 0-H stretching frequency in this region. The free PO 4 ion represents a regular tetrahedron of point group and has four distinct vibratioifal frequencies. In order of increasing frequency these modes are a Raman active doubly degenerate vibration V2 (E), a triply degenerate Raman and infrared active vibration V4 (Fa)* a Raman active totally symmetric vibration v^  (Ai) and a triply degenerate Raman and infrared active vibration V3 (Fa). In aqueous solution the POj" frequencies [10] are Va (E) = 425 on" ,^ V4 (F2) = 573 cm- ,^ Vi (Aj)« 946 cm“* and V3 (Fa) = 1018 cm"*.256 A Habeeburrahman, M  Shanmugham and S GunasekaranThe stretching V3 (F2) band in PbHP04 is centred at around 1020 cm"* and does not appear to be split. The protons thus seem to have a relatively small influence on the distortion of the PO4 group on the low frequency side of the band. There are two weak shoulders at 946 cm^  and 732 cm“  ^ which may represent the weakly allowed Vj (AO band and the OH deformation respectively. The strong and sharp V4 (FO band at 573 cm^ is as well not split. The same is true for the weaker V2 (E) band at 425 cm‘ *.The 0-H vibrations are usually rather weak in Raman spectrum as compared to PO4 internal vibrations and lattice vibrations. A comparison of the infrared and Raman spectra thus allows an additional check on the correctness of the assignments of characteristic group frequencies. No strong Raman scattering was found above 12(X) cm~*, This confirms the present assignment that only 0-H vibrations give rise to infrared absorption in this region. The strong Raman lines at 1018, 946. 573 to 500 and 425 cm’  ^ obviously rcprescnljthe V3 (PO4). Vi^ (P04). V4 (PO4) and V2 (PO4) normal vibrations thus again confiming the assignment of the infrared absorption bands.3 J  Highfrequency H mode :The infrared spectra of PbHP04 is very close to the one observed [11] in KH2PO4. It shows the so called A, B. C band structure fPbHP04 : Va = 2750 cm-^, Vd = 2350 cm , Vc = 1250 ciir  ^1 which is characteristics [11] for medium strong O -H -0 bonds. The main cause of absorption in the rather broad spectral range between 2880 and 1600 cm~' is the 0-H  stretching vibration. The A, B. C peaks are presumably caused by Fermi resonance and combinations with the in plane (5 0-H) and out of plane (v 0-H) deformations. The peak around 1350 cm ' is due to the in plane 0-H deformation vibration. The above data definitely rule out the possibility of a symmetric H bond in PbHP04 . The infrared spectra suggest an off-centre location of the hydrogen.4. ConclusionsA complete vibrational assignment has been made available for ferroelectric phase lead hydrogen phosphate crystal using Fourier Transform infrared and laser Raman spectra in the present investigation. The results obtained in the different spectral regions viz; lattice phonon region, PO4 internal vibration region and the high frequency H mode region have been discussed. A comparison of the infrared and Raman bands made available in the present work confirms the corrcctitess of the vibrational assignment for lead hydrogen phosphate.References11) A V de Carvalho and S R Salimas 1978 J, Phys. Soc. Jpn. 44 238[2] T J Ncgran, A M Glass, C S BhekenKamp, R D Roseniein, R K Osteiheld and R SusoU 1974 Ferroekdrics 6  179[3| R Blinc and 2eks 1974 Soft Modes in Ferroelectrics and Ant^erroeledrics (Amsterdam : Noith-HoUand)[4] M Shanmugham, F D Gnanam and P Ramasamy 1982 Indian J. Pure AppL Phys. 20 579 15) D J Lockwood, N Ohno, R J Nelmes and H Arend 1985 J. Phys. CIS L559Ir^rared and Raman spectroscopic investigations etc. 257f8 | B B Uvrcncic and J Pel/cli 19777 Chem Pkys, 61 3890[9] E E Henry and C B Baddicf 1967 Spectra cMim, Acta bA23 2089[101 G HciTberg 1968 Molecular sj>eclra andMotecular struaure The Infrared and Raman Spectra of Polyatomic Molecules, Vol.2 (Lx>ndon . Van Nosirand)[ 1 1 1  R Blinc, DHadii and Novak 1960 /.lerv Bunsenf>es Phys 64 567

Infrared and Raman Spectroscopic Investigations on Ferroelectric Lead Hydrogen Phosphate Crystals

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Infrared and Raman Spectroscopic Investigations on Ferroelectric Lead Hydrogen Phosphate Crystals

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