Raman Studies of Phase Transitions in Mg(ClO,),~BH,O Crystal

Anshu Agarwal, M. B. Pate1 and H. D. Bist Department of Physics, Indian Institute of Technology, Kanpur, India

Raman spectra of Mg(C104)2.6H,0 single crystal in different polarizations in the temperature range 350-110 K are presented. Three phase transitions were found at 325,272 and 225 K. The transition at 272 K is associated with the rotation of crystallographic a and b axes with respect to the space-fixed coordinate axes X and Y in the a6 plane. The transition at 325 K occurs as a result of reorientation of the C104- ion and that at 225 K as a result of the tumbling motion of the aquo complex.

INTRODUCTION

The divalent metal perchlorate hexahydrates M(C104)2-6H20 (M = Mg, Zn, Ni, Co, Mn or Fe) show a number of phase transitions. The transitions for Mg(C10J2.6H20 (henceforth abbreviated to MgPH) in the temperature range 350-100 K can be summarized as follows:

TI TZ T3 =4

Phase I - Phase 11 - Phase 111 - Phase 1V 335 K 325 K 272 K 225 K

T5 T6 7-7

Phase V - Phase VI - Phase VII - Phase VIII 195 K 168 K 103 K

These transitions have been widely studied using EPR,1*2 infrared,3 piez~electric,~ PMR,6 specific and Mossbauerg methods. The structure in the room tem- perature (RT) phase I11 is known to be orthorhombic pseudohexagonal with space group Ci, (z = 2)," but for other phases the crystal structures are not known. The transition mechanisms in this salt are also not very clear. Svare and Fimland' concluded from their PMR data that the motion of the water ligand is not directly responsible for the piezoelectric phase transition at T2 in this salt. Recently, Jain and Upreti' carried out an EPR study of Mn2+ doped in single crystals of MgPH and suggested that the transitions at Tl and T3 involve changes in the state and the symmetry of the

- I

I

T=350K

L I I 1 1 1 1 I I I I I I ' +w I , 3600 3400 1600 \ZOO 1000 600 400 200 I

WAVENUMBER (cm-' 1

Figure 1. Raman spectra in different polarizations of Mg(CIO,),~GH,O crystal at 350 K.

CCC-O377-0486/85/OQ16-0303 $02.50

@ Wiley Heyden Ltd, 1985 JOURNAL OF RAMAN SPECTROSCOPY, VOL. 16, NO. 5,1985 303

A. AGARWAL, M. B. PATEL A N D H. D. BIST

Mg(C104)2.6H20

T = 300 K

--- . - I I I l,i I I I 1 I

WAVE NU M B ER ( cm -' I 3 3500 { ~ O O 1600 'd1200 1000 ' 600 400 200

' 1 . 1 '

Figure 2. Raman spectra in different polarizations of Mg(CIO,),~GH,O crystal at room temperature (300 K).

water-perchlorate sublattice. From IR studies using the isotopic dilution technique, three transitions have been detected3 at 225, 195 and 110 K; the first two transitions have been associated with the tumbling motion of aquo complex and the last one with the positional ordering of hydrogen atoms.

Previous Raman studies on this salt4 were performed only for the polycrystalline sample, which revealed a single phase transition at 325 K. Recently, we reported the Raman spectra of single crystals of M(C104)26H20 (M=Mg or Zn) at room temperature." In order to determine the phase transition temperatures and clarify the nature of the transitions, we have now studied the Raman scattering from a single crystal of MgPH in the range 350-110 K. The investigation indicated the pres- ence of three phase transitions at 325,272 and 225 K in this temperature range.

EXPERIMENTAL

The preparation of the sample and the growth of single crystals of MgPH have been reported earlier.*' The three

faces of the crystal were easily identified by crystal morphology and the a and b axes were chosen in one of the three equivalent ways in the triply twinned hexagonal crystal. The crystal was fixed on the sample holder of a locally made variable-temperature ce11.I2 For high-temperature studies the crystal was sandwiched between two metal (brass) pieces and kept in the heating block of a high-temperature cell made in our labora- tory.13 Raman measurements were taken on a Spex 1403 double monochromator using the 514.5 nm line from an argon ion laser for exciting the sample. Spectral slit widths were held constant throughout the experiment so as to maintain a resolution of about 5 cm-'.

In a preliminary investigation, it was observed that the intensities of the Raman bands at room temperature before and after going to the low temperatures differed remarkedly. This might have been due to the moisture absorbed by the crystal during the process of cutting and then fixing it on the sample holder. We therefore heated the sample to about 50°C and dried it under vacuum before starting the measurements. With this treatment, the absorbed surface water could be removed while leaving intact the crystalline water, and the measurements were reproducible.

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RAMAN STUDIES OF PHASE TRANSITIONS IN THE Mg(CI04)z~6Hz0 CRYSTAL

Z(XZ1V

I I I I I , I I I I I I I , I

3800 3400 1600 4200 1000 600 400 200 ‘ f d

WAVENUMBER (crn-1) Figure 3. Raman spectra in different polarizations of Mg(C10&.6H20 crystal at 258 K.

m5 3600 3400 1600

T.110 K

I

0 WAVE N u M B E R ( cm-’

Figure 4. Raman spectra in different polarizations of Mg(CIO,),.GH,O crystal at 110 K.

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0 l I [ I 1 I " I I I ' 1 ' 1 ' ' ' I 1 1 1 '

TEMPERATURE (K) Figure 5. Integrated intensity versus temperature curve for the v p mode in various polarizations.

RESULTS AND DISCUSSION

The observed Raman spectra of MgPH in different polarizations in phases I (350 K), I11 (300 K), IV (258 K) and VII ( 1 10 K) are shown in Figs 1, 2 , 3 and 4, respec- tively. A detailed assignment of the vibrational bands in phase 111 (RT) has been reported earlier." Assignment of the bands in other phases is made by comparison with the phase 111 spectra and is given in Table 1 for Z ( X X ) Y polarization.

The band positions of the internal modes remained almost unchanged with temperature variation from 350 to 110 K, whereas the peak intensities changed substan- tially for some of the observed modes. The totally sym- metric modes of C104- (vp) and H20 ( u p ) were the most thermosensitive both at lower and higher temperatures and the bending modes of C104- (v! and vi) showed maximum changes in both their half-widths and intensities at higher temperatures. These bands were monitored continuously with temperature. The plots of the intensity of vp versus temperature in different polariz- ations are shown in Fig. 5. Prominent discontinuities appear at 325 and 272K and a weak discontinuity at 225 K in one or more polarizations. The plots of half- widths and intensities of the v 4 and v i modes and of

the v';N mode, presented in Fig. 6, show sharp discon- tinuous changes at 325 K. These curves therefore indi- cate phase transitions at 325, 272 and 225 K. The plot of integrated intensity of the uy mode versus temperature

Table 1. Raman frequencies for Mg(C104)2-6H20 in Z(XX)Y polarizations at different temperatures

Frequency (cm-'1

350K 300 K 258 K l l O K Assignment

- - - 3566 sh - 3536 m 3540m 3530rn 3528rn v;" 1632 w 1632 w 1634w 1628 w V Y

- - 1137w

- 1090 vw - 1080 vw 1114w 1113rn 1117rn 1117 - rn } 4

938 s 937 s 937 s 937s v: 636 m 637 m 634 m 633 m 4

V T

467 m 469 m 468 m 468 m v1p

191 vw 1 8 5 w 190 w 19ovw V R

130vw - 84vw 82 w 84W VL

- - 60 w 65 vw

- - - 535 vw

354 wv 360 w ? ? M-0

- - - -

-

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RAMAN STUDIES OF PHASE TRANSITIONS I N THE Mg(C10,),.6H20 CRYSTAL

360 12

300 320 340 TEMPERATURE (K)

Figure 6. Variation of the half-widths (-) and integrated intensities (. . . . . .) of v; (A) , vqP(0) and u; (0) modes with tem- perature in YY polarization. The open and filled symbols correspond to the right- and left-hand scales, respectively.

(Fig. 6 ) also shows an inflection at 335 K which coincides with the transition temperature

At 272 K, we found a sharp increase in the intensity of the vy (Fig. 5) and v$ (not shown) bands in the X Y polarization and a corresponding decrease in XX and YY polarizations. The discontinuity at this temperature is weak in the 22 polarization and absent in X Z and Y Z polarizations. The intensity increase in X Y polariz- ation is nearly twice the intensity decrease in each of

the X X and YY polarizations. This can arise either from a flip of the a and b axes by 45" with respect to the space-fixed coordinate axes X and Y in the ab plane or from the system going from the orthorhombic to a monoclinic form. The latter possibility is feeble because we did not observe such spectral changes in the similar system Zn(C104)2.6H20, which is known to go to the monoclinic form at low temperature^.^ Dayal et a].* assumed that the a, b and c axes remain unaltered at this phase transition but the ligand field axes change with respect to the a, b and c axes of the room tem- perature phase on account of the increase in the trigonal distortion of the water octahedron. On the other hand, our results suggest that the ligand field axes change owing to a change in the crystallographic axes during the phase transition without a change in space group symmetry. This explains why this transition could not be observed in polycrystalline IR,3 Raman, and piezoelectric5 studies.

The transition at 225 K appears weakly only in the 22, X Z and YZ polarizations, indicating that one of the three-fold symmetry axes of the ClO,- ion no longer remains parallel to the 2 axis below 225 K. This seems to arise from a tumbling motion of the aquo complex in the X Y plane, as a result of which asymmetric hydro- gen bonding takes place between the different oxygen atoms of the c10,- ion, as also predicted by IR studies3

The minor discontinuity in the intensity of v: mode in all the six polarizations at 325 K (Fig. 5) is accom- panied by a sudden broadening and simultaneous decrease in the intensity of the v 2 and v4 modes of the C104- ion (see Fig. 6). This reveals that this transition is associated with the reorientation of ClO,- groups, thus affecting the various polarizations. A reorienta- tional motion of the C104- ion would also affect the aquo complex because of hydrogen bonding, which is apparent from the plots of half-width and intensity of the v$ mode against temperature (Fig. 6 ) , showing a sharp discontinuity at 325 K. As a result, this may pro- duce monoclinic distortion, as suggested by the EPR study.' The aforementioned view supports the con- clusion drawn by Svare and Fimland5 from a PMR investigation that the transition at T2 in MgPH is trig- gered by small changes in the hydrogen bonding from the M(OHJ6 octahedral complexes.

Acknowledgement

The authors' thanks are due to the Department of Science and Tech- nology for establishing a laser Raman laboratory which made the present work possible.

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Received 15 August 1984

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