Polarized infrared and Raman spectra of diglycine nitrate single crystal

Spectrochimica Acta, Vol. 51A, No. 2, pp. 197-214, 1995

Pergamon 0584-8539(94)E0093-P

Copyrightt~ 1995ElsevierScienceLtd Printed in Great Britain. All rights reserved 0584-8539/95 $9.50+ 0.00

Polarized infrared and Raman spectra of diglycine nitrate single crystal JAN BARAN,~" AUSTIN J. BARNES~ and HENRYK RATAJCZAKt§ t Institute of Chemistry, University of Wroclaw, 50-383 Wroclaw, Poland and Institute of Low Temperature and Structure Research of the Polish Academy of Sciences, 50-950 Wroclaw, Poland 1:Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT, U.K. (Received 18 May 1993; in revised form 7 March 1994; accepted 9 March 1994)

Abstract--Polarized infrared (4000-ca. 350cm -t) and Raman (4000-10cm -t) spectra of diglycine nitrate (DGN) single crystal were measured at room temperature; polarized infrared spectra were also measured at several low temperatures (230, 180, 15 K). In the paraelectric phase, the glycine ion pairs were found to be joined by a symmetrical (Ci) O...H...O hydrogen bond and the nitrate ions exhibited high symmetry (most likely D3h) as a result of effectively free rotation. The ferroelectric phase transition occurs because of inhibition of the rotation of the nitrate ions, with consequent lowering of their symmetry to Cj, as a result of increased N-H...O hydrogen bonding interaction with the +NH3groups of the neighbouring glycine ions. The symmetry of the glycine ion pair also falls to Ct in the ferroelectric phase, but the proton motion in the O---H.--O bond does not play a significant role in the phase transition. INTRODUCTION DIGLYCINE NITRATE ( D G N ) is one of a small n u m b e r of glycine salts which exhibit ferroelectric behaviour [1]. The crystal is monoclinic, with two formula units in the primitive unit cell; the space group in the paraelectric phase is P2t/a, changing to Pa below the Curie t e m p e r a t u r e , To, of 206 K [2, 3]. Dielectric and specific heat measurements suggested that the ferroelectric phase transition is of the o r d e r - d i s o r d e r type, similar to that in triglycine sulphate (TGS); the m a x i m u m value of spontaneous polarization was found to be along the [101] direction [2]. T h e r e are both similarities and differences between the structures of the D G N and T G S crystals. In D G N , there is a very short O 1 - H - " O 3 hydrogen bond ( O . . . O ca. 2.45/k) connecting two glycine ions, denoted G A and G B , with different configurations: the proton is attached to G A , the glycinium ion (+NH3CH2COOH), while G B is apparently in the zwitterion form (+NH3CHECOO-) [3]. A similar hydrogen bond, also with an O . . . O distance of ca. 2.45 A, connects the G I I (zwittterion) and G I I I (glycinium ion) glycine ions in T G S [4-7]. H o w e v e r , the D G N crystal does not contain any counterpart of the G I (glycinium ion) glycine ions, which are responsible for the ferroelectric properties of T G S [8]. Infrared and R a m a n powder spectra [9-11] and N M R studies [12-14] provided further evidence for the presence of two types of glycine ion in D G N . The proposed switching mechanism in the ferroelectric phase would involve the proton in the short O1""O3 hydrogen bond transferring from G A to G B (thus reversing the glycinium ion and the zwitterion), combined with a torsion and translation of the nitrate ion so as to maintain the s y m m e t r y of the crystal. In the paraelectric phase, there has to be either a dynamic or a statistical disorder between the two possible arrangements of the nitrate ions (one rotated by 60 ° with respect to the other). The G A and G B ions should be symmetrically equivalent in the paraelectric phase, in conflict with the diffraction data [3] (note however that the structure was not determined very precisely, with R = 0.126). Dielectric studies showed that Tc is unchanged on deuteration, suggesting that the O1"" Oa hydrogen bond is effectively of the single minimum type and thus cannot play a role as the trigger in the phase transition [15]. The Curie t e m p e r a t u r e was found to have a positive pressure coefficient [16], implying that the ferroelectric transition is caused by an o r d e r - d i s o r d e r r e a r r a n g e m e n t of p e r m a n e n t dipoles rather than a proton tunnelling process as in the case of K D P - t y p e ferroelectrics. Recently, a nuclear quadrupole double resonance study [17] demonstrated that above Tc the proton is effectively in the centre of the hydrogen bond, while below Tc it shifts into an off-centre position. The N Q R data § Author to whom correspondence should be addressed. 197

198

J. BARANet al.

a

029

C4.~

GB

.,,""

Hi0

~)

Fig. 1. Projection of part of the crystal structure of DGN onto the (010) plane, showing the orientation of the optical indicatrix axes.

suggest that the reorientable dipoles in D G N are the NH~ and N O 3 groups, linked by N - H . . . O hydrogen bonds. In the present study, polarized infrared and R a m a n spectra of D G N were recorded in order to investigate in more detail the spectroscopic properties of the strong O~...O3 hydrogen bond and to obtain more information about the mechanism of the ferroelectric phase transition. EXPERIMENTAL

Single crystals of DGN were obtained by slow evaporation of an aqueous solution, containing glycine and nitric acid in the stoichiometric ratio 2:1, at a constant temperature of 30°C.t The crystals obtained were shaped as described by SATO [3], who reported the orientation of the crystallographic axes a, b, c towards the crystal habit. The orientation of the optical indicatrix axes X, Y, Z was found to be at an angle of ca. 26° to the (001) plane with the acute bisectrix parallel to the b axis (Fig. 1). For infrared spectral measurements, plates about 4 mm thick were cut parallel to the (001) and (010) planes. The plates were stuck onto KBr windows with paraffin wax and polished until the measurement of polarized infrared spectra was possible. Spectra were recorded using a Perkin-Elmer 180 spectrophotometer equipped with a wire-grid polarizer, at room temperature and at several low temperatures (just above, 230 K, and below, 180 K, the Curie temperature and at 15 K) using a CTI Cryodyne model 21 closed-cycle refrigerator. For the measurement of Raman spectra, a cube of approx. 4 mm sides was cut with its edges parallel to the X, Y, Z axes of the refractive index indicatrix. Raman spectra were recorded with a Cary 82 spectrometer, using the 514.5 nm line of an argon ion laser. CRYSTAL STRUCTURE AND SELECTION RULES

The D G N crystal is monoclinic in both phases, with space group Pa-C~ in the ferroelectric phase and P21/a-C~h in the paraelectric phase. There are two formula units per unit cell in both phases. t Attempts to obtain crystals of deuterated DGN were unfortunately unsuccessful.

Polarized infrared and Raman spectra of diglycine nitrate

199

Table 1. Analysis of the fundamental vibrations of the DGN crystal in the ferroelectric phase

Local symmetry

Factor group symmetry

Cl

C~

A

A' A"

(

No. of vibrations

Activity

TA

T

R

n~

IR

Raman

2 1

7 8

9 9

57 57

X, Z Y

xx, y y , zz, zx xy, y z

TA, acoustic vibrations; T, translational lattice vibrations; R, rotational lattice vibrations; n~, internal vibrations of the glycine and nitrate ions.

Ferroelectric phase In the space group Pa, positions of local symmetry C~(2) are possible. Thus, since Z = 2, each G A - G B pair and each NO3 ion should occupy a site of Ct symmetry. The results of an analysis of the fundamental vibrations of the ferroelectric phase of the DGN crystal are shown in Table 1. From these data, it follows that each fundamental vibration should be active in both the infrared and Raman spectra.

Paraelectric phase In the space group P21/a, positions of local symmetry 4Ci (2) and C~ (4) are possible. Thus, for Z = 2, each G A - G B pair and each NO~ ion should occupy a site of Ci symmetry. However, the nitrate ion, whose symmetry as a free ion is D3h , cannot have C/ symmetry. Furthermore, according to the crystallographic data, the GA and GB glycine ions are not symmetrically equivalent, even in the paraelectric phase, and thus the G A - G B pair could not occupy a site of C~ symmetry. Since the G A - G B pair and NO3 ion cannot obey the site multiplication factor (4) for the sites of C~ symmetry, the fundamental vibrations of the paraelectric phase of DGN have been analysed on the assumption that the site symmetry is C~. On this basis, each fundamental vibration should be active in either the infrared or the Raman spectrum.

RESULTS AND DISCUSSION

Polarized infrared spectra of the (001) and (010) faces of the DGN crystal are illustrated in Figs 2 and 3, respectively. The bands observed, and their proposed assignments, in the polarized infrared (300 and 15 K) and Raman spectra are summarized in Tables 2 and 3, respectively. The bands in the wave number region 4000-ca. 350 cm-1 originate from the internal vibrations of the glycine ion pair G A - G B and the nitrate ion. The former can conveniently be separated into the vibrations of the very strong O~... 03 hydrogen bond, linking the glycine ions, and the internal vibrations of the glycine ions. The problem of determining the orientation of the transition moments in molecular crystals of the monoclinic system has been widely discussed [18-20]. DGN is not a typical molecular crystal and its spectroscopic properties should be determined by the very strong O~...O3 hydrogen bond, which is very anharmonic and gives rise to an intense continuous absorption centred at ca. 1000cm -~. It could be expected that for all vibrations the axes of the real component of the dielectric tensor in the (010) plane would be approximately parallel to the direction of the maximum or minimum absorption of the vaOHO vibrational mode. The approximate orientation of the transition dipole moments of the internal vibrations of the glycine ions, determined from the bond polar model, allows these directions to be distinguished for the majority of vibrations (Table 4).

J. BARAN et al.

200

(a) T = 300 K

DGN

(001) E II Y(b)

{

~\

i

T-- 230 K

~A/

e, =

¢.

4000

,i

I

I

3500

3000

2500

1500

2000

I

1000

I

500

Wavenumbers (cm -l)

(b) DGN (001) E II a T = 300 K

%, ,

t~

.=.

!: :

[-.,

gt.

4000

i 3500

I 3000

I

I

2500

2000

I

I

I

l

l

I

1500

i

l

i

I

1000

I

I

I

iI 500

Wavenumbers (cm -I) Fig. 2. Polarized infrared spectra of the (001) face of the D G N crystal at various temperatures: (a) EI[Y(b); (b) Ella.

Hydrogen bond vibrations Antisymmetric stretching vibration vaOHO. This mode is characterized by a continuous absorption in the infrared spectrum polarized Ila axis in the (001) plane of the crystal (Fig. 2). In the spectra of the (010) plane of the crystal (Fig. 3), the continuous absorption is most intense in the spectrum polarized iIX' ( - 1 5 ° with respect to the X axis, i.e. at an angle of ca. 41 ° to the crystallographic axis a). It appears clearly over the wave number range ca. 2500-400 cm-1, with its maximum intensity at 1000 + 200 cm-1. In the spectra polarized ilZ' (i.e. perpendicular to X ' ) or ]lY(b) axis, a broad band centred at 1400+100cm -~, masked by other bands, can be discerned. The O1""O3 direction makes an angle of ca. 20° with the X' and a axes. Thus, as in other cases [2126], the transition moment of the vaOHO vibration lies along the C2"'" C4 direction of the hydrogen-bonded COO groups. The continuous absorption underwent little change when the temperature was lowered through Tc and down to 15 K, but transmission windows observed at 1032, 943,890, 828 and 489 cm- ~became deeper. These windows are located close to the positions of the CN stretching, NHa-L rocking, CC stretching, NO3 v2 and COO rocking modes, respectively.

Polarized infrared and Raman spectra of diglycine nitrate

201

(a) DGN (010) E II X

e~

[-

!

4000

I 3500

I

i

I

3000

2500

2000

i

t

i

i

I

..

I

tv

I

i

1500

I

I

,

t

,

1000

l 500

Wavenumbers (cm-l) (b) T = 300 K

[-

V 4000

I 3500

I 3000

~ 2500

t ~ 2000

i

i

I

I I 1500

I

I

I

I I 1000

I

I

I

I 500

Wavenumbers (cm-I) Fig. 3. Polarized infrared spectra of the (010) face of the DGN crystal at various temperatures: (a) EllX; (b) EIIZ. A strong interaction of the vaOHO vibration with the CN and CC stretching modes would be expected since the transition moments of these vibrations are almost parallel to the X ' axis (Table 4). However, anharmonic interaction also occurs with the NH3_L rocking, N O ; v2 and C O O rocking modes, whose transition moments lie approximately perpendicular to the X ' axis.

Deformation vibrations 6 O H O and 7 O H O . The out-of-plane deformation vibration, 7 O H O , could be assigned to bands at 1230cm -1 (l[Y(b)) and 1220cm -] ([[Z') on the basis of their polarization properties and temperature dependence. When the temperature was lowered to 15 K, the bands shifted up to ca. 1240cm -1 and became much sharper. A definite assignment of the in-plane deformation vibration, 6 O H O , would require spectra of the deuterated crystal, and would still be complicated by the presence of the NH3 groups whose deformation modes fall in the same wave number range. Bands at 1556 cm -~ ([[a, [IX') and 1552 cm -t ([[Z'), observed only in the low temperature spectra, are tentatively assigned to 6 O H O . Internal vibrations of the glycine ions C O 0 group vibrations. The diffraction data [3] showed that the G A - G B pairs are of + + the form N H a C H 2 C O O H . . . - O O C C H 2 NH3, with the hydrogen-bonded proton closer

J. BARAN et al.

202

Table 2. Bands observed (cm 1) in the polarized infrared spectra of D G N Plane (001)

Plane (010)

mu

nu

Ell Y ( b ) 300 K

Ella 15 K

300 K

3220 sh

3236vs

3220vs 3180 vs

3120 vs

3118 vs

3040vs

3056 vs

EIIX' 15 K

3236 vs 3178 vs 3098 vs 3080sh

3040 vs

300 K 3236 sh 3164sh

3064vs

EIIZ' 15 K

3240vs 3180 sh 3100vs

2730s 2696s

2706 s

2722 s 2696s

2722 s

2722 s 2696s

2620m

2630s

2620 m

2620 m

2550sh

2550m

2540 m

2638s 2596 sh 2550m

2638 s 2596 sh 2550m

2 436w

2438 w

2436w

2438 w

2396sh

2376 vw

2396sh

2390sh

2220 sh

1900 sh

1900 w

1900 sh

1900 m

1900sh

1780m 1750sh

1754 sh 1740 vs

1730vs

1720vs

1720 vs

1690t 1650sh 1636s 1624 s 1601 sh

1621 vs

1630vs

1510 sh 1500s 1440 s

1587 s 1556 m 1537m 1521 sh 1490 s 1452 s

1603 s 1595 sh 1527m

1490vs

1482 s

1431 s

1432 s

1420s

1422s

1335 vs

1340sh 1322 vs

1410 sh 1401 vs

1335 vs

1419 vs 1412 vs 1394 sh 1380 sh 1375 vs 1350 sh 1328 vs 1317 vs

1307 s 1240s 1130 s 1120 sh

3212 vs 3160vs

3236s

3028s 2708m

2624m 2550 w 2530 w

1220 vs

1240 s 1220 s? 1130 vs 1120 vs

1410 sh

1378 vs

v.NH 3

3028s 2722 s 2696s 2658 s 2620s

2438 vw

2464vw 2438 vw 2422sh

2360w 2260 sh 2222 w 2080 sh 1912m 1898 sh 1858m 1809 sh 1782m 1758m 1735 sh 1720 h 1690 t 1650 sh 1640 s 1618sh

2340 vw

2340vw

1900sh

1900 vw

v.C=O 1702 s

1600 s

1487s

v~NH3 v~CH2

2550 m

2438 w

1587m 1556m 1537m 1500s

Tentative assignments

3120vs

1709s 1642 sh 1632 s 1601 s 1590sh 1552m 1529s 1505m

v~C=O 6allNH3 6a ±NH3 OOHO 6~NH3

1490 s 1436vs

1378 vs

1230s

1624 s 1600sh

15 K

3064 sh

2 720ms 2698 m

2400vw 2360sh

300 K

1439 vs 1420 sh

1412 s 1401s

6CH2 v~C-O vaC-O

1338 vs

1372 vs 1350 sh 1322 s

1378 vs 1366 vs 1340 vs

1372 vs

paraffin?

1331s

v3NO3 toCH 2 tCH2

1300vs

1310 s

1300s

1238s

1220 s

1309s 1292 s 1239 vs

1128 vs

1120 sh

1128s

yOHO rklNH3

Polarized infrared and R a m a n spectra of diglycine nitrate Table 2.

(Continued)

Plane (001)

Plane (010)

B.

mu

EIIY(b) 300 K 1110 s

Ella 15 K

300 K

l106s

EllS' 15 K

300 K

EllZ' 15 K

300 K

1108 vs 1102 vs

ll00vs

1102 s

1056 vw 1028 vw

1025 vw

1052 vs 1032t 990vs

951m 933m

888vw 869 sh 8 29vw

203

932m

943t

903 vw 888vw

827 vw

829 vs

660m

664 m 610vw

650m

575 vw

575 vw

575 m 549 m 520 w

1040 t 990 vs 950 t 936 vs 916 vs 901 sh 891 t 884 vs 829 vs

681 m 664? 653 s 611 m

1044 vs 1032 t 990 vs

912 vs 890 t 870 vs 840 vs 828 tt

1050 vs 1041 t 990 vs 950 t 940vs 936 vs 918 vs 905 vs 893 t 885 vs 835 vs 823 t

1035 vw

1110 s } llOls 1052 w 1040 vw 947 s

652 vs

rCH2 vtNOf vC-N vaOHO

) I

r±NH3

930 s

930 s

888 vw 870 sh

889 vw 871 vw

vC-C

829 s 770 vw

829 s 770 vw 701 vw

v2NO f

656 m

6COO

578 m 540 vw

wCOO

680 vs 666 vs 650 vs

Tentative assignments

15 K

666 vw 650 w

v4NO~6~COO-

601 s

509 vw 341 vs

578 m 541 w 520 w 489 t 475 vw

560? 521 sh 489 t 470 m 340 sh 320 s

575 m 521 m 493 t 471 m

rNH~ rCOO

Abbreviations: vs = very strong; s = strong; m = m e d i u m ; w = weak; vw = very w e a k; sh = shoulder; t = transmission window.

to the 01 oxygen atom of GA. Spectroscopic data also appeared to support the presence of the zwitterion [9-14]. However, the difference in CO bond lengths is very similar for GA and GB [3]. In the P2~/a space group of the paraelectric phase of DGN, the G A - G B pair should occupy a site of Ci symmetry, i.e. [+NHaCH2COO...H-..OOCCH2+NH3] with a symmetric hydrogen bond. In this case, vibrations characteristic of the carboxylate ion, C O 0 - , should not be observed. Unfortunately, the antisymmetric deformation modes of the NH3 group appear in the same region as the most characteristic COOgroup vibration, the antisymmetric stretch. Without measurements of the spectra of deuterated DGN crystals, the presence of bands arising from this mode cannot definitely be excluded. However, comparison of the bands arising from the deformation vibrations of the COO group in the DGN crystal (6COO:660cm -1 liE(b), 650cm - I I I s ' , IIz', wCOO: 575 cm -l IIz'; r c o o 509cm -t IlY(b), 489cm -~ IIx') with the corresponding vibrations in monoglycine nitrate [10] (663,578 and 510cm -~) and a-glycine [20] (696, 607 and 505 cm -l) demonstrates unequivocally the absence of the zwitterion in the paraelectric phase of DGN. The glycine ions should be intermediate in character between +NH3CH2COO- and +NH3CH2COOH. Thus, v C = O and vC-O bands should be expected in the CO stretching region. Coupling between the GA and GB ions will give symmetric (Ag, Raman active) and antisymmetric (A,, infrared active) vibrations vsC=O, vsC-O, vaC=O and vaC-O. Assuming that their transition moments lie parallel to the bonds, the

1499

1614 1601

1655

3008

3056

3150sh

3244

Y(xx)Z

1516

1594

2880 1655

2880sh 1655

1594

2982

3029vs

3150sh

3244

Y(zz)X

2982

3029vs

3109

3235 sh 3215

Z(yy)X

Ag

1594

2880sh 1652

2982

3029

3215

Y(xz)X

1499 1490

1594

1660

2982

3029

3259

1594

1657

2982

3029

Y(zy)Z

1490

1508sh

1594 1583 sh 1553 sh 1535 1523 sh

1660 1628 1617

2982 2960 2944 2927 2908 2900

3029

3100

3184

3240

Z(yz)X

1493

1607

1657 1623

2982

3029

Y(xy)Z

1493 1480

1595

1657

2900sh

2900 sh 1657

2982

3O68 3029

3123

3208

Y(xy)X

2982

3029

Z(xy)X

(cm-~) in the polarized Raman spectra of DGN at 300 K

Y(zy)X

Table 3. Bands observed

6sNH3

6aiNH3

6altNH3

v~C=O

vsCH2

v~NH3 v~CH2

t vaNH3

Tentative assignments

> Z

>

to

123sh

86

123

86

255 195 144 sh

86

86

130 124 119

586

586 343 323 sh

134

705

707

707 682 607 sh 586 350

707 682

908 732

908 732

903 732

1058 vs

908 732

1058 1053 sh

1107

908 732

1124 1109

1124 1109

1318

1432

1058 1053 sh

1318

1447 1430

1318

1447 1430

1058 vs

1105

1318

14~sh 1409

1058 vs

1112

1340 sh 1318 1302 sh 1162

1409

1447

125

90

125 sh 122

90

90

109

125

144 sh

583 334

583

140 sh

707 680

707

707 680

941 903 732

1058

l132sh 1124 1108

1124 1108

90 sh

122

244

707 682 615 583 338

1099 sh 1058 1053 sh 941 908 732

1321 1302

1408 1397

1455

1319 1302

1432

903 732

1058

1109

1323

1432

1455

906 732

1058

1108

1323 1302

1432 1412

1455

105 95 90 sh

124

lattice vibrations

toCOO

6COO

7117 684

)

903

viNOS-

rlNH3 rCH 2

v3NO3 ¢oCH2 tCH2

vsC-O

6CH 2

r~NH3 vC-C

1058 1053 sh

1111 1105

1323 1302

1455 1449 sh 1432 1412

eo

go

.=.

.-t

o.

go

go

e~

r~ g~

go

o.

=.

206

J. BARANet al.

v a C = O m o d e should a p p e a r in infrared spectra polarized [[Z' and IIY(b), while the v a C - O m o d e should a p p e a r in the spectra polarized I[X' and ]lY(b). In the C = O stretching region a b a n d was o b s e r v e d whose wave n u m b e r and intensity varies with polarization. In the spectra m e a s u r e d in the (010) plane, the wave n u m b e r of the b a n d varies f r o m ca. 1780 ([IX') to 1 7 0 2 c m -1 ([[Z') with its m a x i m u m intensity occurring at ca. + 3 0 ° to the axis X ' (1750 c m - l ) . In the spectra polarized HY(b) axis a very strong b a n d a p p e a r e d at ca. 1730 cm -1. Additionally, a transmission window occurred at ca. 1690 cm -1 in the s p e c t r u m polarized HX' axis and there is a b r o a d b a n d n e a r 1900 cm -~ (most intense in the s p e c t r u m polarized at 150 ° to the X ' axis) whose shape, intensity and position vary with polarization. Such b e h a v i o u r has previously been attributed to strong a n h a r m o n i c coupling b e t w e e n a C = O stretching vibration and the v a O H O m o d e of a very strong h y d r o g e n b o n d [23, 24, 27-30]. In the R a m a n spectra (Fig. 4), v s C - - O is o b s e r v e d as a depolarized b a n d at ca. 1655 cm -~, confirming the c o m p l i a n c e with Ci s y m m e t r y . B a n d s which could be attributed to the C - O stretching m o d e a p p e a r in the infrared spectra at 1410 cm -1 ( l l x ' ) and 1420 cm -1 ( l l r ( b ) ) and in the R a m a n spectra (Fig. 4) at 1412-1408 cm -~ ( x x , y y , x y and z y ) . In the low t e m p e r a t u r e , ferroelectric phase the Ci s y m m e t r y of the G A - G B pair is lost. A new b a n d a p p e a r e d in the NH3 d e f o r m a t i o n region at 1640cm -~ (IIX') and 1632 cm -1 (HZ'), reaching its m a x i m u m intensity at a polarization of ca. 60 ° to the X ' axis. This could be attributed to the v a C O O - m o d e , although its polarization b e h a v i o u r is not as predicted for this m o d e ( m a x i m u m at ca. 135 ° to the X ' axis), but in view of the lack of any o t h e r m a j o r changes in the C O O vibrational absorptions on cooling t h r o u g h the p h a s e transition, it is b e t t e r described as Vs C - - O (now active in the infrared because of the lower s y m m e t r y ) .

Table 4. Orientations a of the transition dipole moments for the vibrations of the GA-GB pair in the DGN crystal and assignment of bands observed in the polarized infrared spectra Glycine A Vibrational modes vC=O vC-O

lPaCOO~? rCOO J vsCOO'~ 6COO J wCOO'~ yc=o J vC-C vC-N v~CH2 \ rCn2 J

diCH2 tCH2 wCHz v~NH3 "} 6~±NH3 ~ r±NH3 )

vsNn3 } 6,NH3 rNa3 ~alINH3 \ rlINH3 J

a

Bands observed (cm 1)

Glycine B Y

a

7

83(98) 147(165)

143 85(100) 61 155(170)

121 (135)

137 133 (148) 141

3 (18)

110

81 (96)

67

6 (21) 151(166) 82(97) 98 (113) 178 (14)

95

70 (85)

76

151(166)

123

172(7)

78

153 ~1740 (ZY), 1730(11Y) 60 1410(11X')

, 509 (liE), 489 (llg') 7 (22) 115 ( 660 (IIY), 650 (HX', IIZ') 92 (107) 74 575 (llz')

f

116 14 (29) 123 151(166) 112 85(100) 20

DGN

115 890 (llx'), 888 (11r) 120 1041(30°X), 1032(11X') 131 ] 3028(IIZ')'3022(60°X) [ 1102(11Z')

MGN b 1727, t717

663 578

1596 505 1410 696 607

871 1045

892 1033

918

1111

510

1436(105°X), 1431 (lit) 1445 1300 (120°X) 1315 174 (10) 107 1317 (I[X) 1300 f 3212 (llZ'), 3120 (lit) 88 (103) 130 1600(11z') 1629, 1598 933 (lit), 930 (llz') { 3072 (150°X), 3040 (HY) 151(166) 120 1500(llX'),1490(135°X,l[r, IIz ' ) 1522 521 (llX') 15(30) 140 f 1624(I[X"IIY) [ 1120 (]IZ') 72 (87)

42

~

° a = angle with X axis (X' axis); 7 = angle with Y axis. bRef. [10]. c Ref. [201.

a-Glycinec

1443 1314 1334 1612 910 1503 1523 1130

Polarized infrared and Raman spectra of diglycine nitrate

207

(a) Y(XX)Z

1383

1 ~

~1587 1~ 1 6 1 6 ~

1700

1600

II

1313

1424 / ~'42141J I 1326t 1536 14S61 ; V ~ " v l I #1 1292

............ "1658 "" ...... 1'''''0500 l i l n l i i l t l l l Iiiii 1760

A

42

14071370134731;". ........ Illll I ....

1500

1400

1300

1200

W a v e n u m b e r s (era -l)

(b)

1587

1325 1318

1658

~1657

I

1541

I

1620

V

r

/

143

I I

#

I

1493 1511

~

11383

!

r t n I i i i a I , I t i I , , , , I , , , ", 1760 1700 1600 1500 1~0 1300 Wavenumbers (¢m -1) Fig. 4. Polarized Raman spectra of the D G N crystal:

~(A) Sl-2-D

,26 K; . . . . .

1200

, room temperature.

208

J. BARAN et ai.

(c) Y(ZZ)X

1316

1587 :::l

1650

1408 :I 1,3olg 1433 ~:

_= 1t 1594

r!16,8/

t!

ii

ll:

,447t,,

Y i

l

1760

i

1453r,JI'd/',

,,,1.535,,86 I

I

I

I

1700

]

I

I

1600

I

i

I

i

i

I

i

i

I

,

t

i

,

1500 1400 W a v e n u m b e r s (era -l)

I

i

,

,

)

1300

1200

(d) Y(ZX)Z

1297

1310

i

,3,, trl ,6,8 A

1536

A / /

A,,,3

A 1490

-

1760

,

'I~:'3~: ~, 1321~!i/1250

~,6,,~59o~.J~A.tJV ' : " ,-- _\' , ~ d I / 1298',~

-

--

,,,,~LLyu-,.~

~

v

. . . . . . . . t

lit,!

I

1700

,

i

I

i

l

1600

l

l

t

-~ I

I

l

I

1430 I

i

I

--l

1500 1400 W a v e n u m b e r s (era -I) Fig. 4. (Continued.)

I

l

I

- ...... i

I

1300

I

)

i

i

1200

Polarized infrared and R a m a n spectra of diglycine nitrate

209

(e) 1462 II

Y(XY)Z

111425

1321 11311

,i

v

1486 296

1642 1608 ~1587

1533

"/ 484

1760

1700

1600

1500

1400

1300

1200

W a v e n u m b e r s (cm - t )

(f) 1434

Y(ZY)Z

1431

.q

1586 •

1486

LJ 1412

.L~~o 1760

1700

15;7k ~

1311 1322

1292

v ;:

1600

1500

1400

W a v e n u m b e r s (¢m -t) Fig. 4.

(Continued.)

1300

1200

210

J. BARANet al.

NH3 group vibrations. The orientations of the transition moments for the NH3 group vibrations in D G N , determined from the HERRANZ and DELGADO model [20] (Table 4), are only approximate because of the different structures of the glycine ions in a-glycine and D G N , the presence of the two glycine ions G A and GB in the D G N crystal and the N - H . - . O hydrogen bonding interactions with the nitrate ions in DGN. The NH3 symmetric stretching vibration could be assigned to intense bands in the infrared spectra at ca. 3064 cm-~ (maximum intensity at 165 ° to the X ' axis) and at ca. 3040 cm- 1 (ll Y(b)) and very weak bands of Ag and Bg symmetry in the Raman spectra at ca. 3056 cm -1 (xx) and 3068 cm -1 (xy). An intense band at 3120 cm -1, with a shoulder at 3220cm -~ (][r(b)), and the strong doublet at 3212 and 3160cm -1 (lIz') in the infrared spectra could be assigned to the NH3 antisymmetric stretch. Below To, the 3212-3160cm -~ doublet became an intense, broad band at ca. 3140cm -~ (180K), 3120cm -~ (15 K) with a high frequency component at 3220cm -t (180 K), 3236cm -~ (15 K). In this region of the Raman spectra, there are weak and broad bands extending over a range from 3259 cm -1 (zy) to 3100 cm -1 (yz) (Table 3). Strong bands in the infrared spectra at ca. 1624 cm -~ (llY(b), IIX') and ca. 1600 cm -~ (IIZ') are assigned to the parallel and perpendicular antisymmetric NH3 deformation modes, respectively, on the basis of their polarization properties (Table 4). The Raman spectra (Fig. 4) exhibit very weak bands at 1628 and 1617 cm -~ (yz), 1623 cm -~ (xy) and 1614cm -~ (xx) which could be due to the parallel mode and a much stronger, depolarized band at 1594 cm -~ assigned to the perpendicular mode. Note that these assignments differ from those of HERRANZ and DELGADO [20] for a-glycine. The symmetric deformation mode appears in the infrared spectra at ca. 1490 cm- 1 and is most intense in the spectrum polarized at 150° to the X ' axis; it also appears in the spectra polarized ]1Y(b) and IIZ'. Below To, this absorption splits into two components at ca. 1530 and 1485 cm -~. The Raman spectra (Fig. 4) exhibit a strong doublet at 1499 and 1489 cm -1 (zy) and weak bands at 1499 cm -1 (xx), 1490 cm -1 (yz), 1493 and 1480 cm -1

(xy). The perpendicular NH3 rocking mode was assigned to bands in the infrared spectra at 930cm -~ (lIZ') and 933cm -~ ([[r(b)), and in the Raman spectra at 941cm -~ (xy). Lowering the temperature led to the bands in the infrared spectra shifting to higher wave numbers and splitting into doublets below To: 947, 930cm -~ ([[Z') and 951, 932 cm -~ (llY(b)) at 15 K. The parallel NH3 rocking mode was assigned to a band observed in the room temperature infrared spectra as a shoulder at ca. 1120 cm -~ (IIZ'), although its polarization was not as expected, and weak bands in the Raman spectra at 1124 cm -1 (zz, xz, xy). As the temperature was lowered, bands in the infrared spectra at 1128 cm -~ ([[Z') and 1130 cm -1 (l[Y(b)), due to the rll NH3 mode, separated clearly from the rCH2 absorptions at 1110, 1101 cm -z ([[Z') and 1106cm -1 (lit(b)). The changes in the NH3 group vibrations on cooling below To, particularly the splitting of the 6s and r± modes, provide clear evidence of the non-equivalence of the G A and GB glycine ions in the ferroelectric phase and suggest a change in the N - H . . . O hydrogen bonding interactions. C H 2 group vibrations. Bands originating from CH2 vibrations should appear at similar wave numbers to those in the a-glycine spectrum. The antisymmetric and symmetric CH2 stretching modes should be polarized almost parallel to the Z ' and Y axis, respectively (Table 4). Thus VaCH 2 is assigned to a band at 3028cm -~ in the infrared spectrum polarized [[Z', overlapping the NH3 stretching region. An intense depolarized band appears at the same position in the Raman spectra. The symmetric stretching mode is obscured by paraffin absorption in the infrared spectra but is assigned to a depolarized band at 2982 cm- 1 in the Raman spectra, most intense in the (zz) and (xz) polarizations. Rather surprisingly vaCH2 is more intense than vsCH2 in the Raman spectra. The deformation mode 6CH2 should be polarized as vsCH2 and is thus assigned to bands in the infrared spectra at 1436 cm-~ (maximum intensity at 105 ° to X ' ) and 1431 and 1420cm -~ ([IY(b)). A corresponding depolarized band appears in the Raman

Polarized infrared and Raman spectra of diglycine nitrate

211

Table 5. Internal vibrations of the nitrate ion Activity D3h

"Free" ion~

IR

DGN crystal (300 K)

Raman

IR

Raman

A~ A~'

v~ v2

1049 830

ia Z

xx+yy, zz

--

1058(dp)

ia

E'

v3

1400

X, Y

xx-yy, zz

1340(xx)

E'

v4

720

X, Y

xx-yy, zz

829 (llZ') 1340 (11/') 1338 (llx') 1335 (lit3 pad'

732 (dp) 707 (dp)

a K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds. Wiley, New York (1963). b Obscured by paraffin wax absorption. spectra, most intense in the (zy) and (xy) polarizations. The wagging vibration toCH2 is at 1334 cm -~ in the a-glycine spectrum. For D G N , bands at 1340-1335 cm-~ are m o r e reasonably assignable to NO3 and ~oCH2 is assigned to the band at 1317 cm -~ in the infrared spectrum polarized [[a axis (more prominent at low t e m p e r a t u r e ) and the depolarized band at 1318 cm -1 in the R a m a n spectra. The twisting vibration tCH2 is assigned to an infrared band at 1300cm -~ whose polarization behaviour ( m a x i m u m intensity at 120 ° to X ' ) is as expected (Table 4). Corresponding weak bands appear at 1302 cm -1 in the R a m a n spectra of Bg symmetry. The rocking vibration rCH2 is assigned to bands in the infrared spectra at 1110 c m (liE(b)) and 1102 cm -~ (llz'), at low t e m p e r a t u r e the latter becomes a doublet at 1110 and 1101 cm -~. A weak, depolarized band occurs in the R a m a n spectra at ca. 1108 cm -~. Internal vibrations o f the nitrate ion. The isolated N O 3 ion has D3h symmetry; the vibrational modes and infrared and R a m a n activities are shown in Table 5. Studies of D G N have led to differing views as to the symmetry of the nitrate ion in the paraelectric and ferroelectric phases. The diffraction studies [3] suggested that the symmetry is at least C3 in the ferroelectric phase but it could be lower in the paraelectric phase. On the other hand, studies of the optical spectra [11] pointed to O3h symmetry of the ion in both phases. KHANNA et al. [10] suggested that their observation of all four internal modes of NO3 in infrared powder spectra of D G N was due to large amplitude internal motions, while KRISHNAN and NARAVANAN [9] explained the absence of the four expected NO3 bands in their R a m a n spectra as due to lowering of the symmetry to C2o. Assuming that the nitrate ion retains its O3h symmetry in the D G N crystal, the 3-fold s y m m e t r y axis should m a k e an angle of ca. 106 ° with the Y ( b ) axis and in projection on the (010) plane it should give an angle of ca. 125 ° to the X ' axis. The activities of the modes will be as shown in Table 5. In the infrared spectra, a strong band was observed at 829 cm-~ polarized almost IIz', with no counterpart in the R a m a n spectra. On the other hand, the R a m a n spectra contain a very strong band at 1058 cm -~, depolarized with its greatest intensity at polarization ( z z ) , which has no obvious equivalent in the infrared spectra.t These bands are readily assignable to the v2 and v~ modes of N O 3 and their behaviour demonstrates that the ion has at least D3, and very probably D3h, symmetry. On cooling the D G N crystal below To a sharp new band appeared at ca. 1050 cm -~ in the infrared spectra polarized at 60 ° to X ' and IIz'; it is also visible adjacent to the transmission window even at polarization 150 ° to X ' . This band clearly originates from the nitrate ion v~ vibration and demonstrates that in the ferroelectric phase the symmetry of the ion is lower than Ca, most probably CI.

t Although there were absorptions at 1052cm -j ([[a) and 1044cm -~ ([IX'), these appear to be the high frequency components of derivative type bands [27], associated with the transmission window at 1032cm ~, resulting from interaction between v~OHO and the CN stretching mode.

Ci

C2h

"~B~

m/mu

Cl

Symmetry

x, z

r

ia

ia

IR

ia

ia

-- 1740 ( ± I')

1730 (IIY)

1660 (zy)

1657 (xy, zy)

xy, yz

zz, zx

1655 (yy, zz) 1652 (xz)

vC=O

xx, yy,

Raman

Activity

1410 (llx')

1420 (IIY)

1412(xy, zy) 1408 (xy)

1409(xx, yy)

vC-O

650 ( _1.Y)

660 (HY)

684 (xy)

680 (xy, zy) 682 (xy)

682 (xx, yy)

6COO

575 (Ill')

583 (xy, yz)

586 (xx, yy, xz)

toCOO

Table 6. Correlation diagram for the vibrations of the GA-GB pair in the DGN crystal

~1000 (Jig')

1400 (IIY, IIZ')

vaOHO

1220 (IBZ')

1230 (UY)

~,OHO

Z

hu

Polarized infrared and Raman spectra of diglycine nitrate

213

The NO3 v3 mode appears in a region containing a number of other vibrations: CO stretch, CH2 wag and CH2 twist of the glycine ions. It is reasonably assigned to the intense bands in the infrared spectra at 1340-1335 cm -~ (depolarized) and a shoulder in the Raman spectra at 1340 cm -1 (xx). The v4 mode unfortunately appears in a region obscured by paraffin absorption in the infrared spectra. In the Raman spectra, two weak depolarized bands at 732 and 707 cmcould originate from this mode.

CONCLUSIONS

The continuous absorption due to the vaOHO mode of the strong hydrogen bond linking the GA and GB ions in DGN is observed, centred near 1000 cm -~, in the infrared spectra polarized Ha and I[X', i.e. polarized almost perpendicular to the ferroelectric direction [101]. This, combined with the minimal changes observed in this absorption during the transition from the paraelectric to the ferroelectric phase, shows that it cannot play a significant role in the phase transition mechanism, in agreement with several studies [15-17]. In the paraelectric phase, the symmetry of the G A - G B pair appears to be Ci, i . e . a symmetric O1-" H.-. 03 hydrogen bond, as demonstrated by the satisfactory agreement with the selection rules of the observed bands due to the OHO and COO vibrational modes (Table 6). The vaOHO vibration may not obey the selection rules as a result of anharmonic interactions. The broad band at ca. 1400 cm -~ in the spectrum polarized [[Y(b) would be expected, taking into account the dynamic splitting, but unexpectedly a similar band also appears in the spectrum polarized Ilz' axis. If the O1---O3 hydrogen bond has Ci symmetry, it would be classified as an A-type bonding according to the SPEAKMAN classification [31]. In the ferroelectric phase, the centre of symmetry is lost and the bonding would be classified as pseudo A-type. Having ruled out any role for proton tunnelling in the ferroelectric phase transition, the mechanism must originate in an ordering of the nitrate ions, linked to the +NH3 groups of the glycine ions by hydrogen bonding [16, 17]. The present work demonstrates that NO~- ions have a high symmetry (most likely D3h ) in the paraelectric phase, presumably as a result of effectively free rotation in the crystals (if there were statistical disorder of the NO~- ions, their symmetry would be lower--presumably C1). In the ferroelectric phase, the appearance of the v~ mode of NO3 in the infrared spectra demonstrates a lowering of the symmetry of the ion, most likely to C~. This must be caused by increased interaction ( N - H . . . O hydrogen bonding) with the neighbouring +NH3 groups, whose participation is confirmed by the changes in the infrared spectra on cooling through the phase transition. NMR studies have also suggested a freezing in below Tc of a 'flipping' motion of the glycine ions, coupled to motion of the nitrate ions, although hindered rotation of the +NHa groups persists below the Curie temperature [14]. Acknowledgements--This work was partly supported by KBN (grant 206199101). We are also grateful to British Council for an academic link award.

REFERENCES F. Jona and S. Shirane, Ferroelectric Crystals. Pergamon Press, Oxford (1962). R. Pepinsky, K. Vedam, S. Hoshino and Y. Okaya, Phys. Rev. 111,430 (1958). S. Sato, J. Phys. Soc. Jpn 25, 185 (1968). S. Hoshino, Y. Okaya and R. Pepinsky, Phys. Rev. IIS, 323 (1959). I. Shibuya and T. Mitsui, J. Phys. Soc. Jpn 16, 479 (1961); K. Itoh and T. Mitsui, Ferroelectrics 2, 225 (1971); T. Mitsui and K. Itoh, Acta Cryst. A28, S184 (1972); Ferroelectrics $, 235 (1973). [6] S. R. Fletcher, A. C. Skapski and E. T. Keve, J. Phys. C: SolidState Phys. 4, L255 (1971); K. L. Bye and E. T. Keve, Ferroelectrics 4, 87 (1972)~ S. R. Fletcher, E. T. Keve and A. C. Skapski, Ferroelectrics 8,479 (1974).

[1] [2] [3] [4] [5]

214

J. BARANet al.

[7] M. I. Kay and R. Kleinberg, Ferroelectrics 5, 45 (1973). [8] V. Winterfeldt, G. Shaack and A. KI6pperpieper, Ferroelectrics 15, 21 (1977). [9] R. S. Krishnan and P. S. Narayanan, Crystallography and Crystal Perfection (Edited by G. N. Ramachandran), p. 329. Academic Press, New York (1963). [10] R. K. Khanna, M. Horak and E. R. Lippincott, Spectrochim. Acta 22, 1801 (1966). [11] Y. Sato, J. Chem. Phys. 45, 275 (1966). [12] K. R. K. Easwaran, J. Phys. Soc. Jpn 21, 61 (1966). [13] V. Saraswati and R. Vijayaraghavan, J. Phys. Soc. Jpn 23, 590 (1967). [14] R. Blinc, M. Jam~ek-Vilfan, G. Lahajnar and G. Hajdukovi~, J. Chem. Phys. 52. 6407 (1970). [15] M. Ichikawa, Ferroelectrics 39, 1033 (1981). [16] K. Gesi and K. Ozawa, Japan J. Appl. Phys. 12, 951 (1973). [17] J. Seliger and R. Blinc, Ferroelectrics 78, 223 (1988). [18] G. Turrell, Infrared and Raman Spectra of Crystals. Academic Press, New York (1974). [19] J. Herranz and J. M. Delgado, Spectrochim. Acta 31A, 1255 (1975). [20] J. Herranz and J. M. Delgado, Spectrochim. Acta 32A, 821 (1976). [21] L. Angeloni, M. P. Marzocchi, D. Hadzi, B. Orel and G. Sbrana, Spectrochim. Acta 33A, 735 (1977). [22] L. Angeloni, M. P. Marzocchi, S. Detoni, D. Hadzi, B. Orel and G. Sbrana, Spectrochim. Acta 34A, 253 (1978). [23] V. Videnova-Adrabifska, J. Baran and H. Ratajczak, J. Molec. Struct. 145, 33 (1986). [24] V. Videnova-Adrabifiska, J. Baran and H. Ratajczak, Spectrochim. Acta 42A, 641 (1986). [25] J. Baran, M. M. Ilczyszyn, R. Jakubas and H. Ratajczak, J. Molec. Struct. 246, 1 (1991). [26] M. M. Ilczyszyn, A. J. Barnes, S. N. Bhat and H. Ratajczak, J. Molec. Struct. 269, 23 (1992). [27] M. P. Marzocchi, L. Angeloni and G. Sbrana, Chem. Phys. 12, 349 (1976). [28] B. Orel and D. Hadzi, Chem. Scr. 11, 102 (1977). [29] V. Videnova-Adrabiriska, J. Baran, H. Ratajczak and W. J. Orville-Thomas, Can. J. Chem. 63, 3597 (1985). [30] M. M. Ilczyszyn, H. Ratajczak and A. J. Barnes, J. Molec. Struct. 198, 505 (1989). [31] J. L. Speakman, Struct. Bonding 12, 141 (1972).