Low-temperature infrared studies

3ournaI Elscvier

of Molecular Publishing

S1rucrure

Company,

Amsterdam.

Printed

LOW-TEMPERATURE INFRARED STUDIES VII. THE INFRARED SPECTRUM OF AND MAGNETIC TRANSITION IN AMMONIUM NH,CuCI J

A.

M. IIEYNS

AND

339

in the Netherlands

POSSIBLE ANTIFERROTRICHLOROCUPRATE

C. J. H. SCHUTTE

Unioersity of South Africa, Preforia

(Republic

of South Africa)

(Received July 9th, 1970)

According to the crystal structure et al., the NH4 +-ion in this compound haviour

of the N-H

vibrational

modes

determination

of NH,CuCI,

by Will&t

should be very much distorted.. The bcin NH,CuC13, however, is not in agrec-

ment with this distortion; vibrations which are activated according to predictions by the site group and factor group theories are not observed; bands such as V~ have symmetrical shapes and the combination band of the torsional mode with vJ, viz. v,+ v6, does not appear, even at 17 “K. These facts have Icd to the conclusion that the NH4 +-ion must have considerable rotational freedom in NH,CuCI J _ Durjng the cooling cycles the intensities of the N-H vibrations showed changes at -85 and -45 “K, while the observed copper
antiferromagnetic

transition

occurred.

INTRODUCTION

The crystal structures of the isomorphous salts, NH,CuCIX and KCuCIs have been determined by Willett et al. ‘. These monoclinic crystals belong to the space group P2,,,, with 2 = 4, and contain planar Cu,C16’ dimers which are packed above each other along the crystallographic a-axis, as shown for NHoCuCIs in Fig. 1. The copper-bridging chlorine distances (Cu-Cl, in Fig. 1) of 2.32 A are slightly longer than the copper-terminal chlorine distance (Cu-CI, and Cu-Cl2 in Fig. 1) of 2.25 and 2.26 A respectively. The Cu-Cl, distances in these two compounds are very close to the Cu-Cl ones of 2.30 A in CuCI, ‘. The latter comJ. Mol.

Structure, 8 (1971) 339-349

A. hf. HEYNS. C. J. H. SCHUTTE

Fig. 1. The packing of the Cu2CIe’

chains

in NH.CUCIJ.

pound contains infinite chain molecules formed by planar CuCI, groups sharing are packed in opposite edges T. As can be seen in Fig. 1, the chains in NH,CuCI, such a way that each copper in a chain is bonded to a chlorine above and one below the plane of its own chain, by relatively long bonds (Cu-Cl; = 3.19 A, Cu-Cl; = 2.99 A). The chains in CuCI, are packed in a similar way, the lengths of these bonds being equal to 2.95 A’. The latter bonds suggest that very little covalent bonding could be expected between the copper and chlorine atoms in different chains3 in these compounds. Six of the nine chlorine atoms which surround the cations in NH,CuC13 and KCuCI, lie at the corners of a trigonal prism, while the remaining three arc situated outside the faces of this prism ‘. The positions of the hydrogen atoms were not determined, but considerable distortion can be expected since the covalent copper chlorine bonds have considerable stronger directional character than the ammonium or potassium-chlorine bonds’. It can also be expected that any possible hydrogen bonding in the NH, +-ions will occur with the four nearest chlorine is illustrated in Fig. 2. atoms. The coordination of the NH4 +-ion in NH,CuCI, Antifcrromagnctic transitions are known to occur in several copper halides at lower temperatures, e.g. in CuCI, and CuBr, 4*5 in CuCI, * 2H 06-8, and also in LiCuCI, - 2H209- “. In NH,CuCI, and KCuCl, the dimer itructure of the Cu,CI,’ groups does not lead to diamagnetism, and KCuCl, has a magnetic Willett et al.’ have measured the moment of 1.77 B.M. at room temperature12. magnetic susceptibility of KCuCl, between 70 and 1.5 “K. At about 30 “K a maxiJ. Mol.

Structure,

8 (1971)

339-349

LOW-TEMPERATURE

INFRARED

STUDIES.

VII.

Fig. 2. The coordination of the NH. +-ion in NH.CuCl,. another chain) bond lengths arc denoted by l.

341

The copper-chlorine

(belonging

lo

mum in the susceptibility

occurs, which could be an indication of an antiferromagnetic transition. A study of the reciprocal molar susceptibility (I/l,) of the K,Cu,Cl, dimer at lower temperatures revealed that a maximum in z,,, occurs at 32 OK, and that an ordering process takes place below 17.5 “K, which produces a sharp incrcasc in the susceptibility’ 3. In his study of the antiferromagnetic KCuBr, (which is nearly isomorphous with KCuCIJ (ref. 1))Sinnz4concluded that this compound could bc much better represented by a lattice antifcrromagnctism model than by one of discrete pair-wise antiferromagnetism. Although no magnetic, thermodynamic, or low-temperature

crystallographic

data are available for NH,CuCI,, it might bc expected that this compound, like the isomorphous KCuCl, and other copper compounds containing Cu,CI,’ chains, will undergo an antiferromagnetic transition at lower tcmperaturcs.

EXPERIMENTAL

NH,CuCI, was prepared according to the method by Foote and Walden”. The CuCl, which was used in the preparation was obtained by heating recrystallised AnalaR CuCI, - 2Hz0 in a stream of HCI gas. The average chemical analysis of the compound gave the following results: oA Cu (talc.) = 33.83, % Cu (obtained) = 33.33; O/ ,,, Cl (talc.) = 56.63, o/0 Cl (obtained) = 56.31. Details regarding the low-temperature apparatus used, have already been published16. Spectra were recorded of films and Pctrolcum Jelly mulls of J_ Mol.

Structure.

8 (1971) 339-349

342

A. M. HEYNS,

NH4CuCf3 on several different pressed pellets.

alkali halide windows,

C. J. H. SCHUTTE

and also of KBr and CsBr

RESULlS

(a) N-H vihrutionaf nrodes Considering the space group symmetry of k2,,, and 2 = 4 in NH,CuCI,, the correlations bctwccn the vibrations under T,, symmetry for the NH,*-ion and those under the site and factor group symmetries of Cl, show that all the N-H vibrations will be activated in the infrared spectrum of NH,CUC~~‘~*‘*. Situated on the position of C, symmetry, the NH,, +-ion must be very much distorted, and this must be refiected in the infrared spectrum of the ion by severe splitting of the

3200

3000

2800

1400

i

1;

3

cm-’

Fig. 3. The N-H vibrational modes in the frequency ranges 3400-2600 and I500-1300 cm“ (a) 298 “EC,(b) 17 “K and (c) of the supercooled phase at 17 “K. 3. Mol. Structure, 8 (1971) 339-349

at

LOW-TEMPERATURE

INFRARED

STUDIES.

343

VII.

bands

as well as the activation of inactive modes (under Td) such as vl and v2. The N-H vibrational modes in NH,CuCl, in the frequency ranges of 3400-2600 and 1~1300 cm-’ at (a) 298 “K, (b) 17 “K and (c) of the supcrcooled phase at 17 “K arc shown in Fig. 3. The frequencies and assignments of these modes arc collected in Table 1. TABLE

I

FREQUENCIES

AND

ASSIGNMENTS

OF

RANDS

IN

THE

INFRARED

SPECTRUM

OF

NH&uCI, --__

-298 “K -.-

__._..--._

cm-’

Assignment

17 “K __ cm-

.-

’

Assignment

I 7 oK supercooled _._.-._ __-.---_ Cm-’

Assignment

---

.--.

-3180

Vj

3260

(sh)

3240 3210 3180

(sh) I

2800

(sh)

(sh)

1400

:‘z -t l’a

21’.

1’4

3220

VJ

VI

3135

VI

3050 (sh) 1 3030

vz + v4

306&3070

2800

21.~

2800

(sh)

2Vh

3’44

1415 (sh) 1405 (sh) 1395

)‘&A

1390 (sh)

v.~

3070

r(Cu-Cl)t*

v(Cu-Cl)b**

(sh)

1415 I405 1395 I I389

309 283

v3, “38 ‘#J.

3130 306&3040

“3d

V4.2

“aa )*4.

305

v(Cu-CI)1*

280

v(Cu-Cl)b**

(sh)

vz + a.4

V4C

a’*b

t = terminal. * b =: bridging.

l l

At room

temperature, the N-H stretching frequency v3 occurs at -3180 cm-‘, and a broad shoulder can be seen on the low-frequency wing of this band at 3040-3060 cm- I, which is assigned to the combination band v2 + v+ v4, the N-H bending mode, is a symmetrical band at 1400 cm-‘. The temperature dependence of these bands is shown in Figs. 4 and 5. As can be seen in Fig.4, splitting of v3 and v, gradually develops at lower tem-

peratures (these bands occur respectively at 3210 and 3130 cm-’ at these temperatures). Below -85 “K this splitting becomes more distinct with a concomitant incrcasc in the intensities of these bands, and shoulders also develop on v3 at 3260 (weak), 3240 and 3180 cm-‘. v1 is a well-defined component at 17 OK, while the broad shoulder at 304U-3060 cm-’ has split into three components at 3070, 3050 and 3030 cm-’ respectively. The first overtone of v.+ is observed as a very weak shoulder at ~2800 cm-’ and remains practically unchanged during the cooling cycles. J. Mol.

Structure.

8 (I 971)

339-349

A. M. HEYNS,

Fig. 4. The temperature

dependence

stretching

dependence

vibrations

‘6

36

24

Fig. 5. The tcmpenturc

of the N-H

of the N-H

bending

vibrations

C. J. H. SCHUTTE

in NH4CuCI,.

6

in NHICUCI~.

Two asymmetries at 1405 and 1415 cm-’ appear gradually on v4 at lower temperatures, and below - 85 “K another one develops at 1389 cm - *. There are no indications of the modes v2 and v4 + vg (where v6 is the torsional mode), even zt 17 “K. These bands generally occur in the frequency range 165& 1750 cm-r jrefs. 16, 19 and 20). If the same sample had been used in several cooling cycles, only a supercooled phase was eventually obtained at 17 “K. In this phase at 17 OK, vJ and respectively and none of these bands were v, appeared at 3220 and 3135cm-’ split; while v4 was observed to occur at 1395 cm-’ with weak shoulders, at 1390, 1405 and 1415 cm-’ respectively (Fig. 3(c)). 3. Mol.

Structure,

8 (1971)

339-349

LOW-TEMPERATURE 16

80

INFRARED

-*

STUDIES.

VII.

345

J

\

15-

\ \

14.

L

13-

30

40

50

60

x

70

c

,

80

90

TEMPERATURE IN KELVIN

Fig. 6. The variations of the peak heights of (a) Us, (b) yj and (c) v1 with temperature.

(b) Phase transition itI NH,CuCI, The variations of the peak heights of v4, v3 and vr with temperature are respectively shown in Fig. 6 (a, b and c). In the case of v,+and vs. a progressive strong increase in the intensities of these bands takes place below -85 “K and continues down to -45 “K where either the intensity remains more or less constant (vq) or increases much more gradually (v,). The fundamental band vl, on the other hand, develops into a distinct component below -85 “K while its intensity increases rapidly at -45 “K. (c) Cu-Cl vibrations As we were confined to CsBr and CsI optics in the recording of the lowfrequency vibrational modes of NHdCuCl 3, 250 cm- ’ was approximately the low/. Mol.

Structure,

8 (1971) 339-349

346

A. M. HEYNS,

C. J. H. SCHUTTE

frequency limit of our experiments. We have therefore only recorded two Cu-Cl vibrations at room temperature, viz. the medium strong bands occurring at 309 and 283 cm-t. Adams and Lock3 have located three Cu-Cl vibrations in NH,CuCIj, viz. at 311 cm-’ (Cu-terminal chlorine stretching mode) and at 280 and 230 cm-’ (Cu-bridging chlorine modes). We have adopted this assignment, and found that, in agreement with their observations at liquid nitrogen temperatures,‘little sharpening of these bands occurs down to 17 “K. At this temperature none of these bands arc split, no marked changes have occurred in their intensities, and they arise at 305 and 280 cm-’ respectively.

DISCUSSION

(a) N-H vibrationalmodes

As compared to the N-H stretching region in (NH4)$04 20a*b, this region in NH,CuCl, is characterised by comparatively narrow bands, an absence of multiplicity, weak intensities of the combination bands (e.g. 2vq) and an upward shift of vj (at lower temperatures where vj and vI are split, v3 occurs at 3210 cm-’ in NH,CuCIs and at 3110 cm-’ in (NH,),SO,). In (NH,),SO,, the properties of the N-H stretching region were used to confirm the existence of a considerable amount of hydrogen bonding in the compound, and it can thus be concluded that much less hydrogen bonding occurs in NH,CuCI,. The occurrence of vr at 17 “K as a well-defined satellite on vlr and the relatively weak nature of v2+v4, is in direct contrast with the behaviour of the corresponding modes observed in the spectra of (NH4)2S0420b and (NH4)2Crz0,20c at these temperatures. On the other hand, however, this low-temperature behaviour of v, and v2 + v4 in NH4CuCI,, agrees with that of the corresponding modes in 22 In the latter case, the correct assignment of NH,CIO, and NH,BF, at 17 “K . v1 in the infrared has been confirmed by Raman spectra of NH4BF422. Furthermore, v4 is not only observed as a symmetrical band at room temperature but, even at 100 “K, practically no splitting of this band occurs. In view of the predictions of the site- and factor-group theories, two explanations for the symmetrical shape of v4 can be submitted, viz. (a) the actual site symmetry of the +-ion could be much higher than the predicted Ct , or (b) very little hydrogen NH4 bonding exists in NH4CuC13 and this could mean that the NH,+-ion is reorientating very rapidly at room temperature. The latter possibility is in agreement with the properties of the bands in the N-H stretching region in this compound and, therefore, seems the more plausible one. Although the absence of v4 + vg is no criterion for free or nearly free rotational motion in ammonium salts, even at lower temperaturcs20b, it may serve as 1. Mol. Structure,

8 (1971)

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LOW-TEMPERATURE

INFRARED

STUDIES.

VII.

347

an indication of such motions if it is accompanied by the above-mentioned properties of the N-H vibrational modes. From infrared cvidcnce alone, very little can bc concluded about the mechanism of the reorientational motion of the NHi+-ion in NH,CuCI,. In comparison with the infrared spectra of NH,CIO, and NH,BF, I9 (the NH,+-ions are believed to rotate “freely” in these salts) it can be concluded that the NH,+ions will rather execute fast reorientational motions in NH,CuCI,. In NH,CIO, and NH.,BF4, v3 occurs respectively at 3290 and 3332 cm-’ (ref. 19). indicative of even less hydrogen bonding than in NH,CuCI,. In these salts, v3 is also a much narrower band than in NH,CuCI,, and does not split into two bands (the one being v,) at lower temperaturcs22 (this is before the occurrence of any phase transitions). On the other hand, the NH,+-ions in (NH4)2Cr207 2oc also possess a considerable amount of reorientational freedom at room temperature, but vJ and v2 + v., are observed as separate components of equal intensities while the N-H stretching region is characterised by much broader bands than in NH,CuCIs. From this evidence it seems that the reorientational freedom of the NH,+-ions in NH,CuCI, lies in between those of the NH,‘-ions in NH4C104 and NH4BF4 on the one hand, and those of the NH, +-ions in (NH4)2Cr207 on the other hand.

(6) Phase transition

The transition from the paramagnetic to the antiferromagnetic state is a transition from a disordered state to an ordered state of the spins, and is generally believed to be a transition of the second order, although Stout” prefers to class these transitions as lambda-ones (which occurs in NH,C124). Apart from susceptibility measurements where the susceptibility reaches a maximum at the transition (Curie) point, it can also be detected by means of a study of various nonmagnetic properties, e.g. heat capacity, thermal expansion, deformation of the lattice, etc.25. Neutron diffraction methods are used to determine antiferromagnetic structures”. However, systematic studies of the temperature dependence of the infrared spectra of antiferromagnetic materials have been very much neglected. The changes which occur in the crystal parameters on going from the paramagnetic to the antiferromagnetic state, must surely be reflected in the infrared spectra of such compounds*. Although antiferromagnetic transitions arc cooperative ones and must, therefore, be pronounced, it has been found that the heat capacity vs. temperature curves for such compounds tail off on the high temperature side”. It is still not clear if these tails are the result of the remnants of short-range order of the spins or are caused by structural changes. In their study of the heat capacities of CuCI, at various temperatures, Stout and Chrisholm” observed that, apart from the peak which denotes the antiferromagnetic transition, a further gradual maximum J. Mol. Structure.

8 (1971)

339-349

A. M. HEYNS,

C. J. H. SCHUTTE

also appears on the high-temperature side of this curve. Apart from the existence of relatively strong antiferromagnetic interactions between adjacent copper ions in the chain, much weaker interactions between nearest neighbours also exist in adjacent chains. These latter interactions indicate the beginning of long-range order and are reflected rn the peak of the heat capacity vs. temperature curve at 23.9 “K. The gradual maximum at higher temperatures in this curve, as well as the maximum in the paramagnetic susceptibility at 70 “K, is the result of antiferromagnetic interactions within the chain. Likewise, the maximum in the susceptibility of the K,Cu,CI, dimer in KCuCI, which occurs at 32 OK, has been ascribed to the depopulation of an excited triplet state to a singlet which becomes appreciable at this temperature’ 3. The temperature dependence of the intensities of the N-H vibrations in NH4CuC13 must, on the other hand, reflect the structural changes which occur in going from the paramagnetic to the antiferromagnetic state. No drastic changes occurred in the infrared bands at either -85 or -45 “K, only a gradual development of shoulders and/or satellites takes place at these temperatures, and the changes could only be detected by means of intensity vs temperature plots. (In the supercooled phase, the N-H vibrational bands only exhibit changes in intensities at -85 “K.) It thus seems that the reoricntational motions of the NH,++-ions are and this reaches a maximum at gradually restricted from -85 “K downwards, -45 OK. We propose that a possible antiferromagnetic transition takes place at this temperature. The behaviour of the observed copper-chlorine vibrations during these cooling cycles suggests that the structural changes arc interchain ones. Unfortunately the copper-chlorine (belonging to another chain) modes could not have been detected since they will also reflect the changes which have occurred in the interchain parameters when the transition point is traversed.

1 2 3 4

Sa

Sb 6 7 8 9 10 11

12 13 14 15

R. D. WILL, C. DWICCINS, JR., R. F. KRUH AND R. E. RUNDL~. J. C/mm Whys., 38 (1963) 2429. A. F. WELLS, f. Chem. Sot., (1947) 1670. D. M. ADAMS AND P. J. LOCK, J. Chem. Sue., A (1967) 620. C. G. BARRACLOUGHAND C. F. NC, Truns. Furuduy Sot.. 60 (1964) 836. J. W. S1ou-r AND R. C. CHRISHOLM.1. C/mm Whys.. 36 (1962) 979. J. W. STOIJTAND R. C. CHRISHOW, 1. Phys. Sot. Jupun. 17, Suppl. B-l (1962) 522. R. E. RUNDLE, J. Am. Chem. SW.. 79 (1957) 3372. W. MAEHALL, Phys. Chem. Soli&, 7 (1958) 159. A. 1. STEKHAHOV,E. A. POPOVAAND Yu. F. MARKOV. Opfiku ISpekrroskopiyu, 19 (196s) 583. P. Ii. Vossos, L. D. JENNXNCS AND R. E. RUNDLE, 1. Chem. Phys., 32 (1960) 1590. P. H. Vossos, D. R. FITZWATER AND R. E. RUNDLE, Aclu Crvsr.. 16 (1963) 1037. S. C. ABRAHAMS AND H. J. WILLIAMS, J. Chem. Phys., 39 (1963) 2923. E. COI-TON-FIYTIS. Ann. Chim. (Puris), 4 (1925) 9. G. J. Mlws, B. C. G~RSIXIN AND R. D. WILEIT. /. Chem. Phys., 46 (1967) 401.

E. SINN. Inorg. Nucf. Gem. Letters, 5 (1969) 193. W. H. FOOTE AND P. T. WALDEN, J. Am. Chem.

J. Mol. Srmcmre, 8 (1971) 339-349

Sot.,

33 (1911) 1032.

LOW-TEMPERATURE

INFRARED

STUDIES.

349

VII.

AND A. M. HEYNS, C/tern. Phys. Lerrers, 1 (1967) 487. X-Ray Crystallography, Vol. I, Symmetry Groups, Kynoch Press, Birmingham, England, 1965. 18 E. B. WILSON, J. C. DECIUS AND P. C. CROSS, Molecular Vibratims - The Theory of Infrared and Raman Vibrational Spectra. McGraw-Hill. New York, 1955. 19 T. C. WADDINGTON.f. Chcm. Sot., (1958) 4340. 20a C. J. H. SCH~JTIZAND A. M. HEYNS, Chem. Phys. Lerfers, 1 (1968) 51 I. 20b C. J. H. SCHUITE AND A. M. HEYNS, f. Chem. Phys.. 52 (1970) 864. 2Oc C. J. H. SCHUI-TI?AND A. M. HEYNS, 1. Mol. Srructure. 5 (1970) 37. 21 G. HBF~ZBERCI, Infrared and Roman Spectra of Polyaromic Molecules. Van Nostrand. New York, 1945, p. 167. 22 C. J. H. SCHUITE AND D. J. J. VAN RENSBURG,to bc published. 23 J. W. STOUT. Pure and Appf. Chem.. 2 (1961) 287. 24 A. B. PIPPARD, Chap. 9 in Elements of Classical Thermodynamics, Cambridge University Press, 1957. 25 K. P. BELOV,Chapter 4 in Magnetic Transitions, Translated by W. H. FURRY, Consultants Bureau. New York, 1961. 16 17

C. J. H. SCH~ International

Tables for

J. Mol. Structure,

8 (1971) 339-349