Raman scattering in magnetic US3

~

Solid State Communications, Vol.53,No.9, pp.783-787, Printed in Great Britain.

RAMAN SCATTERING IN

1985.

0038-1098/85 $3.00 + .00 Pergamon Press Ltd.

MAGNETIC US3

A. Zwick, G. Nouvel, M.A. Renucci and G. Mischler Laboratoire de Physique des Solides, Assoei~ au C.N.R.S. Universit4 Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse Cddex, France W. Suski Institute for Low Temperature and Structure Research Polish Academy of Sciences, 50-950 Wroclaw, Poland and D.J. Lockwood Microstructural Sciences Division, National Research Council, Ottawa K IA 0R6, Canada Received 26 November 1984 by E.F. Bertaut

A Raman scattering investigation of magnetic US 3 has been made from 7 to 300 K. Comparison of room temperature spectra with those of the n o n m a gnetic isostructural sulfide HfS 3 allowed the assignment of most of the lines to ~ = 0 optical phonons . Drastic changes take place in the 10 150 cm -I range when lowering the temperature 4own to 7 K : four equal ly spaced lines appear at 54 , 72.5, 91 and 109.5 cm -I. Three of them broaden significantly with increasing temperature and disappear near 50K, at which previous measurements indicate a maximum in the magnetic sus ceptibility and suggest a magnetic phase transition. The stronger fourth line is still observed at 100 K and merges into a phonon line at higher temperature. These four lines are attributed to electronic transitions within the 5f 2 configuration of U 4+. Their temperature dependences ap pear to involve a spin-dependent scattering mechanism and are consistent with antiferromagnetic ordering.

I. INTRODUCTION

grown by chemical transport reaction as described in ref. I. The resulting crystals are dark platelets with (001) surfaces, and their typical dimensions are approximately 3 x I x 0.5 nma3. Due to the second cleavage plane (101), the platelets can be separated in ribbons parallel to the chains i.e. to the b axis, according to the International Tables of X-ray Crystallography (second setting). Because of high sensitivity to air and moisture, the samples were freshly cleaved and immediately immersed in the helium exchange gasof an OXFORD CF 204 cryostat for room and low temperature measurements. In spite of strong optical absorption, the Raman spectra were excited with the 4 880 ~ line of an Ar ion laser as the inherent response function of the spectrometer is practically independent of polarization near this wavelength. Polarization properties were measured in a backscattering geometry. The laser beam was focused onto the sample surface at nearly normal incidence with polarization either parallel or perpendicular to the b axis. Analysis of polarization was performed on the scattered light. In order to remove unwanted inelastic scattering from the entrance window of the cryostat and from the air in front of it, a pseudo-right angle arrangement was used for the temperature study, with incident laser beam at nearly grazing incidence. The polarization was either parallel or practically perpendicular to the surface. This arrangement proved to be practically equivalent to a backscattering configuration inside the absorbing crystal, with in-

PREVIOUS MEASUREMENTS on the uranium trichalcogenide US 3 have revealed interesting magnetic properties and this actinide compound is considered to be a Van Vleck paramagnet at higher temperatures due to tetravalent uranium ions ~1]. Recently, a maximum in the susceptibility versus temperature was observed at 50 K for single crystals of US~ [I] . Moreover, electron paramagnetic resonance (EPR) experiments [2] have been interpreted in terms of impurities, with the disappearance of EPR lines below 50 K suggesting the onset of antiferromagnetie ordering at low temperature in US 3. These interesting properties stimulated the present work, as Raman spectroscopy has proven itself to be one of the most effective methods for studying electronic and magnetic excitations in magnetic compounds [3]. Furthermore, US 3 possesses the "ZrSe3-type'" monoclinic structure ~,5 (space group P 21/m - C~h) of the IV B transltlon metal trlehalcogenl~es. It is therefore expected to show vibrational spectra similar to those of these chain-like compounds, that were widely studied during the past few Mears by means of Raman and infra-red techniques ~ ] .The assignment of phonon scattering in US 3 will thus be achieved by comparison with non magnetic isostructural trisulfides, e.g. HfS3, the one for which most information is available. .

•

•

T

2. EXPERIMENTS Single crystals of US 3 were used for the Raman scattering experiment. These samples were 783

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RAMAN SCATTERING IN MAGNETIC US 3

cident polarization either perpendicular or parallel to the b axis. Raman signals were analyzed with a T 800 CODERG triple monochromator set at 2 cm -I resolution and then processed by a conventional photon counting device. Care was taken to minimize sample heating by utilizing a cylindrical focusing lens and a low excitation power level. Temperatures quoted are those of the sample holder.

Vol. 53, No. 9

=~88o~,

T--300K

US3

z(x )2

i

3. RESULTS

3.1. Room temperature scattering The group theoretical analysis predicts for US 3 twelve Raman active ~ = 0 phonons [7] .These comprise A g and . B g modes, with atomic displacements respectively perpendicular and parallel to the chains. The non vanishing components of the corresponding Raman tensors are given by [8]:

_

m

O0

Z LU I,.--

_z

]

x=sl~s~,

T:300K

I

I

I

I

~. ~ HfS3

Z

< Ig

Ag :

0 b dO

B

:

no"

0 f f 0

g

in the orthogonal set of X Y Z principal axes (X and Y coincide with a and b cristallographic axes, respectively). The mechanical and optical properties of the crystals prohibit the investigation of components other than the XX, YY and XY ones. Fig. I a) and b) show the room temperature spectra of US 3 for incident light polarization respectively perpendicular (X) and parallel (Y) to the chains inside the crystals, without analysis of polarization of the scattered light. The twelve optical modes are thus allowed in the two configurations but only nine of them are observed at 53, 62, 88, 98, 180, 234, 239.5, 243 and 506 cm -I. The very weak feature at 180 cm -I, observed in the Y spectrum, was confirmed by further experiments at lower temperature. Polarization measurements in the backscattering geometry assign the Ag symmetry to phonons associated to the peaks at 88, 98, 234, 239.5 , 243 and 506 cm -I . No clear conclusions could be drawn concerning the symmetry of the other modes due to their rather low scattering efficiency. Three Raman active phonons are thus missing. The same thing occurs for the most absorbing IV B transition metal trichalcogenides, e.g. the selenides and tellurides (the optical band gap decreases from S to Te because of the increasing electronegativity of the chalcogen [9] ), and even for the sulfides when the exciting energy is above the band gap. In particclar, no more than one or two B$ modes could be detected in IVB transition metal trichalcogenides when absorption effects take place. Room temperature spectra for HfS 3 measured in the same geometry are presented in Fig. Ic) and d) together with the symmetries of the lines given by Zwick et al. [6] . All the modes except one of B- symmetry have been assigned by the authors and described in terms of internal vibrations of the chains (5 Ag + 3 Bg) and external modes corresponding to quasirigid chain vibrations (two translational A~ modes along the a and c axes and one libra-tional Ag mode around the b axis). According to this assignment, the missing Bg mode is the translational mode along the b axls. Among the eight internal modes, the Ag highest frequency ,

.

°

fA% II Bg I~ External

Internal

t IAg

1

I~

200 ~00 WAVENUMBER(cm-1)

600

Fig. I. Room temperature spectra for US 3 excited with % = 4880 ~ (a and b) and for HfS 3 excited with % = 5145 A (c and d). The asterisks in c) and d) mark an impurity mode due to Zr originally present in the Hf material used for growing HfS 3. Notation for different configurations stands for ~i ( ~ i ) ~s where ~ and e are respectively the wavevector and polarization of light inside the crystals, the indices i and s refering to incident and scattered light. Symmetries (Ag or Bg) of the lines have been determined by other measurements.

mode (506 cm -I) involves the stretching vibrations of tightly bound S 2 pairs. Analogies in the Raman spectra of US 3 and HfS 3 are noteworthy, the overall and slight lowering in frequencies for US 3 being consistent with a decrease in force constants related to larger lattice parameters and an increase of reduced masses for modes involving motions of the metal atoms. The comparison between the spectra of Fig. I leads to the following conclusions concerning US 3 : (i) The Ag mode at 506 cm -I is the stretching mode of S 2 diatomic "molecules". (ii) The low frequency A~ modes at 88 and 98 cm -I involve translationa~ and / or librational motions of adjacents chains one against the other. (iii) One of the two weak lines at 53 and 62 cm -I must be attributed to a Bg mode.

Vol. 53, No. 9

RAMAN SCATTERING IN MAGNETIC US 3

(iv) The Ag modes at 234, 239.5 and 243 cm -I correspond to internal deformation of the chains. (v) The phonon responsible for the weak structure at 180 cm-1 is a good candidate for an internal Bg mode. 3.2. LOw temperature scattering No significant effect takes place when lowering the temperature down to 100 K. The low-lying frequency lines can no longer be observed, consistent with the change in phonon population, except for the peak at 88 cm -I (modes are labelled by their room temperature wavenumbers). On the other hand, the feature at 180 cm-1 shows up clearly against a broad band extending from 120 to 200 cm -I . Drastic changes occur in the low-frequency part of the spectrum over the temperature range from 7 to 100 K. Four peaks appear in the 10 150 em -| region, with a surprisingly constant spacing at 7 K since they lie at 54, 72.5, 91 and 109.5 cm -I respectively. A fifth, weaker structure at 128 cm -I is even observed for Y polarization. These features broaden markedly with increasing temperature until they disappear when approaching 50 K, with the exception of the most intense llne at 91 cm -I (7 K wavenumber) which evolves continuously, merging into the room temperature phonon line at 88 cm -|. Fig. 2 illustrates the temperature dependence

US 3

X=/,880~

785

of the Y spectrum between 10 and 150 cm -I in the temperature range 7 - 100 K. It gives evidence for some magnetic contribution to the scattering process, consistent with previous measurements [2] suggesting the onset of antiferromagnetic order below 50 K. The line frequencies show no significant shift over the temperature range, which would preclude an assignment to magnon Raman scattering and suggests that these lines are more likely electronic transitions within the 3H 4 (free ion) ground term of the 5 f2 configuration of U 4+. The marked temperature dependence of their intensities would thus result from some spin-dependent scattering mechanism associated with magnetic ordering below 50 K. Due to a relatively poor signal-to-noise ratio, it was impossible to obtain the llne positions, widths and intensities at all temperatures, except for the strongest peak at 91 cm -I as is depicted in Fig. 3. Generally, integrated intensities are normalized to that of a standard phonon line with a temperature dependence determined solely by phonon population effects, because "absolute" intensities from one temperature run to another are difficult to compare. However, as Raman experiments have demonstrated the influence of magnetic order on the phonon spectrum of magnetic compounds [I0], intensities quoted in Fig. 3 have been normalized solely to the power of incoming light. The curves of po-

Z(Y )Z

T(K) 100 s7

_.= C :3

Jd >.

Z i,i I,.-

z Z

t,,,"

18

7 I

h

I

50

100

150

WAVENUMBER (cm-I)

Fig. 2. Temperature dependance of the 10 to 150 cm -I p a x of the Z(Y )Z spectrum in the temperature range 7 - 100 K.

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RAMAN SCATTERING IN MAGNETIC US 3

Vol. 53, No. 9

lc)

¥u 'T"

_m 3=

I

I

I

I

50 TEMPERATURE (K)

•

1

100

(b)

250

c

>U') Z LU I--

150

_z I

I

|

I

50 TEMPERATURE (K)

•

•

•

100

91

1

(a)

Q~ 1,1,1

90 Z I,Xl

t • I

I

I

50 TEMPERATURE (K)

89

I

100

Fig. 3. Temperature dependance of the position (a), intensity (b) and width (c) of the 91 cm-1 line in the temperature range 7 - 100 K. The integrated intensity has been normalized to the power of the incoming light. The linewidth has been corrected for the instrumental response assuming a Gaussian shape for both the spectrometer response function and the experimental structure. Arrows locate the temperature for which previous measurements indicate a maximum in the magnetic susceptibility.

sition and intensity versus temperature are similar in shape, and show a slight decrease with increasing temperature near 50 K. The linewidth exhibits an unexpected peak at about 50 K. A similar maximum of the damping was encountered

in metamagnetic FeCI2-2H20 at the N~el temperature, for a phonon in resonance with the magnon and was attributed to effects of the fluctuations in the longitudinal spin component near T N ~I I . The decrease in linewidth just above

Vol. 53, No. 9

RAMAN SCATTERING IN MAGNETIC US 3

50 K suggests that the excitation responsible for the 91 cm -I line is not purely electronic. As this line merges at higher temperature into a phonon line, the possibility of a coupled electron-phonon excitation cannot be dismissed. Further measurements are required in order to confirm the electronic character of the

low frequency lines at 7 K and to identify the mechanism responsible for their variation with temperature. An investigation of their polarization properties is in progress, while more experiments with an improved signal-tonoise ratio are planned to further characterize their temperature-dependent behaviour.

REFERENCES

I. B. Janus, W. Suski and A. Blaise, Proceedings IV International Conference on Crystal Electric Field and Other Structural effects in f-electron systems, Wroclaw 1981

2. 3.

4. 5.

(Edited by R.P. Guertin, W. Suski and Z. Zo~nierek) p. 539, Plenum Press, NewYork and London (1982). M. Baran, H. Szymczak, B. Janus and W. Suski, Solid State Commun.48, 569 (1983). D.J. Lockwood, in Light Scattering in Solids /IT, ed. by M. Cardona and G. Gbntherod, Springer Verlag, Heidelberg 1982, p. 59. W. Kr~nert and K. Plieth, Z. Anorg. Allg. Chem. 336, 207 (1965). S. Furuseth, L. Brattas and A. Kjekshus, Acta Chem. Scand. A 29, 623 (1975).

6. See for instance A. Zwick, G. Landa, M.A. Renucci, R. Caries and A. Kjekshus, Phys. Rev. B 26, 5694 (1982) and references therein. 7. A. Zwick and M.A. Renucci, Phys. Status Solidi B 96, 757 (1979). 8. R. Loudon, Adv. Phys. 13, 423 (1964). 9. H.W. Myron, B.N. Harmon andF.S. Khumalo, J. Phys. Chem. Solids 42, 263 (1981). 10. E.F. Steigmeier and G. Harbeke, Phys. Kondens. Materie 12, I (1970). 11. L. Graf and G. Schaack, z. Physik B 24, 83 (1976).

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