Quasi-crystalline behavior of NiCl2-water solutions

Solid State Communications, Vol. 18, pp. 765-767, 1976.

Pergamon Press.

Printed in Great Britain

QUASI-CRYSTALLINE BEHAVIOR OF NiCI2-WATER SOLUTIONS M.P. Fontana Istituto di Fisica and G.N.S.M., Parma Italy

(Received 3 October 1975 by R. Fieschi) In this communication we report dynamical evidence for the existence of phonon-like collective excitations in electrolyte solutions at high molar concentrations. The results were obtained by resonant Raman spectroscopy on water solutions of NiCI2 for molar concentration up to 4 M. IN THIS communication we wish to report strong dynamical evidence of crystalline behavior of a liquid in the time scale characteristic of optical phonons. The fluid investigated is a solution of NiC12 in water; for this electrolyte at high molar concentrations, recent elastic neutron scattering experiments have shown unusually strong peaks in the Ni-Ni correlation function, implying an ordered structure over distances much larger than the Ni-Ni shortest average distance. 1 For order to propagate over such large distance, the interactions be. tween the solvated ionic complexes in solution must be sufficiently strong to allow perhaps for collective phonon-like vibrational excitations to propagate. The existence of such excitations and their frequency spectrum should be strongly dependent on the solute concentration and this fact can be used to separate them from "single particle" internal vibrations of the ionic complexes. In order to detect the presence of "quasi-phonons" in solutions at high molar concentrations (typically 1 to 4 molar), we have used the technique of resonance Raman spectroscopy. The solution in fact has a main optical absorption peak at 3950 A, and Ar ion laser lines are situated on its low energy tail. Resonance enhancement of the Raman cross-section connected with the solute is essential for these measurements, since the expected spectrum falls in a frequency region where water itself has strong Raman efficiency. 2 Furthermore the selective character of the resonance effect would enhance those vibrational modes more closely connected with motion of the Ni ions. The solutions were prepared by successive dilutions of a 4 M initial solution, which had been centrifugated for 30 min. to insure good optical quality and homogeneity. Optical absorption spectra a were taken for all solutions used, and the optical density was found to follow Beer's law. Thus the absorbance could be used to check the solution concentration. Raman spectra were taken mainly with 4880 .~ Ar laser line, using a double monochromator with a 4 cm -1 bandpass and photon

counting detection; the recording instrument was a 1024 ch. analyzer. In order to avoid turbulence and inhomogeneities due to laser heating of the solution, excitation power was kept as low as practicable, typically at or below 20 mW. Finally, since in the spectral region used (i.e. on the low energy side of the 4880 laser line) the absorption coefficient is relatively independent of wavelength, the Raman spectra had to be corrected only for overall intensity, when comparing results at different concentrations. In order to separate the broad contributions to the low frequency Raman spectrum due to water and those due to the eventual quasi-crystalline aggregates associated with the Ni ions it was essential to use some form of difference spectroscopy. The technique we used relied on the capabilities of the multichannel analyzer. First the solution spectrum was taken, then the solution was removed from the optical cell using a Pasteur pipette and pure distilled water was put in leaving the optical alignement unaffected. The water spectrum was then stored using the subtract mode in the memory containing the solution spectrum. Since the solution did absorb light (typically 1 cm -1 coefficient at 4 M concentration), the excitation power for the pure water spectrum was lowered to yield equal signal in solution and pure water scattering at the fiduciary shift of 400 cm -1, where only water is expected to contribute. Thus the net memory content at the end of a cycle was proportional to the true difference spectrum. In Fig. 1 we show the difference Raman spectra for four typical concentrations. The spectral intensities are corrected for the variation of the absorption coefficient of the solution with concentration. Thus the integrated spectra should give a qualitative idea of the behavior of the scattering cross-section with Ni +2 concentration. The most significant portion of the spectra is for to s > 100 cm -~. For lower shifts the interpretation of the spectrum is ambiguous due to the very strong scattering of water in that region. It is not clear in fact whether the strong ionic concentration would not alter the very low frequency scattering of water, which is connected with

765

QUASI-CRYSTALLINEBEHAVIOR OF NiC12WATER SOLUTIONS

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Vol. 18, No. 6

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Fig. 1. Difference Raman spectra between solution and pure water at four concentrations: (a) 4 M; (b) 2.75 M; (c) 1.2 M; (d) 0.5 M. The sharp rise of intensity in the (d) spectrum is due to the onset of luminescence (see text). its hydrogen-bonded structure, 2 and thus possibly sensitire to salt-induced destructuring effects. 4 The observed behavior in the very low frequency difference spectra could then be due at least in part to changes in the water relal~ed spectrum. At c = 4 M, the difference spectrum clearly shows structure, and particularly the broad peak at 220 cm -1. To the author's knowledge there are no reported Raman spectra of either NiCI2 or its various hydrated forms. However, for crystals belonging to the same family (such as COC12), the reported Raman spectra show two peaks, located about 150 cm -1 and 230 cm -l respectively. 5 We have taken Raman spectra of the starting material for our solutions, i.e. NiCI2 • 6H20. We have found a variety of peaks, some connected with librations of the water molecules, and some located in the proper regions to be assigned to the Ni-CI network, a Although the difference spectrum lies in the proper frequency region, it differs from that of the powder mainly because of its lack of sharp features. Thus, it reflects a smoothed out onephonon density of states, such as would be expected for an amorphous form of NiC12. In Fig. 1 we also show the behavior of the density of states with concentration. As c decreases, the spectrum broadens and shifts to lower frequency, indicating a general "softening" of the vibrational dynamics connected with the Ni ions. At still lower concentrations (below about 1 M) the difference spectrum reduces to a horizontal line; thus, within the sensitivity of our measurements, there is no observable difference between the spectra of pure water and solution below a certain characteristic concentration range. At the same time that the difference spectrum disappears, a strong yellow luminescence develops. This

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Fig. 2. Semilog plot of luminescence yield vs Ni molar concentration. Below about 0.1 M the yield is independent of concentration. Since the spectral distribution of the luminescence was independent of c, to obtain the yield we have used the intensity emitted at 5300A, with excitation at 4880 A. luminescence can be excited with all the available Ar laser lines. Its behavior with concentration is shown in Fig. 2 where we plot the luminescence yield vs concentration. The observed difference spectrum at the highest concentrations indicates that the ionic complexes related to the Ni ions are interacting strongly and form a quasicrystalline structure, with reasonably long range order. The ensuing collective vibrational excitations have a spectral density which is reflected in the difference spectrum. There seems to be no sharp boundary between the quasi-crystal and the eventual remaining parts of the solution; this is evidenced by the continuous variation of the spectrum with concentration, at least as far as it could be experimentally followed. As the concentration decreases, the coupling among the ionic complexes decreases and so do the collective characteristic frequencies: there is a general softening of the quasi-crystal. At still lower concentrations, the complexes do not interact sufficiently, the local order reduces to that of an ordinary fluid, and the collective vibrations we are concerned with vanish. Incidentally, such behavior excludes that the observed spectrum be due to single particle internal vibrations of the ionic complexes. If this were the case in fact, the spectral features should sharpen up and become more well defined as the inter-complex interaction becomes weaker.

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QUASI-CRYSTALLINE BEHAVIOR OF NiCI2 WATER SOLUTIONS

Since obvious experimental difficulties do not allow the determination of the difference spectra in the interesting region where the collective effects vanish, we cannot state whether the transition is continuous or discontinuous. However, we may use the luminescence behavior, which was found to be essentially complementary to that of the Raman spectra. It is apparent from Fig. 2 that there is no discontinuity in the luminescence drop as c increases. The observed decrease resembles closely what is known as concentration quenching in phosphors, e In these materials luminescence is due to the presence of impurity or color centers, and the luminescence quenching is typically due to inter-center interactions of various kinds which become important above a certain characteristic impurity concentration range, and which generally involve some coupling to lattice vibrations. In our case the luminescent centers are the isolated Ni complexes; therefore the onset of luminescence quenching is directly related to the onset of local long range order and collective vibrational modes, which may

767

provide an additional de-excitation channel. Within this interpretation, the continuous luminescence intensity variation indicates continuity in the transition between the quasi-crystalline state of the solution in the high concentration range and the normal state of the dilute solution. Furthermore from the exponential portion of the luminescence yield vs concentration diagram we obtain a characteristic concentration, 0.3 M, for which the average n.n. distance for Ni ions is approximately 14 A: thus it is about this concentration that the ionic complexes begin to interact significantly. In conclusion, the results reported here corroborate the structural information obtained with neutron diffraction I and offer direct dynamical proof of the presence of collective vibrations connected with the solute in acqueous solutions.

Acknowledgements - The author wishes to thank Drs. Wanderlingh, Maisano and Migliardo for bringing this problem to his attention and for helpful discussions.

REFERENCES

1.

HOWE R.A.,HOWELLSW.S.&ENDERBY J.E.,J. Phys. C7, L l l l (1974).

2.

WALRAFEN G., J. Chem. Phys. 44, 1546 (1966).

3.

A more complete account of both absorption-luminescence and Raman scattering measurements will be published elsewhere.

4.

See for instance ENDERBY J.E., HOWELLS W.S. & HOWE R.A., Chem. Phys. Lett. 21,109 (1974).

5.

LOCKWOOD D.J., "Light Scattering in Solids", Edited by WRIGHT G.B.) Springer-Verlag, NY (1969).

6.

KLICK C.C. & SCHULMAN J.H., Solid State Phys., Vol. 5, Academic Press, NY (1957).