J. Phys, Chem. Solids Vol. SO, No. 7, pp. 7X4-742, 1989 Printed in Great Britain.
0022.3697/89 S3.00 + 0.00 Maxwell F+ergamonMaanillaa plc
RAMAN STUDY OF HIGH PRESSURE INDUCED PHASE TRANSITION IN SODIUM NITRITE R. A. L)ALTERIO~ and F. J. OWENS~: tGeo Centers Inc., Lake Hopatcong, NJ 07849, U.S.A. SEnergetic Materials Lab., ARDEC, Dover, NJ 07801, USA; Dept Physics, Hunter College, City University of New York, U.S.A. (Received 20 September 1988: accepted in revisedjbrm
8 February 1989)
Abstract-The effect of pressure up to 200 kbars on the Raman active internal and external modes in NaNO, has been measured using a diamond anvil cell. The appearance of a new low frequency Raman active lattice mode at 50 kbars indicate8 the existence of a high pressure phase not previously observed. Slightly above this pressure a transfer of intensity is observed between two lattice modes whose frequencies have been shifted close to each other by pressure, indicative of a pressure-induced coupling between the modes. Keywords:
Raman spectroscopy, high pressure, sodium nitrite, lattice modes, phase transition.
INTRODUCTION
Sodium nitrite is a ferroelectric at room temperature having a body-centered orthorhombic unit cell (C::) with one molecule per unit cell [l, 21. At 164°C there
is a transition to a non-polar disordered phase. This transition has been studied by X-ray diffraction and Raman spectroscopy [3-S]. A measurement of the volume of the material as a function of temperature and pressure showed a break in the compression curve at 8.5 kbar and 20°C 161.The transition was not, however, detected by differential thermal analysis [6]. Measurements of the infra-red spectrum of the internal bending mode of NO,- in a diamond anvil cell @AC) showed a splitting of the mode at 14 kbars [7J A more recent high pressure infra-red study indicated that the splitting occurs at 10 kbars [S]. No studies of either the internal or external Raman active vibrations have been reported as a function of pressure. This work reports measurements of the effect of pressure on the Raman active internal NO; and lattice vibrations of NaNO, up to 200 kbars using a diamond anvil cell, in order to examine and obtain further jnfo~ation on high-pressure structural phase transitions. Evidence is presented for the existence of a phase transition not previously observed as well as a pressure-induced coupling between lattice mode vibrations. EXPERIMENTAL
Raman scattering measurements were carried out using a Coderg T800 symmetrically-mounted triple monochrometer. The Raman spectra were excited using the 4880 A line of an argon-ion laser operating at SO-75 mW. The diamond anvil cell is basically of a type described earlier [9]. A 0.3 mm thick Inconel 739
gasket with a 200 pm diameter hole was used to confine the sample, a ruby chip, and the pressure transmitting medium. The pressure is obtained by measuring the shift of the ruby fluorescence which has been previously calibrated against pressure [IO]. Quasi-hydrostatic conditions are verified by monitoring the width of the fluorescence at each pressure. An increase in the width of the fluorescence of a single ruby chip in the diamond cell has been shown by Piermarini er al. [9] to be a good indication of the onset of non-hydrostaticity. Their work showed that the fluid medium used here, an ethanol-methanol mixture, sustains hydrostatic press&e until above 100 kbars where there is a glass transition. RESULTS
At ambient pressure and temperature the unit cell of NaNO, is orthorhomic with four atoms per unit cell. The lattice modes have nine optical vibrations and three acoustical branches. Figure 1 shows the Raman spectrum in the lattice mode region at ambient conditions in finely ground powders of NaNO,. Five lines are observed at frequencies in agreement with previous observations in single crystals [4, S]. Based on single-crystal polarization studies these lines have been assigned to specific vibrations. The line at 121 cm-* is an A, NO; librational vibration. Two overlapping lines at 155 and 164 cm-’ which have been deconvoluted by fitting them to Lorentzian lines, as shown in Fig. 1, are assigned to modes of B symmetry. Polarization studies indicate the line at 121 cm-’ is the orientational vibration of the NO; around the b-axis and the 153 cm-’ line has considerable contribution from the NO; rotation about the c-axis 1111.The lines at 188 and 222 cm-’ are in good agr~ment with modes observed in the crystals and
R. A.
DALTERIO
and F. J. OWENS
Frequency
( I cm-‘)
Fig. 1. Raman spectrum in the lattice mode region of powders curve).
The Lorentzian
deconvolution
of NaNO, at ambient conditions (upper of some of the modes is also shown (lower curves).
assigned to vibrations of B symmetry. The pressure dependence of the three low frequency modes at 121, 155 and 164 cm-’ was measured up to 200 kbars. Figure 2 gives a plot of the pressure dependence of the frequency of the A2 and the B, librational modes. The frequency of the two B modes increases linearly with pressure up to 200 kbar. However, the frequency of the A2 mode displays a linear dependence on pressure up to 60 kbars but above 60 kbars the dependence on pressure weakens and becomes nonlinear. No changes in the lattice mode frequencies were observed at 10 or 14 kbars where previous IR studies of the internal NO, modes showed a splitting of the line. In the same pressure region where the A, mode begins to show non-linearity in the pressure depen-
dence of the frequency, the relative intensity of the B, librational mode to the intensity of the A2 mode begins to change. The intensity of the A2 mode decreases while that of the B, increases. Figure 3 shows a plot of the relative intensity of the B, to A, mode as a function of increasing pressure. Figure 4 shows the Raman spectrum in the external mode region at 101 kbars. The marked change in the relative intensity of the B mode at 194 cm-’ compared with the A mode at 177 cm-’ is evident. Also apparent is a new line at 88 cm-‘. This new line appears in the vicinity of 50 kbars. The pressure dependence of this new line was also measured up to 200 kbars and its frequency increased linearly with pressure. The changes observed on increasing pres35I-
13
230 c
.
I 50
(1)
A2
libratm
(A)
El
libration
I 100 Pressure
Fig. 2. Pressure
.
.
I
I50
l
ZOO
(Kbor)
dependence of the A2 and modes’ frequencies.
.
. I
8, librational
.
.
. .
.
I
I
I
50
100
150
Pressure
1
2000
(Kbar)
Fig. 3. Difference in frequency of the A, and B, lattice modes vs pressure (m) and the pressure dependence of the relative intensity of the B, to A, modes (0).
Phase transition
in sodium
Frequency
nitrite
741
(I cm-‘)
Fig. 4. Raman spectrum in lattice mode region at 101 kbar in the high-pressure phase showing the new low frequency line at 88 cm-‘.
sure were reversible in that on lowering the pressure from 100 to 30 kbars, the spectrum returned to that observed at this pressure when measured as a function of increasing pressure. The 88 cm-’ line disappeared when the pressure was lowered to 30 kbar. At room temperature three Raman lines associated with the internal vibrations of the NO; are observed: a strong line at 1327 cm-’ due to the symmetric stretch, a line at 829 cm-’ with about half the intensity of the symmetric stretch due to the bending mode and a very weak line at 1234 cm-’ due the asymmetric stretch. These frequencies agree with previous IR and Raman observations of the internal modes. In the diamond cell the line at 1327 cm-’ is not observed because of a larger line from the diamond at 1335 cm-’ that overlaps it. The pressure dependences of the 829 cm-’ and the 1234 cm-’ modes were measured. The intensity of both modes decreased on the application of pressure and above 2 kbars the asymmetric stretch was no longer observed. The effect of pressure on the A bending mode frequency is shown in Fig. 5. The frequency begins to show a non-linear dependence on pressure in the vicinity of 10-20 kbars, the same pressure region where the pressure-dependent IR studies show a splitting of the line.
vicinity of 20 kbars. In this work the Raman spectrum of the bending mode did not show any splitting at any pressure up to 120 kbars. However in the vicinity of 10-20 kbars, as shown in Fig. 5, the pressure dependence of the frequency begins to show a non-linear dependence on pressure. No changes, however, were observed in the lattice mode Raman spectrum in the 10-20 kbar region. It is possible there is some small change in the crystal arrangement at this pressure region perhaps involving a slight reorientation of the NO; but there is no major change in the symmetry of the unit cell which would change the Raman spectrum in the lattice mode region. The more pronounced changes in the lattice mode region observed near 60 kbars provide evidence for
DISCUSSION Previous studies of the pressure dependence of the vibrational spectrum of the internal NO; vibrations up to 50 kbars using IR spectroscopy indicated the onset of a splitting * _ of the bending vibration in the
az50/
Pressure (Kbar)
Fig. 5. Effect of pressure on the frequency of the internal A bending mode of NO,
742
R. A.
DALTERIO
a phase transition at this pressure. The evidences for a phase transition are: (1) a marked change in the relative intensity of the B, to A2 Iattice modes; (2) the onset of a non-linearity in the pressure dependence of the A, mode frequency; (3) the appearance of a new Raman line at 88 cm-‘; and (4) the reversibility of the effects. A plot of the difference of the frequency of the A, and B, modes vs pressure shown in Fig. 3 indicates a gradual decrease in the difference which reaches a minimum at 60 kbars and remains constant at about 17 cm-’ until near 100 kbars where the separation starts to increase. As Fig. 3 shows, the region of pressure where the two frequencies are closest corresponds well to the pressure range where the relative intensity of the modes 1s changing. This correspondence of the pressure region where the frequencies of the two modes have a minimum separation and where the relative intensity of the modes exchanges suggests there is a pressure-induced coupling between the modes. A strikingly similar effect was observed in the ferroelectric phase of SrTiO, as a function of an applied electric field where the electric field shifted the two lattice modes close to each other and the two modes exchanged intensities [12]. As in the SrTiO, case there was no change in line widths or shapes in NaNO,. Such effects are characteristic of coupled classical harmonic oscillators. In NaNOz it has been shown that hydrostatic pressure increases the transition temperature from the ferroelectric phase to the paraelectric phase at 16O’C implying that the polarization field in the ferroelectric phase increases with pressure at a given temperature [13]. It is possible therefore that the pressure-induced mode coupling effect is indirectly an electric field-induced mode coupling as in SrTi03. For mode coupling to occur the frequency of the two modes must be close in
and F. J. OWENS
magnitude and the two modes must have the same symmetry. The two modes involved in the coupling were assigned by EIartwig et al. 143 to A, and B, symmetry. Since the appearance of the new line at 88 cm-’ occurs somewhat below the pressure at which the two modes have the smallest frequency difference and where the onset of the intensity exchange occurs the phase transition occurs before the mode coupling effect and thus the mode coupling effect is in the high-pressure phase. The mode coupling effect in the new phase implies one of the modes has changed symmetry such that both modes have the same symmetry as required for mode coupling.
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