Pressure-induced crystalline-amorphous transition in NaVO3 and its recrystallization

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Pergamon

0022-3697(94)E0048-K

J. Phys. Chem. Solids Vol. 55, No. 8, pp. 665~69. 1994 Copyright •' 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-3697/94 $7.00 + 0.00

PRESSURE-INDUCED CRYSTALLINE-AMORPHOUS TRANSITION IN NaVO3 A N D ITS RECRYSTALLIZATION Z. X. S H E N , t C. W. O N G , S. H. T A N G a n d M. H. K U O K Department of Physics, National University of Singapore, Lower Kent Ridge Road, Republic of Singapore 051 i (Received 27 September 1993; accepted in revisedform 14 March 1994) Abstract--Pressure-induced amorphization has been the subject of intense study for the past few years because of its importance in materials science and solid state physics. We reported a crystallineamorphous phase transition at c. 60 kbar in NaVO 3 [Shen et aL, Phys. Rev. B 49, 1433 (1994)], which is the lowest pressure found for such transitions in ionic crystals. Here we report on the details of the transitional region and the re-linking of the VO4 chains upon heating. Keywords: A. non-crystalline materials, C. Raman spectroscopy, C. high pressure, D. phase transitions, D. phonous.

INTRODUCTION Amorphous states are typically prepared by quenching from the melt. Due to the very rapid cooling rate, the substance does not have enough time to rearrange itself and the disordered structure of the high temperature liquid state together with its isotropy are "frozen" and retained in the low temperature state. Although the amorphous state is only metastable and the total energy is higher than its crystalline counterpart, the thermal energy required to overcome the potential barrier for recrystallization may be too large at room temperature. The substance therefore remains "stable" for practical purposes, the applications of such amorphous materials being widespread. It has long been thought that the amorphous state is a unique property of temperature quenching. However, pressure-induced amorphization was reported for ice at 77 K [1] and since then such transitions have been reported for a number of materials, including several ionic crystals, SiO 2 [2-5], CaAI~Si20s [6], AIPO4 [7], Ca(OH)2 [8] and LiKSO4 [9]. Because of the importance in applications of amorphous materials and the fundamental physical questions that pressure-indiced amorphization poses, this has been the subject of many theoretical studies [10-14]. The structures of alkali metavanadates are noteworthy in that they have infinite chains formed by VO4 tetrahedra sharing corners. In this respect, they are similar to the better known and studied silicates, which also have chain structures and are the primary examples of amorphous materials. NaVO3 (or-form) is ferroelectric below 380°C and has the monoclinic

space group C c (Z = 8), with V03 chains parallel to the c axis [15, 16], isostructural with the diopsides, CaMg(Si03) 2. It exists in another polymorphic form (fl-phase, orthorhombic, space group Pnma, Z = 4) [17], which transforms irreversibly to the or-phase at 405°C. EXPERIMENTAL ~t-NaVO3 was prepared from the powder form of the fl-phase (Merck, 99%) by heating to 450°C. A Raman spectrum taken after heating confirmed that the fl--a transformation was complete. High pressure was generated in a gasketed diamond anvil cell with stainless steel gaskets of 200/~m in thickness having a hole 200 p m in diameter and predented. A small amount of sample and a ruby chip were loaded into the gasket hole together with a 4"1 mixture of methanol-ethanol, which acts as a quasi-hydrostatic pressure transmitting medium. The pressure was calibrated by the ruby fluorescence technique [18]. Raman spectra were recorded in backscattering geometry using a Spex double monochromator coupled to a conventional photon counting system. The 514.5 nm line of a Spectra-Physics Ar + ion laser was used as the excitation source and the power at the sample was estimated to be below 250 mW except for the reclaimed samples. All high pressure spectra were taken at room temperature. RESULTS AND DISCUSSION (a) High pressure R a m a n spectra The crystalline-amorphous transition of ct-NaVO3 occurs at 60kbar and is marked by the abrupt

tAuthor to whom correspondence should be addressed. 665

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appearance of a strong and broad peak at c. 800 c m (Figs 1 and 2). This band is wider than those of N a V O 3 in solution or in melt-quenched form in which the samples, especially in solution, must be highly disordered [19]. Figure 1 shows the high pressure Raman spectra in the backbone vibration region of the VO 3 chain up to 168 kbar. All the peaks below 60kbar belong to the s-phase and are reasonably sharp. The frequencies of the R a m a n bands increase with pressure as the corresponding bonds strengthen on compression. The spectrum at 61 kbar is a mixed phase and a more detailed analysis is given below. Most of the sharp bands have disappeared at 106 kbar and only two very broad bands are present at 168 kbar. The spectra near the phase transition region are shown in Fig. 2, which clearly demonstrate the sudden change at 60 kbar. Figure 3 shows the frequency against pressure for the maximum peak position of the broad peak at 807 cm ~. It decreases (softens) with pressure up to about 100 kbar, and only then shows the expected increase. The Rarnan spectra in the 800 cm-~ region after the transition are shown in Fig. 4. The strong band consists of two components, one at 805.2 cm and a shoulder at 738 cm ~ at 61.4 kbar. Another peak begins to appear at 778 cm ~ at 66 kbar and its intensity increases with pressure. At 73.4kbar, the 738 cm-~ peak becomes vanishingly small and there

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is a new broad band underneath the sharp band at 670 cm, '. The 778 cm ' peak overrides the 807 c m - ] peak (which has now softened to 800.6cm -~) and becomes the most dominant in the entire spectrum, while the sharp peaks at 670 and 936 cm ~ are too weak to be detected. The intensities of the broad bands at 670 and 778 cm ~increase with compression while those of the sharp bands decrease. The intensity of the 807 c m - ~ band disappears above c. 95 kbar, so that there are only two bands at 665 and 774 c m - ~ in the 600-850 c m - ~ region. 900

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In our earlier paper [19], we have assigned the spectra just above the transition to three different forms of NaVO3 : the original a-phase as indicated by the sharp bands; an intermediate phase with the infinite chains broken into finite ones suggested by the broader band at 918cm -~ apparently derived from the sharp bands in the 900 c m - ] region; and a new phase with individual VO 3 ions manifested mainly by the strong broad band at 807 cm -~. We assign the two broad bands at 800 and 687 cm -~ at 168kbar as the vI and v3 vibrational modes of the VO~ ions with average C3~ symmetry. A m o n g the four normal modes of vibration of the VOW- ion, the vx and v3 modes are expected to give rise to stronger Raman lines. The frequency changes of the observed R a m a n bands are shown in Fig. 5. It is clear that the R a m a n spectra just after the crystalline-amorphous transition are more complicated and some bands in the 800 c m - l region soften with pressure. We speculate that just after the transition, the VO4 chains break into individual V O f ions but leave the bond angles largely unchanged. The bonding changes gradually under further compression to a final configuration of VO3 ions, so that there are more bands in the pressure range 60--100 kbar. On decompression, the spectra are reversible by and large with some hysteresis, and the amorphous state is retained to 36 kbar. The sample does not

revert to the a-phase on further pressure decrease but transforms to another amorphous state instead. This low pressure amorphous state has no Raman active bands, as shown in spectrum f of Fig. 6, where the sharp bands belong to the a-form. When the reclaimed samples were repressurized, a broad peak

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was found at about 800 cm ~, which also softens with pressure (see spectrum g of Fig. 6), but the intensity of this peak is weak. We suggest that the sample is a mixture of the high pressure amorphous phase and the low pressure amorphous phase, and only the high pressure phase gives rise to the Raman band and hence has a smaller intensity. This interpretation is supported by the increased spectral noise around the 1000 cm -I region, similar to that found for the low pressure amorphous phase in spectra e and f. Thus we see three different phases at one pressure ( ~ 100 kbar) depending on the loading path: a mixture of the high pressure amorphous phase and a small portion of the or-form on increasing pressure; an almost pure high pressure amorphous phase on pressure decrease; and a mixture of the high pressure and low pressure amorphous phases on recompression. (b) Temperature variation of the Raman spectra from reclaimed samples Since an amorphous state is necessarily unstable and the a- and fl-phases are the stable states of NaVO3 at ambient conditions, the reclaimed sample should transform to either the ct- or fl-phase given enough thermal energy. After heating the samples to 330°C, the samples revert to the original a-phase with only a small portion (5%) of the fl-phase left (Fig. 7). We have also used laser heating of the amorphous samples and recorded the Raman spectra. In Fig. 8, we show the Raman spectra normalized with respect to the laser power for the skeleton vibration region of the VO3 chains for a reclaimed sample (pressurized to 70kbar) whose starting material consisted of about a 4:1 mixture of a- and fl-phases. The fl-form undergoes phase transitions in this pressure range but

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Wavenumber Fig. 8. The re-linking of the VO4 chains of the ~t-form in a sample consisting of a mixture of ~- and fl-phases heated by the laser. (a) Spectrum recorded at 120 mW power showing only fl-phase peaks; (b) the ~-phase is evident at 300 mW, shown by peaks indicated by the arrows; (c) the relative intensity of the or-phase increasing with laser power, P = 420mW; (d) spectrum recorded at 120mW after the sample is exposed to 420 mW. This should be compared with spectrum (a) which shows only the fl-phase. these are reversible [20]. The intensities of the fl-phase peaks are nearly constant, so that they act as a kind of standard. The ratio of the peaks of the ~-phase to those of the fl-phase gives an indication of the percentage of the sample transformed back to the or-phase. Below 120roW laser power, the Raman spectrum consists of only fl-phase peaks. The ~-phase begins to appear at about 200 roW. The strongest peak is still smaller than that of the //-phase at 300 roW, but it is almost twice as big as that of the fl-phase at 420mW. This is a clear demonstration that an amorphous-crystalline phase transition occurs upon heating, corresponding in our case to the re-linking of the infinite VOW- chains. Note that the frequencies of the a-phase bands increase with increasing laser power (hence higher temperature) towards the frequencies of the virgin material. Normally, they decrease with temperature due to the weakening of the bonds caused by thermal expansion as is the case for ~ crystals [16]. We take this unusual behaviour as additional evidence for an increased degree of crystallization at higher temperature. After laser heating they show the expected frequency increase when recorded with a lower laser power. The frequencies of the or-peaks recorded at 120 mW after heating at 420 mW are almost identical to those of the

Pressure-induced crystalline-amorphous transition virgin material. Using the data on frequency shifts against temperature on a-NaVO 3 crystals [16], we estimate that the temperature of the sample is about 200°C at 420 mW laser power. It is interesting to note that both the a- and fl-forms are stable phases under ambient conditions, but the recrystallization of the amorphous phase results in overwhelmingly ~t-phase. In our study of the fl-phase up to 170 kbar [20], at least three phase transitions were inferred, but no amorphous phase was found. The difference between these two stable phases of NaVO 3 therefore persists as high as 170 kbar. This may be explained by the very different structures of the two. The ~-phase is monoclinic with corner-linked VO, tetrahedra in the c direction, while the fl-phase is orthorhombic with double chains formed by VO S in the b direction. In the case of the fl-phase, the double chains never completely break up to 170 kbar, indicating that it is harder to break the double chains in the fl-form than the single chains in the a-form. The reverse may also be true, i.e. it is easier to form single chains resulting in the a-form from the disordered amorphous phase.

CONCLUSIONS The crystalline-amorphous transition of a-NaVO3 at 60 kbar, which is by far the lowest formation pressure for an amorphous phase for ionic crystals, is found to be a two-step process. The VOf chains break up abruptly at the transition pressure but leave the bonding angles unchanged. The gradual arrangement of the bonding angles is complete at about 100 kbar. On pressure release, instead of reverting to the or-phase, the high pressure amorphous phase transforms to a low pressure amorphous phase at 36kbar, which has no Raman active bands. Shear instability is thought to be the cause for pressureinduced amorphization in silicates [12, 13]. The amorphization at 60 kbar is preceded by changes in the chain deformation region [19], so that the same mechanism may apply to a-NaVO3 as well.

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Upon heating the reclaimed amorphous samples, gradual restoration of the a-phase is achieved. The fact that they revert to only one of the two stable phases of NaVO3--the starting material--the ~tphase, may be linked to the fact that the double chains in the fl-phase are harder to form (and to break).

Acknowledgement--This work is supported by the National

University of Singapore by research grant RP910682. REFERENCES

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