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Journal of Non-Crystalline Solids 349 (2004) 30–34

A high pressure Brillouin scattering studyof vitreous boron oxide up to 57GPa

Jason Nicholas a,*, Stanislav Sinogeikin b, John Kieffer c, Jay Bass a,b

a Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, 1304 W. Green St., Urbana, IL 61801, USAb Department of Geology, University of Illinois at Urbana-Champaign, 1301W. Green St., Urbana, IL 61801, USA

c Department of Materials Science and Engineering, University of Michigan, 2300 Hayward St., Ann Arbor, MI 48109, USA

Available online 28 October 2004

Abstract

Brillouin spectroscopy has been performed on vitreous boron oxide (B2O3) from ambient pressure to 57GPa at room tempera-

ture. Upon initial compression to 53GPa, the longitudinal and shear sound velocities increase gradually from 3.3km/s to 13.1km/s

and from 1.8km/s to 7.1km/s respectively. Upon decompression, the shear and longitudinal sound velocities follow a different path

than during compression. This path remains smooth until a discontinuity of approximately 2.7km/s in the longitudinal velocity and

2.0km/s in the shear velocity occurs near 2.8GPa. This discontinuity returns the sound velocities to those seen during compression,

and suggests a polyamorphic reorganization of the glass structure. The path dependent nature of the properties and the disconti-

nuity at 2.8GPa can also be seen in the Poisson�s ratio and the index of refraction. A second compression–decompression cycle

to 57GPa produces the same behavior as the first cycle, confirming that the 2.8GPa discontinuity does in fact return the glass

to its original structure. The existence of a sharp transition in glass properties, such as was observed here, provides strong support

for the existence of vitreous polymorphs.

� 2004 Elsevier B.V. All rights reserved.

PACS: 64.70.Pf; 62.50.+p; 78.35.+c

1. Introduction

According the traditional, random-network theory

put forth by Zachariasen in 1932 [1] a glass should not

exhibit a sharp discontinuity in properties when sub-

jected to changes in temperature or pressure, because

the local environment is slightly different in each part

of the glass. However, for glasses which contain fourcoordinated cations, such as silica [2,3], germania [4],

ice [5], and yitria meta phosphate (YMP) [6], it has been

shown that a glass can quite suddenly transform from

0022-3093/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2004.08.258

* Corresponding author. Current address: Materials Science and

Engineering Department, University of California, 210 Hearst Memo-

rial Mining Building, Berkeley, CA 94703, USA. Tel.: +1 510 486 6898;

fax: +1 510 486 4881.

E-mail address: jdnicholas@lbl.gov (J. Nicholas).

one structure to another under the influence of pressure.

The observed discontinuities in macroscopic observables

such as density, refractive index, sound velocity, etc.,

suggest that a given structure is stable only over a lim-

ited range of pressures – a behavior reminiscent of crys-

talline systems. This has led to the development of the

idea of �vitreous polymorphs� in tetrahedrally coordi-

nated glasses.The effect of pressure on glasses with trigonally coor-

dinated cations is less well characterized, with vitreous

boron oxide being the only material studied to moderate

pressure [4,7,8]. These B2O3 studies showed that trigo-

nally coordinated glasses exhibit different structures dur-

ing compression and decompression but return to

something close to the melt-quenched glass structure

upon decompression to room pressure. In the study pre-sented in this paper, we subjected vitreous boron oxide

J. Nicholas et al. / Journal of Non-Crystalline Solids 349 (2004) 30–34 31

to the highest pressures ever achieved for this material,

57GPa. In addition to the behavior seen by previous

studies, we observed a sharp transition in the glass prop-

erties between 2 and 3GPa upon decompression. We

suggest that this transition indicates the presence of vit-

reous polymorphs (also known as polyamorphs) in trig-onally coordinated B2O3 glass.

2. Experimental procedure

The melt-quenched B2O3 sample for this experiment

was prepared by heating reagent grade boron oxide

(Alfa Aesar) in a muffle furnace at 1000C overnight inair. To shape the glass into pieces convenient for incor-

poration into a diamond anvil cell, two polished plati-

num plates, spaced 30lm apart, were dipped into the

B2O3 melt. After allowing 5–10min for capillary forces

to draw the B2O3 between the platinum plates, the plates

were quickly removed from the oven, placed in a vac-

uum dessicator, and moved into a nitrogen-purged glove

box.After prying the platinum plates apart inside the

glovebox, the B2O3 was loaded into the sample chamber

of a piston cylinder diamond anvil cell. The gasket for

this diamond cell run was made from rhenium and the

sample chamber was 185lm wide. After adding several

20lm rubies to the sample chamber for pressure meas-

urement, the diamond cell was sealed, pressure was ap-

plied, and the diamond cell was removed from theglovebox. The purpose of the lengthy heating schedule

and glovebox loading was to prevent the glass from

absorbing water and potentially turning into boric acid

(H3BO3) and/or crystallizing while under pressure [9].

Determination of pressure dependent properties was

done at room temperature, which is significantly below

the glass transition temperature of about 290 �C. During

the course of the experiment the pressure on the glasswas determined by the fluorescence shift of the R1 ruby

line using a liquid nitrogen cooled CCD and the

514.5nm line of an Ar laser. The pressure dependence

of the ruby fluorescence has been calibrated up to pres-

sures of 100GPa [10]. For the measurements presented

here, no pressure transmitting medium was used. The

B2O3 specimen was compacted to fill up the orifice of

the metal gasket between the diamond anvil, to act asits own pressure-transmitting medium. Although hydro-

staticity is not ideal in this setup, comparison with meas-

urements done using Fuorinert� and liquid Ar as

pressure-transmitting media showed very good agree-

ment. Ultimately, even the use of such pressure-trans-

mitting fluids provides hydrostatic conditions only

over a limited pressure range, i.e., up to 4GPa for Fluor-

inert� and up to 9GPa for Ar, which is much smallerthan the maximum pressures we targeted in this study.

The 514.5nm line of the Ar laser was also used to excite

Brillouin scattering within the glass. For the Brillouin

measurements a 50� scattering geometry, checked with

an MgO standard prior to the experiment, was em-

ployed. The Brillouin signal was analyzed with a

Sandercock [11] six-pass tandem Fabry–Perot interfer-

ometer. A full description of the Brillouin setup is givenin Sinogeikein et al [12].

For several pressures where both the longitudinal and

shear sound velocities were not obscured by the dia-

mond anvil shear peak, i.e. the velocities were less than

11,000m/s, Brillouin scattering was collected in both a

50� forward scattering geometry and a 180� back scat-

tering geometry. Comparison of the forward scattering

geometry, which directly measures the sound velocity,and the back scattering geometry, which measures the

product of the sound velocity and the index of refrac-

tion, yields the index of refraction. For compressional

B2O3 velocities greater than 11,000m/s, which occurred

at pressures greater than 23GPa during compression

and 11GPa during decompression, an index of refrac-

tion of 1.76 was assumed and the longitudinal velocities

were calculated from the backscattering geometry alone.This assumption seemed valid since the refractive index

leveled off at higher pressure.

During the first pressure cycle, the sample was al-

lowed to equilibrate for at least 12h, and during the sec-

ond pressure cycle the sample was allowed to equilibrate

for at least 4h before Brillouin spectra were collected.

3. Results

The refractive index is shown as a function of pres-

sure in Fig 1. This plot shows that below 20GPa the

glass exhibits different properties during compression

and decompression. Further, this plot suggests that the

compression and decompression data are probably

merged above 23GPa. The exact pressure dependenceof the refractive index is however unknown. Lastly

and most importantly is the sudden drop in refractive in-

dex that is observed between 2 and 4GPa upon

decompression.

The sound velocity is plotted versus pressure in Fig. 2

and shows behavior similar to that of the refractive

index. Upon initial compression to 53GPa, the longitu-

dinal sound velocity gradually changes from 3.3km/s to13.0km/s and the shear sound velocity gradually

changes from 1.8km/s to 7.1km/s. Above 23GPa,

changes in the sound velocities are minimal. (Note that

the shear velocity was directly observed, while the longi-

tudinal velocity was calculated from the back-scattered

Brillouin spectrum assuming a refractive index of

1.76.) Upon decompression, the velocities follow a dif-

ferent velocity–pressure path than during compression.At pressures greater than 2.8GPa the sound velocities

changed gradually. However, between 2.8 and 2.1GPa

Fig. 1. Refractive Index versus Pressure. Solid symbols indicate data

taken during the first cycle, while open symbols indicate data taken

during the second cycle. Black symbols represent data taken on

pressure increase, and gray symbols represent data taken on pressure

decrease. Arrows indicate direction in which the measurements were

taken. (Vertical error bars are too small to resolve.)

Fig. 3. Poisson�s ratio versus Pressure. Solid symbols indicate data

taken during the first cycle, while open symbols indicate data taken

during the second cycle. Black symbols represent data taken on

pressure increase, and gray symbols represent data taken on pressure

decrease. Arrows indicate direction in which the measurements were

taken.

32 J. Nicholas et al. / Journal of Non-Crystalline Solids 349 (2004) 30–34

the sound velocities exhibit a pronounced discontinuity

and return to the values seen during compression. In

the course of this transition, the longitudinal velocity

drops from 7.8km/s to 5.1km/s and the shear velocity

drops from 4.6km/s to 2.6km/s. A second compres-

sion–decompression cycle reproduces the behavior ob-

Fig. 2. Shear and Longitudinal Velocities of B2O3 versus Pressure. Squares re

Solid symbols indicate data taken during the first cycle, while open symbols in

taken on pressure increase, and gray symbols represent data taken on pressu

taken. Ambient pressure data was obtained outside the DAC from a differe

served during the first cycle within the experimental

error.

Fig. 3 shows the behavior of the Poisson ratio as a

function of pressure. The Poisson ratio was calculated

from the sound velocities using the equation

r ¼ V 2p�2V 2

S

2ðV 2p�V 2

SÞ. Like the refractive index and sound veloc-

present shear velocities and diamonds represent longitudinal velocities.

dicate data taken during the second cycle. Black symbols represent data

re decrease. Arrows indicate direction in which the measurements were

nt B2O3 sample. (Vertical error bars are too small to resolve.)

J. Nicholas et al. / Journal of Non-Crystalline Solids 349 (2004) 30–34 33

ity, the Poisson ratio exhibits a gradual change during

compression. Unlike the velocities of sound, the Poisson

ratio values during compression and decompression dif-

fer only below approximately 20GPa. However, near

3GPa upon decompression, an abrupt change is again

observed.

4. Discussion

The smooth, gradual changes in the sound velocity

during compression suggest a gradual rearrangement

of the structure with pressure. Given that it has been

shown that rings of three coordinated boron break-upwith pressure [7], and that crystalline boron oxide has

trigonal borons at pressures below �3GPa and tetrago-

nal borons at pressures greater than �3GPa [13] it

seems likely that the structural change seen upon com-

pression is the result of an ever-increasing number of

four coordinated boron atoms.

The most striking feature seen upon decompression is

the abrupt transition near 3GPa. At first glance, onemight think that this transition is the result of crystalli-

zation. However, it would be quite a coincidence if the

sound velocities of the crystal were the same as those

of the glass during compression. Furthermore, the fact

that the sample can be repeatedly cycled around the

same sound velocity–pressure loop indicates that the

structure obtained after the transition is that of the orig-

inal glass. We therefore conclude that during the transi-tion seen upon decompression boron converts from

four- to three-fold coordination. This interpretation is

not only based on the observation that the sound veloc-

ities return to those of initial, three coordinated boron

structure, but also because the transition occurs at the

same pressure as the phase boundary between three

and four coordinated boron in the crystalline system.

Hence, our explanation for the behavior of B2O3 un-der pressure is that a reversible polyamorphic transition

takes place, which involves the changeover between

three-and four-coordinated boron [14]. What is remark-

able is that this structural transition is continuous upon

compression and abrupt upon decompression, and as a

result exhibits significant hysteresis. We envisage this

process to occur as follows. Because of the disorder in

an amorphous structure, every structural unit does notexperience the increasing pressure equally. Transitions

from three- to four-coordinated boron therefore occur

sporadically, they remain localized, and it takes a signif-

icant pressure change before they engulf the entire spec-

imen. During the gradual release of pressure, B2O3 at

first remains stuck in its high-density state and then sud-

denly reverts to its low-density structure. The delay in

the reverse transition can be understood based on ther-modynamic and kinetic reasons. Thermodynamically,

four-coordinated boron represents a stable unit at high

pressure, as indicated by the crystalline phase diagram.

Kinetically, atomic mobility is reduced in the high-den-

sity configurations, and thus prevents the necessary

structural rearrangements.

In order to understand the decompression behavior,

it is informative to observe the trends displayed by Pois-son ratio, as shown in Fig. 3. Initially, the Poisson ratio

follows the same path as upon compression, indicating

similar aspect ratio in the volume changes. Below

20GPa, however, the Poisson ratio on decompression

rapidly drops below that on compression. In this regime,

the cross contraction perpendicular to the axis of defor-

mation is much reduced in comparison to the elongation

in that direction. This is a sign of instability, such aswould result from a strong degree of disconnect within

the structure characterized by highly directional bond-

ing. It is furthermore associated with a rapid volume in-

crease. Once the structure has gained the necessary

activation volume for structural rearrangements to oc-

cur, the reverse transformation takes place.

5. Conclusions

In addition to presenting the refractive index, sound

velocity, and Poisson�s ratio to pressures in excess of

three times the previous limit, this paper shows the clear-

est evidence to date of vitreous polymorphism in a trig-

onally coordinated glass. The case of a transition

between a vitreous B2O3 polymorph that is comprisedof mainly trigonally coordinated boron, and a vitreous

B2O3 polymorph that is comprised of mainly tetrahed-

rally coordinated boron is quite compelling and may

serve as a model for the behavior of all three coordi-

nated glasses.

Acknowledgments

This research was supported by the NSF grant #

DMR-0230662. We also would like to thank J. Palko

for fruitful discussions and A. Hofmeister for IR analy-

sis of our sample.

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