# Coesite Formation at Ambient Pressure and Low Temperatures

Authors: J. R. Martínez; A. Vázquez-Durán; G. Martínez-Castañón; G. Ortega-Zarzosa; S. A. Palomares-Sánchez; Facundo Ruiz
Journal: Advances in Materials Science and Engineering
 (2008)

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## Abstract

Partial crystallization of silica xerogel in the form of coesite has been obtained at low-pressure conditions and temperatures of∼565∘C, in samples containing chlorophyll aggregates dispersed in amorphous silica. Silica xerogel samples were prepared by the sol-gel method using an ethanol:H2O:TEOS molar ratio of 4:11.6:1 and loaded with extracts from frozen spinach leaves. The silica xerogel microstructure of the powders was studied as a function of annealing temperature. It was found that partial crystallization of the glass matrix in the form of coestite was obtained at lower pressure than those specified by the phase diagram. Chlorophyll aggregates were added to the starting solutions which, upon thermal treatments, form small colloidal particles in the glass matrix. The presence of coesite is corroborated by the Rietveld refinement method.


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## Body

## 1. Introduction

The crystallization of silica
glasses has been investigated very soon after the discovery of glass. The rate
and mechanism of crystallization vary with the chemical composition of the
glass, temperature, and atmosphere. It has been observed that the presence of
alkali metal, water, and oxygen accelerates the crystallization of glass. It is well known that the incorporation of metallic
species in the silica xerogel matrix promotes the devitrification process at
relatively low temperatures with the presence of partial crystallization in
form of quartz or cristobalite [1–3]. The structural changes in the xerogel matrix are
induced by the incorporation of particles and by the thermal treatment;
however, the appearance of specific
crystalline form of SiO2 is not clear.Silicon dioxide,SiO2,
may occur in several crystalline forms of which quartz is the most stable.
However there are other polymorphs that may occur as minerals and/or as
synthesized compounds. The most common polymorphs of silicon oxide are quartz,
cristobalite, and tridymite. Dense and high-pressure forms of silica are the
stishovite, olivine, and coesite. In natural form, the coesite is present in
methamorphism extreme in the underground of earth or in impact meteor sites.Coesite, a high-pressure polymorph ofSiO2,
was first synthesized by Coes Jr [4], and was discovered subsequently as
a mineral at Meteor Crater, Arizona [5].Since that year, synthesis of coesite has been reported by some
authors; coesite synthesized under high pressure, that is, 6.5 GPa and 1375 K (1100C°) for the crystal was studied by Levien and Prewitt [6]. Coesite crystals were synthesized using a girdle solid-media,
high-pressure apparatus where powdered crystals of quartz and silicic acid, SiO2nH2O, were used as starting materials [7]. It was also synthesized in a single experiment at   700C° and 3.2 GPa using an end-loaded piston-cylinder apparatus, using opal-A as the starting material [8], and in some other high-pressure experiments  [8–14].Coesite has eight unique Si–O bonds and five unique
Si–O–Si groups in the unit cell [15], and it is an
ideal material for studying the properties of the SiO bonds [15, 16]. Coesite is a
framework silicate in which each silicon atom is bonded to four oxygen atoms
forming a silicate tetrahedron, and each oxygen atom is bonded to two silicon
atoms forming a SiOSi disiloxy group. The structure of coesite is composed of
SiO4 tetrahedrons that are linked into four-membered rings. The
rings are then linked together into a chain-like structure, to form the
so-called double crankshaft chain.In this work, we report that the sol-gel-made amorphousSiO2 bulk
samples containing extract of spinach are partially crystallized into coesite,
obtained at much lower pressures than those specified by the phase diagram. For
to achieve this, chlorophyll aggregates need to be added to the starting
solutions which, upon thermal treatments form small colloidal particles in the
glass matrix.


## 2. Experimental Method

The standard method to extract chlorophyll is by crude extraction from
frozen spinach leaves by simple grinding and mixing with a suitable solvent. In order to obtain the
organic compounds extracted from spinach leaves embedded in the xerogel matrix,
a precursor material composed of TEOS, water, ethanol, and pigments extracted
from spinach leaves was prepared [17]. The
ethanol to TEOS and water to TEOS molar ratios were 4:1 and   11:1, respectively.
This composition has proven to give good quality SiO2 coating. These quantities correspond to a high water/TEOS ratio,
needed to enhance the hydrolysis [18] to assure a close amorphous structure for
the as-prepared SiO2 powder, and to allow us to compare the results
with some previous work [18–21]. The
TEOS was dissolved in ethanol using magnetic stirring for 15 minutes. The
suspension of pigments in water was added to the ethanol-TEOS solution to form
the starting material.The X-ray diffraction (XRD)
patterns were obtained using a GBC-Difftech MMA diffractometer using nickel
filtered Cu Kα(λ=1.54Å) radiation. The infrared (IR) spectra
were recorded with an FT-IR
spectrometer Nicolet system model Avatar 360 using the diffuse reflectance (DR) mode, for which 0.05 g of powder
sample was mixed with 0.3 g of KBr. DTA measurements were
carried out in a TA Instruments system, model Q600, at constant rate of
10C°/min inside DTA analyzer in nitrogen atmosphere up to 600C°.X-ray data
refinement was made using the program Maud in order to corroborate the presence
of coesite and calculate the amorphous/crystalline fraction of the composites
[22].The program Maud was developed to analyze diffraction spectra and
obtain crystal structures, quantity, and microstructure of phases along with
the texture and residual stresses. It applied the RITA/RISTA method as
developed by Wenk et al. [23] and Ferrari and Lutterotti [24].The analysis was
started assuming the structure of cubicSiO2 (cubic, spacegroup P213) for the
amorphous phase. For the crystalline phase of silica, the structure of coesite
(monoclinic, spacegroup C 12/c1) was assumed.The Rietveld
analysis of amorphous phases is based on an improved microcrystallinity model that
includes microstrains. Using the Rietveld method for glass modeling supposes
that crystal structure may represent a mean model for a glass or a nanocrystalline
material. The disorder is statistically introduced by microstrain effect,
leading to strong line broadening on the diffraction pattern. This method has
been tested in ceramic ware materials where it is very important to assess the
product quality by checking the amorphous silica content [25].


## 3. Results and Discussion

In order to
investigate the structural evolution of the compound, the as-prepared powder
was heated at a constant rate of 10C°/min inside a DTA analyzer in nitrogen
atmosphere. Due to the fact that we only want to focus on the formation of the coesite phase, we show the DTA data in the range
of 500C° to 600C°; in such range, the analyzed sample gives endothermic peaks at
508C° and 549C°, shown in Figure 1, and exothermic peaks at 535C° and 565C°.
After 400C° (not shown in the figure), a very broad endothermic process occurs
due to decomposition of chlorophyll species, whereas it initiated the formation of phases of
oxide and hydroxide magnesium transformation of gel to glass and the formation
of crystalline phases of SiO2, as coesite.Figure 1
DTA curve of chlorophyll
species embedded in a silica matrix heat-treated in nitrogen in the temperature
range from 500C° to 600C°.According to
the DTA data and X-ray diffraction pattern, not shown in the figures, at
temperature above 600C° the coesite phase disappears and there begin to appear the cristobalite phase and the tridymite at temperatures above 800C°
[26].Figure2 shows
the X-ray diffractograms for the prepared samples. Embedded in amorphous silica
xerogel, the chlorophyll species decomposed and interacted with the silica
matrix promoting the formation of hydroxide magnesium species, quenching centers, or
nonfluorescing aggregates due to denaturation of photosystem promoted by
chlorophyll decomposition, and partial
crystallization of the matrix, as discussed below.Figure 2
X-ray diffraction patterns of the chlorophyll-containing silica xerogel
samples heat-treated at the indicated temperatures.Using both the
results of DTA and X-ray diffraction, it can be seen that at temperatures lower
than 600C°, the composite powders are in amorphous form. The chlorophyll
species decompose at temperatures of ∼222C°, its decomposition conduces to the
formation of magnesium hydroxide species and organic compounds that produce
quenching centers and partial crystallization of SiO2 in the form of coesite,
corresponding to the exothermic peaks at 565C°. The exothermic peak at 535C°
corresponds to the formation of tetramagnesium trioxide dihydroxide. The
diffractogram clearly shows the presence of these phases at 600C°.In Figure2, for the samples heat-treated
below 600C°, only the broad band corresponding
to the SiO2 gel is present, which is extended to higher 2θ-degrees
due to the presence of chlorophyll species. In the sample heat-treated at 600C°, besides the band assigned to silica, we observe
narrow peaks, corresponding to coesite and tetramagnesium trioxide dihydroxide
the last one centered at ∼40.5°.The infrared absorption spectra in the range from
400cm−1 to 4000cm−1 are
shown in Figure 3.
The bands at 457cm−1, 800cm−1, and 1078cm−1 are associated to absorption bands related to
particular vibrational modes of the oxygen (O) atom with respect to the silicon
(Si) atoms, which
they bridge [27, 28]. The principal band, located at about 1078cm−1,
has a strong absorption shoulder at its high-energy side that is more pronounced to conform to the temperature increased.Figure 3
Infrared absorption spectra of the
chlorophyll-containing silica xerogel samples.Several reports have been
published concerning the high-energy shoulder of the stretching band, in the
frequency range of 1150 to 1300cm−1. For instance, in previous
work, IR spectra have been reported in which these subbands can have an
amplitude comparable or larger than the main stretching band at 1078cm−1 [18, 29]. The enhanced intensity of these subbands has been interpretated as
due to the formation of a chain- or ring-like structure. This band suffers
significant modification with increasing annealing temperature. An explanation
of this behavior will be proposed in the next parts of this section.A band centered
at 950cm−1, assigned to silanols (Si–OH), is present at room
temperature; for temperatures above 400C°, the band is incorporated to the main
stretching band conform the temperature is increased. We can also observe a band at 1600–1690cm−1, assigned to the deformation of molecular water and to
alkenyl C=C stretch. This band is found in the spectra of the
as-prepared sample, upon this temperature, the water is eliminated and only the
alkenyl and C=C–C stretching and bending vibrations are present. It indicates that organic species, typical of green
leaves, still exist in the structural conformation.For samples heat-treated
below 400C°, the IR spectrum shows a strong absorption band at the range 1270–1560cm−1.
This band is associated with a combination of vibrations of the nitrate and CH
species. The principal contributions are due to the CH group; from the figure
we can observe that according to the increment of temperature, the band
undergoes noticeable changes, indicating the evolution of chemical species. It
is possible to see that, for samples heat-treated at 600C°, the band is
partially incorporated to the main stretching band. In these temperatures, the
composite shows an important phase transformation. Their presence at high temperature
indicated, among the presence of organic compound in the composite, a
preference linear structure of the matrix [30].The band in the range of 2250
to 3000cm−1 corresponds to the fundamental stretching vibrations of
different hydroxyl groups [31]. The band is composed of a superposition of
SiO–H stretching vibrations. The weak band observed at ∼2800cm−1 corresponds to C–H stretching vibrations. This band is related to the band at
the 1250 to 1550cm−1. It is noticeable, according to the previous
discussion, that for temperatures higher than 600C°, these contributions are
present.The presence of these IR
bands suggests that the structure includes polymerized species predominantly
with oligomers. The formation of this kind of polysiloxanes is related to the
presence of the IR bands at the range of 1250 to 1550cm−1 and 2800–3000cm−1;
these bands tend to diminish by heat treatment
indicating the evolution of the OR groups due to the chlorophyll decomposition.
At the same time, the Si–O–Si chains in formation are terminated by OH species,
as indicated by the presence of free surface silanols band (3724cm−1)
Si–OH, as an evidence of the linear character. The permanence of these groups
at elevated temperatures promotes, surprisingly, the partial devitrification
with the formation of coesite phase in ambient pressure conditions; this has
not been reported previously.The IR absorption spectra of
the sample heat-treated at 600C° show slight features associated with the coesite phase at the range from
440cm−1 to 700cm−1, in agreement with the X-ray
results. Thus, the quantity of coesite present in the sample is very low.Four-membered ring
species are obtained in the early stages of the sol-gel process. In previous
work, we have
studied by means of quantum
chemical calculations the vibrational frequency of oligosilsesquioxanes, which are composed of 4-fold
siloxane rings and cyclic trisiloxanes. In particular, we have obtained that a
four-membered ring family is present in silica xerogels in the early stages of
gelation process which remains during the conformation of the final structure.
Three four-membered configurations are possible and the more stable configuration
is the slightly deformed configuration [32]. That is close related with the double crankshaft chain
formed by linking vierer rings that
form the coesite structure.As can be discussed, this four-membered slightly deformed species can be
included with oligomers, that promotes partially the formation of double
crankshaft chain characteristic of the coesite phase. At temperatures lower
than 600C°, the amorphous structure of the SiO2 matrix is closely related with
the quartz-like
amorphous. For the same formation parameters used in this work, we reported in
previous work that for the silica xerogel matrix without incorporation of
particles, we obtain close structures in which a transformation of quartz-like
amorphous to low-cristobalite amorphous phase occurs at about 600C° [33].It is known that the coesite phase can be obtained from quartz phase
under high pressures. According to the present results, the sample promotes the
formation of amorphous quartz-like for the matrix, obtaining oligomers species
with four-membered sligthly deformed rings; under heat treatment the amorphous
structure of the matrix evolutes to low-cristobalite amorphous phase but
previously, the matrix promotes the partial devitrification in the form of coesite due
to the presence of the organic compound in which the four-membered rings are then linked
together into a chain-like structure, resulting in the so-called double
crankshaft chain characteristic of coesite.In order to corroborate the presence of coesite, X-ray data refinement
was made using the program Maud. It has been
shown that the method works also well to calculate the amorphous/crystalline
fraction in composites with two crystalline phases, as in the case of the
sample heat-treated at 600C° [21]. In any case, very good
agreement between experimental and calculated amorphous/crystalline fraction of
composites was obtained [21, 22, 34, 35]. Figure 4 shows
the X-ray diffraction pattern of the sample heat-treated at 600C° along with the fitted
spectrum, using the Maud program. The computed fraction of coesite phase is
1.25% wt, and the cell parameters are a=7.137079Å,b=12.409115Å, and β=120.50162°; these values are very close to
those reported by Kroll and Milko [36].Figure 4
Experimental X-ray diffraction pattern of the sample heat-treated at 600C°
together with the fitted spectrum. The difference is showed in the bottom of
the figure.In summary, for the
sample heat-treated
at 600C° we observed, among the magnesium oxide species, the presence of low-cristobalite
amorphous phase and coesite phase.
The presence of these two phases at this temperature is confirmed by the Rietveld refinement for
amorphous SiO2. Using the above refinement process, the Bragg
pattern and the peak position of the diffractograms correspond to the phases
identified according to the spatial group C 12/c1 for the coesite and P213 for the
low-cristobalite.The spatial group
used for low-cristobalite is representative of the amorphous phase of the
sample at this heat treated temperature. This fact is justified by the results
shown in the X-ray diffractogram of Figure2, for the amorphous band associated
to the SiO2. The main feature of these patterns is a broadband
located at the left part of the diffractogram. According to the treatment
temperature, this band suffers a sligth shift to lower 2θ-degrees. For the
sample heat-treated
at 400C°, the position of the center is around 23.8°, and for the sample heat-treated at 600C° the
position of the center is at 21.5°. It is important to note that cristobalite
has its main diffraction peak at this position (21°).In conclusion, we
can assure the formation of the coesite phase, in this kind of samples prepared
at ambient pressure and low temperatures. The existence of the phase is
determined by using DTA results combined with X-ray diffractometry and Rietveld
refinement method. This is a proven method to analyze and quantify the phases,
both crystalline and amorphous, in a compound.As it can be seen
in Figure2, there are some peaks produced by a crystalline structure that
according to the refinement of its structure corresponds to the coesite. It is
possible to quantify the phases, in this case, due to the large intensity of
the peaks produced by any silicon oxide, even in so small quantities.The diffraction
pattern is composed by both amorphous and crystalline phases; due to the small
quantity of coesite phase, some of the minor intensity peaks are screened by
the amorphous phase. The refinement process is capable of separating the
contribution of both phases. The results show very good fit between the refined and experimental patterns as it is shown in the bottom part of  Figure4. The principal peak does not match any other
peaks of silicon oxide and it is in agreement with the structural parameters
reported by Kroll and Milko [36].The IR results
show the formation of four-membered slightly deformed
species included with oligomers, that promotes partially the formation of
double crankshaft chain characteristic of the coesite phase.


## 4.conclusions

This work shows that sol-gel-made amorphousSiO2 bulk samples can be crystallized into the coesite phase at
temperatures of 565C° and at ambient pressure conditions. This is much lower
than those specified by the phase diagram where the coesite is obtained at high
pressures. The coesite phase had not been reported in this kind of xerogel
matrix and mainly in low-pressure conditions. In order to achieve this phase,
chlorophyll aggregates need to be added to the starting solutions. Embedded in the amorphous silica xerogel, the chlorophyll
species decomposes and interacts with the silica matrix promoting the formation
of hydroxide magnesium species, quenching centers or nonfluorescing
aggregates due to denaturation of the photosystem promoted by chlorophyll
decomposition, and partial crystallization of the
matrix. At 600C°, both the crystalline coesite phase and the amorphous phase
coexist which was confirmed by the Rietveld refinement method.



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