Amelia Fitzsimmons
26 March 2009
2009 Student Colloquium
Selenium and Tellurium Binary Glasses
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Abstract:
Glass has had many uses throughout history; most notably, it was used in the cathedrals
to tell the story of Christianity to an illiterate society. Binary glasses formed from
selenium and tellurium, two heavy and non-radioactive chalcogens, are of interest to
modern investigators because of their useful properties. Both elements can form
homoatomic bonds, which allows the formation of ring structures, composed solely of
the one element. This property is integral to the structure of the glasses that are
formed. Glass is an alternative solid state to the commonly known crystalline state.
While the atoms in a crystal are highly ordered in a repeating pattern, the atoms in a
glass have a less rigid structure. Selenium and tellurium binary glasses have many useful
properties. Because they transmit light in the 3-5 micrometer range, they have been
investigated for use in infrared missile guiding systems, night vision equipment, and
thermal imaging. Using the chalcogens-based glasses in these applications requires that
the vitreous form be stable; a major obstacle to overcome when synthesizing these
glasses in the spontaneous glass-to-crystal transformation. Because the crystalline form
is so highly ordered, the compound will spontaneously shift from the glassy form to the
crystalline form. Finally, glass can be connected to faith, as it has historically been
important in western religious education.
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Chalcogens are the column of elements with one less valence electron than the
halogens: oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po)
compose this group. The most well studied chalcogen is oxygen. Chalcogens have six
valence electrons, which means that in ionic interactions, they will most often gain two
electrons to have a -2 charge. The bonding interactions of some chalcogens are well
studied, such as those of oxygen and sulfur, which often form covalent interactions with
hydrocarbons. Marie Curie extensively studied polonium, but selenium and tellurium
are the least studied of all the chalcogens. Because they are in the same group, all of the
chalcogens have a similar structure with equal numbers of valence electrons. The
heavier elements have more neutrons and protons in their nuclei, but their valence
interactions are essentially the same as those of the lighter chalcogens.
Since antiquity, it has been known that silicon dioxide compounds can be heated
and then cooled to form a glass structure different from the crystal structure of silicon
dioxide normally found in nature. It was originally believed that these glasses were a
type of solid material, but the windowpanes in very old buildings, like cathedrals, are
thicker at their bases than they are at the tops. While it was originally thought that the
glass was slowly flowing down the windowpanes, it was later discovered that the
method of glassblowing used in the middle ages resulted in one end of the glass sheet
being naturally thicker than the other. The common practice was to put the thicker end
at the base of the windowpane, because putting it the other way around would put
undue stress on the thinner part of the glass (Physics of Glass). As an intermediate
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between the crystalline solid state and the liquid state, glasses have many uses in
technology and research. Containers made of glass, while fragile, can often withstand
high temperatures up to 4500 F, or 8420 C (Pegasus Borosilicate). Fiber optics, wires
made of glass, can be used to transmit light the same way that insulated copper wires
transmit electricity.
A recent topic of investigation in the field of glasses is compounds other than
oxygen and the properties of the glasses that they can form. The natural choice for
compounds to investigate was chalcogen-containing compounds, because while they
are considered to be a separate class of elements from oxygen, they are in the same
group in the periodic table. Many of these studies take place at the Laboratory of
Glasses and Ceramics at the University of Rennes, France. Selenium (Se)-and Tellurium
(Te)-based glasses are the main focus of the investigations because they are the
heaviest workable chalcogens. Polonium is unsuitable for any sort of materials science
because it is radioactive.
Although chalcogens are in the same periodic group as oxygen, there are some
fundamental differences in the way that the lower chalcogens bond and the way that
oxygen bonds. Oxygen can form compounds of the structure A-O-A, with the bond
angles between the oxygen and the other element being almost anything from 80 to
180 degrees, depending on the size of the other element and the type on bond. The
heavier chalcogens, however, have a much narrower range of bond angles available to
them because of their larger, more diffuse s and p orbitals. Another contributing factor
is the increased s-p hybridization present in the heavier chalcogens. The energy spacings
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between the s and p orbitals in the chalcogens are drastically smaller than the energy
spacings between the s and p orbitals in oxygen. A common period trend within groups
is that as the size of the element increases, the energy spacings between the s and p
orbitals decrease. The increased level of hybridization in Se and Te limits the range of
bond angles available to Se-and Te-containing compounds (Chalcogenides: Solid State
Chemistry).
Another property of the heavier chalcogens not shared by oxygen is the ability to
form homoatomic bonds (Chalcogenides: Solid State Chemistry). This allows Se and Te
to form rings or chains composed solely of the one element. The ability of the heavier
chalcogens to form homoatomic bonds is foundational to the study of Se-and Te-based
glasses.
The glass (or vitreous) forms of any are fundamentally different from their
crystal forms. The crystal forms have a rigid geometrical structure that is repeated
multiple times. Every atom is in the same position each time the pattern is repeated,
resulting in a highly ordered compound. Glasses form when the compound is heated
and then quickly cooled. The result is an amorphous solid with no defined structure. The
repeating pattern present in the crystal structure is absent, and instead, the
arrangement of atoms within the glass is fairly random. As expected because they are in
the same group, Se and Te form crystal structures with similar geometries. Se and Te
form hexagonal chain spirals, an arrangement that relies heavily on the delocalization of
electrons in the Te compounds. [Figure 1]
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Figure 1-side and bird's eye views of the hexagonal chain structures (Bureau, B. et al)
Because Te has larger, more diffuse orbitals than Se, it would be expected that
its bond lengths would be slightly longer than those found in the Se compounds.
Usually, when one element has larger orbitals than another element, the element with
the larger orbitals will have longer bond lengths. However, the Te bond lengths were
found to be shorter than the Se bond lengths, which shows that the bonds between the
Te chains are resonant, with delocalized electrons accounting for the bond lengths being
shorter than expected (Chalcogenides: Solid State Chemistry). Resonance in bonds
simply means that the bond between the atoms does not exist as a pure single bond or
a pure double bond, but is an intermediate form between the two. Electrons are shared
around all the atoms linked in the bond, and circulate freely around the atoms in the
bonds (Bruice, P). The absence of this delocalization in the Se compounds results in a
much lower melting temperature for the Se compounds than for the Te compounds
(Bureau, Danto, et al). The Se compounds melt around 2170 Celsius and form a viscous
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liquid, which indicates that the atoms are still polymerized into chains. However, when
the Te compounds melt, they require temperatures around 4500 Celsius and acquire the
fluidity of a liquid metal. This shows that the bonds in the Te compounds are
homogenous, while the bonds in the Se compounds forming the chains are stronger
than the bonds holding the chains together in the hexagonal conformation. The electron
delocalization that is present in the Te compounds makes all of the Te-Te bonds
resonant and identical. The Se compound melts at a lower temperature and also
requires a lower rate of cooling in order to cause the liquid compound to enter its
vitreous state. The Te compounds require an extremely high rate of cooling and have a
low ratio of product to reactant formation. In addition, the Te glass quickly converts to
its crystalline form around 1000 F (Bureau, Danto, et al). Because the Te glasses shift to
the crystalline form more easily than the Se glasses do, the Te glasses are less stable
than the Se glasses, making them less useful for materials science and industrial
applications. For practical uses, the Se glasses are more commonly used because the
vitreous form is stable. A benefit of the instability of the Te glasses, however, the Te
compounds make it possible to observe the glass to crystal transition in chalcogens. In
addition, it is advantageous to investigate methods of stabilizing the Te-based glasses
because they are able to transmit light in the longer wavelengths of the infrared part of
the spectrum (Hocdé, S et al). Compounds which transmit in the 3-5 micro-meter region
of the spectrum have been investigated for uses in thermal imaging systems, forwardlooking infrared, and night vision systems (Carts, Y).
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Because Te-based binary glasses, or glasses composed of two different elements,
have the ability to transmit IR light (Hocdé, S. et al), these types of glasses are of
interest to researchers working with Fourier Transform Infrared Spectroscopy (FTIR)
instruments and have been suggested for use in space exploration applications (Bureau,
D., Danto, et al). One type of glass that is currently being investigated is formed from a
tetrahedral arrangement of four Te atoms around a germanium atom. The tellurium
atoms form bridges, with two telluriums linking two tetrahedrons together. [Figure 2]
Figure 2-double tetrahedral Te/Ge arrangement (Nichols, A)
A difficulty in synthesizing glasses of this kind is that they have to be cooled
extremely quickly, which makes it impractical to make glass pieces larger than chocolate
chips (Bureau, Danto, et al), which have no real use. Because of this, the authors of this
study decided to make Ge/Ga/Te and Ge/Te/I glasses, because halogens have been
previously found to increase the stability of the glass complexes Making glasses out of
Ge/Te composites is a standard practice, because the Ge adds stability to the glass form
and makes it less likely to shift to the crystalline form (Bureau, B. et al). When these
glasses were compared with the Ge/Te glasses, the ternary glasses (glasses composed of
three different elements) were found to have a higher degree of interconnection of
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atoms in the compound, which means that the glass phase is more stable than the
crystalline phase for that compound. This is important because the main difficulty in
synthesizing chalcogens-based glasses is trying to find a combination of elements that
will stay in the vitreous phase and not transition to the crystal phase, which is more
ordered and more stable. In addition, the Ge/Te/I glass combination was found to be
able to transmit IR light over 2-28 micrometers in the infrared (IR) range, which is
considered very good. The best ternary glass studied so far was the Te/As/Se glass,
because it can transmit IR light from 1-18 micrometers and does not crystallize until it
reaches 1370 Celsius (Bureau, B., Danto, et al). This glass is being investigated for use in
fiber optics because of the stability of its vitreous phase and its ability to transmit IR
light.
The Te/As/Se ternary vitreous system has been further investigated for use in IR
optical fibers. This glass was chosen because it does not convert easily to its crystalline
form and because it can transmit a relatively large range of IR light (Hocdé, S. et al). The
larger the range of light wavelengths that a glass can transmit, the more versatile it is
for fiber optics applications. Crystalline forms are not as useful for IR applications,
because they do not transmit light the same way that the vitreous forms do (DET Physics
Archive). The structure of these glasses is a snowflake [Figure 3], with the light dots
representing As atoms and the dark dots representing either Te or Se atoms.
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Figure 3-the snowflake arrangement of the As/Te/Se glasses used in fiber optics (Hocdé, C et al)
Because of the structure of the constituent glass complexes, the optical fibers made
from this material can be tapered at the ends, which makes many applications possible
for these optical fibers (Hocdé, S. et al). One application of note is for the use of these
fibers as sensors in laparoscopic procedures. Because these fibers carry light in the IR
range, they can provide additional information during laparoscopic surgery while
maintaining as minimally invasive procedure as possible. Of specific interest to the
authors in the Hocde, et al study is the possible use of these fibers in improving
laparoscopic biopsies of tumors used to determine malignancy.
Nuclear Magnetic Resonance (NMR) can be used to determine the sequence of Se
and Te atoms in a binary Se-Te glass. Isotope is a term used to describe the condition
where two atoms of the same element can have different numbers of neutrons from
each other. They will both still be the same element, but a number is placed after the
chemical symbol to indicate the difference (Brown, et al). Se-77 and Te-125 NMR are
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used in these studies, but only the spectra from Se-77 NMR are conclusive. The Te-125
spectra contain too much noise (Bureau, B. et al) to be of any practical use in
determining details about the glass. This did not greatly hamper the characterization of
the glasses because the data from the Se-77 spectra was sufficient to determine the
arrangement of the Se and Te atoms in the vitreous matrix. The goal of the Bureau, et al
study of 2005 was to sequence the Se and Te atoms in a vitreous Se/Te compound and
to determine if there was a pattern to the arrangement of the atoms in the glass.
Because chalcogens-based glasses can quickly interconvert between the vitreous phase,
which is a less ordered phase, and the crystalline phase, which has a very high degree of
order, researchers are interested in quantifying the level of order present in the vitreous
phase. Many vitreous materials are useful in IR, but convert quickly to the crystalline
phase and remain in the crystalline phase, rendering the material useless. The
characterization research was aimed at better understanding the level of order in the
vitreous phase in hopes of better understanding the nature of the glass-crystal
transition so that the transition can be better controlled. From this study, the authors
concluded that the arrangement of atoms in the Te/Se binary glass is primarily random,
with the Te-Se bond being slightly more favorable than the Te-Te or the Se-Se bond
(Bureau, B. et al). This could be due to the slight difference in polarities between Se and
Te, making them more likely to bond with each other than to bond with another atom
of the same element. This study also illustrated the usefulness of Se-77 NMR in
determining the arrangement of atoms in a Se-based vitreous material, which could be
of use in future research. The department of defense funds research on making heat
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and water resistant “windows” of IR transmitting material. Chalcogenide glasses have
been investigated for this use, as have synthetic sapphire and diamond crystals (Carts,
Y).
Chalcogens-based vitreous materials, specifically the ternary Ge/Se/Te and the
As/Te/Se glasses, are an important topic of current research. From the purely
theoretical perspective, it is intriguing to take an ancient concept, such as glass, and try
to recreate it in a completely new way. The nemesis of the scientist interested in
synthesizing new glasses is the ubiquitous glass to crystal transition, in which the
vitreous material quickly converts from its vitreous form to its more ordered crystalline
form. However, chalcogens-based compounds that are highly stable in the vitreous form
have been synthesized. These compounds have already been discovered to be useful as
fiber optics that transmit light in the 1-18 micrometer region of infrared light.
Besides being of interest in materials science, medical, and fiber optical
applications, glass has been historically significant in Western religion. Glass has
traditionally played an important part in the Christian religion, especially in medieval
Europe. The cathedrals were built during a time when most people were illiterate, and
the Roman Catholic Church commissioned artisans to design scenes from the Bible in
stained glass to adorn the cathedrals in place of plain, clear glass windows. The pictures
in the stained glass windows that people saw when they attended mass were
sometimes the only way that they could ‘read’ the Bible. Priests often used the stained
glass to explain stories from the Bible and Catholic doctrine (Cunningham, L).
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Glass served a different purpose during the Byzantine period. In Byzantium, the
dominant Christian philosophy was the mysticism of light, immortalized in the Hagia
Sofia. The Byzantine builders incorporated tall ceilings with windows placed almost on
the ceilings, like skylights, to let in and amplify as much sunlight as possible. They
believed that the majesty of God was visible in the immense columns of sparkling light
that fell on the portraits of Christ, Mary, and the apostles that adorned their churches.
Just as the proper attitude of the medieval Catholic was penitence for sin, the proper
worship attitude of the Byzantine Christian was gazing upon the icons and the columns
of glowing sunlight and experiencing a sense of awe at being in the presence of God
(Cunningham, L).
As a translucent solid with the ability to refract light, glass has played a unique
role in history and in science. It has been used universally for windows in building
construction, and the medieval Catholic Church saw the value in glass as an artistic
medium to tell Bible stories to an illiterate populace. Glass has been investigated
because on the molecular level its organization is different from that of crystals. For
some compounds, such as selenium- and tellurium-based binary glasses, there are two
separate solid forms possible: a highly ordered crystalline form, and an amorphous
glassy form. Because the glassy forms and crystalline forms have different structures,
they have different uses in manufacturing and industry. It is of interest to scientists to
be able to control the glass-to-crystal transformation, because some applications, such
as fiber optics, require that the compound remain in the glassy form. Due to their
amorphous nature, glassy forms are less stable that crystalline forms and are prone to
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spontaneous conversion to the crystalline form, with different molecular compositions
having varying levels of stability. Silicon dioxide, the chemical formula for common
window glass, is very stable in its glassy form, and was used by medieval artisans to
create the stained glass windows that adorn cathedrals.
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