During my stay in Arizona for the Tucson shows I tried to keep abreast of the latest battles between rooftop solar vs. the power companies. As in many states, monopolistic power companies seem frightened of the increasing number of individual homes installing solar panels. There are substantial “for and against” arguments coming from both sides; however, with Arizona being a “sunshine state” solar panels would seem a reasonable option. But, there is little cooperation between the two camps. As a political junkie, I am fascinated, on a daily basis, with Arizona politics!
At any rate, in one of the numerous articles on
solar power I stumbled on the term perovskite structure as it pertained to solar
panels. This piqued my interest since
perovskite is a mineral, calcium titanium oxide [CaTiO3]. I wondered how this not-so-common mineral
could be plentiful enough to be used in the solar industry. But, further exploration and reading
indicated my lack of understanding about: 1) the mineral perovskite; and 2) the perovskite
structure. I blame this little bit of
ignorance on my propensity for breaking glassware in third semester chemistry
and deciding that a chemistry major was not on my radar screen. In addition, there were the rumors (mostly
true I think) about a future college course, physical chemistry (P Chem in the
vernacular), being a “killer course”, something that my college GPA did not need
at that time!
What I recently learned is that the term perovskites
refers to a family of crystals, most of which are made synthetically, with the
same type of crystal structure as the “real mineral” perovskite. These
compounds have a chemical structure of ABX3 where A and B are two
different cations of different sizes and the anion X bonds them together. It
forms a cubic symmetry.
The photo voltaic perovskite structure compound that
first interested researchers was methylammonium lead tri-iodide although today
tin is being examined as a less toxic replacement for lead. “Solar cell manufacturers face a tricky
trade-off between performance and cost. Most commercial solar cells rely on
slabs of crystalline silicon that are more than 150 micrometers thick and take
a lot of energy to produce (efficacy of 1-23 percent). Thin-film solar
cells—those containing just a few micrometers of such semiconductors as copper
indium gallium selenide (CIGS)—have lower material costs, but they are also
less efficient. Cells using crystalline gallium arsenide, on the other hand,
can reach 30 percent, but the materials involved are too costly for
utility-scale solar power. Perovskites could resolve this quandary by matching
the output of silicon cells (efficiency now at 19.3 percent) at a lower price
than that of thin-film CIGS: Their ingredients are cheap bulk chemicals, and
the cells can be built using simple, low-cost materials" (Peplow, 2014).
Kulkarni and others (2012) noted that besides photo voltaics,
perovskite materials exhibit many interesting and intriguing and commonly
observed features/properties: colossal ferroelectricity, superconductivity, charge ordering, spin dependent transport, high thermopower
and the interplay of structural, magnetic and transport properties. Sounds interesting.
But, back to the mineral perovskite. In rummaging through some dusty trays at
Tucson Show mineral dealer, I came across a small box labeled perovskite and
containing a specimen with some small cubic crystals. Wow, a little serendipity with a great
price--$4. So, I scooped it up for my
collection.
The mineral perovskite actually belongs to the
Orthorhombic Crystal System but usually appears as crystals with a cubic outline (BTW, that is something that still confuses me—the crystals appear cubic
but are orthorhombic. If it quacks like
a duck………..:) These small cubes come in
a variety of colors from black to brown to shades of yellow, orange and red. My
cubes have an adamantine to sub-adamantine luster, are fairly hard at ~5.5
(Mohs) and seem partially translucent (ranges from transparent to opaque). At times some cubes have a metallic luster
and look similar to galena.
Perovskite has several relatives listed in MinDat as
belonging to the Perovskite Mineral Group with common substitution of Fe, Nb, Ce,
La.
Anthony and others noted perovskite forms as an
accessory mineral in alkaline mafic rocks, as nepheline syenites, kimberlites,
carbonatites, and can additionally form in calcium-rich skarns (such as Magnet Cove, Arkansas) and is a common
accessory mineral in calcium and aluminum rich inclusions within carbonaceous
chondrites.
My specimen was collected from the Malenco Valley,
Valtellina, Sondrio Province, Lombardy, Italy.
MinDat lists 177 valid minerals collected from the locality. The area is in the Italian Alps but I cannot
locate references describing the specific geology. The best known minerals from the locality are probably gemmy green
“garnets.”
The USGS has noted that titanium is an important
element mined for a variety of purposes and occurs primarily in the minerals
anatase, brookite, ilmenite, leucoxene, perovskite, rutile, and sphene.
Of these minerals, only ilmenite, leucoxene, and rutile have significant
economic importance.
So, I have learned much from this little exercise—mostly
that while perovskite seems like an innocuous mineral, it actually is quite
complex. For example, “the stability of
perovskite in igneous rocks is limited by its reaction relation with sphene. In volcanic rocks perovskite and sphene are
not found together” (Veksler, 1990).
And, perovskite also gives its name to a complex structure that is proving
to be quite critical in many new industries.Life can provide great learning experiences!
REFERENCES
CITED
Anthony, J.W. R.A Bichard, A. Bideaux, K. W. Bladh, and M.
C. Nichols, Eds., Handbook of Mineralogy, Mineralogical Society of America,
Chantilly, VA 20151-1110, USA. http://www.handbookofmineralogy.org/.
Kulkarni, A., F.T.
Ciacchi, S. Giddey, C. Munnings, 2012, Mixed ionic electronic conducting perovskite anode for direct carbon fuel
cells: International
Journal of Hydrogen Energy, v. 37,
no. 24.
Navrotsky A., 1998, Energetics and crystal chemical systematics among Ilmenite, Lithium Niobate, and Perovskite Structures: Chemical Materials, v. 10, no. 10.
Navrotsky A., 1998, Energetics and crystal chemical systematics among Ilmenite, Lithium Niobate, and Perovskite Structures: Chemical Materials, v. 10, no. 10.
Peplow, M., 2014, Perovskite is the new black in the
solar world: IEE Spectrum, 25 June.
Veksler, I.V., and M.P. Teptelev, 1990, Conditions
for crystallization and concentration of perovskite-type minerals in alkaline
magmas: Lithos, v. 26.
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