PEROVSKITE: CALCIUM TITANIUM OXIDE

Western front of the Santa Catalina Mountains as seen from Catalina State Park, Oro Valley, Arizona (northernmost Tucson).  The massive granite "Cathedral" is left of center while folded gneiss is prominent on the left.

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.

Structure of a perovskite with a chemical formula ABX3. The red spheres are X atoms (usually oxygens), the blue spheres are B-atoms (a smaller metal cation, such as Ti4+), and the green spheres are the A-atoms (a larger metal cation, such as Ca2+). Pictured is the undistorted cubic structure.  Public Domain drawing and explanation. Original from Navrotsky, 1998.

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.

Specimen containing cubes of perovskite.  Maximum width 1.6 cm.

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.

Photomicrograph, cube of perovskite (P); width of crystal ~2 mm.  Some overgrowth of calcite (C).

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.

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.