Ordering matters: Not all brownmillerites are created equal

What is it?

Early in the blog we were introduced to perovskite, one of the simplest and most common mineral structures. A vast number of crystals are derived from the perovskite structure, which typically contains two different metals (A and B) and a counteranion, such as oxygen (O), in the ratio ABO­3.

The brownmillerite structure (blogged on March 5) is one such derivative of perovskite, created by removing one-sixth of its oxygen atoms in rows. The B-metal atoms nearest the “missing” oxygens now have only four oxygen neighbours, which form a tetrahedral shape around them; the other B-metals have a full set of six oxygen neighbours, which form an octahedral shape. (Why octahedral? Six corners = eight faces!) Each tetrahedral shape is linked at the corners to two others, making long parallel chains of tetrahedra which extend throughout the crystal in one direction.

Removal of the black-boxed O atoms from perovskite (left) produces the brownmillerite structure (right).

Removal of the black-boxed O atoms from perovskite (left) produces the brownmillerite structure (right).

Now it gets complicated…

The tetrahedra, with their two missing oxygen corners, turn out to be a bit too big for the space they’re in, so they twist slightly to make a better fit. Because each tetrahedron is linked in a chain, the whole chain has to twist the same way, either left (L) or right (R). The question is: if the first chain twists to the left, which way will the next chain try to go? The same way (L)? The opposite (R)? Or will it choose an orientation at random, without any reference to its neighbour?

In fact, there are many different ways that the chain-twists can be ordered in brownmillerite materials. Researchers in the 20th Century grouped brownmillerites into three categories based on their interchain relationships:

1) all chains the same type

2) layers of L alternating with layers of R

3) all chains completely random

In the early 2000s, however, A. Abakumov and coworkers began to notice features in their electron diffraction images that were inconsistent with these three simple models. Subsequent re-investigations using modern high-quality diffraction data revealed a range of more complicated patterns in many known brownmillerites, most of which had previously been contentious or identified as “random”-type structures.

Just three of the possible L-R chain ordering arrangements in brownmillerites

Just three of the possible L-R chain ordering arrangements in brownmillerites

Besides creating some interesting problems for crystallographers to solve, the brownmillerite superstructure can actually provide us with information about the physical properties of these useful materials. For example, it was recently shown that oxide-ion conductivity in Sr2Fe2O5 is triggered by the “loosening” of some of its oxygen atoms at temperatures above 600 °C, where thermal energy allows the chains to move and randomise. At lower temperatures, however, a complex ordering pattern is observed among the chains, demonstrating that there isn’t enough energy to allow the oxygen atoms to move around. (Auckett et al. Chemistry of Materials 25 (2013) p.3080-3087) In this way, crystallography might be used to predict which brownmillerites could potentially conduct oxide ions at lower (industrially useful) temperatures.

Where does it come from?

The common brownmillerite superstructures are described by Abakumov et al. in the Journal of Solid State Chemistry 174 (2003) p.319-328, and in other publications.

This month introduces us to the most iconic of red gems: Ruby, the birthstone of July.

What is a ruby?

Trick question – rubies don’t exist! Well, they do, though technically they are no more than red sapphires. Both ruby and sapphire are names given to gem-quality stones of the mineral corundum (Al2O3). Although the word “sapphire” is commonly used to imply blue gems, it can also be used to refer to the whole range of coloured and colourless corundums (from green or yellow to peachy-orange and purple), while “ruby” refers strictly to red or pink stones. Sapphire will be blogged later this year as the birthstone of September.

The crystal structure of corundum consists of aluminium atoms surrounded by six oxygen atoms in a slightly irregular “octahedral” shape. Each oxygen atom is shared between four of these octahedra – some of which are joined at the corners, some along edges, and some on the faces – so that the ratio of aluminium to oxygen atoms is 2:3.

The corundum structure of ruby (and other sapphires). Blue spheres: aluminium; red spheres: oxygen. (ICSD-10425; drawn using VESTA)

The corundum structure of ruby (and other sapphires). Blue spheres: aluminium; red spheres: oxygen. (ICSD-10425; drawn using VESTA)

Pure Al2O3 is colourless; the many colours it displays in nature are caused by trace metal impurities. Rubies are red due to the presence of chromium, while blue sapphires contain tiny amounts of titanium and iron. Rather than occurring as distinct chunks or particles distributed through the crystal, these metals are actually incorporated into the crystal structure on an atomic scale – meaning that a percentage of the aluminium atoms are simply replaced by the impurity element at random points throughout the whole crystal. (A strongly coloured red ruby might have about 1% of its aluminium replaced with chromium.) This kind of atom-for-atom substitution is known as a “solid solution”, because the corundum structure can be thought of as a liquid that “dissolves” an impurity. Of course, dissolving a metal in a gem requires far more energy than dissolving a sugar cube in water! This is why rubies and many other solid solutions are only formed at very high temperatures, such as those found under the earth.

Natural ruby has been known and valued for its ornamental uses since ancient times, though many red gemstones were indiscriminately termed “rubies” before scientific tests allowed man to distinguish between true rubies and other red gems. Rubies have historically been mined in parts of south Asia (Afghanistan to Sri Lanka), Africa, Australia, and south-east Asia, especially Myanmar, from which the richly-coloured but gorily-named “pigeon’s blood” rubies originate. (Pigeons evidently have redder blood than the rest of us!) The largest mined ruby in the world, the Liberty Bell Ruby, was stolen in a high-profile heist in 2011 and remains unrecovered.

In addition to its aesthetic appeal, ruby finds important technical applications as a laser material, so optical-quality specimens remain in high demand. The majority of laser rubies are now grown synthetically, often using the Vernueil flame-fusion technique, though hydrothermal or melt-crystallisation methods (Czochralski, floating-zone growth) can also be employed.

Here’s one I prepared earlier… Synthetic ruby (1% Cr) grown by the floating-zone method

Here’s one I prepared earlier… Synthetic ruby (1% Cr) grown by the floating-zone method

Where does the structure come from?

William and Lawrence Bragg (the father-and-son team of Nobel Prize and crystallographic Bragg’s Law fame) studied the structure of corundum in 1917. Although they correctly predicted the formula Al2O3 and the six-fold coordination of aluminium, many of their proposed bond lengths were inaccurate. Linus Pauling and Sterling Hendricks published the corrected corundum structure in 1925.

Square planar ferrites: sneaking up on unlikely atomic geometries

What does it look like?

SrFeO2 (ICSD-173434) drawn with VESTA; green: Strontium atoms, brown: Iron atoms, red: Oxygen atoms.

SrFeO2 (ICSD-173434) drawn with VESTA; green: Strontium atoms, brown: Iron atoms, red: Oxygen atoms.

What is it?

For years, chemists studying minerals and metal oxides have used “solid-state” reactions to make their compounds, forcing dry powders to react with each other by roasting them in furnaces at temperatures up to 1800 °C. The extreme conditions provide the atoms with enough energy to move about and form new crystal structures, but they also give the chemist limited control over the final product, because the atoms tend to settle into their “favourite” arrangements upon cooling. Now, scientists are exploring new methods for transforming crystals at lower reaction temperatures, where the atoms are unable to move as freely, as a way of creating unnatural and exotic crystal structures.

The “infinite layer” oxide SrFeO2, first reported in Nature by Tsujimoto and coworkers in 2007 [1], contains a very unusual structural feature: iron surrounded by four oxygen atoms in a square arrangement, each square sharing corners with its neighbours. These two-dimensional sheets of FeO2 are held apart by layers of strontium atoms, which sit in the “windows” of the square network. The reason it’s considered a rarity is that iron almost always prefers a three-dimensional tetrahedral geometry when bonded to four oxygen atoms – in fact, the only natural mineral known to contain iron-oxygen squares is gillespite (BaFeSi4O10), whose squares are not networked but isolated from each other by rings of silica.

Tsujimoto et al. were able to create their remarkable iron oxide network by heating SrFeO3, a cubic perovskite, to a very modest 280 °C in the presence of calcium hydride (CaH2). The relatively low reaction temperature allowed the CaH2 to draw some oxygen out of the perovskite structure while the metals stayed in place. Voilà – a brand new ferrite! Similar methods have since produced a whole set of closely related structures containing calcium, cobalt and manganese [2,3], which have been studied for their interesting magnetic properties.

SrFeO2 (below), created by the removal of selected oxygen atoms (in red) from the cubic perovskite SrFeO3 (above). Image source: Tsujimoto et al. [1]

SrFeO2 (below), created by the removal of selected oxygen atoms (in red) from the cubic perovskite SrFeO3 (above). Image source: Tsujimoto et al. [1]

The moral of the story: sometimes, softly does it.

Where does it come from?

The above information was sourced from Tsujimoto’s Nature article [1] and the other references below. Images were generated using the structure file deposited in ICSD (Ref. 173434) by C. Tassel et al., J. Am. Chem. Soc. 130 (2008) p.3764-3765.

[1] Y. Tsujimoto et al, Nature, 450 (2007) p.1062-1066.

[2] C. Tassel et al., J. Am. Chem. Soc., 131 (2009) p.221-229.

[3] L. Seinberg et al., Inorg. Chem. 50 (2011) p.3988-3995.