Surreal Microscopic Environments – Negative Crystals

A few months ago a story did the rounds about a “negative crystal floating in space” [1] which was accompanied by a rather spectacular image (Image 1).

Image 1: Negative crystal in spinel. Image by Danny Sanchez

Image 1: Negative crystal in spinel. Image by Danny Sanchez

This picture was described as a negative spinel and was one of the most perfectly formed crystal inclusions found and photographed by Danny Sanchez using photomicrography techniques [2]. This unique photography technique allows small regions within crystals to be photographed with exceptional clarity, creating extraordinarily surreal images of crystal inclusions. Negative crystals aren’t the only type of crystal inclusion however, and they certainly aren’t the only ones to produce stunning images.

Crystal Inclusions

Generally speaking, negative crystals are a specific kind of crystal inclusion. Inclusions are when a foreign material is trapped within a host crystal. The foreign substance can be solid, liquid or gaseous and can become trapped within the crystal either during its growth or once the host crystal has formed. The exact method of inclusion formation depends on the type of inclusion and the conditions under which the crystal was grown [3].

Solid inclusions

The most well known solid inclusions are ancient insects or plant life suspended in amber. However, solid inclusions can also include different types of gemstones embedded inside another crystal, typically quartz (Image 2).

Image 2: Rutile on hematite in quartz. Image by Danny Sanchez

Image 2: Rutile on hematite in quartz. Image by Danny Sanchez

These mineral inclusions either form simultaneously with the host crystal or were pre-existing and the host crystal has grown around it, encasing the foreign material. In the case of quartz which usually forms in hydrothermal conditions (basically a hot water soup of dissolved minerals) the included gem existed in the solution prior to the growth of the quartz and over time the quartz grows around it (Image 3).

Image 3: Purple and blue fluorite in quartz. Image by Danny Sanchez

Image 3: Purple and blue fluorite in quartz. Image by Danny Sanchez

Alternatively, as many crystal growth conditions are under high temperature and pressure conditions and in aqueous media, liquid inclusions highly saturated with dissolved minerals can form. As the crystal cools these minerals begin to precipitate forming a second crystal inside the host.

Liquid inclusions

As mentioned, many famous crystals (such as the Naica crystal caverns) were formed in hot, mineral-rich water solutions. As a result, sometimes this solution can become trapped inside the crystal during growth. A rather unique liquid inclusion which occurs under pressurised conditions is liquid CO2, which remains trapped in liquid form due to the pressure maintained inside the inclusion.

Quite often liquid inclusions come paired with a bubble of gas, such as in Image 4 in which liquid petroleum and a bubble of methane are trapped in quartz.

Image 4: Petroleum and methane bubble in quartz. Image by Danny Sanchez

Image 4: Petroleum and methane bubble in quartz. Image by Danny Sanchez

Gaseous inclusions

Finally, “negative crystals” form when a pocket of air is trapped within a crystal. These gaseous inclusions can be a specific gas, depending on the conditions the crystal grew in, or just plain old air. These inclusions can form either due to crystal growth which occurs in multiple directions that then intersect or because crystal growth in a particular direction is inhibited temporarily. Since gaseous inclusions are enclosed by crystal faces the shape will reflect the crystal habit (or defined external shape of the crystal) of the host and they are always oriented parallel to the host crystal (Image 5). While they may look like a typical gemstone, it is actually an inversion with air “inside” the crystal boundaries and crystal material on the “outside”. Hence these inclusions are named “negative crystals”.

Image 5: Negative crystals in amethyst. Image by Danny Sanchez

Image 5: Negative crystals in amethyst. Image by Danny Sanchez

A special aspect of any type of inclusion is that the foreign material is suspended in time. In this way, the inclusion contains a “piece of the past” which provides insight into what the earth and the environment were like when the crystal grew, often millions of years ago. For example, the discovery from air inclusions in amber that the oxygen content in air reached 35% during the Cretaceous period, before suddenly dropping to near the current level of ~21% [4].

There are many other forms of inclusions in addition to those shown. Quite often they can produce stunning visual effects and increase the value of the gemstone. For example, star sapphires which produce a unique six-rayed star effect under certain light sources contain tiny inclusions of aligned needle shaped rutile (Image 6).

Image 6: Star sapphire under a direct light source. Image from http://www.gemselect.com/.

Image 6: Star sapphire under a direct light source. Image from http://www.gemselect.com/.

Ordinarily, most inclusions are microscopic in size and can only be properly visualised under a microscope. For the majority of images featured here highly specialised and expensive equipment is required.

More images of crystal inclusions can be found in Gübelin and Koivula’s Photoatlas of inclusions in gemstones [5] or at http://www.dannyjsanchez.com/.

[1] Hooper, R.; New Scientist 2014, 2975, 24.
[2] The Art of Photomicrography: Gemstone Inclusions by Danny Sanchez <http://www.dannyjsanchez.com/&gt; (Accessed 11/14)
[3] Benz, K. W.; Neumann, W. Introduction to Crystal Growth and Characterization; Wiley, 2014
[4] Kump, L.R.; Kasting, J.F.; Robinson, J.M. Global and Planetary Change 1991, 5, 1.
[5] Gübelin, E.J.; Koivula, J.I. Photoatlas of Inclusions in Gemstones; Vol 1-3, Opinio Publishers

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The world’s most underappreciated gemstone – Red Spinel

Ruby may be the most iconic gemstone, but often this status has been at the expense of the gemstone spinel. Thanks to the similarity of red spinel to ruby and its extremely high quality, spinels have historically been overlooked and incorrectly labelled as rubies. Some of the most historically iconic rubies aren’t even rubies! Spinels are so often confused for other gemstones that they were never even established as a birthstone (poor spinel).

What is Red Spinel?

The spinel structure has been described in an earlier post and MgAl2O4 is no different. Spinels consist of a nearly ideal cubic close-packed array of oxygen atoms with one eighth of tetrahedral sites occupied by magnesium and one half of octahedral sites filled aluminium. The AlO6 octahedra share edges with each other and are corner sharing with the MgO4 tetrahedra.[1]

Figure 1: Crystal structure of MgAl2O4. The Mg atoms sit within the yellow tetrahedral and the Al atoms sit within the blue octahedra. Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software.

Figure 1: Crystal structure of MgAl2O4. The Mg atoms sit within the yellow tetrahedral and the Al atoms sit within the blue octahedra. Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software.

Pure red spinel is colourless, so to obtain red spinel small quantities of chromium are substituted for aluminium, which is also the reason why rubies are coloured red. Only around 1 % of chromium substituted for aluminium is required within the crystal structure to generate the brilliant red colour. However, the higher the concentration of chromium, the darker the shade of red obtained. There are actually a range of colours that spinel can adopt depending on the foreign element substituted into the basic structure, from orange and pink to purple and deep blue. In fact, blue spinels, which are so coloured due to the presence of iron or cobalt (rarer), are often used as a substitute, and mistaken, for sapphire.[2]

Figure 2: Spinel gemstones come in a diverse range of colours. (Image obtained from http://www.gia.edu/spinel)

Figure 2: Spinel gemstones come in a diverse range of colours. (Image obtained from http://www.gia.edu/spinel)

While rubies and sapphires are the more famous and well known gemstones, spinels are equally comparable in appearance and arguably better due to the deeper, richer colours which are more readily obtainable (especially compared to ruby). The heightened brilliance of spinels is a result of being singularly refractive. This means that the gemstone only has a single refractive index, which measures the amount by which light is slowed (and thus bent) when entering the gemstone. In comparison, the overwhelming majority of gemstones exhibit birefringence which results in the splitting of light when it enters the crystal (see below). The most well known singularly refractive gemstones are garnet, diamond and spinel.

Figure 3: An example of a crystal exhibiting birefringence. A singularly refractive crystal would only produce a single image. (Image obtain from http://www-g.eng.cam.ac.uk/CMMPE/lcintro4.html)

Figure 3: An example of a crystal exhibiting birefringence. A singularly refractive crystal would only produce a single image. (Image obtain from http://www-g.eng.cam.ac.uk/CMMPE/lcintro4.html)

A Checkered Past

The high quality of spinels and fact that they are compositionally similar to rubies has meant that historically they were treated as the same crystal. As a result, there are many infamous rubies which are actually just red spinels. The most well known of these is the Black Prince’s ruby which is set in England’s Imperial State Crown and displayed in the Tower of London. This particular gem has been passed from several Moorish and Spanish Kings before reaching Edward, Prince of Wales (the Black Prince) in 1367. The gem has survived fires, attempted theft and World War II bombing raids to become one of the centerpieces of England’s Crown Jewels.[3]

Spinel Applications

Spinels are not just aesthetically pleasing; they also have some interesting applications. The singularly refractive nature of spinel in combination with the relative ease of producing synthetic spinel has led to its use as high strength window material in military applications. In addition, the relative ease and low cost associated with its manufacture, spinel has come to replace the previous state-of-the-art material, sapphire.[4]

Finally, since spinels are so underappreciated, it is much cheaper than ruby! So it is possible to buy polished and cut spinel gemstones which look almost identical to rubies at a fraction of the cost. So there is some benefit to being the underdog of the gemstone world.

MgAl2O4 spinel is number 1010129 in the Crystallographic Open Database.

[1] Passerini, L. Gazzetta Chimica Italiana 1930, 60, 389

[2] Shigley, J. E.; Sloclzton, C. M. Gems and Gemology 1984, 20, 34.

[3] Spinel <http://www.gia.edu/spinel&gt; (Accessed 09/14)

[4] Mroz, T. J.; Hartnett, T. M.; Wahl, J. M.; Goldman, L. M.; Kirsch, J.; Lindberg, W. R. SPIE Proceedings 2005, 5786, 64.

Growing crystals for your PhD – The trials and tribulations Part 2

In yesterday’s post I reminisced about a not-so-successful crystal growth of the defect perovskite material Sr0.8Ti0.6Nb0.4O3. However, my crystal growing adventures did not end there! I had begun a new crystal growth which would prove to be much more successful. In fact, it was to be my first crystal. The material in question was Sr3TiNb4O15.

Sr3TiNb4O15 and Sr0.8Ti0.6Nb0.4O3 are actually very closely related. In fact they are a part of the same series derived from SrTiO3. If substitution of niobium for titanium is continued beyond the Sr0.8Ti0.6Nb0.4O3 composition, the defect perovskite structure is no longer stable and a tungsten bronze type structure is formed instead. The exact composition at which a pure tungsten bronze type structure resulted was Sr0.6Ti0.2Nb0.8O3, which corresponds to Sr3TiNb4O15.[1]

Figure 1: Crystal structure of Sr3TiNb4O15 generated in VESTA. Structural information can be found in reference 1.

Figure 1: Crystal structure of Sr3TiNb4O15 generated in VESTA. Structural information can be found in reference 1.

Initially, Sr3TiNb4O15 was an impurity to me. It was something I was trying to avoid in order to synthesise a highly defect perovskite structure. However, Sr3TiNb4O15 itself is interesting as a relaxor ferroelectric material. Traditional ferroelectric materials exhibit a permanent reversible dipole moment which makes them extremely useful in capacitors, non-volatile data storage and other applications. Relaxor ferroelectrics differ slightly compared to traditional ferroelectric materials due to their complex compositions. Rather than exhibiting a strong ferroelectric response at a specific temperature (like traditional ferroelectrics), the response is spread out over a range of temperatures. This means that the dielectric constant of the material will be very close to its maximum for a wide range of temperatures, making them much more useful in applications.

Much like traditional ferroelectrics, the crystal symmetry plays a large role in determining the properties of the material. Specifically, the crystal symmetry can tell us whether a permanent dipole moment is possible within a structure. The best way to determine the structure of a crystal is diffraction from a single crystal! So once more I set out to grow crystals using the floating zone furnace.

Contrary to my rather lofty plans for Sr0.8Ti0.6Nb0.4O3, I did not require an especially huge crystal. Anything sub-millimetre scale would have been great. However, in comparison to the defect perovskite, a crystal of Sr3TiNb4O15 was really quite easy to grow. The main difficulty encountered was that the crystal growth was a little bit temperamental and unless it was “baby-sitted” it would tend to destabilise after about an hour. So I spent a nice quiet afternoon in the basement of the chemistry building slowly growing my crystal. Every now and then I would adjust some growth parameters to ensure that the melt wouldn’t destabilise. Eventually I obtained a lovely transparent yellow crystal.

Figure 2: Close up of the tungsten bronze crystal.

Figure 2: Close up of the tungsten bronze crystal.

Although the resulting column was comprised of multiple crystals, the crystal fragments were big enough for both X-ray and neutron diffraction experiments (however I was very hesitant to break up my crystal to do these!). While the work to determine the exact symmetry of the structure has now been passed down to another student, I will always be proud of my first ever successful crystal growth.

[1]                  T. A. Whittle, W. R. Brant & S. Schmid (2013). S. Schmid, R. L. Withers, R. Lifshitz (Eds.), Aperiodic Crystals, Proceedings of the 7th Conference on Aperiodic Crystals. Dordrecht: Springer. pp. 179 – 186

Growing crystals for your PhD, – The trials and tribulations Part 1

Will Brant tells us about growing Sr1-xTi1-2xNb2xO3 crystals for his PhD

For this post (and the next) I decided to do something a little different and talk about stories from my PhD which are in line with a crystal growing theme. Growing nice large crystals is not easy; there are so many factors which can influence the outcome of the growth of one specific type of crystal that some may consider it an art form. I found this out the hard way through my forays into trying to grow two seemingly similar, but ultimately very different, oxide materials. That is, Sr0.8Ti0.6Nb0.4O3 and Sr3TiNb4O15 (or Sr0.6Ti0.2Nb0.8O3).

The first of the two crystals I attempted to grow, Sr0.8Ti0.6Nb0.4O3, is a defect perovskite structure derived from the cubic perovskite SrTiO3. To get to Sr0.8Ti0.6Nb0.4O3 from SrTiO3 the TiO+4 cations are incrementally substituted with Nb+5 cations. In order to balance the difference in charge between titanium and niobium, vacancies are produced on the strontium position. (see structure below)

Figure 1

Sr0.8Ti0.6Nb0.4O3 was originally created in an attempt to produce new kinds of A-site deficient perovskites for lithium ion conduction applications. While Sr0.8Ti0.6Nb0.4O3 itself is not a very good lithium ion conductor it did exhibit some interesting short range structural distortions.[1] By growing a large single crystal, these distortions could be studied in greater detail.

My crystal growing method of choice was the floating zone furnace. To grow a crystal using this method, the ends of two sintered powder rods of a near-pure target material are melted using the focused heat from high power lamps. These molten regions are brought into contact with one another to produce a molten zone suspended between the two powdered rods. As the rods are moved vertically the molten zone travels through the polycrystalline powder. As the melt leaves the hot region crystals begin to form as the material cools. Over time, if all goes well, the growth of one of these crystals will predominate and a large, high quality single crystal will result.

The floating zone furnace.

The floating zone furnace.

The process began by preparing a pure Sr0.8Ti0.6Nb0.4O3 powder. This took around two to three weeks of repeated grinding and sintering. Once a rod of the polycrystalline material was prepared the growth could begin. By begin in this sense I mean about four trial growths were attempted to establish the best conditions. The hard work and long hours finally paid off however, as by the fifth attempt a stable growth had begun! However, this came to a grinding halt as a bubble began to form inside the melt. The bubble become larger and larger as the growth continued before finally bursting sending molten material flying out from the melt zone.

So what went wrong? Ironically, the presence of a few vacancies, while not in sufficient quantity to allow for lithium diffusion, encouraged entirely different, unintentional ion diffusion. That is, of oxygen ions! At high temperatures and slightly reducing conditions, oxygen is removed from the material. This gas became trapped in the melt gradually accumulating to the point where eventually a bubble of O2 burst and ruined everything.

It was time for round 6: remove as much oxygen as possible before attempting crystal growth. Given that oxygen rapidly left the material at high temperatures, this involved heating both the powder and then the sintered rod up to 1350 °C for extended periods of time. This time around, no bubbles formed and it appeared as though the melt and crystal growth were actually stable.

Lump!

Lump!

What did I get for my effort? A large lump of super dense highly fused polycrystalline ceramic. So, not a crystal. The lump was so large and dense that I couldn’t actually break it into smaller pieces despite my enthusiastic efforts to do so (RIP mortar and pestle). This lump of perovskite now sits in my desk. By this point I had decided to abandon trying to grow Sr0.8Ti0.6Nb0.4O3 crystals because another material was proving to be much more successful, Sr3TiNb4O15

To be continued…..

What’s in a name? – Bridgmanite

Figure 1: Crystal structure of the distorted perovskite MgSiO3. Image generated in VESTA.

Figure 1: Crystal structure of the distorted perovskite MgSiO3. Image generated in VESTA.

Something very significant happened last month; a mineral known to exist for a very long time was finally acknowledged and given a name. That was silicate perovskite, the most commonly occurring mineral on the planet. Silicate perovskite, now known as bridgmanite, takes on a few elemental forms as it can, for example, incorporate magnesium, iron or calcium on the perovskite A-site. Of these the magnesium containing silicate perovskite, MgSiO­­3, is considered to be the most abundant mineral on the planet. Despite this abundance, naturally occurring silicate perovskites had never been directly observed as it can only be found in the lower mantle, as inferred from high temperature sound velocity data.[1, 2] The only way the structure was able to be studied was from silicate perovskite created synthetically using a laser-heated diamond anvil cells, which recreates the conditions present in the lower mantle.[3, 4] However, since a naturally occurring form had never been directly observed or studied the mineral was never formally recognised with its own name.

Figure 2: A fragment of the 4.5 billion year of Tenham meteorite containing shock veins of bridgmanite. Chi Ma via the American Geophysical Union , Accessed 7/7/14

Figure 2: A fragment of the 4.5 billion year of Tenham meteorite containing shock veins of bridgmanite. Chi Ma via the American Geophysical Union <http://blogs.agu.org/geospace/files/2014/06/bridgmanite_label.jpg&gt;, Accessed 7/7/14

So what is in a name? What was required for the most common mineral on earth to finally be recognised as a mineral and given a name? For the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA), the composition and structure of a naturally occurring sample must be known. Ironically, the final composition and structure of the most common mineral on earth was finally determined from a meteorite. That is, a 4.5 billion year old meteorite which landed 135 years ago in our very own backyard, near the Tenham Station in western Queensland.

The bridgmanite was formed inside the meteor following collisions with other asteroids in space. The high pressures and temperatures experienced in these collisions generated “shock veins” of the mineral through the meteor (Figure 2). The presence of silicate perovskite in these veins was determined following five years of work by Dr Chi Ma from California Institute of Technology and Prof Oliver Tschauner from the University of Nevada-Las Vegas.[5] The mineral was identified using synchrotron X-ray diffraction mapping and the composition were confirmed using a high resolution scanning electron microscope.

The mineral was named after the physicist Percy Bridgman who won a Nobel prize for high pressure physics. This name pays homage to the large body of high pressure research which has been performed on synthesising and understanding silicate perovskites.

[1] M. Murakami, S.V. Sinogeikin, H. Hellwig, J.D. Bass, J. Li, Earth and Planetary Science Letters, 256 (2007) 47-54.

[2] M. Murakami, Y. Ohishi, N. Hirao, K. Hirose, Nature (London, U. K.), 485 (2012) 90-94.

[3] L.-G. Liu, Geophys. Res. Lett., 1 (1974) 277-280.

[4] L.-G. Liu, Geophys. Res. Lett., 2 (1975) 417-419.

[5] J. Wendel, Eos, Transactions American Geophysical Union, 95 (2014) 195-195.

 

Selenite – Crystal Cathedrals

What is it?

It is the dream of all crystallographers to develop a technique which can grow the largest and highest quality crystals possible. Often this is extremely difficult to do due to all of the competing factors which can drive crystal growth, and so it is often considered an art form. The rarity of large single crystals is what makes the Naica crystal caves truly extraordinary.

The great crystal cavern of NAICA mines.

The great crystal cavern of NAICA mines.

Buried 300 meters below the surface, and discovered at a time when we people believed that they had seen every spectacular sight the earth had to offer, are the selenite crystal caves (Figure 1). No, this is not a carefully constructed Photoshop; those are real people climbing over pure gypsum crystals, otherwise known as selenite. In fact these are the largest of any naturally occurring crystals measuring up to 12 m in length, 4 m in diameter and 55 tons in weight. However, before you rush to pull out your passport and start booking flights, know that these caves are not open to the public. The caves are restricted partially for preservation purposes (instigated by a mining company of all things) but also because conditions within the cave are oppressive. Due to their location above an underground magma chamber and the presence of a large quantity of ground water, temperatures can sit at a constant 58 °C and 90 to 99 % humidity. Without special protection humans can only survive in these conditions for a few minutes. Incidentally, these are ideal conditions for crystal growth. Prior to their discovery, the caves were filled with hot, mineral rich water for 500,000 years. During this time the temperature hardly varied and the crystals were allowed to slowly grow to the size they are today.[1]

Researcher in protective gear among the crystals.

Researcher in protective gear among the crystals.

So if the caves are closed to public access, what are people doing there? A huge research project is being undertaken covering fields from medical research to regional geology. Some examples of the research being performed include, investigating how the crystals became so large and how old they are, studying pollen and microorganisms trapped inside the crystals, developing technology which enables humans to work in extreme conditions for long periods of time and building an understanding of the physiological effects of exposure to these extreme conditions over a long period of time. However, the longer the crystals are exposed to the air the more faded and brittle they become, eventually cracking under their own weight. As a result there are constant discussions on whether research should continue in order to understand this unique natural resource or whether the caves should be re-flooded with water in order to preserve their current form.

What does it look like?

Structure was generated in VESTA

Structure was generated in VESTA

As mentioned, the crystals are a pure form of gypsum known as selenite. The crystal structure of selenite/gypsum (CaSO4•2(H2O)) has been featured before on this blog in its “desert rose form”. The main difference between the desert rose crystals and selenite is the presence of sand impurities in desert rose. Another view of the structure is provided below where calcium is shown in blue, SO4 tetrahedra are yellow, oxygen is red and hydrogen is white.

Where did the structure come from?

The image was generated from a structure in the paper Schofield P. F., Knight K. S., Stretton I. C. American Mineralogist 81 (1996) 847-851 and can be found on the American Mineralogical database.

Images from:

Cave of Crystal Giants – National Geographic Magazine, Available from: <http://ngm.nationalgeographic.com/2008/11/crystal-giants/shea-text&gt; [November 2008]

[1]          NAICA PROJECT/CRYSTALS’ CAVE, Available from: <http://www.naica.com.mx/english/internas/interna4_1.htm&gt; [Accessed May 2014]

LiFePO4 – The Unexpected Battery Success Story

What does it look like?

 

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

Two of the basic requirements of positive electrode materials for lithium ion batteries are that they must be both good electronic and good ionic conductors. This enables the material to both accept electrons from the external circuit and insert lithium ions from the electrolyte.

LiFePO4 is not a good electronic or ionic conductor and yet it is the most promising positive electrode materials introduced in the last 10 years. Its success came as a result of a thorough understanding of the structural changes during lithium insertion and clever manipulation of the particle shapes and morphology.

The structure of LiFePO4, which is a naturally occurring mineral called triphylite, was first explored by Bjoerling et al in 1938.1 The significance of LiFePO4 as a positive electrode material was then reported by Padhi et al.2 The LiFePO4 olivine structure is formed by a hexagonal close packed oxygen array with Li+ and Fe2+ occupying half of the octahedral sites and phosphorus occupying 1/8 of the tetrahedral sites in between the layers. Each FeO6 octahedron is corner linked to other FeO6 octahedra to form zig-zag planes running parallel to the c-axis within alternating a-c planes. In addition, one edge of the FeO6 octahedron is shared with one PO4 tetrahedron and two edges are shared with two LiO6 octahedra. The LiO6 octahedra form linear chains of edge shared octahedra between layers of FeO6 octahedra and share edges with two PO4 tetrahedra (see Figure).

The electrochemical properties of LiFePO4 are strongly influenced by its structure. For example, the inductive effect of the PO4, leads to higher voltages enabling normally uninterested cations, such as iron, to be used successfully in a positive electrode material. But there are still problems, the PO4 is also responsible for the drastically reduced electronic conductivity in these materials. Further, the lithium diffuses through the material via one dimensional channels which greatly inhibits the ionic conductivity.

Both of these shortcomings were overcome by preparing nanoscale LiFePO4 particles coated in a thin layer of carbon black. The increase in electronic conductivity was understood to be due to the thin carbon black layer providing a pathway for electrons from the particles to the current collector. The drastic increase in ionic conductivity, however, was not expected from a reduction in particle size.

This is because the reaction was observed to proceed via a two phase conversion reaction between LiFePO4 and FePO4. Thus, the main energy barrier to lithium extraction is the seeding and growth of the second phase. A solid solution reaction, where there is a gradual conversion from LiFePO4 to FePO4 via LixFePO4 (0<x<1), on the other hand benefits from a reduction in particle size due to shorter diffusion pathways.3 It was hypothesised and then eventually proved, that the lithium insertion reaction proceeds via a pseudo-solid solution type reaction where the reaction front between the two phases is single phase LixFePO4.

This behaviour is highly unusual and the main reason why LiFePO4 has become a significant new battery material. So, in future, it is likely that batteries for electric vehicles as well as portable electronic devices will contain LiFePO­4 as the positive electrode material.

Where did the structure come from?

LiFePO4 which is #1011090 in the Crystallography Open Database.

 

  1. Bjoerling, C. O., Westgren, A. Minerals of the Varutrask pegmatite. IX. X-ray studies on triphylite, varulite, and their oxidation products. Geologiska Foereningens i Stockholm Foerhandlingar 60, 67-72 (1938).
  2. Padhi, A. K., Nanjundaswamy, K. S. & Goodenough, J. B. Phospho-olivines as positive-electrode materials for rechargeable lithium batteries. Journal of the Electrochemical Society 144, 1188-1194 (1997).
  3. Malik, R., Zhou, F. & Ceder, G. Kinetics of non-equilibrium lithium incorporation in LiFePO4. Nat. Mater. 10, 587-590 (2011).