Pique the Geek 20101226: Rare Earth Metals

This time we have decided to get back to geeky science and technology.  I had thought about writing about Winter Solstice celebrations through the eons, but that has passed now.  Next year for sure.

The Rare Earth Metals are a group of elements that are extremely closely related in atomic number (henceforth called Z), mass, and chemical properties.  I would wager that most folks who are not technical have never heard of them, except maybe on Mythbusters (the neodymium magnets are the strong ones that they use now and then), let alone touched one to their knowledge.

But almost everyone uses them on a daily basis, and most have indeed touched at least one.  Please come with us and let us explore these interesting and essential elements.

First, we have to get Geeky.  I have maintained many times that if one truly understands the Periodic Table that one understands about half of chemistry, lots of physics, and much of quantum mechanics.  This is no different.  Here is one periodic table (although I prefer the old convention for column labeling) that is representative.  As I have explained before, the periodic table shows relationships betwixt the various elements because of their electronic structures.

If you look closely at this table, you will see that on the sixth row a white space that says “57 - 71”.  If you look even further down, you will see those elements in a row of their own.  There is a reason for this, and it is fundamental for quantum mechanics and for the properties of those elements.

In the elements above them, all electrons are able to fit into s (2 electrons each), p (six electrons each), and d (14 electrons each) orbitals.  However, once we get to Z of 57, the energetics require that new orbitals be used, the f ones.  Because of the subtle interactions of electron physics, these orbitals are inside of the outmost orbitals.  Here is the critical thing:  the outmost orbitals, aka the valance orbitals, of electrons control the major part of the chemistry of the elements.  Thus, with more inner f orbitals changing electronic filling, all of the rare earth elements have similar chemistries.

It is important to list the electronic structures of each of these, so here they are.  I am only listing the outermost orbitals, since all of them have in common the Xenon core electronic structure, so it is of no matter.

Z = 57, Lanthanum (La), [Xe]5d¹6s²

Z = 58, Cerium (Ce,) [Xe]4f¹5d¹6s²

Z = 59, Praseodymium (Pr), [Xe]4f³6s²

Z = 60, Neodymium (Nd), [Xe]4f⁴6s²

Z = 61, Promethium (Pm), [Xe]4f⁵6s²

Z = 62, Samarium (Sm), [Xe]6s²4f⁶

Z = 63, Europium (Eu), [Xe]4f⁷6s²

Z = 64, Gadolinium (Gd), [Xe]4f⁷5d¹6s²

Z = 65, Terbium (Tb), [Xe]4f⁹6s²

Z = 66, Dysprosium (Dy), [Xe]4f¹⁰6s²

Z = 67, Holmium (Ho), [Xe]4f¹¹6s²

Z = 68, Erbium (Er), [Xe]4f¹²6s²

Z = 69, Thulium (Tm), [Xe]4f¹³6s²

Z = 70, Ytterbium (Yb), [Xe]4f¹⁴6s²

Z = 71, Lutetium (Lu), [Xe]6s²4f¹⁴5d¹

The reason for going into so much detail is to point out that every element has a filled 6s orbital, and for most of them, it is the most energetic one.  These orbitals are listed in order of increasing energy, and you can see that only Samarium and Lutetium have orbitals of greater energy than the 6s ones, and that is because of an extremely complex interaction of quantum mechanics and relativity.  Since the outermost orbital defines chemistry to a large degree, these elements behave almost identically, and until only a few decades ago made it nearly impossible to separate them and study their properties.

In the late 1940s and early 1950s new tools were introduced to separate such similar elements as their salts, mostly involving ion exchange chromatography.  By using suitable substrates and solvents, it became possible to isolate each element in a pure form from an extremely complex mixture of those elements.  That was important, because all of these elements are almost always found in association with each other, and separation is essential.  With the Geeky stuff behind, we shall examine the uses of these materials.

Why are rare earth metals important?  They are used in hundreds of applications, from exotic industrial uses to something that practically everyone who smokes (and many who do not) use multiple times per day.  We shall address the last one first.  In all cigarette lighters, except for the ones that use a piezoelectric ceramic for the spark, sparks are produced by a so-called “flint”, which is a mixture of rare earth metals (the entire spectrum, since it is too expensive to separate the components for this use) with some hardening agents (rare earth metals are soft like lead).  When the steel rake on the wheel passes over the misch metal, some particles are filed off, and the friction heats them enough to burn.

For a bit of trivia, when actual flint and steel are used to start fires, the very hard flint knocks off bits of steel which ignites.  This is the opposite in a lighter, where the steel wheel erodes the “flint”.  In disposable lighters the “flint” is usually fairly long so it will last until the butane supply is exhausted, whilst in refillable ones new “flints” have to be inserted from time to time.

If lighters were the only use for rare earth metals, we could do without them, since as just mentioned a piezoelectric material can be used in place of the “flint”.  However, the uses of rare earths are so varied and ingrained in modern life that we would find it difficult to function without them.

The most common rare earth metal is cerium, and if you have a self-cleaning oven then there is cerium(III) oxide in the coating, acting as a catalyst to assist the high temperatures of the oven to reduce baked on splashes to wipeable ashes.  It also is used in catalytic converters in automobiles to oxidize carbon monoxide to carbon dioxide.  Cerium(IV) oxide is perhaps the best polishing agent for precision optics, as it is very hard but available in such a fine state of division that glass can be polished to within a wavelength of light, aka optically flat.  Back in the graduate school days I would often use it to polish potassium bromide windows for infrared spectrometry.

Cerium has preservative properties for glass and clear plastics, in large part due to its extremely large ultraviolet absorption properties.  Clear plastics that will be used in sunlight often contain cerium to prevent darkening, and the glass in TeeVee screens (for the old cathode ray types) has cerium in it to keep it from being damaged from the hard UV and soft X-rays that are produced when the electron beam hits it.  Another widespread use for cerium is in phosphors for lighting and TeeVee applications.  Cerium is also one of the few substances other than water that is more dense in the molten state than in the solid state.

The first rare earth element, lanthanum, is becoming quite a strategic metal.  (Some authorities count scandium (Z = 21) and yttrium (Z = 39) as rare earth metals, because of similar chemistries to the rare earths, but they do not have any f orbitals).  One of the biggest and newest uses in in nickel metal hydride batteries, and with the growing popularity of electric and hybrid automobiles, the use of the metal is increasing.  Too bad that China accounts for over 90% of the production of rare earth metals these days.  Glass doped with lanthanum is very transparent in the infrared range, so long distance fibre optics depend on it.  Like cerium, it is also used in phosphors for lighting.  However, the most important lighting application is the extremely high intensity carbon arc lighting, commonly used for motion picture industry both to light the set and to project the image at the cinema.  The lanthanum added to the carbon electrodes increases the emission of light and also smooths the arc, so flickering does not occur.  It is also used in optical glasses to increase the index of refraction of the glass, allowing for thinner lenses to do the same job as thicker ones without it.

Praseodymium, along with lanthanum, is used in carbon arc lighting for the same reasons.  However, lanthanum is cheaper and more abundant.  A very important use is in the old fashioned welder’s lenses where it absorbs UV.  Modern, electronic lenses use liquid crystals for that now.  It is used in some solid state lasers and some rare earth magnets as well.  A critical use for the metal is to make extremely strong alloys with magnesium for aircraft engine applications where light weight is important.

The next rare earth metal, neodymium, is used along with praseodymium in welder’s lenses.  As a matter of fact, it was only fairly recently that praseodymium and neodymium were separated, and the mixture of the two elements was though to be a single element, didymium, and it was that which was added to the glass.  Neodymium is critically important.  When you hear the term rare earth magnet, the ones containing neodymium are almost always the ones meant.  Neodymium magnets are the most powerful permanent magnets known, so strong that it is possible to hurt yourself rather badly when fooling around with them if they are very large due to crushing or impact.  These magnets are getting cheap now.  I saw a TeeVee advert for a vacuum cleaner that uses neodymium magnets in its motor the other day.  Another critical application for this element is in solid state lasers, where it is probably the most important laser source, from tiny to huge applications.  Neodymium magnets are used in electric automobile motors, so this is a third rare earth metal used in the automobile industry.

Promethium, the next rare earth, is intensely radioactive.  Its half life is so short that only around a pound exists naturally, mostly due to spontaneous fission of uranium.  It is usually recovered from reprocessing spent fuel rods from nuclear reactors.  Its uses all depend on its intense radioactivity for the longest lived isotope, Pm-145, half life of almost 18 years, and a pure beta emitter.  The betas are not particularly energetic, but there are lots of them.  Commercial uses include thickness gauges based on beta absorption and nuclear batteries.

The next rare earth metal is samarium, and one of its largest uses is also in magnets, although those are not quite as strong as the neodymium ones.  They also are more resistant to higher temperatures and are also more resistant to demagnetization.  One of the most important uses for samarium is in nuclear reactor control rods, because it absorbs neutrons very well, and is transformed in large part to other samarium isotopes which also are good absorbers.  Thus, samarium control rods have a longer useful life than those made of other materials.

Europium is next, and until LCD and LED TeeVee screens was (and is) used for the red phosphor.  Phosphors continue to be a large application for it, as it is important in compact fluorescent lamps for its red contribution.  Another form of europium gives a blue light, so when used with a green phosphor (more on that later), a light that approaches the appearance of white is obtained.  Interestingly, it is used in Euro notes as an anticounterfeiting agent because of its fluorescence.  I suspect that it is used in US notes as well, but Treasury is pretty tight lipped about security measures.

Gadolinium is sort of oddball amongst rare earth metals in that it is quite toxic, whilst most others are not very toxic.  Interestingly, it is used as a contrast agent in magnetic resonance imagining scans, but in a form that lessens its toxicity to a large degree.  Still, I see those adverts on the TeeVee for trial attorneys (is it not a hoot that the Fox “News” Channel seems to carry a disproportionate number of those, whilst they rail against trial lawyers) to contact if you suspect or imagine that you were harmed by them.  It is also used as a phosphor in some fluoroscopes because it emits in a range sensitive to the human eye and is efficient as well.  It also is used in some applications to control nuclear fission reactions since it absorbs neutrons better than any other stable element, but it is expensive compared to some others.  It is also used in MASERS (the microwave analogue of a Laser).

Terbium is the element to which I alluded a while ago about compact fluorescent lamps.  Since it has a green fluorescence, adding it to the red and blue europium phosphors gives a good approximation of white light.  It is also the green phosphor in CRT TeeVee tubes.  A very interesting use of an alloy of terbium is to turn any rigid, flat surface into a loudspeaker.  It turns out that this alloy actually changes size when exposed to a magnetic field (and when exposed to a changing magnetic field, changes size rapidly).  When attached to a flat surface, the signal from an amplifier is converted to a changing magnetic field, making the allow change size as does the field.  This change in size is expressed as audible sound if the magnetic field fluctuates at frequencies detectable by the human ear.

Wow, it is almost time to post and we still have six elements to go.  We shall finish this discussion next week, and I shall include some more background information as well.  Well, you have done it again!  You have wasted many einsteins of perfectly good photons reading this earthy piece.  And even though Lindsey Graham quits whining about having to do his job when he reads me say it, I always learn much more than I could possibly hope to teach by writing this series.  Therefore, please keep the comments, questions, corrections, and other feedback coming.  Remember, no scientific or technical issue is off topic here.  I shall stick around as long as comments merit it tonight, and shall return tomorrow after Keith’s show for Review Time.

Warmest regards,

Doc

Crossposted at Docudharma.com and at Dailykos.com