By Ron B. Davis Jr., Georgetown University
Finding a mixture of rare earth metals on Earth isn’t such a chore. The bigger challenge is separating them from one another and purifying them. In fact, it happened more than once, that some of the greatest chemical minds of their time thought they had produced a pure sample of a new element, only to later discover that their creation was merely another mixture of the rare earth elements.
Praseodymium and Neodymium
Dimitri Mendeleev made such a mistake in his 1869 table of the elements. Back around 1842, a near-twin of lanthanum had been discovered, which was accordingly named DI-dymium. So, Mendeleev followed the best science of his day and indicated an elemental symbol of ‘Di’ for an expected element with an atomic mass of 138, just between barium and cerium. However, Mendeleev’s choice of DI-dymium to fill this slot would prove to be a double mistake.
First, the in-between element for his predicted slot was actually lanthanum. Second, it was finally discovered in 1885 that didymium, far from being an element, was actually a mixture of other rare earth metals—none of which had the predicted atomic mass 138!
An Austrian scientist cracked the separation of two of these previously hidden elements, which he named praseo-didymium (or green didymium) and neo-didymium (or new didymium). These names were later mercifully shortened to their current names, praseodymium and neodymium.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
A Group of Quintuplets
Interestingly, the didymium mixture is still used today. As it turns out, a combination of neodymium and praseodymium strongly absorbs the intense yellow light emitted when sodium is heated to high temperatures, as happens during glass blowing. Protective eyeglasses are manufactured with didymium doped into the eyeglass lenses. This is in order to protect glass blowers from eye damage that can otherwise result from the intense yellow light created by sodium in the glass.
And yet, even the separation of didymium into two elements was not the end of the story. A total of five elements were hiding in the didymium. Far from being a ‘twin’ to lanthanum, so-called didymium turned out to be a group of quintuplets.
After neodymium and praseodymium, next came samarium. Subsequently, samarium and didymium samples were found to include small amounts of gadolinium. And, to kick off the 20th century, a 5th element was found hiding in samarium that was named europium.
Discovering Rare Earth Metals
Now, before one assumes that europium must be too rare to be important, just look at any screen. It is the red emission spectrum of europium that made the breakthrough to color television possible in the 1960s. Understandably, europium has remained a cornerstone of vibrant reds in color monitors ever since.
Yttrium and terbium too help to make up the green and blue phosphors that make for full-color displays. And if we turn our device around, we would probably be looking at lanthanum! Lanthanum oxide is also used in the lenses of many modern cell phones, where it improves the optical quality of our snapshots.
Discovering these rare earth metals or ‘hidden’ elements, however, was only half the battle. Cerium, for example, was discovered in 1803, but it was not prepared as a pure metal until 1875.
Misch Metals
In fact, given how difficult it was historically to separate rare-earth elements, when it’s not especially important which metals are used, some applications have simply made use of so-called ‘misch metal’ that leaves the group unseparated. The most common version of misch metal consists of 50% cerium, 25% lanthanum, and 25% from other lanthanides.
Inserting a ‘misch metal’ metal into the flame of a lantern gives off a combined emission spectrum of white light. Adding ‘misch metal’ to iron, nickel, or aluminum can make a stronger alloy. Also, an alloy that is two-thirds ‘misch metal’ and one-third iron has traditionally been used to make the lighter flint in cigarette lighters—it’s the rare earth that has the valuable but safe property of emitting sparks when scratched.
Hollywood even used to exploit this ‘scratch-n-spark’ ability for special effects in movies. However, around 1950, researchers at Iowa State pioneered an ion-exchange purification method that made purified rare earths more widely accessible. And a second method followed shortly thereafter, even more successful, which involved colliding two different streams of fluids.
Fluid Separation
The fluid separation method starts with two solvents that do not dissolve in each other—a lot like oil and water. Flow the two fluids in opposite directions, called counter currents, separates the heavier elements, which move in one direction, from the lighter one, that move in another direction. One then just has to repeat this separation step—maybe dozens of times—until they reach the desired degree of purification.
The impact of these new methods was akin to how bauxite processing opened the door for aluminum as an industrial metal, at the beginning of the 20th century. The sudden availability of two distinct techniques, capable of bulk separation, finally made the hidden metals no longer ‘rare’ for practical purposes.
Optical and Electromagnetic Properties
Chemically, all these elements are very similar. The f-subshell electrons are buried very deep within the atom’s electron cloud, while they all have an identical outer shell.
But magnetically, the huge size of the f-shell allows for an unprecedented range of optical and electromagnetic properties, thanks to the fact that magnetism, at its core, is driven by the presence of unpaired electrons.
The same types of complex interactions of unpaired d-subshell electrons that make iron, cobalt and nickel ferromagnetic also take place among f-subshell electrons in rare earth metals.
In fact, the f-subshell is a perfect environment for large numbers of unpaired electrons to accumulate.
With seven orbitals able to hold a combined total of fourteen electrons, f-subshells can have a wide variety of magnetic and spectral properties. It is the profound impact on the magnetic properties of the lanthanides that makes each rare-earth element distinctive, and often virtually irreplaceable in one modern technology or another.
Common Questions about Rare Earth Metals
It was discovered in 1885 that didymium was actually a mixture of other rare earth metals.
The most common version of a misch metal consists of 50% cerium, 25% lanthanum, and 25% from other lanthanides.
Around 1950, researchers at Iowa State pioneered an ion-exchange purification method that made purified rare earths more widely accessible.
