Yttrium(III) Oxide, “Yttria” (Y2O3)

Formula: Y2O3

Melting Point: 2425ºC

Density: 5.01 gr/cc

Solubility: Alcohols, bases, acids, et cetera. Stable in air and water.

Yttrium(III) oxide (or Ytrium) is named Yttria (or Ytria), following the pattern of naming the most important industrial oxides with a feminine ending, just as with Alumina (Al2O3) and Silica (SiO2). It is one of the lesser-known yet important oxides, especially in metallurgy. It is probably more famous than the pure metal it comprises, and has many applications not only in metal alloys but also in cermets and ceramics. It is poisonous and can be used as a pesticide, although this is rarely the case as it is expensive and has specific uses in metallurgy.

It naturally appeared in nature as the main oxide of the metal Yttrium in the early 2000s, a metal often considered a “rare earth,” i.e., a member of the Lanthanides (although it is actually a transition metal). The first sample of Yttria was synthesized (not “discovered” since the oxide was obtained artificially) in 1794 by the Finn Johan Gadolin (from whose surname a Lanthanide we know today as Gadolinium would receive its name in his honor). Initially, it had no application beyond being the ore of the metal in question, Yttrium, until the properties of the oxide were investigated, leading to the conclusion that it had good properties, suitable for use as a dopant/alloying agent in synthetic compounds. In a way, it can be said that Yttria is much more important than pure metallic Yttrium itself, which has few uses in this form. This case is unique in the Periodic Table, besides some frequent exceptions among the Lanthanides: even Tantalite does not eclipse, in all its splendor, metallic Tantalum itself.

It is a white solid, chemically inert to air and water (though not to alcohols, acids, et cetera), very hard, and with good mechanical properties. It is never used pure, but rather as a dopant or compound in other materials that do not fall into the classification of crystals or ceramics but rather “garnets” (although they are not true garnets, they are called that to distinguish them from the rest). Such is the case of YIG (Yttrium-Iron-Garnet) or YAG (Yttrium-Aluminum-Garnet) with uses in microwave ovens or parts of high-precision acoustic transmission devices.

It is obtained synthetically (the natural form is almost always impure, as it contains other replacement metals, almost always Lanthanides) and is added as a main component in the high-pressure and heat synthesis of glassy compounds like the aforementioned “garnets” (not to be confused with jewelry garnets) used in industry for various purposes. In this case, it cannot be said to be a “dopant” as it is a fundamental part of the compound. For example, in YAG (Yttrium-Aluminum-Garnet) with formula Y3Al5O12, the initial amount of Yttria is around 59% by weight during the synthesis process. In Cubic Zirconia, a diamond substitute in jewelry, the amount is 5%, although it is considered a dopant and not a binder since its function is to fix the crystalline structure of the Zirconia, which is thermodynamically unstable. Nevertheless, 5% is a significant amount. Anyway, I don't make the rules, so if it's considered a “dopant,” I won't question or try to change that condition. It is also a dopant, perhaps more accurately described as such, in other compounds where the amount it represents is typically 0.05-0.01%, more than enough to improve the compound's performance. The general notion of Yttria is that it forms neither ceramics nor is it an alloying agent, since it is itself a compound of two elements, not just a single pure one.

It is added during the formation process to stabilize (or “fix”) the phase (crystalline structure), in the same way that Nickel and Manganese stabilize the gamma phase Austenite in steels (not necessarily, but almost always stainless steels). In Cubic Zirconia, up to 5% is used during the formation of the piece. Cubic Zirconia is not thermodynamically stable (in fact, it does not exist in nature), so it must be “doped” with Yttria. Other materials that contain Yttria as a dopant are refractory metals sold for industrial purposes requiring materials highly resistant to extreme temperatures and pressures. Yttria improves mechanical properties and increases resistance to spalling (oxidation at high temperatures). General chemical resistance is not altered for better or worse. There are two ways to add the oxide: the most frequent by far is when it is mixed in powder form with the powder of the original material; that is, Cubic Zirconia powders and Yttria powders are mixed at a ratio of 95:5 (in % by weight) and shaped by high-pressure and temperature molding in an inert atmosphere.

In “garnets” like YAG, Yttria is melted directly with other compounds, thereby forming another substance with its own crystalline structure.

The primary and pivotal use of Yttria is to “stabilize” the crystalline structure of the compounds to which it is added. In these cases, it is considered a “dopant” rather than strictly an alloying agent, although the difference between these concepts, in this context, is rather “loose.” Possibly the best way to differentiate a dopant compound from one considered an integral part of a whole (be it an alloy, cermet, composite, et cetera) is that the dopant is only added to something that is already considered a compound itself. This means, for example, that when we buy ceramic parts like Zirconia (ZrO2), the purity index of Zirconium oxide itself is not 99% but can be as low as 90%, since the rest is made up of one or more compounds that are themselves combinations. Another reason to differentiate a “dopant” from an “alloying agent” could be the quantity; more than 10% could not be considered a dopant due to its large presence by mass. Finally, it is possible that the true difference between dopant and alloying agent depends on the material itself; that is, alloying agents for metals and dopants for inorganic non-metallic compounds in general.