What Is the Difference Between 99.9%, 99.99% and 99.999% Gadolinium Oxide?

Gadolinium oxide (Gd2O3) is commonly sold in different purity grades such as 99.9%, 99.99% and 99.999%.

At first glance, the difference may look very small, but in some applications the impurity level can significantly affect performance.

A simple way to understand the grades is:

99.9% = 3N purity
99.99% = 4N purity
99.999% = 5N purity

The remaining percentage consists of impurities, which may include other rare earth elements, iron, calcium, silicon, sodium, aluminum and other trace elements.

For many general ceramic, research and industrial applications, 99.9% or 99.99% may already be sufficient.

Higher-purity 99.999% Gd2O3 is more commonly considered when impurity control is especially important, for example in:

- Optical materials
- High-purity ceramics
- Thin films
- Electronic materials
- Specialized phosphors
- Research requiring strict impurity control

However, higher purity does not automatically mean better performance in every application.

Other specifications can be equally important:

- Particle size
- D50 / D90
- BET surface area
- Crystal phase
- Moisture
- Loss on ignition
- Individual rare earth impurities
- Fe, Si, Ca and other non-rare-earth impurities

For buyers, it is often better to ask for a detailed COA rather than looking only at the headline purity percentage.

For example, two products both labeled "99.99% Gd2O3" may still have very different impurity profiles and particle-size distributions.

Question for the community:

Which purity grade do you normally use, and which impurity limits matter most in your application?