tracer diffusion
How do you watch a single kind of atom wander through a solid when the solid is already full of identical atoms? You give a few of them a name-tag. Tracer diffusion is the experiment (and the coefficient it measures) in which a small amount of a labelled species — usually a radioactive or a rare stable isotope of an atom already in the crystal — is deposited on the surface and allowed to diffuse in. Because the tracer is chemically identical to its host, it explores exactly the same jumps, but you can now find where it went by measuring the isotope's profile.
In practice you paint a thin layer of the isotope on a polished face, anneal at temperature for a known time, then slice off successive layers and count the tracer in each. The concentration falls off with depth in a shape (a Gaussian) set by Fick's second law, and its width gives sqrt(D times t), from which the tracer diffusion coefficient D is read straight off. Because the tracer is a dilute label riding a lattice at chemical equilibrium — no concentration gradient of the real species, no chemical driving force — tracer diffusion measures self-diffusion cleanly, the pure atomic hopping rate, uncontaminated by the thermodynamic factors that complicate a chemical interdiffusion measurement.
Tracer coefficients are the gold-standard data for the atomic mechanisms: measured separately for the cation and the anion (by using an isotope of each), they reveal which ion is intrinsically faster and pin down activation energies for the vacancy and interstitial routes. The one honest subtlety worth knowing is the correlation factor: a tracer atom moving by the vacancy mechanism is slightly more likely to jump back where it came from (the vacancy it just used is still sitting there), so its measured diffusion is a known fraction — the correlation factor, less than one — of the naive rate. That small, well-understood correction is itself a fingerprint of the mechanism.
To learn how oxygen moves in zirconia, researchers expose a crystal to oxygen-18 gas or paint on an 18-O layer, anneal, then profile the isotope with depth; the Gaussian tail gives the oxygen tracer diffusion coefficient — the clean self-diffusion rate of oxygen, cation chemistry aside.
Tracer diffusion: label a few atoms with an isotope and follow them into the solid; the depth profile gives the pure self-diffusion coefficient, one species at a time.
A tracer coefficient is not identical to the chemical interdiffusion coefficient. Tracer diffusion measures pure atomic hopping at equilibrium (times a correlation factor below one), whereas an interdiffusion coefficient also carries a thermodynamic factor from the real concentration gradient.