Dear Frédéric Lacour,

You may look at the relevant info below:

Article Production of high purity TeO2 single crystals for the study...

https://www.newport.com/n/how-to-clean-optics

https://worldwidescience.org/topicpages/t/teo2+single+crystals.html

https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.1028.696&rep=rep1&type=pdf

Dear Frédéric Lacour , TeO2 is fairly insoluble in many solvents, so if your AR coating can withstand the solvent you could use a resist resin for protecting the optical faces while treating the orthogonal ones and when finish these processes you could remove the resin with the solvent. Alternatively you could use some thermoplastic or a wax to protect the optical faces and once finished the work on the orthogonal faces, remove the wax or polymer by heating. May be some tape would do the work too. All these protective systems will be only useful depending on the orthoganl faces treatment conditions.

Other sort of treatments with ceramic slurries could be applied as well, and removed at the end by mechanical or chemical means at the end while the AR coat allows it.

Hope this helps. Good luck with your work and best wishes!

Dear Frédéric Lacour as already mentioned by Manuel Gómez tellurium dioxide (TeO2) is insoluble in water and virtually all organic solvents. Thus, depending on the size of your crystals you could use something like acrylic varnish to protect individual crystals faces. Just use a varnish which is easily removed by dissolving e.g. in aceton or another volatile organic solvents.

Good luck with your work!

Dear Len, Manuel and Frank,

Thank you for your answers that confirm that I was thinking in the good way. Nevertheless, if you have some more specific advices or experiences, don't hesitate to share.