The word "diaconicon" refers to a room in a church where liturgical objects and vestments are stored. It's a relatively obscure term, so it's understandable that you may not be familiar with its synonyms. Some possible alternatives include "vestry," "sacristy," or "sanctuary." These terms all refer to the same basic concept of a designated space within a church for storing religious items. Depending on the specific denomination or faith tradition, there may be nuances of meaning that differentiate these terms from one another, but in general they are interchangeable. If you're ever unsure what term to use, "diaconicon" is perhaps the most specific, but any of these synonyms will likely be understood by your audience.