Which field of study are you pursuing?  History?  Theory?  Computation?  Design methods? Social / Behavioral systems?  Building systems?

A doctorate is not a professional degree, and most admissions committees are not looking for architectural design skills per se.*  (Many talented applicants won't even have an architecture degree.)  Departments are looking for research and writing skills, and one of the best arrows for your quiver would be a piece of original research, clearly written, fully cited, based on both primary and secondary sources.  A revised and improved masters-level course paper could suffice, if it's very good.

* Exceptions might include Computation and/or Design Methods research, where a design background would be helpful.  But excellent documentary and writing abilities are still required.

You should also search "PhD" in the forum.  This subject has already been widely covered, and there's some good (as well as questionable) material in those threads.

Citizen, thanks for your response. My area is history / theory. I have a few solid original research projects / seminar papers which could be worked up, and I was wondering about publishing articles - but I'm not entirely clear on what would count? If it's a journal from an architecture school which solicits articles, would that work? Or would it need to be something more academic? It sounds like the more academic journals are fairly hard to be published by? Any thoughts? Thanks!

I'm not an authority on this, and I'm sure each school's admissions committee has its own preferences.  Not everyone has a publication under their belt when they apply.

Others on here have experience on this as well.  If none chime in, make sure to search the threads.  Good luck.

Have you considered any international Universities like Cambridge or Oxford?  UK universities tend to take younger students who are serious about research as a career choice for PhD.  They will not be particularly interested in your design work, but rather they will be interested in your research work and ambitions.  Built environment research is a valid career choice, although less commonly known amongst US architecture schools I'd say!