I agree with Rick. But 14 CFR Part 61 does spell this out…you just have to follow the trail in the various sections to the answer.

14 CFR Part 61.51(e) Logging pilot-in-command time…(3) An authorized instructor may log as pilot-in-command time all flight time while acting as an authorized instructor.

The key words are "authorized instructor".

14 CFR Part 61.1 Applicability and definitions…(b)(2) Authorized Instructor means - …(ii) A person who holds a current flight instructor certificate issued under part 61 this chapter when conducting ground training or flight training in accordance with the privileges and limitations of his or her flight instructor certificate;…

The key words in this section are "…flight training in accordance with the privileges and limitations…"

Flight instructor privileges are detailed in Part 61.193 Flight instructor privileges. "A person who holds a flight instructor certificate is authorized within the limitations of that person's flight instructor certificate and ratings to give training and endorsements that are required for and relate to:
(a) a student pilot certificate;
(b) a pilot certificate;
(c) a flight instructor certificate;
(d) a ground instructor certificate;
(e) an aircraft rating;
(f) an instrument rating;
(g) a flight review, operating privilege, or recency of experience requirement of this part;
(h) a practical test; and
(i) a knowledge test."

You may log PIC time as a flight instructor when providing flight training for the above functions.

If the reason for the flight is outside of the above list then Rick's suggestion of using the provisions of part 91.109 for a "safety pilot" is a good one.