A CE-560XL First Officer reported 'PITCH MISTRIM' warning was received in climb. The flight crew ran the checklist; declared an emergency; and diverted to a suitable alternate.

Narrative

PIC was PF; I was PM. Enroute climbing to FL400 PITCH MISTRIM caution light and master caution illuminated. I identified then canceled the master caution. PIC took firm grip on control yoke and disconnected the AP anticipating possible pitch changes. No abnormal pitch changes occurred. He attempted to operate the electric; then manual; pitch trim controls and found both to be non-responsive. All items on the pitch MISTRIM abnormal checklist were completed. Passengers were informed that we would be diverting due to the flight control issue; to remain seated with seat belts and harnesses on and the company would be notified. The emergency was declared [an airport] had been determined to be a good diversionary airport. The abnormal checklist for Jammed Elevator Trim was initiated. While PIC continued to fly. I handled communications and sent a message through to Dispatch informing them that an emergency involving flight controls had been declared and we were diverting. I pulled charts; got the ATIS; set up the FMS and radios for [the] runway and calculated landing distance for Flaps 15 per the checklist. Through 10;000 feet PIC indicated that the trim seemed to be functioning normally. Passengers were briefed again including time to landing; that we expected a normal landing and roll out; to remain seated with belts fastened and that emergency vehicles would be standing by. Weather was VMC; day and clear. We turned onto a long final and completed the Jammed Elevator Trim checklist; then performed a Flaps 15 landing with no further issues.