Provided is a high-pressure reactor suitable for a high-pressure process using supercritical ammonia grow bulk crystal of group III nitride having lateral dimension larger than 2 inches or to form various transition metal nitrides. The reactor has nutrient distributed along the reactor's longitudinal axis and seed material positioned at the reactor's inner wall and along the reactor's longitudinal axis. Nutrient diffuses through supercritical ammonia from the reactor's longitudinal axis and deposits on the seed material positioned by the reactor's inner wall. Both the nutrient and seed material are heated by the same heater. Material growth can primarily be due to material diffusion through supercritical ammonia. This configuration and methodology reduce convective movement of supercritical ammonia due to temperature differential, providing a more quiescent environment in which group III nitride or transition metal nitride is formed.