What is 'autogenous pressure'?
Autogenous pressure synthesis is a form of solvothermal synthesis (also known as “hydrothermal synthesis” if the solvent is water). In most publications I could find, the two terms (“autogenous pressure” and “—thermal synthesis”) are used together, making it clear what it is: you seal your solution in a stainless steel autoclave, and heat it up. The pressure thus generated is called autogenous pressure because you don't apply it externally, but it simply derives from the closed nature of the container and the $(\rho,P,T)$ relationship for the fluid (its equation of state).