Dear Arnaud,
If you increase temperature, yes, Lithium hydride decomposes LiH -> Li+ + H-. Therefore by this way, you should INCREASE pressure by H- releasing then it's the opposite that what has been observed BTE Dan.
My bad ... you are right this time He found firstly a decrease of pressure not an increase. But we don't know how the Li was ... I mean alone or with H. From wikipedia https://en.wikipedia.org/wiki/Lithium_hydride : I quote "
LiH is produced by treating lithium metal with hydrogen gas:
2 Li + H2 → 2 LiH
This reaction is especially rapid at temperatures above 600 °C. Addition of 0.001–0.003% carbon, or/and increasing temperature or/and pressure, increases the yield up to 98% at 2-hour residence time"
The reactor was at 580°C, so if it crossed the 600°C threshold, a lot of LiH is formed from Li and H. Afterwards when the temperature increase further then LIH decomposes