Crystals in the frame.
The dilithium articulation frame holds the dilithium in alignment with the colliding matter/antimatter stream and is housed inside the matter-antimatter reaction assembly. The frame is housed behind the dilithium chamber hatch. (TNG: "The Drumhead")
Doctor Leah Brahms inspected the dilithium articulation frame of the USS Enterprise-D in 2367 and discovered that Geordi La Forge had modified the articulation frame. (TNG: "Galaxy's Child")
The frame suspends the dilithum in the reaction chamber using a field effect to keep the crystal faces in alignment with the matter/antimatter streams. The correct alignment of the crystal faces and planes maximizes the efficiency of the plasma conversion that takes place. By the 2360s, the theta-matrix compositor was integrated with the articulation frame, allowing recrystallization to take place in the warp core with the core online. (TNG:"Relics")