A Center Controller reported an aircraft experiencing icing descended below the Minimum IFR Altitude.

Narrative

Aircraft X is a C206 that called for a pop-up IFR north of ZZZ just inside ZZZ [Center] airspace. After coordination an IFR clearance was given. After a few minutes Aircraft X was at 110 asked for lower; I advised I was unable for their routing due to terrain; but could give a vector towards lower terrain. Aircraft X requested the vector and it was given. After another few mins Aircraft X asked again for lower; now stating they were encountering icing. By this time they had entered an MIA (Minimum IFR Altitude) of 105; so I issued a decent to 105. Before exiting the 105 MIA Aircraft X descended to 102. I issued a low altitude alert; but forgot to say 'check your altitude immediately'; gave the MIA and when they could expect lower (90 seconds). Aircraft X acknowledged; briefly showed level at 105 before again showing 102. As soon as they were in the next MIA of 100 I issued 100 and then 090 a minute later. Aircraft X was able to exit icing conditions; canceled IFR and remained for flight following with no further issue.I have reviewed and practiced the low altitude alert phraseology for next time. There is a memory aid card at the sector; but its location varies from sector to sector so I did not spend the time to look for it until after the event. A standard location at our facility could be useful.When the aircraft requested lower I believe I did an adequate job explaining how lower could be available and when they could expect it; but always something to keep in mind and improve upon.