Selective generation of trigonal (C3&ngr;) symmetry at Er3+ions in CdF2by oxygen firing increases the green (4S3/2→4I15/2) radiative efficiency from 1.9 to 52% while causing qualitative changes in the optical absorption and emission associated with these ions. Measurements of the parameters involved in the operation of an infrared quantum counter suggest that this oxygen‐fired material could be used to construct an uncooled infrared detector having a minimum detectable power of 3.6×10−14W at 1.53 &mgr;m, appreciably better than currently available detectors sensitive at this wavelength.