In1-xTlxSb is proposed as promising infrared material. A number of opt
ical and structural properties are studied within local density-functi
onal theory. The alloy at x = 0.09 is estimated to have a gap of 0.1 e
V. Although T1Sb is found to favor the CsCl structure, the zinc blende
alloy is stable for low x values. A phase diagram is calculated to es
timate the regions of stable phases and explore the conditions for gro
wing narrow-gap In1-xTlxSb alloys.