A method is provided for producing an article which is transparent to near-wave IR, mid-wave and Long-wave multi-spectral and IR wavelength in the region of 0.4 μm to 16 μm. The method includes the steps of (a) Producing ultra-fine powder of CaLa2S4 via SPLTS process, (b) followed by pretreatment of the ultra-fine powder under inert and reducing gas conditions including H2 or Argon or N2 or H2/H2S, H2S, and mixtures there of (c) followed by sieving the powder in 140 mesh screen and cold pressing the powder at 7000 psi for 7 min. into a disk shaped green body (d) then Cold-Isostatic Pressing (CIP) at 40,000 psi for 5 min in a rubber mold (e) finally sintered article of CaLa2S4 disk of 25.4 mm diameter with ultra-high density containing cubic phase of CaLa2S4 to yield IR transmission of a peak value of 57% within the IR wavelength range of 2 μm to 16 μm, either by using microwave sintering followed by hot isostatic press or spark plasma sintering followed by hot isostatic press or vacuum sintering at (3×10−6 torr) followed by hot isostatic press or hot press sintering followed by hot isostatic press and finally followed by mirror polished IR article, is obtained.