Currently available materials such as ZnS or ZnSe which have excellent optical properties in the 8-12 μm spectral band have been shown to be susceptible to rain impact damage. In recent years interest has been shown in a family of rare earth ternary compounds which offer the potential of improved thermal and mechanical properties while retaining similar transmission characteristics to ZnS and ZnSe. An examination has been carried out of the densification of SrLa2S4, CaLa2S4, MgSc2S4, CdEr2S4 and La2S3 using a hot pressing technique. The phase composition of the products has been studied using X-ray diffraction and Raman spectroscopy whilst the optical properties have been assessed by transmission spectrophotometry. The measured hardnesses of these rare earth sulphides are significantly in excess of those of other wide-band gap optical materials such as ZnS and ZnSe, although the values of coefficients of expansion are greater. The full value of these new emerging materials cannot be determined until improved material has been fabricated for measurement of other properties such as rupture modulus, Young's modulus and thermal conductivity. But the potential improvement offered by these materials in comparison to impurity hardened ZnS and ZnSe is discussed on the basis of currently available data.
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