In view of discrepancies in the available information on the hardness of lithium niobate, a systematic study of the hardness has been carried out. Measurements have been made on two pure lithium niobate crystals with different growth origins, and a Fe-doped sample. The problem of load variation of hardness is examined in detail. The true hardness of LiNbO₃ is found to be 630 ± 30 kg/mm². The Fe-doped crystal has a larger hardness of 750 ± 50 kg/mm².

Microhardness measurements were undertaken on twelve rare earth garnet crystals. In yttrium aluminium garnet and gadolinium gallium garnet, there was no measurable difference in the hardness values of pure and nominally Nd-doped crystals. The hardness values were correlated with the lattice and elastic constants. An analysis of hardness data in terms of the interatomic binding indicated a high degree of covalency.

Vickers hardness measurements have been made on polycrystalline blanks of CsCl_𝑥Br_(1–x) and single crystals of NH₄Cl_𝑥Br_(1–x). The composition dependence of hardness is highly nonlinear in both systems and follows an empirical model that includes a lattice contribution and a disorder contribution. The Gilman–Chin parameter (𝐻/𝐶₄₄) has been calculated and its significance discussed.

Vickers and knoop hardness measurements were carried out on CsBr and CsI single crystals. Polycrystalline blanks of CsCl, CsBr and CsI were prepared by melting and characterized by X-ray diffraction. Vickers hardness measurements were carried out on these blanks. The hardness values were correlated with the lattice constant and the Schottky defect formation energy.

Efforts are made to improve the hardness of rubidium halide crystals by

Systematic microhardness measurements have been made on rubidium halide mixed crystals (RbBr–RbI and KI–RbI) and rubidium halide crystals doped with Sr²⁺ ions. The composition dependence of the hardness of mixed crystals follows the law $\Delta H_V$ = $K\ x$ (1–𝑥), where $\Delta H_V$ is the enhancement in hardness, 𝐾 a constant and 𝑥 and (1 – 𝑥) the concentrations of the first and second component of the mixed crystals, respectively. The hardness of doped crystals increases with the concentration 𝐶 of the dopant according to the law, $\Delta H_V$ = $k\ C^m$, where 𝑘 and 𝑚 are constants. The relative efficacy of the two methods of hardening is discussed.