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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 LiNbO 3 is found to be 630 ± 30 kg/mm 2. The Fe-doped crystal has a larger hardness of 750 ± 50 kg/mm 2 .

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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.

Anderson and Nafe [J. Geophys. Res. 70, 3951 (1965)] pointed out that while log K-log V plots for a large number of crystals have a slope of −1, the plot for oxide compounds has a slope of −4. Recent data on the bulk modulus of eight oxides with a NaCl structure are examined. It is observed that the log K-log V plot for these crystals has a slope of −1 and not −4. From this plot it is estimated that these oxides have an effective ionic charge of about 0.8. The data on lead chalcogenides are also examined. It is shown that from the two widely different values of the bulk modulus of PbSe, the physically acceptable value can be sifted out from the log K-log V plot.