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 .
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.
Lattice strains have been produced in cadmium telluride by grinding for different durations. X-ray diffractograms have been obtained at different stages of grinding. The lattice constants, particle size, lattice strain and Debye-Waller factors have been determined. From the correlation between the strain and the effective Debye-Waller factor, the Debye-Waller factor for zero strain has been estimated.
Hardness of Crystals with NaCl Structure and the Significance of the GILMAN-CHIN Parameter*)The microhardness of PbTe, PbSe, EuS and EuSe has been determined. The microhardness vs lattice constant plot for chalcogenides is a curve similar to the one for alkali halides. The Gilman-Chin parameter (HIC,,) has been calculated for a large number of crytals with NaCl structure. There is no unique value representing the entire group of crystals, the values ranging from 0.01 to 0.20.The Gilman-Chin parameter broadly correlates with the effective ionic charge (e*) indicating that it is a measure of the ionicity. Empirically, it is observed that the product (HIC,,) . (e*)' is approximately constant.
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