A large Goldanskii-Karayagin (G. K.) effect has been observed in the recoilless absorption measurements of the 103 eV γ ray of 153Eu in a single crystal of Eu2Ti2O7. The size of the effect is consistent with results obtained previously in a Eu2Ti2O7 powder sample. G. K. effects were observed in the Mössbauer effect of the 105 keV γ ray transition of 155Gd in Gd2Ti2O7, Gd2Sn2O7, Gd2CrSbO7, Gd2SnTiO7, Gd2RuzO7 and Gd2Ir2O7. The quadrupole interaction parameters in these compounds change drastically and linearly with the size of the unit cell. The G. K. effect was also observed in the Mössbauer spectrum of the 123 keV γ ray of 154Gd in Gd2Ti2O7. The dependence of the G. K. effect on various parameters is discussed

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ion moves in a 3-dimensional set of harmonic potentials. The zero-field-energy splitting and the dipole moment, calculated from this model, agree with the values obtained in a number of other experiments. The results further support the concept that the cm. of the OH~ is displaced from the alkali-halide lattice site.electrons, which are polarized by their interaction with the 4/ electrons of the rare-earth ions. The main object of the present measurements was to investigate the dependence of the effective magnetic field acting on the nucleus of the nonmagnetic ion on the properties of the rare-earth ion with which it forms an intermetallic compound. In order to achieve this, measurements of the effective field acting on Ir nuclei in various rareearth intermetallic RIr 2 compounds were carried out.The investigated RIi2 compounds belong to the family of the Laves phase compounds of the RM 2 type, where R is a lanthanide ion and M can be one of several possible other ions. These compounds have a cubic unit cell and the point symmetry of the rare-earth sites in them is also cubic. Bozorth et al. have carried out magnetiza-PHYSICAL REVIEW Recoilless-absorption measurements of the 73-keV y rays of Ir 193 have been carried out using an Os 193 source in metallic form and absorbers of Ir metal, Feo.99Iro.01, various rare-earth-iridium (RIr 2 ) intermetallic compounds, and a few trivalent and tetravalent Ir salts. From the measurements, the assignment of spin § to the 73-keV level was confirmed, and the ratio of the magnetic moment of this level to the magnetic moment of the ground level was found to be 3.0 ±0.1. The small value obtained for the magnetic moment of the first excited state can be explained by the core-excitation model of de Shalit. The results yielded a value of 52 _ 037j_0.06 for the E2/M1 mixing ratio of the 73-keV transition. The absolute values of the internal magnetic fields acting on the Ir nuclei in iron and in the RIr2 compounds were deduced. The dependence of these fields on the rare-earth element was found to follow approximately the predictions of the Kasuya-Yosida theory on conduction-electron polarization, though an exact fit between this theory and experiment was not obtained. The electric field gradients acting on the Ir nuclei in the hexagonal Os metal lattice, in some Ir salts, and in the various RIr 2 compounds were deduced. Isomer-shift measurements indicate that the sign of A (r 2 ), the difference in the mean-square charge radii between the first excited state and ground state of Ir 193 , is positive.

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