Electron-Paramagnetic-Resonance Absorption by Trivalent Neodymium Ions in Single Crystals of Lanthanum Trichloride and Lanthanum Ethyl Sulphate in Zero Magnetic Field

Measurements of magnetic-dipole transition frequencies of ${}_{}{}^{143,145}{}_{}{}^{}\mathrm{Nd}_{}^{3+}{}_{}{}^{}$ occurring substitutionally for ${\mathrm{La}}^{3+}$ in La${\mathrm{Cl}}_3$ were made for the first time in the absence of an external magnetic field. The sensitive [${10}^{13}$ spins/ (Mc/sec linewidth)], versatile, cavity-type, variable-frequency (1000-4600 Mc/sec) spectrometer developed for these experiments is described. Measurements of magnetic-dipole transition frequencies of ${}_{}{}^{143,145}{}_{}{}^{}\mathrm{Nd}_{}^{+3\mathrm{}}{}_{}{}^{}$ in La${\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{S}{\mathrm{O}}_4\right)}_3$·9${\mathrm{H}}_2$O made at low field by Bleaney were repeated at zero magnetic field. The results are summarized in terms of a least-squares best fit of the spin Hamiltonian $H=A{S}_z{I}_z+B(S_x{I}_x+S_y{I}_y)+P[{I_z}^2-\frac{1}{3}I(I+1\mathrm{}\left)\right]$ to the data. The spin-Hamiltonian parameters are independent of temperature between 4° and 15°K. Theoretical interpretation of the spin-Hamiltonian parameters yielded $\frac{{}_{}{}^{143}{}_{}{}^{}\mu _N^{}{}_{}{}^{}}{{}_{}{}^{145}{}_{}{}^{}\mu _N^{}{}_{}{}^{}}=1.60892\mathrm{}\pm 0.00010$, ${}_{}{}^{143}{}_{}{}^{}g_n^{}{}_{}{}^{}〈r^{-3\mathrm{}}〉=(11.59\mathrm{}\pm 0.10)\times {10}^{24}$ ${\mathrm{cm}}^{-3\mathrm{}}$, and ${}_{}{}^{143}{}_{}{}^{}\mu _N^{}{}_{}{}^{}=(1.10\mathrm{}\pm 0.10)$ nm for the La${\mathrm{Cl}}_3$ crystal; and $\frac{{}_{}{}^{143}{}_{}{}^{}\mu _N^{}{}_{}{}^{}}{{}_{}{}^{145}{}_{}{}^{}\mu _N^{}{}_{}{}^{}}=1.60886\mathrm{}\pm 0.00029$, ${}_{}{}^{143}{}_{}{}^{}g_n^{}{}_{}{}^{}〈r^{-3\mathrm{}}〉=(11.56\mathrm{}\pm 0.10)\times {10}^{24}$ ${\mathrm{cm}}^{-3\mathrm{}}$ and ${}_{}{}^{143}{}_{}{}^{}\mu _N^{}{}_{}{}^{}=(1.10\mathrm{}\pm 0.10)$ nm for the La ${\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{S}{\mathrm{O}}_4\right)}_3$·9${\mathrm{H}}_2$O crystal assuming $〈r^{-3\mathrm{}}〉=(36.9\mathrm{}\pm 4.5)\times {10}^{24}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The close agreement of $g_n〈r^{-3\mathrm{}}〉$ for the two crystals implies that $〈r^{-3\mathrm{}}〉$ is independent of the host for these two crystals.