Nuclear Hyperfine Studies of a Dilute Europous Compound: Europium Hexammine

The M\"ossbauer effect of the 21.6-keV γ-ray transition in ¹⁵¹Eu has been used to investigate the compound Eu(NH₃)₆. Previous magnetic-susceptibility measurements had indicated that the Eu ion might not be in a 4f⁷ divalent configuration, but in a 4f⁹ or 4f⁷5d² condensed state at low temperatures. The present studies strongly support the configuration 4f⁷, ionic state ⁸S7/2. In contrast to earlier inferences, Eu(NH₃)₆ is not magnetically ordered at temperatures as low as 1.2 K. Several unsual features are found to be associated with europium hexammine. The compound has an extremely low Debye temperature of 43 K. It is the only known Eu compound which exhibits magnetic relaxation effects over a wide range of temperatures. The M\"ossbauer-resonance linewidth (corrected for sample thickness) is approximately constant at 8 mm/sec from 4.2 to 67 K. The broad linewidth indicates spin-spin relaxation and the temperature independence implies an Eu²⁺ dipole-dipole relaxation mechanism. Below 4.2 K, the linewidth increases rapidly, probably signifying changes in population of the ionic crystal-field levels. Between 67 and 77 K, the linewidth and recoilless fraction decrease significantly, suggesting a phase change which may be associated with hindered rotation at the NH₃ sites. The isomer shift, - 13.6 mm/sec at 20 K, appears to change slightly with temperature, corresponding to an increasing s-electron density at the Eu nucleus below about 10 K. From experiments in applied magnetic fields, the limiting hyperfine field is found to be approximately - 322 kOe, a value typical for 4f⁷ core polarization. The spectra obtained in moderate applied fields can be fitted very well using relaxation theory. The exchange field at the Eu ion is less 0.5 kOe. This implies that the magnetic ordering temperature is less than 0.1 K, one of the lowest for Eu₂₊ compounds and unsual because of the material's metallic conductivity. A spin-spin relaxation rate of about 50 ×{} 10⁶ sec^-1 is observed. This is in good agreement with a calculated value 88 ×{} 10⁶ sec^-1, based upon dipolar interactions and no exchange.