Variation of the order-disorder phase transition and of the magnetic ordering with deuterium concentration in CeD_x (2.2⩽x⩽3.0)

The crystal and magnetic structures of polycrystalline CeD_x(2.2<or=x<or=3.0) were determined in the temperature range from 1.3 to 380K by means of conventional and high-resolution time-of-flight neutron diffraction measurements. At high temperatures the structures correspond to FCC (Fm3m) symmetry, with a statistical distribution of deuterium on tetrahedral and octahedral sites (displaced along the directions (111) from the ideal position ¹/₂, ¹/₂, ¹/₂). Whereas CeD_x for x>or=2.90 remains approximately cubic down to 1.5K, second-order phase transitions to ordered structures with tetragonal symmetry (basic translations a_t approximately a_c, c_t approximately 2a_c, space group I4; in contrast to Titcomb and co-workers (1974)) occur at temperatures T_c=290(3) and 330(5)K for x=2.18 and 2.29 respectively. Between x=2.29 and x=2.96 the magnetic ordering changes from ferromagnetism (T_c=3.7(2)K, ordered magnetic moment mu (Ce³⁺)=1.1(1) mu _B at 1.3K) to FCC II antiferromagnetism (T_N=4.2(2)K, mu (Ce³⁺)=0.6(1) mu _B at 1.3K, mu //(111)).