No abstract

Determination of the crystal structure of Nd2Fe14B, a new ternary phase, is reported. It has recently been demonstrated that permanent magnets having large co ercivities and energy products can be formed from this phase, underscoring its potential technological impor tance. We relate the crystal structure and intrinsic mag netic properties by considering analogies with previously known rare earth transition metal materials.

New neutron-diffraction data collected at 200 K with deuterated p-terphenyl, and published X-ray data collected at room temperature with protonated p-terphenyl, have been used to determine the characteristics of the double-well potential for the librations of the central ring about the long molecular axis, in the disordered phase. A picture of the double well is obtained simply by halving the atoms outside the molecular axis and imposing constraints on their geometries and thermal parameters. The R values compared with that obtained with the simple-well model, are significantly improved. The deuterated benzene rings, compared to the protonated ones, show a systematic and hard-to-explain distortion: all the internuclear C-C bond lengths parallel to the long axis are shortened and the others lengthened, the mean bond lengths being the same in the two compounds. The double-well potential-barrier height is found to be about 0.6 kcal mo1-1 at 200 K and at room temperature. This value is in good agreement with that given by potential-energy calculations. The rotation angle between the two wells is about 26 °. Only for the internal g mode is the thermal energy kT sufficient to bring the rings near the top of the barrier. For the central-ring librations the doubly peaked distribution function is an argument for an order-disorder regime. However, the smallness of the double-well barrier height suggests that the p-terphenyl structural phase transition is near the boundary between the order-disorder and displacive regimes.

The perovskite La1/3Sr2/3FeO3−δ was investigated by neutron diffraction, magnetic and Mössbauer spectroscopy measurements. La1/3Sr2/3FeO3−δ undergoes magnetic ordering at T = 190–200 K accompanied by charge disproportionation. Magnetic peaks due to charge ordering are observed below 200 K. The charge ordering is gradually developed below 200 K along with a charge disproportionation, 2Fe4+ ⇒ Fe3+ + Fe5+. La1/3Sr2/3FeO3−δ shows an antiferromagnetic structure at low temperature. Magnetic moments of about 3 and 1.3 μB were obtained from the neutron diffraction data refinement for Fe3+ and Fe5+ at 15 K, respectively, which suggest that both Fe ions are in a low spin state. These values are significantly lower than those reported by Battle et al for La1/3Sr2/3FeO2.98. Mössbauer spectra indicate that full charge ordering might be reached below 20 K with no Fe4+.

We have studied the crystal structure of fully deuterated BH3NH3 using powder neutron diffraction at different temperatures. It is evident that an order-disorder phase transition occurs around 225K. At low temperature, the compound crystallizes in the orthorhombic structure with space group Pnm21 and gradually transforms to a high temperature tetragonal structure with space group I4mm above 225K. At 16K, the BD3–ND3 unit stacks along the c axis with a tilt angle of about 16° between the N–B bond and the c axis. As the temperature is increased, the BD3–ND3 groups start to reorient along the c axis and the deuterium atoms become disordered, leading to the tetragonal phase transition.