M Hagen

A neutron diffraction study of the order - disorder phase transition in sodium nitrate

The temperature dependence of the critical scattering above and the order parameter below have been measured for sodium nitrate using time-of flight neutron diffraction. Sodium nitrate undergoes a structural phase transition from a low-temperature rhombohedral structure to a high-temperature structure which is characterized by the disappearance of superlattice reflections at positions corresponding to the Z point of the high-temperature Brillouin zone. In the critical region below the order parameter displays a crossover behaviour. For temperatures the temperature dependence of the order parameter can be described by a power law in the effective reduced temperature with and an effective critical temperature . However, for temperatures the temperature dependence of the order parameter follows a power law in the reduced temperature with . At temperatures the critical scattering takes the usual form of a Lorentzian lineshape, and the correlation length and susceptibility can be described by power-law equations in the reduced temperature with the exponents and respectively. There is a second Lorentzian lineshape in the diffraction pattern above which is much wider than the Lorentzian describing the critical scattering and is essentially independent of temperature.

Inelastic neutron scattering, phonon softening, and the phase transition in sodium nitrate,

Neutron scattering measurements have been performed on sodium nitrate at temperatures between 16 and 553 K, using both time-of-flight and triple-axis spectrometers, focusing on the phonon modes with wavevectors between the Brillouin zone centre and the F zone boundary point. At the F point there is a significant softening in energy of a transverse acoustic phonon branch on heating. Unlike the case of the related material calcite, there is no continuum of inelastic scattering localized in reciprocal space about the F point, and with a strong temperature dependence. Instead we find a component of inelastic scattering that is broad in both energy and wavevector, and with only a weak dependence on temperature.

Pulsed neutron diffraction study of Zr-rich PZT

Pulsed neutron diffraction investigations have been performed in the ferroelectric PZT system (PbZr1-xTixO3) doped with 1 wt% of Nb2O5, as a function of both temperature and composition. The study has been made in a wide range of temperatures encompassing the three known phases in Zr-rich PZT: ferroelectric low temperature (R3c), ferroelectric high temperature (R3m) and paraelectric (Pm3m). The combination of the temperature and the composition dependence of the structural parameters allowed the determination of the special relationship, recently pointed out by Corker et al, between the octahedral strain () and the tilt angle () in the ferroelectric low temperature phase (FL) of PZT. The strain-tilt coupling coefficient has been found to decrease linearly with Ti content and the composition at which and decouple has been found to be x0.30.

Orientational order-disorder phase transition in calcite

Abstract We have performed inelastic neutron scattering experiments on calcite in the ordered phase, focusing on a zone boundary F-point that shows X-ray diffuse scattering. At this position we have observed an incipient soft mode along with a column of inelastic scattering ranging in energy between the soft mode energy and zero energy. The intensity of this scattering varies dramatically with temperature, and is described by an Arrhenius relation with an activation energy of 1035 K.

Time-resolved studies on a millisecond time scale by elastic neutron scattering: transient properties of the ferroelectric phase transition in Rb2ZnCl4

Abstract Using time-resolved neutron scattering, the evolution of the phase transition between the paraelectric incommensurate (IC) phase and the ferroelectric commensurate (C) phase in Rb 2 ZnCl 4 has been observed on a microscopic scale. The transformation was driven by a strong rapidly cycling electric field of 8 kV/cm applied along the ferroelectric a -axis of the crystal. The response of the sample was monitored by measuring satellite reflections corresponding to the IC- and the C-phases. Synchronization of the electric field to the time structure of the ISIS pulsed neutron source allowed us to observe field-induced structural changes on a time scale of milliseconds . Characteristic relaxation times were found to depend strongly on temperature.

Anomalous dynamical effects in calcite

We present the results of an inelastic neutron scattering study of calcite. A continuum of excitations is observed, coupled with a soft phonon mode. Similar excitation spectra have been found in a number of other materials, due to electronic effects. However, calcite is an insulator, and we suggest that our results can instead be explained by a rather unexpected solution of the conventional picture that arises from coupling between phonon and relaxation modes.

A novel double-crystal analyser system for the PRISMA spectrometer at ISIS

Abstract The crystal analyser spectrometer PRISMA has proven to be particularly successful at surveying extended areas of (κ, e) space, efficiently collecting high-quality inelastic and critical scattering data sets from single-crystal samples. In the inelastic mode, mechanical constraints upon the 16 analyser-detector arms limit the final energy to Ef > 12 meV. To overcome this constraint, a novel double-crystal analyser system has recently been installed on the PRISMA spectrometer. Neutrons scattered from the sample are analysed by five groups of paired pyrolytic graphite analysers allowing final energies 2.25 meV

A study of the magnetic critical scattering from by time-of-flight neutron diffraction

The temperature dependences of the magnetic critical scattering above and the staggered magnetization below have been measured for the antiferromagnet using time-of-flight neutron diffraction techniques. undergoes a paramagnetic to antiferromagnetic phase transition at its Neel temperature and was expected to be characterized by the universality class of the d = 3 Ising model. The transition, which occurs at , appears to be that of a continuous second-order transition. From an analysis of the critical scattering data, the exponents and have been deduced. These values are in general agreement with the theoretical values for the d = 3 Ising model of and . From the temperature dependence of the staggered magnetization below the exponent has been determined, which is also consistent with the theoretical value of for the d = 3 Ising model.

The resolution function for a pulsed source multi-analyser spectrometer

Abstract We have formulated an expression for the resolution function of a pulsed source indirect geometry multi-analyser spectrometer and tested it by comparison with inelastic measurements carried out on the PRISMA spectrometer at ISIS. Our formulation of the resolution function includes the assymetry of the incident neutron pulse.

A study of the magnetic critical scattering from the longitudinally modulated antiferromagnets thulium and erbium

The temperature dependence of the magnetic critical scattering above TN and the staggered magnetization below TN has been measured in the longitudinally modulated antiferromagnets thulium and erbium using neutron diffraction. The transitions which occur at TN=57.65+or-0.10 K in thulium and at TN=86.04+or-0.20 K the erbium appear to be continuous second-order transitions. As longitudinally modulated antiferromagnets thulium and erbium would be expected to belong to the d=3, n=2 unversality class of the d=3 XY model.