The I'8-I'7 crystalline-electric-field (CEF) transition of Ce86 has been identified near 530 K (46 meV, 372 cm ) with use of inelastic neutron and polarized Raman scattering. From the anomalous temperature behavior of the transition energy observed in Raman scattering we deduce a I 8 ground state split by 20 cin ' (30 K). The novel CEF level scheme yields a consistent and unified interpretation of so far seemingly unrelated thermal, elastic, and magnetic data.
Coherent inelastic neutron scattering provides detailed information about the correlated atomic displacements in the vibrational modes of a glass. As an example, we show the wave-vector dependence of the scattering function of vitreous Si02 corresponding to different peaks in the onephonon density of states. Pending the availability of results from computer simulations, we give a qualitative comparison with the Sen-Thorpe model. PACS numbers: 63.50.+x, 61.12.FyThe intermediate-range order in insulating and semiconducting glasses is a subject of current interest and controversy. '2 In addition to the use of structural probes like diffraction and extended x-ray absorption fine structure, dynamical information obtained from inelastic neutron scattering and Raman and ir spectroscopy is being used to test the validity of different models. 3 These spectroscopies measure essentially a one-phonon density of states, modulated by a matrix element which, except in the case of certain modes of special symmetry, '2 is assumed to have a regular variation over the frequency range investigated.Since the density of states is essentially a statistical quantity, representing the relative number of vibrational modes in a particular frequency region irrespective of the types of motion involved, the information obtained in this way is quite limited and often inadequate to provide precise identification of particular modes. For example, the high-frequency doublet in Si02 at fee=130 -150 meV (1 meV=8 cm ') has been interpreted as either an optical mode split by the macroscopic electric field3 4 or as "bulk" and "surface" Si-0 stretching modes associated with clusters in the glass. 2 The purpose of this Letter is to show that accurate coherent inelastic neutron-scattering (CINS) measurements provide information about the correlated atomic displacements in modes at a given frequency and hence offer the opportunity to make precise tests of different models for the intermediate range structure and dynamics. CINS measures the scattering function in terms of the wave-vector transfer Q and the energy transfer Ewhich in the conventional harmonic phonon expansion can be written S(g E) S(0) +S(1) +S(m) The first term in Eq. (1) represents the elastic scattering: S 0 (Q,E) =S,)(Q)i)(E) = -g 2 exp[ -( W;+ W)J]e px[ig (R; -R~)], IJwhere W; is proportional to the mean-square displacement (u; ) about the equilibrium site R;, and b, is the scattering length of atom i The second term represents the one-phonon scattering which on the neutron-energyloss side is given by S(')(Q, E) =
The instrumental optimization conditions for most small-angle scattering experiments in which the data are azimuthally symmetric require that the scattered flight path be equal to the incident flight path. This is in contrast to a recent analysis which shows that under some conditions the incident and scattered flight paths are in a ratio of two to one. The equal flight-path condition is also valid for experiments measuring sharp (Bragg-like) peaks, or where the intensity is required at specific scattering vectors, as in low-angle diffraction of ordered or semiordered systems. The implications of the optimization conditions on the resolution and count rates at the detector are discussed for both types of experiment, and the dependence of the resolution on the spectrometer geometry is considered.
The design, development and performance of the time‐of‐flight (TOF) small‐angle diffractometer (SAD) at the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory are described. Similar TOF‐SANS instruments are in operation at the pulsed neutron sources at Los Alamos National Laboratory, USA, at Rutherford Appleton Laboratory, England, and at KEK, Japan. These instruments have an advantage by comparison with their steady‐state counterparts in that a relatively wide range of momentum transfer (q) can be monitored in a single experiment without the need to alter the collimation or the sample‐to‐detector distance. This feature makes SANS experiments easy and very effective for studying systems such as those undergoing phase transitions under different conditions, samples that cannot be easily reproduced for repetitive experiments, and systems under high temperature, pressure or shear. Three standard samples are used to demonstrate that the quality of the SANS data from SAD is comparable with those from other established steady‐state SANS facilities. Two examples are given to illustrate that the wide q region accessible in a single measurement at SAD is very effective for following the time‐dependent phase transitions in paraffins and temperature‐ and pressure‐dependent phase transitions in model biomembranes.
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