NEUTRON SCATTERING FROM LANGATATE

This work used neutrons to study the structure of langatate, particularly the position of the oxygen atoms within the crystal, as a function of temperature.  Neutrons were required to resolve the oxygen positions because the relative intensity of neutron scattering off of oxygen is comparable to that of the other constituent elements (La, Ta, Ga), unlike X-rays which scatter off of the heavy elements much more intensely (X-ray scattering power scales with the number of electrons in the atom).   In addition to collecting standard diffraction spectra with many Bragg peaks (see image bottom left), we collected very high resolution data enabling us to measure the pair distribution function (PDF) which tells us about the interatomic distances for every pair of atoms (bottom right).  Data were collected at the two time-of-flight neutron sources in the United States: the Spallation Neutron Source at Oak Ridge National Laboratory (instrument: POWGEN) and the Los Alamos Neutron Science Center at Los Alamos National Laboratory (instrument: NPDF).

Click on any image to enlarge:

Neutron diffraction spectrum showing Bragg peaks corresponding to many different crystalline planes. The black markers are measured data points while the red trace shows the fit to our model using Rietveld refinement.

Example pair distribution function (PDF) of langatate generated from high resolution neutron scattering experiments.  The function G(r) provides a measure of the likelihood that any two atoms in the crystal will be separated by a given distance.  The black markers represent the experimental PDF while the red trace shows a fit to the data using Reverse Monte Carlo modeling.