Thermal stability of long range order in oxides

Abstract

Diffraction effects have been observed in many oxide phases which have led researchers to believe that unusual long range order may occur. Such diffraction effects are generally observed first as unexplained anomalies in room temperature x-ray diffraction powder patterns. Later work on single crystal x-ray diffraction may help to explain the anomalies or even cause greater confusion. Much of the confusion occurs because the diffraction effects are often non-integral or non-rational and cannot be accounted for by the classic crystallographic definitions. Single crystal neutron diffraction examination can be an important tool in oxide investigations. It can supplement the information obtained on oxygen positions from x-ray diffraction which is often inadequate due to the difference in scattering power between oxygen and the generally heavier cations. With the lack of suitable large single crystals, neutron powder diffraction total profile analyses can also be a powerful tool, providing a satisfactory structural model is available for refinement. Electron microscopy utilizing high resolution lattice images is also becoming a very important technique and is often the only way to obtain information on individual extended defects and the structural nature of grain boundaries. Although not yet widely used, it is likely that EXAFS and total profile analysis of diffraction from synchrotron radiation may someday play important roles in our understanding of these phenomena. The major problem which occurs in the investigation of long range order in oxide crystals arises from the fact that the material under investigation is often not in equilibrium or even in a metastable (quenched-ln) condition. This is compounded by the investigator's avoidance of the possible structural consequences of such non-equilibrium° For instance, random intergrowth of two or more phases at a unit cell level may cause anomalously large temperature factors in a crystallographic refinement. Poorly equilibrated phases are the rule rather than the exception in commercial and research quality ceramic oxide materials because such specimens are less expensive and time consuming to prepare (and often have interesting and useful physical properties). It is the purpose of this paper to examine some of the effects that are produced in oxides as a result of high temperature equilibration processesj how these effects are manifested by x-ray, neutron and electron diffraction and how these effects change under changing thermal conditions.