Thermal evolution of stillwellite, CeBSiO5, a natural prototype for a family of NLO-active materials

Abstract

Stillwellite, ideally CeBSiO5, is a rare mineral, however, its discovery stimulated the intense studies of a new class of stillwellite-like materials with remarkable electrical and optical properties. With the use of the modern non–ambient single crystal X-ray diffraction, this work sheds some light on the evolution of stillwellite crystal structure during the process of high-temperature transformations noted for the most of these practically important compounds. Here a sample of natural stillwellite [stillwellite-(Ce)] from the Darai-Pioz alkaline massif (Tadjikistan) has been studied by means of electron microscopy, IR spectroscopy, single crystal and powder X-ray diffraction (XRD) in a wide temperature range from −180 up to 1200 ​°C. The dielectric properties are also measured in the range from −180 up to 500 ​°C. The low (LT) and the high temperature (HT) structure deformation and transformation are described from single XRD data including the detail analysis of deformation of cationic polyhedra on heating. The disorder-to-order phase transition occurs in the stillwellite structure at about 450–500 ​°C. Both LT (disordered) and HT (ordered) modifications are characterized as relatively low expansion materials: αvolume ​= ​15 and 19 ​× ​10-6 °C-1 for LT and HT phases, respectively. The high-temperature transformation of stillwellite involves some negative thermal expansion (αvolume ​= ​−11 ​× ​10-6 ​°C-1) caused by order-disorder process and presumably by the partial oxidation of Ce3+ to Ce4+. The dielectric properties show a maximum in the region of transformation as well, but the study of dielectric hysteresis loops did not confirm the ferroelectric order of stillwellite structure at room temperature. The high-temperature behavior and the decomposition process of stillwellite are dictated by the partial oxidation of cerium.