CRYSTALLOGRAPHIC STUDY, BY RIETVELD METHOD, OF BARIUM MAGNESIUM TANTALUM OXIDES BASED CERAMICS FOR USE AS DIELECTRIC RESONATOR - X-RAY DILATOMETRY AT LOW-TEMPERATURE

Sintering at high temperature for a long time greatly improves the die lectric loss quality Q of Ba3MgTa2O9 dielectric resonator ceramics. St arting material perovskite powder and grinded dielectric resonator cer amic of this compound have been studied by X-ray diffraction and using Rietveld's method. Sintered phase at 1550-degrees-C is completely lon g-range ordered, starting powder calcined at 1200-degrees-C is almost completely long-range disordered. The observed ordering is a well know n 1/2 ordering on the octahedral sites of some perovskites ; Mg-Ta-Ta- Mg octahedra layer sequence is observed along the [111] direction of t he pseudo-cubic cell. Electrostatic repulsion between two successive T a planes causes the extension of the cubic cell which becomes trigonal . Crystallographic symmetry lowering reduces the number of degeneracie s of the phonon spectrum and thus reduces dielectric loss. At low temp erature below 200 K, dielectric loss of Ba3MgTa2O9 ceramic increases f or decreasing temperatures in contrast with theoretical models. Dilato metric study of the cell versus temperature seems to show abnormal beh aviour of the cell parameters between 150 K and 70 K.