Origin of improvement of Q based on high symmetry accompanying Si–Al disordering in cordierite millimeter-wave ceramics

Cordierite (Mg2Al4Si5O18) is a candidate for millimetre-wave dielectrics because of the low dielectric constant ɛr of 6.19. This study has been focused on the effects of substituting Ni for Mg in octahedron. In the case of octahedral substitution, the quality factor Qf of the Ni-substituted analogue with single phase was improved from 39,900 to 99,110 GHz up to x = 0.1 in (Mg1−xNix)2Al4Si5O18. Though Ni-substituted cordierite is single phase up to x = 0.1, compositions substituted with more than x = 0.15 have a secondary phase. The origin of improvement of Q comes from changing to the high symmetry which is from orthorhombic to hexagonal. Here, cordierite with orthorhombic crystal system Cccm (no. 66) transforms to indialite with hexagonal crystal system P6/mcc (no. 192). It is confirmed, based on the crystal structure obtained by Rietveld method, that the configuration of the (Si4Al2)O18 ring changes to an equilateral hexagonal ring from a distorted ring according to the substitution Ni for Mg. The volumes and covalencies of SiO4 and AlO4 tetrahedra also change to similar values according to the substitution. The crystal structure shows a tendency to higher symmetry of a hexagonal ring accompanying Si–Al disordering. In this case, contribution of high symmetry to Q might be greater than that of ordering.