Makio Ohkawa

Phase relation of Na1−xKxMgF3 (0 ≤ x ≤ 1) perovskite-type solid-solutions

Na{sub 1-x}K{sub x}MgF{sub 3} (0{<=}x{<=}1) solid-solutions were synthesized and the phase diagram of NaMgF{sub 3}-KMgF{sub 3} system was determined by high temperature X-ray powder diffraction experiments and differential thermal analysis (DTA). This system is characterized by a complete series of solid solutions, with a minimum in the solidus at 30 mol% KMgF{sub 3} and 1279 K. No immiscibility gap was found. The crystal system changes from orthorhombic to cubic at x=0.22 at room temperature. The volume change of the unit cell as a function of composition shows a large deviation (excess volume) from Vegards law for solid solution. The transition temperatures of NaMgF{sub 3} and Na{sub 0.9}K{sub 0.1}MgF{sub 3} from orthorhombic to cubic are 1043 and 723 K, respectively. The transition temperature decreases rapidly by the effect of replacing Na by K. Axial ratios of b/a and c/{radical}2a in orthorhombic NaMgF{sub 3} and Na{sub 0.9}K{sub 0.1}MgF{sub 3} decrease linearly with temperature toward the transition and then discontinuously changes to cubic at the transition point.

A new high-pressure strontium germanate, SrGe2O5

The Sr-Ge-O system has an earth-scientific importance as a potentially good low-pressure analog of the Ca-Si-O system, one of the major components in the constituent minerals of the Earths crust and mantle. However, it is one of the germanate systems that has not yet been fully examined in the phase relations and structural properties. The recent findings that the SrGeO3 high-pressure perovskite phase is the first Ge-based transparent electronic conductor make the Sr-Ge-O system interesting in the field of materials science. In the present study, we have revealed the existence of a new high-pressure strontium germanate, SrGe2O5. Single crystals of this compound crystallized as a co-existent phase with SrGeO3 perovskite single crystals in the sample recovered in the compression experiment of SrGeO3 pseudowollastonite conducted at 6 GPa and 1223 K. The crystal structure consists of germanium-oxygen framework layers stacked along [001], with Sr atoms located at the 12-coordinated cuboctahedral site; the layers are formed by the corner linkages between GeO6 octahedra and between GeO6 octahedra and GeO4 tetrahedra. The present SrGe2O5 is thus isostructural with the high-pressure phases of SrSi2O5 and BaGe2O5. Comparison of these three compounds leads to the conclusion that the structural responses of the GeO6 and GeO4 polyhedra to cation substitution at the Sr site are much less than that of the SrO12 cuboctahedron to cation substitution at the Ge sites. Such a difference in the structural response is closely related to the bonding nature.