title: A new high-pressure strontium germanate, SrGe2O5.
abstract: 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 Earth's 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 SrGeO3high-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 SrGeO3perovskite single crystals in the sample recovered in the compression experiment of SrGeO3pseudowollastonite 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 GeO6octahedra and between GeO6octahedra and GeO4tetrahedra. The present SrGe2O5is thus isostructural with the high-pressure phases of SrSi2O5and BaGe2O5. Comparison of these three compounds leads to the conclusion that the structural responses of the GeO6and GeO4polyhedra to cation substitution at the Sr site are much less than that of the SrO12cuboctahedron to cation substitution at the Ge sites. Such a difference in the structural response is closely related to the bonding nature.
authors.affiliation: Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Ube 755-8611, Japan.
authors.forename: Akihiko
authors.initials: A
authors.lastname: Nakatsuka
authors.affiliation: Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan.
authors.forename: Kazumasa
authors.initials: K
authors.lastname: Sugiyama
authors.affiliation: Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan.
authors.forename: Makio
authors.initials: M
authors.lastname: Ohkawa
authors.affiliation: Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan.
authors.forename: Osamu
authors.initials: O
authors.lastname: Ohtaka
authors.affiliation: Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Ube 755-8611, Japan.
authors.forename: Keiko
authors.initials: K
authors.lastname: Fujiwara
authors.affiliation: Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan.
authors.forename: Akira
authors.initials: A
authors.lastname: Yoshiasa
keywords: SrGe2O5; cation size; covalency; crystal structure; high-pressure phase; materials science; single-crystal X-ray diffraction; strontium germanate