Site Preference of Cations and Structural Variation in MgAl2–xGaxO4 (0 ≤ x ≤ 2) Spinel Solid Solution

The crystal structures of MgAl_2–xGa_xO₄ (0 ≤ x ≤ 2) spinel solid solutions (x = 0.00, 0.38, 0.76, 0.96, 1.52, 2.00) were refined using ²⁷Al MAS NMR measurements and single crystal X‐ray diffraction technique. Site preferences of cations were investigated. The inversion parameter (i) of MgAl₂O₄ (i = 0.206) is slightly larger than given in previous studies. It is considered that the difference of inversion parameter is caused by not only the difference of heat treatment time but also some influence of melting with a flux. The distribution of Ga³⁺ is little affected by a change of the temperature from 1473 K to 973 K. The degree of order‐disorder of Mg²⁺ or Al³⁺ between the fourfold‐ and sixfold‐coordinated sites is almost constant against Ga³⁺ content (x) in the solid solution. A compositional variable of the Ga/(Mg + Ga) ratio in the sixfold‐coordinated site has a constant value through the whole compositional range: the ratio is not influenced by the occupancy of Al³⁺. The occupancy of Al³⁺ is independent of the occupancy of Ga³⁺, though it depends on the occupancy of Mg²⁺ according to thermal history. The local bond lengths were estimated from the refined data of solid solutions. The local bond length between specific cation and oxygen corresponds with that expected from the effective ionic radii except local Al–O bond length in the fourfold‐coordinated site and local Mg–O bond length in the sixfold‐coordinated site. The local Al–O bond length in the fourfold‐coordinated site (1.92 Å) is about 0.15 Å longer than the expected bond length. This difference is induced by a difference in site symmetry of the fourfold‐coordinated site. The nature that Al³⁺ in spinel structure occupies mainly the sixfold‐coordinated site arises from the character of Al³⁺ itself. The local Mg–O bond length in the sixfold‐coordinated site (2.03 Å) is about 0.07 Å shorter than the expected one. Difference Fourier synthesis for MgGa₂O₄ shows a residual electron density peak of about 0.17 e/ų in height on the center of (Ga_0.59 Mg_0.41)–O bond. This peak indicates the covalent bonding nature of Ga–O bond on the sixfold‐coordinated site in the spinel structure.     

Crystal Structure of the High Pressure Phase(II) in CuGeO3

Crystal structures of the ambient pressure and temperature phase (phase I) and high pressure phase (phase II) in CuGeO₃ were studied by means of the high pressure single‐crystal X‐ray diffraction method in a diamond anvil cell using high power X‐ray generator and imaging plate detector. The pressure dependence of the atomic displacements in the phase I was investigated under the hydrostatic pressure of 0.1 MPa and 2.9 and 3.9 GPa. The lattice is particularly compressive in the b direction. In phase I the rippled layers are formed by the corner‐shared chains of GeO₄ tetrahedra and edge‐linked planar CuO₄. Major effects of pressure, directly related to the shortening of the b‐axis, consist of an enhanced folding of the rippled layers towards the b‐direction and of a shortening of the weak Cu–O bond. The crystal structure of phase II is monoclinic, a = 4.935(57), b = 6.754(14), c = 6.208(11) Å, β = 92.67(3)°, space group; P2₁/c. The transition from phase I to II involves a corrugated arrangement of the both cation with some oxygens around the c‐axis. Ge ion at the transition point of 6.4 GPa changes its coordination number from four‐fold to five‐fold, and Cu ion occupies a position of seven‐fold site. The structure of phase II is explained as a slab structure having unique edge‐ and corner‐sharing arrangements of GeO₅ and CuO₇ polyhedra. The average Ge–O and Cu–O distances in phase II is 1.92 and 2.17 Å, respectively, at 6.5 GPa.     

One‐Pot Synthesis of Novel Branched Polylactide Through the Copolymerization of Lactide with Mevalonolactone

Polylactide, which is a biodegradable and bio‐absorbable polymer having low immunogenicity and good biocompatibility, has been mainly studied for biomedical applications. Branched polymers have different rheological and mechanical properties compared with their linear counterparts owing to their molecular architectures. We synthesized novel biodegradable polylactide having a branched structure composed of metabolically degradable and/or absorbable materials only. The branched polylactide was obtained from a one‐pot copolymerization of L ‐lactide using metabolic intermediate dl ‐mevalonolactone as a bifunctional comonomer having both lactone ring and pendant hydroxy group. The glass transition point, melting point and crystallinity of the branched polylactide are lower than those of linear polylactide.