Terutoshi Sakakura

Determination of the local structure of a cage with an oxygen ion in Ca12Al14O33

The crystal structure of mayenite (12CaO·7Al(2)O(3)) has been investigated by single-crystal synchrotron diffraction with high resolution and accuracy, using a four-circle diffractometer equipped with an avalanche photodiode detector (APD) detector installed at PF14A in Tsukuba, Japan. Analysis revealed random displacements of ions by the electrostatic force of the O(2-) ion (O3) clathrated in two out of 12 cages. O3 ions are located at general positions close to the \bar 4 site at the centre of each cage. The difference-density map revealed two large peaks corresponding to displaced Ca ions. The positive ions close to O3 are displaced and one-to-one correspondence was found between one of the four equivalent O3 ions and the displaced ions. When an O3 ion is present in the cage the Al ion at the 16c position moves 0.946 (3) Å toward the O3 ion. One of the Al-O bonds is broken and a new Al-O3 bond is created. The result is an AlO(4) tetrahedron that is quite deformed. The three O1 ions and the O2 ion of the destroyed AlO(4) tetrahedron are forcibly displaced. O1 and O2 have two and one displaced ions, respectively. The local structure of the cage occupied by one of the four O3 ions was determined by calculating the electrostatic potential and electric field in the deformed cage, although the position of one of the displaced O1 ions was not clearly identified.

Structural evolution of FeCO3 through decarbonation at elevated temperatures

The structural evolution of siderite (s), FeCO3, through decarbonation at elevated temperatures has been investigated by the in-situ single-crystal X-ray diffraction technique using an area detector. When the crystal was heated above 255 °C, the transparent crystal turned colour in faint black from surface, indicating that the decarbonation commenced. The spinel-type magnetite (m), Fe3O4, first appeared in coexistence with the FeCO3 parent crystal. The orientation relationship between the rhombohedral FeCO3 and the cubic Fe3O4 can be described as [111]m // [001]s, and [2-1-1]m // [120]s. On further heating, additional diffraction spots appeared at 411 °C. They were indexed on the basis of the corundum-type hematite (h), (α-Fe2O3). The rhombohedral α-Fe2O3 unit cell had the same orientation relationship with the parent rhombohedral FeCO3 one, i.e., [001]h // [001]s, and [100]h // [100]s. On further heating the parent FeCO3 phase disappeared completely at 464 °C. The formation of iron oxides in FeCO3 depended on not only temperature but also the holding time. The structural relationships among FeCO3, Fe3O4, and α-Fe2O3 are discussed.

Structural phase transitions in KNbO3 and Na0.5K0.5NbO3

The in-situ high-temperature single-crystal X-ray diffraction experiments on KNbO3 (KN) and Na0.5K0.5NbO3 (NKN) revealed a series of structural evolution associated with two transitions among orthorhombic, tetragonal and cubic forms. Upon heating, the pseudocubic unit cell volume collapsed discontinuously at these transition points, whereas the reverse took place upon cooling, indicating that both transitions were of the first-order. The coordination numbers of Nb showed a decreasing tendency with decreasing temperature, i.e., 6 in cubic, 5+1 in tetragonal, and 4+2 in orthorhombic. The difference in ionic size between Na and K gave the Na atom an attribute of rattling in the cavity surrounded by corner-sharing NbO6 octahedra in NKN. The small Na not only vibrated with large amplitude, but also shifted further from the centre of the cavity than K. The coordination number of Na was always small compared with K at all temperatures, and showed an decreasing tendency upon cooling, i.e., 8+4 in tetragonal and cubic, and 7+5 in orthorhombic. The structural phase transition of KN occurred in a similar way as NKN, though the transition temperatures were shifted slightly toward the high temperature side.

Apatite-type SrPr4(SiO4)3O

Single crystals of the title compound, strontium tetrapraseodymium tris(silicate) oxide, SrPr4(SiO4)3O, have been grown by the self-flux method using SrCl2. The structure is isotypic with the apatite supergroup family having the generic formula IX M12 VII M23(IV TO4)3 X, where M = alkaline earth and rare earth metals, T = Si and X = O. The M1 site (3.. symmetry) is occupied by Pr and Sr atoms with almost even proportions and is surrounded by nine O atoms forming a tricapped trigonal prism. The M2 site (m.. symmetry) is almost exclusively occupied by Pr and surrounded by seven O atoms, forming a distorted pentagonal bipyramid. The Si atom (m.. symmetry) is surrounded by two O (m.. symmetry) and two O atoms in general positions, forming an isolated SiO4 tetrahedron. Another O atom at the inversion centre (.. symmetry) is surrounded by three M2 sites, forming an equilateral triangle perpendicular to the c axis.

A general expression of the polarization factor for multi-diffraction processes.

A general expression of the polarization factor of multi-diffracted beams is formulated. By assigning the diffracted beam direction of each diffraction process as the y axis of a Cartesian coordinate system, the polarization factor of multi-diffraction processes can be easily calculated for polarized and unpolarized beams without being limited by the number of diffraction processes. The method can be applied to processes with more than three scattering events such as multiple diffraction and extinction.

Filled skutterudite structure of europium ruthenium polyphosphide, EuRu4P12

The crystal structure of EuRu4P12 is isotypic with filled skutterudite structures of rare earth transition metal polyphosphides: RFe4P12 (R = Ce, Pr, Nd, Sm and Eu), RRu4P12 (R = La, Ce, Pr and Nd) and ROs4P12 (R = La, Ce, Pr and Nd). The Ru cation is coordinated by six P anions in a distorted octahedral manner. The partially occupied Eu position (site occupancy 0.97) is enclosed by a cage formed by the corner-shared framework of the eight RuP6 octahedra.

(CaxNd11-x)Ru4O24 (x = 4.175)

Single crystals of the title compound, calcium neodymium ruthenate, (CaxNd11-x)Ru4O24 (x = 4.175), have been grown by the flux method. The structure consists of two crystallographically independent RuO6 octahedra, which are isolated from each other and embedded in a matrix composed of the Ca and Nd atoms. There are seven M sites which accommodate the Ca and Nd atoms with different populations. Four M sites at general positions are enriched with Nd, whereas the remaining three M sites on twofold rotation axes are enriched with Ca. The coordination numbers of O atoms to the M sites range from 6 to 9. The mean oxidation state of Ru was estimated at +4.79 from the composition analysis. The title compound is non-centrosymmetric and potentially multiferroic.

Tripraseodymium penta­iron(III) dodeca­oxide, Pr3Fe5O12: a synchrotron radiation study

The title compound, pentairon tripraseodymium dodecaoxide (PrIG), has an iron garnet structure. There are two Fe site symmetries. One of the Fe atoms is coordinated by six O atoms, forming a slightly distorted octahedron, and has site symmetry. The other Fe atom is coordinated by four O atoms, forming a slightly distorted tetrahedron, and has site symmetry. FeO6 octahedra and FeO4 tetrahedra are linked together by corners. The Pr atom is coordinated by eight O atoms, forming a distorted dodecahedron, and has 222 site symmetry. The O atoms occupy the general positions.

Trineodymium(III) penta-iron(III) dodeca-oxide, Nd(3)Fe(5)O(12).

The title compound, Nd3Fe5O12 (NdIG), has an iron garnet structure. One of the Fe atoms is coordinated by six O atoms in a slightly distorted octahedral geometry and has site symmetry. The other Fe atom is coordinated by four O atoms in a slightly distorted tetrahedral geometry and has site symmetry. The FeO6 octahedron and FeO4 tetrahedron are linked together by corners. The Nd atom is coordinated by eight O atoms in a distorted dodecahedral geometry and has 222 site symmetry. The O atoms occupy general positions.

Trineodymium(III) penta­iron(III) dodeca­oxide, Nd3Fe5O12

The title compound, Nd3Fe5O12 (NdIG), has an iron garnet structure. One of the Fe atoms is coordinated by six O atoms in a slightly distorted octahedral geometry and has site symmetry. The other Fe atom is coordinated by four O atoms in a slightly distorted tetrahedral geometry and has site symmetry. The FeO6 octahedron and FeO4 tetrahedron are linked together by corners. The Nd atom is coordinated by eight O atoms in a distorted dodecahedral geometry and has 222 site symmetry. The O atoms occupy general positions.