Katsuhiro Kusaka

Oriented graphite single-crystal inclusions in diamond

Abstract A crystallographic study of 13 specimens of diamonds with euhedral single-crystalline graphite inclusions in their centres is presented. All inclusions belong to the hexagonal graphite modification (space group P63/mmc; a0 ≈ 2.46 Å, c0 ≈ 6.70 Å) and are up to 300 μm in diameter. Comparison of the measured c-lattice parameters of the graphite crystals with lattice parameters of graphite at elevated isostatic pressures indicates remnant pressures of up to 2.6 GPa acting on the inclusions. All samples exhibit distinct orientation relations between graphite and diamond. In 12 samples the direction [001] of graphite (G[001]) approximately parallels one of the directions 〈111〉 of cubic diamond (D〈111〉). The largest deviation is about 4°. A further division of these 12 samples is: (a) The three G〈100〉 directions have angles of about 4° with three D〈110〉 directions. This orientation relation is observed in five samples. (b) The angle between the same directions is about 34° in six samples. (c) In one sample this angle is about 16°. In a 13th specimen G[001] approximately parallels one of the three D〈100〉. The deviation is of about 6°, and one of the G〈100〉 directions nearly parallels one of the D〈110〉. These orientation relations are analysed with a simplified application of the “coincidence site lattice (CSL)” concept. A 14th specimen is different to all others, as it exhibits a large (about 300 μm edge length) region with a sharp hexagonal borderline filled with a “patchwork” of tiny graphite “islands”. Several parallel lamellae of up to 5 μm thickness result in a hexagonal pyramidal form. This graphite inclusion is suggested to be protogenetic with respect to the diamond.

Commensurate structure of Ca2CoSi2O7, a new twinned orthorhombic structure

The crystal structure of the commensurate phase of Ca2CoSi2O7, dicalcium cobalt disilicate, has been derived from the modulated structure described in (3 + 2)-dimensional space. The structure is orthorhombic P21212; a = 23.510 (4), b = 23.510 (4), c = 5.025 (1) A (at 170 K), Z = 18. Since the crystal is twinned and the apparent diffraction symmetry is 4/mmm, the parameters were refined by a newly developed least-squares program for the refinement of twinned crystals. The structure is essentially similar to the known structure of the melilite group, but with regular arrangement of the bundles along [001] formed with four arrays of the sixfold coordinated Ca polyhedra and an array of CoO4 tetrahedra. The distribution of the bundles found in the present structure is different from that reported by Riester et al. [(2000), Z. Kristallogr. 215, 102–109].

Two-dimensional incommensurately modulated structure of (Sr0.13Ca0.87)2CoSi2O7 crystals

The incommensurate structure of (Sr(0.13)Ca(0.87))(2)CoSi(2)O(7) at room temperature has been determined from single-crystal X-ray diffraction data. The compound has a non-centrosymmetric tetragonal basic cell of a = 7.8743 (4) and c = 5.0417 (2) A with the space group P4;2(1)m. The refinements of the basic structure converged to R = 0.038 for 757 main reflections. The two-dimensional incommensurate structure is characterized by the wavevectors q(1) = 0.286 (3)(a* + b*) and q(2) = 0.286 (3)(-a* + b*), where a*, b* are the reciprocal lattice vectors of the basic structure. With the (3 + 2)-dimensional superspace group P(p4mg)(P4;2(1)(m)), the refinements converged to R = 0.071 for 1697 observed reflections (757 main and 940 satellite reflections). The structure is described in terms of displacement of the atoms, rotation, distortion of CoO(4) and SiO(4) tetrahedra, and the partial ordering of the Sr and Ca atoms accompanied with the modulation. Correlated evolution of these features throughout the crystal gives rise to various oxygen coordination around Ca/Sr. Comparison of the derived modulated structure to that of Ca(2)CoSi(2)O(7) clarified that the partial substitution of Ca by large alkaline-earth atoms such as Sr should decrease the distortion of the polyhedra around the cations.

Variation of large-scale regularity in modulated structures of Ca2CoSi2O7 studied by a simulation method.

The period of two-dimensional modulation in Ca2CoSi2O7 varies with temperature (q = 0.285-1/3). The change in the modulated structure with the variation of q has been clarified by the construction of the structures using various q values and the modulation amplitudes determined at 293 K [q = 0.2913 (1), tetragonal]. The features of the modulated structures are characterized by the formation of CaO6 polyhedra and the variable distribution of bundles along the c-axis, composed of four arrays of CaO6 polyhedra and an array of CoO4 tetrahedra. The formation of octagonal arrangements of the bundles is a typical feature of the structures in the incommensurate phase. Large-scale regularities with sizes much larger than the modulation wavelength are also formed in the structures.

Martensitic Transformation of Ni2.18Mn0.82Ga Single Crystal Observed by Synchrotron Radiation White X-Ray Diffraction

A Heusler-type Ni2.18Mn0.82Ga single crystal shows a shape memory effect. It makes a thermo-elastic martensitic transformation at around 340 K, which is coincident with a Curie temperature. We made a synchrotron radiation white X-ray diffraction of the single crystal by changing the temperature from 400 K to 103 K. We observed the change of Laue spots following the transformation. As a result of experiment, the single crystal shows one cubic Heusler structure at 400 K. The direction of the crystal growth is along cubic [010] direction. In the process of decreasing temperature, many tetragonal structures with small volume of different axial direction become to appear. At room temperature the transformation almost finishes and two tetragonal Heusler structures which are twin each other remain. One of them is nearly the same as the structure at the beginning room temperature. The direction of the crystal growth is [010] of this tetragonal structure. We found that the twinning plane of the tetragonal structure is (011) plane.