Kazuaki Iishi

Lattice dynamics of corundum

The short-range force constants and the contribution of the potential energy to the elastic constants have been calculated from explicit expressions for the elastic constants and for the optical frequencies. The large elastic constants of corundum are attributable to the contribution of the large short-range stretching forces. The frequencies of the phonon spectrum and the contribution of the potential energy to the frequencies have also been calculated based on a rigid-ion and polarizable-ion model. The effective charge of an oxygen ion is found to be −0.955e. The electronic polarizability of an aluminium and oxygen ion is found to be 0.05 and 1.36 Å3, respectively.

The modulated structure of synthetic Co-åkermanite, Ca2CoSi2O7

Diffraction patterns of synthetic Co-akermanite, Ca 2 CoSi 2 O 7 , reveal distinct satellite reflections at room temperature. An incommensurate modulation in this material has been determined by five-dimensional refinement pf the structure. The basic structure is tetragonal P42 1 m, unit-cell dimensions a=7.8417 (6), c=5.0249 (3) A, V=308.99 A 3 , Z=2, M r =307.26, D m =3.307 (4) g cm -3 at 293 K, D x =3.302 g cm -3 , Mo Kα with graphite monochromator, μ=47.7 cm -1 , F(000)=302, R=0.049 for 734 unique reflections

Serpentine minerals hydrothermally synthesized and their microstructures

Abstract Serpentine minerals were synthesized from the 2Mg 2 SiO 4 (natural olivine)+3H 2 O system under various conditions for periods of 30 min to 30 days to investigate: (1) the influence of pH and other additives, (2) the influence of the reaction temperature and (3) the behavior of the initial stage of serpentinization. Growth patterns and structures of the synthesized products were examined by means of X-ray, electron diffraction and lattice imaging electron microscopy.

Phononspectroscopy and lattice dynamical calculations of anhydrite and gypsum

Detailed experimental and theoretical studies of the k=0 vibrational spectra of anhydrite and gypsum are reported. The dielectric constants and the infrared reflection and Raman spectra of the single crystal have been measured. The frequencies of the phonon spectrum, the contribution of potential terms to potential energy and detailed mode assignments have been determined based on a polarizable-ion model. The relative experimental intensity of the spectra, the observed crystal field effects, the rotatory lattice modes of the H2O molecule and the mixed character of translatory and rotatory modes of the Ca, SO4, and H2O groups are discussed based on the theoretical vibrational modes and the potential energy distribution. The principal moments of inertia of the water molecule and the Lennard-Jones potential constants for nonbonded oxygen-oxygen interactions are presented.

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].

Preparation of low-temperature Li3VO4 single crystal by floating zone technique

Abstract Single crystals of the low temperature phase β II Li 3 VO 4 were prepared by using a floating zone technique. Three problems, micro-cracks, accumulation of bubbles and the coloration of crystals, were faced during single crystal growth. These problems were investigated in relation with the initial molten zone composition, gas atmosphere and the purity of starting materials, respectively. The obtained crack-free, colorless and transparent β II Li 3 VO 4 single crystal had a diameter of 5 mm and was 5 mm long.

Local structure of (Ca, Sr)2 (Mg, Co, Zn) Si2O7 melilite solid-solution with modulated structure

The local structure around Co, Zn and Sr atoms in incommensurately modulated, melilite-type X2T1 T22O7 (X=Ca and Sr, T1=Mg, Co and Zn, T2=Si) solid-solutions has been investigated by EXAFS analyses. The modulated structure was confirmed in Ca2-xSrxCoSi2O7 solid-solutions with X=0.0 to 0.6 and for both Ca2Mg1-YCoYSi2O7 and Ca2Mg1-YZnYSi2O7 solid-solutions over the whole compositional range at room temperature.The actual bond-distances determined by the EXAFS method for the T1 site (Co-, Zn-O) in the modulated structure are longer than the mean bond-distances obtained from the X-ray diffraction method. This is attributable to the libration of the T1 tetrahedra. In the Ca1-XSrXCoSi2O7 solid-solution both the Sr-O and Co-O distances by the EXAFS method for the X-site increase from Ca end-member to Sr end-member. These increases are respectively 0.8% and 0.6%. This means the local expansions of the tetrahedral sheets and of the XO polyhedra are well matched. In the modulated Ca2Co1-YMgYSi2O7 and Ca2Zn1-YMgYSi2O7 solid-solutions, the actual Co-O and Zn-O distances for the T1-sites are nearly constant in the whole compositional range. The compositional variations of the local structure around the cations in the solid-solution are different for the X and T1 sites.It is concluded that the local geometric restriction for the size of substituted cation in X site is larger than that in T1 site. The dimension of the tetrahedral sheet puts restriction on the size of the cations situated at the interlayer X sites. In other words, the different behavior of the local geometric restriction between the X and T1 sites is an important feature of the melilite structure and is also related to the modulated structure.

Electron microscopy studies of akermanites (Ca1−xSrx)2CoSi2O7 with modulated structure

Single crystals of akermanite (Ca1−xSrx)2Co-Si2O7 solid solution were grown in nitrogen by the floating zone method using a lamp-image furnace. The grown crystals were 6 mm in diameter by 50 mm in length. Microprobe analyses indicate uniform strontium content x except in the initially crystallized part. Synthetic crystals with x from 0.0 to 0.3 give, at room temperature, satellite reflections and circular diffuse scatterings in the electron diffraction pattern, which are related to an incommensurate phase and microdomains, respectively. With increasing Sr content the wavelength of a modulation increases and the intensity of satellites decreases, but the intensity of circular diffuse scattering increases up to x=0.15 and then decreases until eventually the satellites and the circular diffuse scatterings disappear at x = 0.3. The circular diffuse scattering is explained by the cluster model for the transition state, proposed by De Ridder et al. (1976).

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.

Ionicity scale and piezoelectricity of crystals with zincblende- and wurtzite-type structure

Abstract We have tried to determine the ionicity of crystals with a dynamical effective ionic charge defined by the observed TO - LO splitting. We find excellent agreement between our results and the ionicity scale derived by Phillips from band gap theory. We confirm that the observed piezoelectric constants can be well accounted for on the basis of the Born lattice-dynamical treatment by adding the concept of charge transfer, and we have made an estimate of the latter.