Tohoku University · Materials Science
Professor Yuzuru Miyazaki's research lab specializes in the synthesis, structural characterization, and thermoelectric properties of complex oxide and silicide materials, particularly composite crystals with modulated structures. The lab focuses on understanding the interplay between crystallographic complexity—such as incommensurate superstructures and misfit layers—and their impact on electronic and thermal transport properties. Key research directions include the development of high-performance thermoelectric materials through cation substitution and nanostructuring, with an emphasis on optimizing the figure of merit (ZT) in materials like Ca₃Co₄O₉ and higher manganese silicides (HMS).
Figures are computed from collected data and may differ slightly.
Electric resistivity, thermoelectric power and thermal conductivity of a polycrystalline sample of the composite crystal [Ca 2 CoO 3.34 ] 0.614 [CoO 2 ], also known as Ca 3 Co 4 O 9 , have been measured below 300 K. Metallic conductivity accompanied by large thermoelectric power has been observed down to 50 K. At 300 K, the sample exhibits a thermoelectric power of S = 133 µV·K -1 , resistivity of ρ= 15 mΩ·cm and thermal conductivity of κ= 9.8 mW·K -1 ·cm -1 . The resulting dimensionless figure
We have determined the crystal structure of the composite crystal [Ca 2 CoO 3 ] 0.62 CoO 2 , known as Ca 3 Co 4 O 9 , by a superspace group approach. Structural parameters were refined with a superspace group of C m (0 1- p 0) using powder X-ray and neutron diffraction data. The compound consists of two interpenetrating subsystems of a CoO 2 sheet and a distorted triple-layered NaCl-type Ca 2 CoO 3 block, being incommensurate parallel to the b -axis. The CoO 2 sheet is composed of a CdI 2 -type
The crystal structure of a polycrystalline sample of higher manganese silicide (HMS) has been determined by means of the $(3+1)$-dimensional superspace group approach. The structural parameters were refined with a superspace group of $I{4}_{1}/amd(00\ensuremath{\gamma})00ss$ using powder neutron-diffraction data collected at 295 K. The compound belongs to a composite crystal family consisting of [Mn] and [Si] subsystems, with an irrational $c$-axis ratio (misfit parameter) of $\ensuremath{\gamma
A new composite crystal cobaltite, [Ca2(Co0.65Cu0.35)2O4]0.624CoO2, has been synthesized and its thermoelectric properties below 300 K have been investigated. The compound consists of a CdI2-type CoO2 conduction sheet and a quadruplicated rock-salt-type [Ca2(Co0.65Cu0.35)2O4] layer alternatively stacked along the c-axis. Metallic conductivity accompanied by large thermoelectric power (S) has been observed down to 100 K. At 300 K, the sample exhibits S=150 µV·K-1 and resistivity of ρ=15 mΩ·cm. Th
A partially Fe-substituted solid solution of a higher manganese silicide, (Mn 1- x Fe x )Si γ , has been prepared by means of arc-melting and a subsequent annealing process. According to the linear relationship between x and lattice parameters, the solid solution can be prepared up to x = 0.35. The compounds consist of two tetragonal subsystems of [Mn 1- x Fe x ] and [Si], with an irrational c -axis ratio γ= c Mn / c Si ∼1.7. With increasing x from 0 to 0.35, equivalent to increase electron carr
The dissipation of MnSi layered precipitates during solidification is critical for further enhancement of the thermoelectric properties of the higher manganese silicides. We have investigated the effects of partial substitution of V in Mn sites and of Ge in Si sites on the crystal structures and thermoelectric properties of these silicides in detail. As previously reported, a small amount of V-substitution is quite effective in completely dissipating the MnSi striations; in contrast, a small pro
We have determined the crystal structure of Bi-substituted and Bi-free misfit layered cobalt oxides [Ca 2 CoO 3 ] 0.62 CoO 2 , by a (3+1)-dimensional superspace group approach. Structural parameters have been refined with a superspace group of C 2/ m (1 p 0) s 0 using powder neutron diffraction data. Bismuth atoms are found to substitute for both Ca and Co atoms in the rock salt-type [Ca 2 CoO 3 ] subsystem. The resulting structural formula is expressed as [(Ca 0.90 Bi 0.10 ) 2 (Co 0.95 Bi 0.05
We have determined the crystal structure of novel thermoelectric compound [Ca2(Co0.65Cu0.35)2O4]0.63CoO2 by a superspace group approach. Structural parameters have been refined with a superspace group of C2/m(1 p 0)s0 using powder neutron diffraction data collected at 293 K. The compound consists of mutually interpenetrating triangular CoO2 layers and distorted four-layered rock salt-type Ca2(Co0.65Cu0.35)2O4 slabs, which are incommensurate parallel to the b-axis. The CoO2 layer is composed of t
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