酸素欠損を含むCa3Co4O9の 中性子回折測定による結晶構造解析 3P-08 日本中性子科学会第8回年会＠名古屋大学 横国大工1, 防衛大理工2, Lille大3 中津川 博1, 長澤 兼作1, 岡本 庸一2, Oliver Mentre3

Introduction Since the discovery of a large thermoelectric power in the layered compounds NaxCoO2 and Ca3Co4O9,　misfit-layered cobalt oxides particularly have attracted considerable interest as candidates for thermoelectric (TE) materials.　A reacent precise structural study by Masset et al.　revealed that the crystal structure of Ca3Co4O9 consists of an alternate stacking of a distorted three-layered rock salt (RS)-type Ca2CoO3 block layer (BL) and a CdI2-type CoO2 conducting sheet parallel to the c-axis.　However, detailed study of the temperqatute dependence of oxygen nonstoichiometry δ and its effect on the crystal structure, magnetic transitions and TE properties have not been reported. In this study, we investigate the neutron powder diffraction technique and also the measurements of TE properties, magnetic susceptibility and photoacoustic spectroscopy (PAS) to clarify both the TE properties and the valence state of Co ions.　

Conventional research Y.Miyazaki, Solid State Ionics 172, 463 (2004) R.Asahi et al., Phys. Rev. B66, 155103 (2002) J.Sugiyama et al., Phys. Rev. B68, 134423 (2003) J.Shimoyama et al., Jpn.J.Appl.Phys. 42, L194 (2003)

Sample preparation 試料の合成 ：a conventional solid-state reaction method 試料の合成　 ：a conventional solid-state reaction method Sample A : Ca3Co4O9 Sample B : Ca3Co4O9-δ calcined in air at 920℃ for 12h calcined in air at 920℃ for 12h sintered in pure O2 at 920℃ for 24h sintered in pure O2 at 920℃ for 24h 3 times 3 times furnace cooled to room temperature furnace cooled to room temperature density: 2.595g/cm3 (55.3%) annealed in pure Ar at 740℃ for 48h quenched into cold water density: 2.549g/cm3 (54.6%)

Experimental details 中性子回折測定 ：HERMES (λ=1.8205Å), 7°≦2θ≦156.9° 変調構造解析 中性子回折測定　 ：HERMES (λ=1.8205Å), 7°≦2θ≦156.9° 変調構造解析　 ：PREMOS 91, 超空間群 C2/m(1 p 0)s0 変調関数： 磁化率測定　 ：SQUID, ZFC & FC, H = 1000Oe, T = 4～380K 平均場近似： 酸素欠損量　 ： [Ca2Coy1O3.08-δ]b1/b2 Coy2O2 δ= 0.09 電気抵抗率・熱起電力測定　 ：T = 80～900K 光音響分光測定　 ：λ = 400～1600nm 5nm step at RT

modulated structure analyses 格子定数と対称操作　 3次元表記 [CoO2] sheet : 回折ピーク (hkl), 格子定数 (a, b1, c), 並進・回転 C2/m RS-type BL : 回折ピーク (hml), 格子定数 (a, b2, c), 並進・回転 C21/m 4次元表記 [RS]b1/b2 [CoO2] : 回折ピーク (hklm), 格子定数 (a, b1, b2, c), σ=(0, b1/b2, 0) 並進・回転 C2/m (1 p 0) s0 反射条件　 : h + k + m = 2n 変調関数M(t)の制約条件　 [CoO2] sheet : (2n+1)b2* : Co1: Ay=Bx=By=Bz, O1: Ay=Bx=Bz 2nb2* : Co1: Ax=Ay=Az=Bx=Bz, O1: Ay=Bx=Bz RS-type BL : (2n+1)b2* : Ca, O3: Ay=Bx=Bz, Co2, O2: Ay=Bx=By =Bz, 2nb2* : Ca, O3: Ay=Bx=Bz, Co2, O2: Ax= Ay=Az=Bx =Bz,

Rietveld analysis (PREMOS 91) Sample A Rwp= 5.37% 0010 0020 0030 Lattice constants : a = 4.83353(4)Å, b1 = 2.82380(7)Å, c = 10.8455(3)Å 　　　　　　　　　　　β= 98.141(5)°, b2 = 4.55757(8)Å 　　　　　　　　　　　b1/b2 = 0.61958(5) ideal density: 4.6960(3)g/cm3

crystal structure (PRJMS) O1 Co1 [CoO2] sheet O1 c c Ca, O3 RS-type block layer Co2, O2 Ca, O3 O1 [CoO2] sheet Co1 O1 b1 a b2

interatomic distances (MODPLT)

Rietveld analysis (PREMOS 91) Sample B Rwp= 4.69% 0010 0020 0030 Lattice constants : a = 4.83804(6)Å, b1 = 2.82533(1)Å, c = 10.86027(2)Å 　　　　　　　　　　　β= 98.135(8)°, b2 = 4.55128(9)Å 　　　　　　　　　　　b1/b2 = 0.62077(6) ideal density: 4.6722(9)g/cm3

crystal structure (PRJMS) O1 Co1 [CoO2] sheet O1 c c Ca, O3 RS-type block layer Co2, O2 Ca, O3 O1 [CoO2] sheet Co1 O1 b1 a b2

interatomic distances (MODPLT)

magnetic susceptibility Sample A FC [Ca2Co3.48+O3.08]0.62Co3.18+O2 Ca3Co3.923.29+O9.46 ZFC ZFC Co13.18+ Co23.48+ Co13.18+

magnetic susceptibility Sample B [Ca2Co3.41+O2.99]0.62Co3.11+O2 Ca3Co3.923.22+O9.33 FC ZFC ZFC Co13.11+ Co23.41+ Co13.11+

photoacoustic spectroscopy (PAS) 1.55eV 1.24eV 1.04eV Sample A R.Asahi et al., Phys. Rev. B66, 155103 (2002) Sample B Y.Miyazaki, Solid State Ionics 172, 463 (2004)

electric resistivity LS Sample B HS LS Sample A HS J.Sugiyama et al., Phys. Rev. B68, 134423 (2003) Sample A HS

Seebeck coefficient Sample B Sample A J.Sugiyama et al., Phys. Rev. B68, 134423 (2003)

Summary Acknowledgments ● Sample A : [Ca2Co3.48+O3.08]0.62Co3.18+O2 Sample B : [Ca2Co3.41+O3.08－δ]0.62Co3.11+O2 ( δ=0.09 ) ● a, b1, c増加、b2減少、変調構造(Co-O)緩和 ● a1g band → eg band への励起エネルギー減少 　　→ electron dope によるフェルミレベルの上昇？ ● Spin 転移 (LS→IS→HS)温度の低温側へのシフト Acknowledgments This study was partly supported by Takahashi Industrial and Economic Research Foundation.