Browsing by Author "Kamiyama, T"
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- ItemOn the structure of α-BiFeO3(American Chemical Society, 2013-03-04) Wang, H; Yang, CX; Lu, J; Wu, MM; Su, J; Li, K; Zhang, JR; Li, GB; Jin, T; Kamiyama, T; Liao, FH; Lin, JH; Wu, YCPolycrystalline and monocrystalline α-BiFeO3 crystals have been synthesized by solid state reaction and flux growth method, respectively. X-ray, neutron, and electron diffraction techniques are used to study the crystallographic and magnetic structure of α-BiFeO3. The present data show that α-BiFeO3 crystallizes in space group P1 with a = 0.563?17(1) nm, b = 0.563?84(1) nm, c = 0.563?70(1) nm, α = 59.33(1)°, ? = 59.35(1)°, ? = 59.38(1)°, and the magnetic structure of α-BiFeO3 can be described by space group P1 with magnetic modulation vector in reciprocal space q = 0.0045a* ? 0.0045b*, which is the magnetic structure model proposed by I. Sosnowska(1) applied to the new P1 crystal symmetry of α-BiFeO3. © 2013 American Chemical Society
- ItemThe stability of Na-doped Bi-2(NbCr)O7-x pyrochlores: the non-existence of "(BiNa)(NbCr)O-6"(Elsevier, 2008-04) Thorogood, GJ; Kennedy, BJ; Avdeev, M; Kamiyama, TWe show that the pyrochlore described as "(BiNa)(NbCr)O-6" is in fact a non-stoichiometric pyrochlore with an approximate composition of (Bi1.33Na0.67)(Nb1.33Cr0.67)O7-x. Refinement of this structure using constant wavelength powder neutron diffraction data reveals the presence of vacancies in the anion sites coupled with displacive disorder of the Bi and Na cations. This is necessary to achieve a satisfactory bonding arrangement for both the Bi and Na cations that occupy the pyrochlore A-type sites. Attempts to prepare other pyrochlores in the series Bi2-xNax(NbCr)O-6 were unsuccessful and it appears that the pyrochlore is only stable only over a very narrow composition range. The structure of the pure Bi pyrochlore Bi-2(NbCr)O-7 is also described. © 2007, Elsevier Ltd.
- ItemStructural origin of the anisotropic and isotropic thermal expansion of K2NiF4-type oxides(American Chemical Society, 2015-04-02) Kawamura, K; Yashima, M; Fujii, K; Omoto, K; Hibino, K; Yamada, S; Hester, JR; Avdeev, M; Miao, P; Torii, S; Kamiyama, TK2NiF4-type LaSrAlO4 and Sr2TiO4 exhibit anisotropic and isotropic thermal expansion, respectively; however, their structural origin is unknown. To address this unresolved issue, the crystal structure and thermal expansion of LaSrAlO4 and Sr2TiO4 have been investigated through high-temperature neutron and synchrotron X-ray powder diffraction experiments and ab initio electronic calculations. The thermal expansion coefficient (TEC) along the c-axis (αc) being higher than that along the a-axis (αa) of LaSrAlO4 [αc = 1.882(4)αa] is mainly ascribed to the TEC of the interatomic distance between Al and apical oxygen O2 α(Al–O2) being higher than that between Al and equatorial oxygen O1 α(Al–O1) [α(Al–O2) = 2.41(18)α(Al–O1)]. The higher α(Al–O2) is attributed to the Al–O2 bond being longer and weaker than the Al–O1 bond. Thus, the minimum electron density and bond valence of the Al–O2 bond are lower than those of the Al–O1 bond. For Sr2TiO4, the Ti–O2 interatomic distance, d(Ti–O2), is equal to that of Ti–O1, d(Ti–O1) [d(Ti–O2) = 1.0194(15)d(Ti–O1)], relative to LaSrAlO4 [d(Al–O2) = 1.0932(9)d(Al–O1)]. Therefore, the bond valence and minimum electron density of the Ti–O2 bond are nearly equal to those of the Ti–O1 bond, leading to isotropic thermal expansion of Sr2TiO4 than LaSrAlO4. These results indicate that the anisotropic thermal expansion of K2NiF4-type oxides, A2BO4, is strongly influenced by the anisotropy of B–O chemical bonds. The present study suggests that due to the higher ratio of interatomic distance d(B–O2)/d(B–O1) of A22.5+B3+O4 compared with A22+B4+O4, A22.5+B3+O4 compounds have higher α(B–O2), and A22+B4+O4 materials exhibit smaller α(B–O2), leading to the anisotropic thermal expansion of A22.5+B3+O4 and isotropic thermal expansion of A22+B4+O4. The “true” thermal expansion without the chemical expansion of A2BO4 is higher than that of ABO3 with a similar composition. © 2015 American Chemical Society
- ItemThermal expansion properties of Ag2O crystal structure by powder neutron diffraction(The Physical Society of Japan, 2015-7-12) Ishikawa, Y; Danilkin, SA; Avdeev, M; Kamiyama, T; Sakuma, TThermal expansion property of Silver(I)-Oxide (Ag2O) was structurally investigated by neutron powder diffraction from 3 to 400 K, and measured by X-ray powder diffraction from 20 to 498 K. Temperature dependency of lattice parameter by neutron diffraction indicates the negative thermal expansion (NTE) below room temperature, and it changes to positive thermal expansion (PTE) from 300 to 400 K. And this result is consistent with X-ray diffraction and TMA measurement. Moreover, the result by X-ray diffraction shows that lattice parameter significantly decreases above 400 K. Since the result by TG measurement shows the mass loss remarkably in this temperature region, it is considered that decreasing lattice parameter above 400 K is caused from thermal decomposition. ©2015 The Physical Society of Japan, Creative Commons Attribution 4.0 (CC-BY 4.0)