Structures of LuFeCoO4 and LuFe2O4

Isobe, M.; Kimizuka, Noboru; Iida, Junji; Takekawa, Shunji

doi:10.1107/s0108270190004784

Cited by 70 publications

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“…8 at room temperature are given in Table I. We observe that the average Fe-O (apical) bonds of the FeO 5 bipyramids are significantly lower than the average crystallographic Fe-O distance (ß2.08Å) given for the hexagonal structure from diffraction studies [5,6], although it approaches the value of the shorter Fe-O bond along the c axis. However, the correspondingly high σ 2 factors indicate the presence of a spread of distances.…”

Section: B Exafs Spectramentioning

confidence: 88%

“…We assign the two peaks to the 3d 5 and 3d 6 configurations of the mixed valence iron state. The Fe in the hexagonal RFe 2 O 4 ferrites is surrounded by an oxygen bipyramid of triangular basis with three oxygen atoms (d Fe-Obasal = 2.00Å) located in the ab plane and one above (d Fe-O1ap = 1.95Å) and one below (d Fe-O2ap = 2.2Å) along the c axis [5,6]. Thus, the polarized E c and E//c XANES spectra represent the projected density of empty p states in the basis and along the axis of the bipyramid, respectively.…”

Section: A Xanes Spectramentioning

confidence: 99%

“…The exchange-correlation part of the potential was taken as the real Hedin, Lundqvist, and Von Barth potential, and the spectra were convoluted using an appropriate Lorentzian function. The high-temperature rombohedral crystal structure of LuFe 2 O 4 [6] was employed to obtain the atomic positions. Figure 4 compares the calculated XANES spectra obtained for a cluster radius of 6Å around the Fe atom (about 70 atoms) to the corresponding experimental polarized XANES spectra for both, E//c and E c polarizations.…”

Section: A Xanes Spectramentioning

confidence: 99%

“…The crystal structure of these compounds belongs to the rhombohedral system with space group R-3m [5,6]. In the equivalent hexagonal setting, it can be described along the c axis as a stacking of [Fe 2 O 4 ] Ý bilayers, each consisting of two triangular [FeO] Ý planes, separated by close-compact [RO 2 ] Ý layers.…”

Section: Introductionmentioning

confidence: 99%

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Strong local lattice instability in hexagonal ferritesRFe2O4(R= Lu,Y,Yb) revealed by x-ray absorption spe

et al. 2014

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We present here an x-ray absorption study of the RFe 2 O 4 series (LuFe 2 O 4 , YFe 2 O 4 , YbFe 2 O 4 , and LuCoFeO 4 ) at the Fe K-edge. Extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectra were measured at temperatures ranging from 100 K to 390 K crossing the charge ordering (CO) transition temperatures on both isotropic and oriented powder samples. Unpolarized and polarized x-ray absorption spectra with the x-ray polarization parallel and perpendicular to the hexagonal c axis were obtained separately. The XANES spectra show almost no dependence with either polarization or temperature. This indicates that, in contrast to its average crystallographic structure, the local electronic and geometrical state of the Fe atom is barely anisotropic, and it remains the same above and below the proposed CO. Moreover, the linear combination of two spectra corresponding to the pure ionic states Fe 2+ and Fe 3+ does not fit to the experimental XANES of either LuFe 2 O 4 , YFe 2 O 4 , or YbFe 2 O 4 , discarding total ionic segregation in favor of an intermediate mixed valence state. The maximum charge disproportionation compatible with the XANES spectra is 0.5 ± 0.1 electrons in the whole temperature range. The polarized EXAFS spectra have allowed us to analyze the local structure separating the different contributions mixed up in the polycrystal. Best-fit results show that the five oxygen atoms of the first coordination shell are at nearly the same distance but with a large Debye-Waller factor. Neither the interatomic distances nor the Debye-Waller factors of the first oxygen coordination shell change with temperature, indicating that the dynamical structural distortion of the high-temperature symmetric hexagonal phase freezes upon cooling down. Thus, the local structure instability of the mixed valence is in the origin of the structural transitions.

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Section: B Exafs Spectramentioning

confidence: 88%

Section: A Xanes Spectramentioning

confidence: 99%

Section: A Xanes Spectramentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Strong local lattice instability in hexagonal ferritesRFe2O4(R= Lu,Y,Yb) revealed by x-ray absorption spe

et al. 2014

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“…Above 320 K, it has a rhombohedral crystal structure (space group R3m) that can be described as an alternating stacking of [ 2 ] ∞ layers leads to a pseudobidimensional (2D) structure for the Fe-O sublattice. 9 LuFe 2 O 4 undergoes successive phase transitions. 2D charge correlations are observed below 500 K, while the ferroelectric phase transition is proposed to coincide with a 3D Fe 3+ /Fe 2+ CO below 320 K. 4,5,10 This is followed by ferrimagnetic order below T N ∼ 240 K. 2,[11][12][13] The general claim about ferroelectricity in LuFe 2 O 4 has been based on the following experimental results: (i) the observation of giant dielectric constant at temperatures above 150 K, [4][5][6] and (ii) the measurement of the pyroelectric current after field polarization.…”

Section: Introductionmentioning

confidence: 99%

Intrinsic electrical properties of LuFe2O4

et al. 2013

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We here revisit the electrical properties of LuFe 2 O 4 , compound candidate for exhibiting multiferroicity. Measurements of dc electrical resistivity as a function of temperature, electric-field polarization measurements at low temperatures with and without magnetic field, and complex impedance as a function of both frequency and temperature were carried out in a LuFe 2 O 4 single crystal, perpendicular and parallel to the hexagonal c axis, and in several ceramic polycrystalline samples. Resistivity measurements reveal that this material is a highly anisotropic semiconductor, being about two orders of magnitude more resistive along the c axis. The temperature dependence of the resistivity indicates a change in the conduction mechanism at T CO ≈ 320 K from thermal activation above T CO to variable range hopping below T CO . The resistivity values at room temperature are relatively small and are below 5000 cm for all samples but we carried out polarization measurements at sufficiently low temperatures, showing that electric-field polarization curves are a straight line as expected for a paraelectric or antiferroelectric material. Furthermore, no differences are found in the polarization curves when a magnetic field is applied either parallel or perpendicular to the electric field. The analysis of the complex impedance data corroborates that the claimed colossal dielectric constant is a spurious effect mainly derived from the capacitance of the electrical contacts. Therefore, our data unequivocally evidence that LuFe 2 O 4 is not ferroelectric.

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Synthesis, Crystal Structure and Charge Distribution of InGaZnO4. X-ray Diffraction Study of 20kb Single Crystal and 50kb Twin by Reticular Merohedry

Nespolo

Sato

Osawa

et al. 2000

Cryst. Res. Technol.

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Cryst. Res. Technol. 352000 2 151-165Synthesis of InGaZnO 4 at 20Kb and 50Kb produced a single crystal and a twinned crystal respectively. The X-ray diffraction crystal structure refinement (R1 = 0.015 and 0.019 respectively) and the Charge Distribution analysis are reported. Space group is  R3m; cell parameters in hexagonal axes are a = 3.2990 (2)Å, c = 26.1013(25)Å (20kb single crystal) and a = 3.3051(2)Å, c = 26.1029(19)Å (50kb twinned crystal). The cell volume is 246.01(3)Å 3 and 246.94(3)Å 3 respectively. The In is in regular octahedral coordination, whereas Ga/Zn are in trigonal bipyramid co-ordination. Charge Distribution on cations (2.94 and 2.95 respectively vs. 3.0 for In, and 2.53 vs. 2.5 for Ga/Zn) shows that the structure is well refined. Charge Distribution on oxygens (-1.96 and -2.04 for O1 and O2, vs. -2.0) excludes the presence of valence unbalance effects. A possible structural role of the trigonal bipyramidal coordination is discussed.

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Structures of LuFeCoO4 and LuFe2O4

Abstract: U33 0.008 (1) 0-008 (1) B,,q 0-7 (1) 0-9 (1)

Cited by 70 publications

References 0 publications

Strong local lattice instability in hexagonal ferritesRFe2O4(R= Lu,Y,Yb) revealed by x-ray absorption spe

Strong local lattice instability in hexagonal ferritesRFe2O4(R= Lu,Y,Yb) revealed by x-ray absorption spe

Intrinsic electrical properties of LuFe2O4

Synthesis, Crystal Structure and Charge Distribution of InGaZnO4. X-ray Diffraction Study of 20kb Single Crystal and 50kb Twin by Reticular Merohedry

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