RESUMEN
A polycrystalline sample of SrLa2FeCoSbO9 has been prepared in a solid-state reaction and studied by a combination of electron microscopy, magnetometry, Mössbauer spectroscopy, X-ray diffraction, and neutron diffraction. The compound adopts a monoclinic (space group P21/ n; a = 5.6218(6), b = 5.6221(6), c = 7.9440(8) Å, ß = 90.050(7)° at 300 K) perovskite-like crystal structure with two crystallographically distinct six-coordinate sites. One of these sites is occupied by 2/3 Co2+, 1/3 Fe3+ and the other by 2/3 Sb5+, 1/3 Fe3+. This pattern of cation ordering results in a transition to a ferrimagnetic phase at 215 K. The magnetic moments on nearest-neighbor, six-coordinate cations align in an antiparallel manner, and the presence of diamagnetic Sb5+ on only one of the two sites results in a nonzero remanent magnetization of â¼1 µB per formula unit at 5 K.
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Polycrystalline samples of Ln2CoGe4O12 (Ln = Gd, Tb, Dy, Ho or Er) and LnBCoGe4O12 (B = Sc or Lu) have been prepared and characterised by a combination of magnetometry, 155Gd Mössbauer spectroscopy and, in the case of Tb2CoGe4O12 and TbScCoGe4O12, neutron diffraction. The holmium- and erbium-containing compositions remain paramagnetic down to 2 K, those containing dysprosium behave as spin glasses and the terbium and gadolinium-containing compounds show long-range magnetic order with transition temperatures below 4 K in all cases. The data can be rationalized qualitatively in terms of the interplay between magnetic anisotropy and crystal field effects.
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Polycrystalline samples in the solid solution ZrMn2-xCoxGe4O12 (x = 0.0, 0.5, 1.0, 1.5 and 2.0) have been prepared using the ceramic method and characterised by a combination of magnetometry, X-ray diffraction and neutron diffraction. They all adopt the space group P4/nbm with a â¼ 9.60, c â¼ 4.82 Å and show long-range magnetic order with transition temperatures, TC, in the range 2 ≤ TC/K ≤ 10. The underlying magnetic structure is the same in each case but the ordered spins lie along [001] when x = 0.0 and in the (001) plane for all other compositions. In all cases the magnetically-ordered phase is a weak ferromagnet although the magnitude of the spontaneous magnetisation and the strength of the coercive field are composition-dependent. The magnetic structure can be rationalized by considering the strengths of the interactions along the distinct M-O-Ge-O-M superexchange pathways in the crystal and the observed magnetic structure is entirely consistent with the predictions of ab initio calculations.
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Polycrystalline samples, prepared by a solid-state route, of compositions in the solid solution CeMn2-xCoxGe4O12 (x = 0.0, 0.5, 1.0, 1.5, and 2.0) were characterized by X-ray diffraction, neutron diffraction, and magnetometry. They adopt space group P4/nbm with a ≈ 9.78 and c ≈ 4.85 Å and become anti-ferromagnetic (x = 0.0, 1.5, 2.0) or weakly ferromagnetic (x = 0.5, 1.0) at 4.2 ≤ T ≤ 7.6 K. The ordered moments lie along [001] when x = 0.0 and in the (001) plane otherwise. The unit cell doubles along [001] when x = 1.5 and 2.0 order anti-ferromagnetically, but the doubling is lost when a first-order metamagnetic transition to weak ferromagnetism occurs on the application of a 10 kOe magnetic field. The ordered moments at 1.6 K for x = 0.0 and 2.0 are 4.61(2) and 2.58(2) µB, respectively; the corresponding effective moments in the paramagnetic phase are 5.91 and 5.36 µB.
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Non-equilibrium molecular dynamics has been used to model cation diffusion in stoichiometric Li3N over the temperature range 50 < T/K < 800. The resulting diffusion coefficients are in excellent agreement with the available experimental data. We present a detailed atomistic account of the diffusion process. Contrary to the conclusions drawn from previous studies, our calculations show that it is unnecessary to invoke the presence of a small concentration of intrinsic defects in order to initiate diffusion. The structure can be considered to consist of alternating layers of composition Li2N and Li. As the temperature increases an increasing number of cations leave the Li2N layers and migrate either to the interlayer space or to the Li layer. Those that move into the interlayer space form Li2 dimers with cations in the Li2N layers and those that move into the neighboring layer form dimers with cations therein. The two types of dimer are aligned parallel and perpendicular to [001], respectively and have lifetimes of â¼3 ps. The vacancies so created facilitate rapid diffusion in the Li2N layers and the interlayer cation motion results in slower diffusion perpendicular to the layers.
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The use of non-equilibrium molecular dynamics facilitates the calculation of the cation diffusion constant of Li2O at temperatures too low to be accessible by other methods. Excellent agreement with experimental diffusion coefficients has been obtained over the temperature range 873 < T/K < 1603. Diffusion below 1200 K was shown to be dominated by a concerted nearest-neighbour hopping process, whereas in the high-temperature superionic region an additional mechanism involving a six-coordinate interstitial cation site in the anti-fluorite structure becomes increasingly dominant. Our model thus accounts for the transition from the superionic regime to the non-superionic regime.
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A polycrystalline sample of La3Ni2SbO9 has been synthesized using a standard ceramic method and characterized by neutron diffraction and magnetometry. The compound adopts a monoclinic, perovskite-like structure with space group P2(1)/n and unit cell parameters a = 5.0675(1), b = 5.6380(1), c = 7.9379(2) Å, ß = 89.999(6)° at room temperature. The two crystallographically distinct six-coordinate sites are occupied by Ni(2+) and a disordered distribution of Ni(2+)/Sb(5+), respectively; the Ni(2+) and Sb(5+) cations occupy the disordered site in a 1:2 ratio. Both ac and dc magnetometry indicate the presence of a spontaneous magnetization below 105 K. A magnetization of 1.5 µB per formula unit was measured at 2 K in a field of 40 kOe. However, no magnetic scattering was observed in neutron diffraction data collected at 5 K. It is proposed that, as a consequence of the cation disorder, La3Ni2SbO9 behaves as a relaxor ferromagnet, analogous to a relaxor ferroelectric, with magnetic domains too small to be detected by neutron diffraction forming below 105 K.
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The structure proposed for Li(11)Nd(18)Fe(4)O(39-δ) (Chen et al. Inorg. Chem. 2012, 51, 8073) on the basis of diffraction and Mössbauer spectral data is compared to that determined previously for Nd(18)Li(8)Fe(5)O(39) (Dutton et al. Inorg. Chem.200847, 11212) using the same techniques. The Mössbauer spectrum reported by Chen et al. has been reinterpreted. The newly refined spectral parameters differ significantly from the published values but are similar to those reported for Nd(18)Li(8)Fe(5)O(39). The relative areas of the three components indicate that iron cations occupy the 2a, 8e, and 16i sites in space group Pm3n, in disagreement with the model determined from neutron diffraction by Chen et al. in which only the 2a and 8e sites are so occupied. The relationship between Li(11)Nd(18)Fe(4)O(39-δ) and Nd(18)Li(8)Fe(5)O(39) is discussed, and it is proposed that the sample prepared by Dutton et al. is a kinetic product whereas the sample prepared by Chen et al. is the thermodynamically preferred product.
RESUMEN
Polycrystalline samples of Ln(18)Li(8)Rh(5-x)Fe(x)O(39) (Ln = La, Nd; 0.5 < or = x < or = 5) have been synthesized by a solid-state method and studied by a combination of dc and ac magnetometry, neutron diffraction, and Mossbauer spectroscopy. All compositions adopt a cubic structure (space group Pm3n, a(0) approximately 12 A) based on intersecting 111 chains made up of alternating octahedral and trigonal-prismatic coordination sites. These chains occupy channels within a Ln-O framework. At low values of x, iron preferentially occupies the smaller (2a) of the two distinct octahedral sites as low-spin Fe(IV). The Rh(III) on the larger (8e) octahedral site is replaced by high-spin Fe(III). Nd-containing compositions having x > 1 show spin-glass-like behavior below approximately 5 K. La-containing compositions having x > 1 show evidence of a magnetic transition at approximately 8 K, but the nature of the transition is unclear. This contrasting behavior demonstrates that, although the structural chemistry of the two systems is essentially the same, the magnetic character of the Ln cations plays an important role in determining the properties of these compounds.
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Compositions in the series Ni(2-x)Co(x)GeMo(3)N (0 < or = x < or = 2), Co(2)Ge(1-x)Ga(x)Mo(3)N (0 < x < or = 0.7), Co(2-x)Fe(x)GeMo(3)N (0 < or = x < or = 2), and Co(2-x)Fe(x)Ge(0.5)Ga(0.5)Mo(3)N (0 < or = x < or = 0.8) have been synthesized by the reductive nitridation of binary oxides and studied by appropriate combinations of magnetometry, transport measurements, neutron diffraction, and Mossbauer spectroscopy. All of these compositions adopt the cubic eta-carbide structure (a approximately 11.11 A) and show a resistivity of approximately 10(-3) Omega cm. No long-range magnetic order was observed in Ni(2-x)Co(x)GeMo(3)N, although evidence of spin freezing was observed in Co(2)GeMo(3)N. The introduction of gallium into this composition leads to the onset of antiferromagnetic ordering at 90 K in Co(2)Ge(0.3)Ga(0.7)Mo(3)N. The magnetic structure consists of an antiferromagnetic arrangement of ferromagnetic Co(4) groups, with an ordered magnetic moment of 0.48(9) micro(B) per cobalt atom. The same magnetic structure is found in Co(0.5)Fe(1.5)GeMo(3)N and Co(1.2)Fe(0.8)Ge(0.5)Ga(0.5)Mo(3)N. The former orders above room temperature with an average moment of 1.08(3) mu(B) per transition-metal site, and the latter at 228 K with an average moment of 1.17(4) micro(B) per site. The magnetic behavior of these compounds is discussed in terms of the electron count within each series.
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For the first time, the chemistry in H(2) gas of a perovskite-like material, Pr(2)Sr(2)CrNiO(8), has been monitored at temperatures up to approximately 700 degrees C, in situ, by neutron powder diffraction.
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Polycrystalline samples of Nd(18)Li(8)Fe(5-x)M(x)O(39) (M = Mn, 0 < x < or = 4; M = Co; 0 < x < or = 3) have been synthesized using a solid-state method and have been studied using a combination of dc and ac magnetometry, neutron diffraction, and Mössbauer spectroscopy. All compositions adopt a cubic structure (space group Pm3n, a(0) approximately 11.9 A) based on intersecting <111> chains made up of alternating octahedral and trigonal-prismatic coordination sites. These chains occupy channels within a Nd-O framework. The trigonal prismatic site is occupied by Li(+) and, in some cases, high-spin Fe(3+). When M = Mn, the smaller of the two distinct octahedral sites in the structure is occupied by a disordered distribution of Mn(4+) and low-spin Fe(4+), but in Nd(18)Li(8)Fe(4)CoO(39), this site is occupied exclusively by Fe(4+). The larger of the octahedral sites is occupied by a disordered distribution of M(3+), high-spin Fe(3+), and, in some cases, Li(+). Unusually, the Mn(3+) cations in Nd(18)Li(8)Fe(5-x)M(x)O(39) adopt a low-spin state. This is attributed to the presence of an internal chemical pressure generated by the Nd-O framework. Competing magnetic superexchange interactions lead to the formation of a spin-glass-like phase in all compositions below T(f) approximately 5 K; the transition temperature decreases with increasing cobalt content but is relatively insensitive to the composition when M = Mn.
RESUMEN
Polycrystalline samples of Nd(18)Li(8)Fe(5)O(39) and Nd(18)Li(8)Co(4)O(39) have been synthesized using a solid-state method and studied by a combination of neutron diffraction, direct and alternating current magnetometry, and, in the case of Nd(18)Li(8)Fe(5)O(39), Mossbauer spectroscopy. Both compounds adopt a cubic structure (space group Pm3n, a(0) approximately 11.9 A) based on intersecting 111 chains made up of alternating octahedral and trigonal-prismatic coordination sites. The Fe(4+) cations in Nd(18)Li(8)Fe(5)O(39) are found on only the smaller of the two distinct octahedral sites in the structure; Fe(3+) and Li(+) are disordered over the larger octahedral site and the trigonal-prismatic site. The Nd(3+) cations occupy sites between the chains. The smaller octahedral site is fully occupied by cobalt in Nd(18)Li(8)Co(4)O(39), with 25% of the larger octahedral sites being vacant; Li(+) is only found on the prismatic sites. Nd(18)Li(8)Fe(5)O(39) shows spin-glass-like behavior with a spin-freezing temperature of 5.75 K, whereas Nd(18)Li(8)Co(4)O(39) appears to order antiferromagnetically at 2.3 K. In both cases, the magnetic coupling involves the Nd(3+) sublattice.
RESUMEN
Polycrystalline samples of Ln18Li8Rh5O39 (Ln = La, Pr) have been synthesized by the ceramic method and characterized by X-ray and neutron diffraction. The compounds crystallize in the cubic space group Pmn, with a0 approximately 12.1 Angstroms. The unit cell contains four intersecting 111 chains, each comprised of an alternating sequence of face-sharing RhO6 octahedra and LiO6 trigonal prisms. The octahedra located at the points of intersection contain Rh4+, whereas the remainder contain Rh3+; the compounds thus contain a charge-ordered arrangement of the two cations. The polyhedral chains are enclosed in tunnels formed by the Ln-O sublattice. The magnetic properties of the two new compounds are discussed briefly: both are paramagnetic over the temperature range 5 < TK < 300.
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Variable temperature X-ray and neutron powder diffraction techniques have been used to identify structural phase transitions in Cu-rich A(3)A'BO(6) phases. A transition from monoclinic to rhombohedral symmetry was observed by X-ray diffraction between 700 and 500 K in Sr(3)Cu(1-x)M(x)IrO(6) (M = Ni, Zn; 0 < or = x < or = 0.5). The temperature of the phase change decreased in a linear manner with Cu-content and was essentially independent of the nature of M. Ca(3.1)Cu(0.9)MnO(6) was shown to pass from a rhombohedral phase to a triclinic phase on cooling below 290 K; the structure of the triclinic phase was refined against neutron diffraction data collected at 2 K. Ca(3.1)Cu(0.9)RuO(6) undergoes a transition between a disordered rhombohedral phase and an ordered monoclinic phase when cooled below 623 K. Neutron diffraction has been used to determine the structure as a function of temperature in the range 523 < or =T/K < or = 723 and hence to determine an order parameter for the low temperature phase; the second-order transition is shown to be incomplete 100 K below the critical temperature.
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We describe the results of an inelastic neutron scattering measurement of the magnetic excitations in SrCaGaMnO(5+δ), a quasi-two-dimensional compound whose structure consists of layers of MnO(6) octahedra separated by layers of GaO(4) tetrahedra (the brownmillerite structure), and Ca(2.5)Sr(0.5)Mn(2)GaO(8), a bilayered brownmillerite. In both materials, a band of magnetic scattering appears below the magnetic ordering temperature which can be associated with magnon excitations. Our measurements allow us to provide an estimate for the intraplane exchange constant in both materials, which we find to be 3.4(4) meV for SrCaGaMnO(5+δ) and 2.2(4) meV for Ca(2.5)Sr(0.5)Mn(2)GaO(8).
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The temperature dependence of the crystal structure and electronic properties of brownmillerite-like Ca(2.5)Sr(0.5)GaMn(2)O(8) has been studied by neutron powder diffraction and muSR spectroscopy. The results show that short-range 2D magnetic order begins to develop within the perovskite-like bilayers of MnO(6) octahedra approximately 50 K above the 3D Néel temperature of approximately 150 K. The bilayers show a structural response to the onset of magnetism throughout this temperature range whereas the GaO(4) layers that separate the bilayers only respond below the 3D ordering temperature. XANES spectroscopy shows that the sample contains Mn(3+) and Mn(4+) cations in a 1:1 ratio, and the behavior in the region of the Néel transition is interpreted as a local charge ordering. Electron diffraction and high-resolution electron microscopy have been used to show that the local microstructure is more complex than the average structure revealed by neutron diffraction, and that microdomains exist in which the GaO(4) tetrahedra show different orientations. It is argued that the bonding requirements of diamagnetic gallium control the electronic behavior within the perovskite-like bilayers.
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We have developed a new technique, which is complementary to other procedures, that will have wide applicability for generating new feasible framework structures with defined microporous architectures from the knowledge of only the unit cell dimensions, constituent elements and by defining forbidden regions within the unit cell.
Asunto(s)
Algoritmos , Cristalografía , Modelos Genéticos , Modelos Moleculares , Zeolitas/químicaRESUMEN
Polycrystalline samples of La4LiMnO8 and La3SrLiMnO8 have been studied by a combination of X-ray diffraction (XRD), neutron diffraction (ND), 6Li MAS NMR, electron microscopy (EM), and magnetometry. Room-temperature XRD and ND measurements suggest that both compounds have the K2NiF4 structure, with a disordered arrangement of Li and Mn over the six-coordinate sites. However, MAS NMR and EM demonstrate the presence of local 1:1 Li:Mn order on these sites, and EM shows that although cation order is well-developed in each xy sheet of corner-sharing octahedra, the sheets are stacked randomly along z. The structures are best described as paracrystalline, and many of the concepts of conventional crystallography are inapplicable. Magnetometry and low-temperature ND experiments show that, despite their paracrystallinity, the two compounds are ordered antiferromagnetically with susceptibility maxima at 26 and 18 K, respectively, and with ordered magnetic moments of 3.61(6) and 2.3(1) muB per Mn cation at 2 K. Anisotropic peak broadening reveals a 2D character in the magnetic behavior of both compounds, and La3SrLiMnO8 is well-modeled as a quadratic layer S = 3/2 Heisenberg antiferromagnet.
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A polycrystalline sample of a new phase in the Sr-Fe-Mn-O system has been prepared by standard solid-state techniques. Characterization at room temperature by X-ray diffraction, high-resolution electron microscopy, Mössbauer spectroscopy and neutron diffraction has led to it being described as a 15-layered, rhombohedral (15R) perovskite [space group R&thremacr;m: a = 5.4489(1) Å, c = 33.8036(7) Å] with a previously unobserved structure. The pseudo close-packed SrO(3) layers have a (cchch)(3) stacking sequence such that the occupation of the interstitial 6-coordinate sites by Mn (or Fe) leads to the formation of Mn(2)O(9) units which are linked to each other either directly by a common vertex, or indirectly via a single, vertex-sharing MnO(6) octahedron. The stoichiometry of the compound was determined to be SrMn(0.915(5))Fe(0.085(5))O(2.979(3)). The face-sharing sites are occupied by 0.957(3)Mn/0.043(3)Fe while the exclusively corner-linked sites show a higher Fe occupation; 0.745(4)Mn/ 0.255(4)Fe. A neutron diffraction experiment carried out at 3 K indicated the presence of long-range magnetic order with the Mn(4+) cations aligned antiferromagnetically with an ordered moment of 2.26(3)&mgr;(B)/Mn(4+). Both the neutron and the susceptibility data are consistent with the Fe cations remaining magnetically disordered to 3 K. The latter data show T(N) = 220 K, and suggest that some spin frustration is present at low temperatures.
