Showing papers by "J. Paul Attfield published in 2014"
TL;DR: It is shown that an A- and B-site-ordered quadruple perovskite oxide, CaCu3Fe2Re2O12, has spin-polarized conduction electrons and is ferrimagnetic up to 560 K and spin polarization is confirmed by an observed low-field magnetoresistance effect in a polycrystalline sample.
Abstract: There are only a few transition metal oxides whose conducting electrons show a strong spin polarization at sufficiently high temperatures for spin electronics applications. Here, the authors find that CaCu3Fe2Re2O12 has such spin-polarized conducting electrons and is ferrimagnetic up to 560 K.
78 citations
TL;DR: In this paper, the B-site cation ordering of 1322 CaCu3Fe2Nb2O12 perovskites has been investigated by synchrotron X-ray diffraction structure analysis.
Abstract: Cation ordering in the 1322 perovskite CaCu3Fe2Nb2O12 has been investigated by synchrotron X-ray diffraction structure analysis, high-resolution electron microscopy, and magnetic measurements. All of the samples have full 1:3 ordering of Ca and Cu cations at the perovskite A-sites, but the Fe and Nb cations at the B sites show different degrees of ordering depending on the synthesis conditions. A slow-cooled sample has almost complete rock-salt-type 1:1 cation ordering at the B sites, but a rapidly cooled sample has no long-range B-cation arrangement. The ordered compound shows a large saturated magnetization (5.7 μB/formula unit (fu)) below 170 K due to the ferrimagnetism of the A′-site Cu2+ and B-site Fe3+ spins, whereas the B-site disordered form has a small magnetization (<1 μB/fu at 5 K). The degree of B-site cation order thus controls the magnetic behavior of 1322 CaCu3Fe2Nb2O12 perovskites.
31 citations
TL;DR: The rare earth vanadium oxynitride perovskites RVO3−xNx (R = La, Pr, Nd; 0 ≤ x ≤ 1) have been prepared by treatment of RVO4 precursors in NH3 as mentioned in this paper.
Abstract: The rare earth vanadium oxynitride perovskites RVO3−xNx (R = La, Pr, Nd; 0 ≤ x ≤ 1) have been prepared by treatment of RVO4 precursors in NH3. The ammonolysis reactions proceed through an initial reduction of RVO4 precursors to RVO3 perovskites that are progressively nitrided and reoxidised to RVO3−xNx materials with varying proportions of V4+/V3+. Variable temperature neutron diffraction studies show that NdVO2N and PrVO2.24N0.76 have orthorhombic Pbnm symmetry between 3.5 and 300 K, but LaVO2.11N0.89 undergoes a broad Pbnm to Rc structural transition over this temperature range. In LaVO3−xNx samples the proportion of the Rc phase at room temperature increases with the nitrogen content. The partial anion order indicative of planes of cis-VN2 chains found in NdVO2N is also present in PrVO2.24N0.76, and is thus robust to the disorder created by non-stoichiometry. Magnetic measurements reveal spin freezing transitions at low temperatures in LaVO2.09N0.91 and PrVO2.24N0.76, but NdVO2N is paramagnetic.
22 citations
TL;DR: X-ray and neutron powder diffraction studies show that the materials adopt orthorhombic perovskite superstructures in which the RuO6 octahedra are tetragonally compressed, and the Mott insulator properties of the materials are driven by strong spin-orbit coupling.
Abstract: An extensive series of new LnRuO3 perovskites has been synthesized at high pressure. These ruthenium(III)-based oxides are ruthenium deficient, and high-pressure samples have compositions close to LnRu0.9O3. These phases stabilize ruthenium(III) which is very unusual in oxides. X-ray and neutron powder diffraction studies show that the materials adopt orthorhombic perovskite superstructures in which the RuO6 octahedra are tetragonally compressed. These distortions, and the Mott insulator properties of the materials, are driven by strong spin-orbit coupling.
17 citations
TL;DR: In this article, the perovskite CaCrO3−δ superstructure phases (δ = 0.33, 0.4, and 0.5) have been discovered through soft low temperature reduction of the CaCr O3 made at "hard" high pressure-temperature conditions.
Abstract: New CaCrO3−δ superstructure phases (δ = 0.33, 0.4, and 0.5) have been discovered through ‘soft’ low temperature reduction of the perovskite CaCrO3 made at ‘hard’ high pressure–temperature conditions. Their structures form a related series based on stacking of octahedral and tetrahedral chromium oxide layers.
12 citations
TL;DR: This work presents a novel, compact pressure cell with a large sample volume in which load is generated by a bellow, and special fiber ports added to the cryogenic center stick allow for in situ pressure determination using the ruby pressure standard.
Abstract: Pressure generation at cryogenic temperatures presents a problem for a wide array of experimental techniques, particularly neutron studies due to the volume of sample required. We present a novel, compact pressure cell with a large sample volume in which load is generated by a bellow. Using a supply of helium gas up to a pressure of 350 bar, a load of up to 78 kN is generated with leak-free operation. In addition, special fiber ports added to the cryogenic center stick allow for in situ pressure determination using the ruby pressure standard. Mechanical stability was assessed using finite element analysis and the dimensions of the cell have been optimized for use with standard cryogenic equipment. Load testing and on-line experiments using NaCl and BiNiO3 have been done at the WISH instrument of the ISIS pulsed neutron source to verify performance.
11 citations
TL;DR: The 3C1:2-Ba3NaRu2O9 was synthesized by heating the hexagonal perovskite 6H-Ba 3 NaRu 2O9 at 900 °C under a pressure of 9 GPa.
Abstract: The new phase 3C1:2-Ba3NaRu2O9 was synthesized by heating the hexagonal perovskite 6H-Ba3NaRu2O9 at 900 °C under a pressure of 9 GPa. 3C1:2-Ba3NaRu2O9 adopts the 1:2 B site-ordered cubic perovskite type structure [space group Pm1, cell parameters a = 5.8068(3) and c = 7.0652(5) A] but a significant cation inversion between Na and Ru sites is found. No magnetic ordering transition is observed down to 2 K and 3C1:2-Ba3NaRu2O9 displays Pauli paramagnetism indicative of a metallic oxide with strong electron-electron correlations.
11 citations
TL;DR: The 6-layered hexagonal polymorph of BaCrO3 has been synthesized at 900 °C under a pressure of 9 GPa as mentioned in this paper, and the hexagonal structure (space group P63/mmc, cell parameters a = 5.3809(9) and c = 13.123(3) A at 300 K) is stable on cooling down to 5 K.
Abstract: The 6-layered hexagonal polymorph of BaCrO3 has been synthesized at 900 °C under a pressure of 9 GPa. The hexagonal structure (space group P63/mmc, cell parameters a = 5.3809(9) and c = 13.123(3) A at 300 K) is stable on cooling down to 5 K. Ferrimagnetism with a small saturated magnetization of approx. 0.1 μB is observed below a Curie transition at TC = 192 K, and neutron diffraction shows that a simple collinear ferrimagnetic spin structure with moments parallel to c is adopted.
9 citations
TL;DR: In this article, a series of LnRuO3 perovskites (Ln: La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y) is synthesized from stoichiometric mixtures of Ru, RuO2, and Ln2O3 at a pressure of 10 GPa for Ln: Pr,Nd,Sm, and 11 GPA for ln: Eu.
Abstract: A series of LnRuO3 perovskites (Ln: La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y) is synthesized from stoichiometric mixtures of Ru, RuO2, and Ln2O3 at a pressure of 10 GPa for Ln: Pr, Nd, Sm, and 11 GPa for Ln: Eu, Gd, Tb, Dy, Ho, Y (1200 °C, 15 min).
6 citations
TL;DR: A mean-field approximation approach revealed that this unusual behavior in the ordered phase is an L-type ferrimagnetism driven by the nonequivalent magnetizations of the two ferromagnetic Cu(2+) spin sublattices in the G-type spin structure.
Abstract: A-site-ordered perovskite Ce1/2Cu3Ti4O12 has been found to crystallize in two different forms, one with random and the other with ordered Ce/vacancy distribution at the A site of the prototype AA'3B4O12 structure. The random phase is isostructural with CaCu3Ti4O12, and the ordered phase is a new ordered derivative of the AA'3B4O12-type perovskite with two crystallographically distinct Cu sites. Although both phases form a G-type antiferromagnetic arrangement of Cu(2+) spins below 24 K, their magnetisms are quite different. A typical antiferromagnetic transition is observed in the random phase, whereas a small ferromagnetic moment appears below 24 K in the ordered phase, which rapidly decreases upon further cooling. A mean-field approximation approach revealed that this unusual behavior in the ordered phase is an L-type ferrimagnetism driven by the nonequivalent magnetizations of the two ferromagnetic Cu(2+) spin sublattices in the G-type spin structure. This unusual ferrimagnetism is a direct consequence of the Ce/vacancy ordering.
6 citations
TL;DR: The magnetism of the A-site-ordered perovskite is studied comprehensively by means of neutron powder diffraction experiments and theoretical calculations as mentioned in this paper, and it has been shown that a rhombohedral 60-ifmmode 60-degree spin structure emerges on the cubic lattice below a 44-K N\'eel transition.
Abstract: The magnetism of the A-site-ordered perovskite $\mathrm{LaM}{\mathrm{n}}_{3}{\mathrm{V}}_{4}{\mathrm{O}}_{12}$ is studied comprehensively by means of neutron powder diffraction experiments and theoretical calculations. Magnetic neutron diffraction results show that a rhombohedral 60\ifmmode^\circ\else\textdegree\fi{} spin structure emerges on the cubic lattice below a 44-K N\'eel transition. Ab initio electronic structure calculations confirm that high-spin ${\mathrm{Mn}}^{2+}$ moments are localized while V $3d$-band states are itinerant, and that the noncollinear 60\ifmmode^\circ\else\textdegree\fi{} spin structure is more stable than collinear ferromagnetic or G-type antiferromagnetic alternatives. Effective Heisenberg model calculations reveal that the appearance of such a nontrivial spin structure can be attributed to significant next-nearest-neighbor and third-nearest-neighbor magnetic interactions.
TL;DR: An unprecedented reentrant mechanism in which HfNCl host layers transform to an alternative geometry and then revert to their original structure, has been discovered during the synthesis of superconducting Na0.5HfNCL by ultra-slow electrochemical intercalation of sodium as discussed by the authors.
Abstract: An unprecedented re-entrant mechanism in which HfNCl host layers transform to an alternative geometry and then revert to their original structure, has been discovered during the synthesis of superconducting Na0.5HfNCl by ultra-slow electrochemical intercalation of sodium. Both the host and final product contain hexagonal β-HfNCl layers although the stacking sequence changes during a complex reaction sequence involving four intermediate phases. Restacking occurs through a displacive mechanism in which β-type layers transform to a different arrangement, most likely the rectangular layers found in the α-HfNCl polymorph, and then retransform to β-layers. These results reveal that intercalation reactions may proceed by very different mechanisms to those expected in the conventional ‘slab-gliding’ picture.
TL;DR: In this article, a 2.3C1:2-Ba3NaRu2O9 was obtained by pressurized sintering of 6H-Ba 3NaRu 2O9 in a two-stage Walker-type press (900 °C, 9 GPa).
Abstract: 3C1:2-Ba3NaRu2O9 is obtained by pressurized sintering of 6H-Ba3NaRu2O9 in a two-stage Walker-type press (900 °C, 9 GPa).