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[EN] High pressure X-ray diffraction, Raman scattering, and electrical measurements, together with theoretical calculations, which include the analysis of the topological electron density and electronic localization function, evidence the presence of an isostructural phase transition around 2 GPa, a Fermi resonance around 3.5 GPa, and a pressure-induced decomposition of SnSb2Te4 into the high-pressure phases of its parent binary compounds (alpha-Sb2Te3 and SnTe) above 7 GPa. The internal polyhedral compressibility, the behavior of the Raman-active modes, the electrical behavior, and the nature of its different bonds under compression have been discussed and compared with their parent binary compounds and with related ternary materials. In this context, the Raman spectrum of SnSb2Te4 exhibits vibrational modes that are associated but forbidden in rocksalt-type SnTe; thus showing a novel way to experimentally observe the forbidden vibrational modes of some compounds. Here, some of the bonds are identified with metavalent bonding, which were already observed in their parent binary compounds. The behavior of SnSb2Te4 is framed within the extended orbital radii map of BA(2)Te(4) compounds, so our results pave the way to understand the pressure behavior and stability ranges of other "natural van der Waals" compounds with similar stoichiometry., This work has been performed under financial support from the Spanish MINECO under Project MALTA-CONSOLIDER TEAM network (RED2018-102612-T) and Project FIS2017-83295-P, from Generalitat Valenciana under Project PROMETEO/2018/123. This publication is a product of the "Programa de Valoracion y Recursos Conjuntos de I+D+i VLC/CAMPUS and has been financed by the Spanish Ministerio de Educacion, Cultura y Deporte, as part of "Programa Campus de Excelencia Internacional". Supercomputer time has been provided by the Red Espanola de Supercomputacion (RES) and the MALTA cluster. J.A.S. acknowledges a "Ramon y Cajal" fellowship (RYC-2015-17482) for financial support, and E.L.D.S. acknowledges Marie Sklodowska-Curie Grant No. 785789-COMEX from the European Union's Horizon 2020 research and innovation program. We also thank ALBA synchrotron and DIAMOND light source for funded experiments.

Using in situ high pressure Raman spectroscopy, two structural changes were observed in a sample of the composition LiLa5O5(VO4)2. Taking this into account and by combining different conditions, three new compounds were further obtained from high pressure–high temperature synthesis. Their crystal structure description was done using the antiphase approach, which implies the presence of oxygen-centered [OLn4] building units, where Ln is La for (1) β-LiLa5O5(VO4)2 and (2) β-LiLa2O2(VO4) or Nd for (3) LiNd5O5(VO4)2 compounds. (1) crystallizes in the triclinic space group P[1 with combining macron] with unit cell parameters of a = 5.8167(15) Å, b = 12.2954(28) Å, c = 18.7221(69) Å, α = 102.03(2)°, β = 98.76(2)°, and γ = 103.54(2)°; a 3D structure was deduced from the ambient pressure polymorph. (2) also crystallizes in P[1 with combining macron] with a = 5.8144(7) Å, b = 5.8167(7) Å, c = 8.5272(1) Å, α = 98.184(7)°, β = 100.662(7)° and γ = 92.579(7)°. It shows a 2D structure with [La2O2]2+ layers surrounded by [LiO4] and [VO4] tetrahedra sharing corners and edges. (3) exhibits a 3D architecture isotypic with AP-LiLa5O5(VO4)2. The crucial role of high pressure in such types of synthesis and materials is also discussed. 49

In this work we report on the microwave assisted synthesis of nano-sized Gd2Ti2O7 (GTO) and Ho2Ti2O7 (HTO) powders from the RE2Ti2O7 pyrochlore family (RE = rare earth). Synchrotron X-ray powder diffraction was used to study RE–Ti cationic anti-site defects with concentrations that decrease in both samples with increasing temperature starting from 1100 °C, and the defects disappear at 1400 °C. SQUID magnetometry measurements revealed that GTO shows a predominantly anti-ferromagnetic structure, whereas HTO exhibits magnetic saturation and a ferromagnetic component at low temperature. Impedance spectroscopy data revealed strongly increased ionic oxygen vacancy conduction in HTO ceramic pellets as compared to GTO, which may be associated with a higher degree of oxygen vacancy disorder. This argument was supported by Raman spectroscopy data.

Multiferroic materials exhibit two or more ferroic orders and have potential applications as multifunctional materials in the electronics industry. A coupling of ferroelectricity and ferromagnetism is hereby particularly promising. We show that the synthetic melanostibite mineral MnFeSbO (R (Formula presented.) space group) with ilmenite-type structure exhibits cation off-centering that results in alternating modulated displacements, thus allowing antiferroelectricity to occur. Massive magnetoelectric coupling (MEC) and magnetocapacitance effect of up to 4000 % was detected at a record high temperature of 260 K. The multiferroic behavior is based on the imbalance of cationic displacements caused by a magnetostrictive mechanism, which sets up an unprecedented example to pave the way for the development of highly effective MEC devices operational at or near room temperature., The authors thank MINECO for funding through projects MAT2013‐44964‐R, MAT2013‐41099‐R, MAT2015‐71070‐REDC, MAT2014‐52405‐C02‐02, CTQ2015‐67755‐C2‐1‐R (MINECO/FEDER) and FPI (BES‐2013‐066112) and Ramon y Cajal (RyC‐2010–06276) fellowships, and Comunidad de Madrid through S‐2013/MIT‐2753 grant.

The A-site ordered perovskite oxide CdCu3Mn4O 12 has been synthesized for the first time in polycrystalline form under high pressure (7 GPa) and high temperature (1000 °C) conditions, required to stabilize the small Cd and Cu cations at the A positions of the perovskite. The crystal structure has been studied by X-ray powder diffraction at room temperature. This oxide crystallizes in the cubic space group Im3̄ (no. 204) with the unit-cell parameter a = 7.2179(5) Å at 300 K. The MnO6 network is extremely tilted, giving rise to a square planar coordination for Cu2+ cations. The magnetic characterization shows that this compound is ferrimagnetic with an ordering temperature TC = 347 K, well above room temperature. A metallic behavior is displayed between 10 and 300 K. Negative magnetoresistance (MR) of 15% is achieved at 10 K for H = 9 T; MR is still significant at room temperature, displaying values above 7% for H = 9 T. Density functional theory calculations carried out on the density of states lead to electronic and magnetic features in good agreement with the obtained experimental results. © 2014 American Chemical Society., We thank the financial support of the Spanish Ministry of Science and Innovation to project MAT2010-16404, of the Comunidad de Madrid to project S2009PPQ-1551 (QUIMAPRES), and of the Spanish Consolider Ingenio 2010 Program (project CDS2007-00045). J.S.-B. acknowledges receipt of a Ramón y Cajal Fellowship (RyC-2010-06276) from the Ministerio de Economía y Competitividad.

Perovskite hydrides containing light elements are attractive for hydrogen storage. We describe a new series of composition NaKMgH (0 ≤ x ≤ 1) prepared by an original synthesis method based on the direct reaction of simple hydrides under high-pressure and high-temperature conditions. Mixtures of MgH, NaH and KH were enclosed in gold capsules and treated in a piston-cylinder hydrostatic press at moderate pressures of 2 GPa and temperatures around 775 °C. Well-crystallized hydride samples were analyzed by x-ray and neutron powder diffraction, collecting excellent patterns even in non-deuterated samples that provided an accurate description of the crystal structure features. For x ≤ 0.75 the perovskites are isostructural with GdFeO-type oxides, characterized by the same tilting scheme of the MgH octahedra. For the end-member KMgH the structure is cubic, adopting the canonical aristotype of the perovskite structure. The analysis of the tolerance factor across the series accounts for the stability of those samples in the studied compositional range., This research was supported by the MICINN and MINECO under projects MALTA-Consolider Team (CSD2007-00045, MAT2015-71070-REDC), MAT2013-41099-R and CTQ2015-67755-C2-1-R (MINECO/FEDER). A. A-C. acknowledges support from a MINECO-FPI grant (BES-2013-066112).

New complex perovskites of the series CaCu(MnRe)O have been prepared from citrate precursors under moderate pressure conditions of 2 GPa and 1000 °C, in the presence of KClO as oxidizing agent to stabilize Mn/Mn mixed valence. The polycrystalline samples have been characterized by x-ray diffraction, neutron powder diffraction (NPD), magnetic, and magnetotransport measurements. The crystal structures are cubic, space group Im-3. The unit-cell parameters increase from a = 7.2379(2) Å for the parent (x = 0) compound to a = 7.2420(4) Å for CaCu(MnRe)O. Both oxides adopt a superstructure of the perovskites ABO with long-range 1:3 ordering for Ca and Cu ions at the A sublattice. For the compound doped with Re (x = 0.1), the result of a NPD study shows that Re ions are randomly located at the octahedral positions, being the (Mn,Re)O octahedra strongly tilted, with superexchange (Mn,Re)-O-(Mn,Re) angles of 142.03°. Neutron diffraction data clearly show that some Mn ions are located together with Cu at the square-planar 6b positions. The magnetic structure determined from low-temperature NPD data unveils a ferromagnetic coupling between (Mn,Re) spins at octahedral positions and weak antiferromagnetism with (Cu,Mn) spins. Interestingly, an enhancement of the magnetoresistance effect is observed for the Re-doped compound, well beyond that found for the parent perovskite., We thank the financial support of the Spanish Ministry of Economy and Competitiveness to the projects MAT2013-41099-R and RyC-2010-06276.

Whether $\mathrm{CaCr}{\mathrm{O}}_{3}$ is a Mott insulator or a correlated metal is still controversial. We have performed measurements of magnetization, specific heat, and thermal conductivity on $\mathrm{CaCr}{\mathrm{O}}_{3}$ samples selected from many batches of high-pressure synthesis. The single-crystal $\mathrm{CaCr}{\mathrm{O}}_{3}$ sample exhibits an unprecedentedly sharp transition at a N\'eel temperature ${T}_{N}\ensuremath{\approx}90\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The critical behavior of specific heat cannot be rationalized by the renormalization group theory for a second-order magnetic transition. More surprisingly, the thermal conductivity $\ensuremath{\kappa}$ exhibits an anomalous drop on cooling through ${T}_{N}$, which is opposite to all known influence on $\ensuremath{\kappa}$ from either spin or orbital ordering. We have argued, on the basis of anomalies found in all three measurements and structural data, for the coexistence of itinerant \ensuremath{\pi}-bonding electrons in a $c$-axis band and localized $xy$ electrons in $xy$ orbitals responsible for type-C antiferromagnetic order below ${T}_{N}$ and the occupation of a pure, localized $xy$ orbital undergoing a Bose-Einstein condensate at ${T}_{N}$.

Magnesium base alloys are very attractive for hydrogen storage due to their large hydrogen capacity, small weight and low-cost. We have designed a new synthesis method for the ternary metal hydride perovskite system Na1−xLixMgH3, based on the direct reaction of simple hydrides under high-pressure and moderate-temperature conditions. Well-crystallized samples were obtained in a piston-cylinder hydrostatic press at moderate pressures of 2 GPa and temperatures around 750 °C from mixtures of MgH2, NaH and LiH enclosed in gold capsules. X-ray and neutron powder diffraction analysis were used to identify the purity of the samples and provide an accurate description of the crystal structure features (GdFeO3 type). Na1−xLixMgH3 hydrides series (0 ≤ x ≤ 0.18) show an orthorhombic symmetry with space group Pnma (No. 62). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been carried out to determine the hydrogen desorption temperatures.

Many transition-metal oxides in elevated valence states [e.g. Mn(V), Co(IV), Ni(III), Cu(III) ] present a metastable character and, given the difficulty of their synthesis, have been relatively little studied. However, they are very interesting materials presenting strong electronic correlations that are bound to exotic properties such as superconductivity, metal behavior, metal–insulator transitions or colossal magnetoresistance. The metastability of these compounds requires special synthesis conditions such as the application of high pressure. In the last years, we have prepared and investigated a good number of materials belonging to several families such as RNiO3 (R = rare earths), Ba3Mn2O8 , (Ba,Sr)CoO3 , La2(Ni,Co)O4+δ , etc. In the study and correct characterization of these oxides it has been decisive the use of elastic neutron diffraction, most of the times in powder samples. This technique has allowed us to access the structural details typically related to the octahedral tilting in perovskite structures, the oxygen stoichiometry and order–disorder of the oxygen sublattice, the distinction between close elements in the Periodic Table, the resolution of magnetic structures and, in general, the establishment of a correlation between the structure and the properties of interest. This letter is organized around the binomial "high-pressure synthesis" and "characterization by neutron diffraction" and illustrated with some selected examples among the metastable materials above mentioned.

A combination of high-pressure single crystal X-ray diffraction and high-pressure SQUID magnetometry has been used to study two oxo-bridged mixed-valent MnIII/MnIV dimers. [Mn2O2(bpy)4](ClO4)3·3CH3CN, (1·3CH3CN; bpy = 2,2ʹ-bipyridine) has been compressed to 2.0 GPa whilst [Mn2O2(bpy)4](PF6)3·2CH3CN·1H2O, (2·2CH3CN·1H2O) could be measured crystallographically up to 4.55 GPa. The PF6 salt of [Mn2O2(bpy)4]3+ has never been reported before while 1 has been reported as a hydrate and in a different crystallographic space group. The application of hydrostatic pressure imposes significant distortions and modifications in the structures of both complexes. In particular, in complex 1·3CH3CN the Mn-Mn separation is reduced by the contraction of some of the Mn-O bond lengths, whilst in 2·2CH3CN·1H2O the Mn-O-Mn bridging angles and the Mn-O bond lengths are substantially unchanged. Interestingly 2·2CH3CN·1H2O also shows a constant contraction in nearly all the Mn-N bonds. The magnetic behaviour of the complexes has been measured up to 0.87 GPa for 1·3CH3CN and 0.84 GPa for 2·2CH3CN·1H2O.

The complex perovskite ThCu3Mn4O12 has been prepared at moderate pressures of 2 GPa. With respect to the parent compound CaCu3Mn4O12, the replacement of Ca2+ by Th4+ involves a double electronic injection that leads to a substantial increment of TC, up to 370 K. The crystal structure was refined in the space group Im3̄ from NPD data collected with λ = 1.33 Å at r. t. An additional NPD pattern recorded at 1.8 K with λ = 2.42 Å allowed to refine the magnetic structure, which displays a ferrimagnetic coupling between Mn3+/Mn4+ and Cu2+ spins, aligned along the c direction. The refined magnetic moments at the Mn and Cu substructures of 2.5 and −0.5 μB, respectively, account for the observed saturation magnetisation at 2 K, of 7 μB/f. u. A semiconducting behaviour is observed between 10 and 350 K which can be correlated with the appearance of a gap in the conduction band for the ~50% Mn3+/50% Mn4+ mixed valence observed in the B substructure of this perovskite.

We report high-pressure X-ray diffraction and electrical measurements of the topological insulator SnBi2Te4 at room temperature. The pressure dependence of the structural properties of the most stable phase of SnBi2Te4 at ambient conditions (trigonal phase) have been experimentally determined and compared with results of our ab initio calculations. Furthermore, a comparison of SnBi2Te4 with the parent compound Bi2Te3 shows that the central TeSnTe trilayer, which substitutes the Te layer at the center of the TeBiTeBiTe layers of Bi2Te3, plays a minor role in the compression of SnBi2Te4. Similar to Bi2Te3, our resistance measurements and electronic band structure simulations in SnBi2Te4 at high pressure suggest that this compound exhibits a pressure-induced electronic topological transition or Lifshitz transition between 3.5 and 5.0 GPa. (C) 2016 Published by Elsevier B.V., We thank Dr. Philipp Urban for preparing the sample. This work has been performed under financial support from Spanish MINECO under projects MAT2013-46649-C4-2-P, MAT2015-71070-REDC and CTQ2015-67755-C2-1-R and from Spanish Ministerio de Educacion, Cultura y Deporte as part of "Programa Campus de Excelencia Internacional/Programa de Valoracion y Recursos Conjuntos de I + D + i VLC/CAMPUS" through projects SP20140701 and SP20140871. One of the experiments were performed at MSPD-BL04 beamline at ALBA Synchrotron with the collaboration of ALBA staff. J.A.S. thanks "Juan de la Cierva" fellowship program for funding. A. A.-C. and J.S.-B. are also grateful to Spanish MINECO for the FPI (BES-2013-066112) and Ramon y Cajal (RyC-2010-06276) fellowships. We acknowledge Diamond Light Source for time on beamline I15 under Proposal EE9102.

RNiMnO (R = Tb, Ho, Er, Tm) perovskites have been prepared by soft-chemistry techniques followed by high oxygen-pressure treatments; they have been investigated by X-ray diffraction, neutron powder diffraction (NPD), and magnetic measurements. In all cases the crystal structure is defined in the monoclinic P2/n space group, with an almost complete order between Ni and Mn cations in the octahedral perovskite sublattice. The low temperature NPD data and the macroscopic magnetic measurements indicate that all the compounds are ferrimagnetic, with a net magnetic moment different from zero and a distinct alignment of Ni and Mn spins depending on the nature of the rare-earth cation. The magnetic structures are different from the one previously reported for LaNiMnO, with a ferromagnetic structure involving Mn and Ni moments. This spin alignment can be rationalized taking into account the Goodenough-Kanamori rules. The magnetic ordering temperature (T) decreases abruptly as the size of the rare earth decreases, since T is mainly influenced by the superexchange interaction between Ni and Mn (Ni-O-Mn angle) and this angle decreases with the rare-earth size. The rare-earth magnetic moments participate in the magnetic structures immediately below T., We acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness through the projects MAT2013-41099-R and CSD2007-00045 and funding of the Universidad Complutense de Madrid and Banco Santander through the project UCM2014-971703. J.S.-B. acknowledges receipt of a Ramón y Cajal Fellowship (RyC-2010-06276) from the Spanish Ministry of Economy and Competitiveness.

This work was supported by the National Science Foundation (DMR MIRT 1122603) and Robert A. Welch Foundation (F-1066) in the U.S. J.A.A., R.M.C., J.S.B., and M.T.F. thank the Spanish MINECO Projects No. MAT2013-41099-R and No. RyC-2010-06276 for financial support.

We have synthesized two new series of metastable oxides, namely REMn2O5 and RECu3Mn4O12 (RE = rare earths) under moderate pressure conditions. A novel series of ferrimagnetic oxides has been obtained by replacing Mn3+ by Fe3+ in the parent REMn2O5 compounds (RE = Y, Dy, Ho, Er, Tm, Yb). The crystal structure has been studied by neutron powder diffraction (NPD); it contains chains of edge-linked Mn4+O6 octahedra connected via dimeric groups of Fe3+O5 square pyramids. The magnetic susceptibility and the thermal evolution of the NPD patterns reveal the onset of a ferrimagnetic structure below TC ≈ 165 K, characterized by the propagation vector k = 0. Immediately below TC, the Fe3+ and Mn4+ moments lie in an antiparallel arrangement along the c-axis direction. At lower temperatures, the magnetic moment of the rare-earth cations also participates in the magnetic structure, adopting a parallel arrangement with the Fe3+ spins. Some new derivatives of CaCu3Mn4O12 have been prepared at moderate pressures of 2 GPa by replacing Ca2+ by RE3+ cations in the series RECu3Mn4O12 (RE = Pr, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb); the concomitant electronic injection leads to a substantial contribution to TC. The crystal structures of the new materials were refined in the space group Im3̅ from NPD data for the non-absorbing RE cations. The unit cell parameters are considerably expanded with respect to CaCu3Mn4O12, as a result of the electronic injection. The r. t. magnetic structure displays a ferrimagnetic coupling between Mn3+/4+ and Cu2+ spins; at low temperatures there is an antiferromagnetic coupling of the RE3+ moments with the Mn substructure, which substantially reduces the susceptibility and the saturation magnetization.

A stoichiometric calcium vanadium (IV) oxide with formula CaVO3 has been prepared by soft-chemistry procedures, followed by annealing under reducing conditions (H2/N2 flow). This material has been studied by X-ray and neutron powder diffraction (NPD), thermal analysis, magnetic and magnetotransport measurements. CaVO3.0 perovskite crystallizes in the orthorhombic Pbnm (No. 57) space group, with the GdFeO3-type structure. The unit-cell parameters are a=5.3219(1) A , b=5.3427(1) A and c=7.5472(1) A . In this distorted perovskite the VO6 octahedra are tilted by 10.1° in order to optimize the Ca–O bond-lengths. A bond valence study from NPD data confirms the tetravalent oxidation state for V cations. The perovskite is fully oxygen stoichiometric, as demonstrated from thermal analysis and the refinement of the oxygen occupancy factors. The magnetic susceptibility is predominantly Pauli paramagnetic-like, although a non-negligible temperature-dependent component due to isolated V4+ spins is patent at low temperatures. The transport measurements show a metallic behavior between 2 and 300 K; at low temperatures a positive magnetoresistance as large as 14% for H=9 T is interpreted as the result of quantum interference effects.

The insulating character of undoped manganites RMnO 3 is related to the Jahn-Teller (JT) distortion of the MnO 6 octahedra, which can be reduced by applying hydrostatic pressure. We analyze the dependence of the Raman phonons with pressure up to 10 GPa, for PrMnO 3 and TbMnO 3 . The variation of the stretching and bending modes indicates that the compressibility of Mn-O bonds is higher than R-O ones in PrMnO 3 , but smaller in TbMnO 3 . The variation of the tilt mode frequency, in TbMnO 3 , can be explained by an increase of the octahedra tilt angle, which is consistent with a larger compressibility of the Tb-O bonds. Therefore, while PrMnO 3 evolves towards the structure of the metallic-ferromagnetic doped perovskites, TbMnO 3 does not.

We have studied and compared the optical properties of SrAl1.7B0.3O4:Eu, R (R=Nd, Dy) pigments that present long-lasting phosphorescence obtained by different synthesis techniques. Samples obtained by ceramic methods, in our laboratories and by an industrial process, present better phosphorescent properties than those obtained by sol–gel technique. Raman spectra show that grinding produces severe damage of the lattice. We have obtained and analyzed the Eu3+ crystal field luminescence indicating that Eu3+ is found in quite different sites comparing ceramic and sol–gel samples. Codoping, with Nd or Dy is necessary in order to reduce the Eu3+ content, in all cases. The green luminescence band, obtained under UV illumination, can be fitted to two and three components in ceramic and sol–gel samples, respectively, due to different Eu2+ sites. Eu–Dy samples present the longest and the most efficient phosphorescence. The time evolution of the afterglow is well described by a t−1 law, up to about 2 h, indicating that the recombination process is achieved by electron–hole tunneling.

We have studied the crystallographic structure, the oxidation states and the electronic structure of the ternary metal hydride Mg2FeH6 prepared by high-pressure and high-temperature reaction from the simple hydrides. This procedure provides a well-crystallized sample essential to perform Neutron Powder Diffraction (NPD) experiments. By NPD we accessed to subtle crystallographic details of Mg2FeH6, such as the hydrogen occupancy and the metal-hydrogen distances and angles; a full hydrogen composition with six hydrogen atoms per unit formula is determined. No magnetic structure was observed by NPD at 2 K. The oxidation states of the atoms inside the material and the electronic structure of Mg2FeH6 have been evaluated by x-ray absorption near-edge spectroscopy (XANES). Unexpectedly, the Fe K-edge spectrum shows a zero oxidation state of Fe atoms in agreement with Mössbauer data. Our results suggest a metallic nature of the hydride in contrast with the ionic character previously reported for this compound., We thank the financial support of the Spanish Ministry of Economy and Competitivity to the project MAT2013-41099-R, and for beamtime provision.

In this work we present magnetic and transport measurements as a function of temperature of polycrystalline pure and doped RMnO 3 (R=La, Nd and Ho) samples. We have performed AC transport measurement (in the range from DC to 2 MHz) in pellets of La 0.7 Ca 0.3 MnO 3 annealed at several temperatures, which present different connectivity between grains keeping the same mean grain size (about 1 μm). A large increase of resistance is observed at temperatures below the ferromagnetic order temperature where a metallic regime is expected. The behavior with temperature and frequency of the resistance of low temperature annealed pellets can be explained taking into account that two kinds of conduction paths are present in the sample: low resistive metallic ones, where the carriers percolate through well connected grains and highly resistive channels for high intergrain boundaries that behave as capacitors. In order to obtain the conduction mechanisms for non-doped RMnO 3 samples, we have measured the resistance in a large temperature range up to 1400 K. We present a discussion about the deviation of the measured resistance from any kind of polaronic conduction at high and low temperature. The differences in the activation energy with the rare earth ion are also discussed.

Pressure-induced switching of a fast-relaxing single-molecule magnet to a slow-relaxing isomer is observed for the first time by using a combination of high pressure single-crystal X-ray diffraction and high pressure magnetic measurements.

We have studied the series of nominal composition BiCu3(Mn 4-xFex)O12 (x=0, 1.0, 2.0) where Mn is replaced by Fe cations in the ferrimagnetic perovskite BiCu3Mn 4O12. These compounds have been prepared from citrate precursors under moderate pressure conditions (3.5 GPa) and 1000 °C in the presence of KClO4 as oxidizing agent. All the samples have been studied by x-ray and neutron powder diffraction (NPD) at room temperature and 4 K. The crystal structure has been defined in a cubic Im3̄ (No. 204) space group with a 2a0 × 2a0 × 2a0 unit-cell. The doubling of the unit-cell occurs due to the ordering of Bi 3+ and Cu2+ cations over A sites of the AA′3B4O12 structure. The A-site accommodates 12-fold coordinated Bi3+ ions and, at the A'-site, Jahn-Teller Cu2+ ions form pseudo-square planar units aligned perpendicular to each other. Mn4+/Fe3+ cations randomly occupy the centre of slightly distorted octahedra. These materials have also been characterized by magnetic and magnetotransport measurements. We found that all the samples are ferrimagnetic and show a progressive decrease of TC as the Fe content increases, since Fe ions disturb the ferromagnetic interactions within the B magnetic sublattice. In fact, the Curie temperature diminishes from TC=360 K (x=0) to TC=219 K (x=2). The magnetic structures, studied by low-temperature NPD data, correspond to an antiferromagnetic arrangement of spins at 8c and 6b sites; the ordered moments are in excellent agreement with those obtained from the saturation magnetization at 4 K. A significant magnetoresistant effect is determined for the x=1.0 oxide, with low-field values as high as 5% at 300 K and 1 T. © 2013 Elsevier Inc., We thank the financial support of the Spanish Ministry of Science and Innovation to the project MAT2010-16404 and of the Comunidad de Madrid to the project S2009PPQ-1551.

Intermetallic compounds with nominal formula CaNi 5− x Cu x ( x = 0, 1, 2.5) have been prepared in order to investigate their hydrogenation properties. The samples were obtained by arc-melting and were deuterated in a Sieverts reactor. For x = 0 and 1, we have found that the fast kinetics and the different shape of the curve (non sigmoidal) in the second absorption process indicate an improvement of the hydrogen absorption due to the activation of the alloys. The deuterium desorption spectra are similar for x = 0 and 1 whereas for x = 2.5 the desorption ranges a broader temperature interval (∼100–350 °C) indicating a certain degree of chemical inhomogeneity or amorphization intrinsic to the parent sample or induced by the deuterium absorption. The formed deuterides were passivated in the presence of air in order to carry out a neutron diffraction study, allowing us to determine the deuterium positions in the samples. While in CaNi 4 CuD y the deuterium is randomly distributed over seven different positions, in CaNi 5 D y the deuterium only occupies five of them. This wider distribution in CaNi 4 CuD y can explain its higher stability, and therefore, its higher desorption temperature for deuterium.

The compounds of the title series have been prepared from citrate precursors under moderate pressure conditions (P = 2 GPa) and 1000 °C in the presence of KClO4 as oxidizing agent. The crystal structures are cubic, space group Im3̄ (No. 204); the unit cell parameters linearly vary from a = 7.3272(4) Å (R = La) to a = 7.2409(1) Å (R = Lu) at room temperature. A neutron or synchrotron X-ray diffraction study of all the members of the series reveals an interesting correlation between some structural parameters and the magnetic properties. The electron injection effect upon replacement of Ca2+ with R3+ cations in the parent CaCu3Mn4O12 oxide leads to a substantial increment of the ferrimagnetic Curie temperature (TC). An essential ingredient is supplied by the internal pressure of the R3+ cations upon a decrease in size along the rare-earth series, from La to Lu: the concomitant compression of the MnO6 octahedral units for the small rare earths provides progressively shorter Mn-O distances and improves the overlapping between Mn and O orbitals, thereby promoting superexchange and enhancing TC by 50 K along the series. This interaction is also reinforced by a ferromagnetic component that depends on the local distortion of the MnO6 octahedra, which also increases along the series, constituting an additional factor, via intersite virtual charge transfer t-e orbital hybridization, for the observed increment of TC. © 2010 American Chemical Society., We thank CICyT for the financial support of the project MAT2007-60536.

The possibilities of the use of Eu3+ in extracting information of the pressure effects on the nature of its crystal site in the NH4.Eu(SO4)2 catalytic host are closely inspected through the study of emission spectra for applied pressures up to 87 kbar. The phenomenological crystal field analysis of these spectra reveals clear discontinuities, at approximately 30 kbar, the sharper ones, and then at approximately 70 kbar, in crystal field strength trends, which taken together with structure-based simulations of crystal field interactions indicate well-defined pressure-induced anisotropic distortions in Eu3+ local environments.

In2VO5, containing magnetically frustrated zig-zag chains, shows a remarkable magnetic crossover at 120 K between paramagnetic states with positive (17 K) and negative (-70 K) Weiss temperatures. Magnetic moment and entropy data show that the V4+ S = 1/2 spins condense into S = 1 triplet dimers below the crossover. A further freezing of the antiferromagnetically coupled triplet dimers into a global singlet state is observed at 2.5 K, with no long range magnetic order down to 0.42 K and in fields up to 9 T. No structural V-V dimerization is observed by high-resolution X-ray diffraction down to 10 K, but a subtle lattice anomaly evidences a spin-lattice coupling in the triplet dimer state. This is assigned to longitudinal oxygen displacement modes that reduce frustration within the chains and so couple to the spin dimer fluctuations.

We have carried out neutron powder-diffraction measurements on zinc paratacamite Zn$_x$Cu$_{4-x}$(OH)$_6$Cl$_2$ with $x=1$, and studied the heat capacity in fields of up to 9 T for $0.5 \leq x \leq 1$. The $x=1$ phase has recently been shown to be an outstanding realisation of the $S=1/2$ kagom\'{e} antiferromagnet. A weak mixing of Cu$^{2+}$/Zn$^{2+}$ between the Cu and the Zn sites, corresponding to $\sim 9$% of all Cu$^{2+}$ for $x=1$, is observed using neutron diffraction. This ``antisite disorder'' provides a consistent explanation of the field dependence of the heat capacity for $0.8 \leq x \leq 1$. From comparison of the derived Cu$^{2+}$ occupancy of the Zn sites for $x = 0.8... 1$ with the magnetic susceptibility, we argue that for $x = 0.8... 1$ zinc paratacamite is a spin liquid without a spin gap. The presence of unpaired but nevertheless strongly interacting spins gives rise to a macroscopically degenerate ground state manifold, with increasingly glassy dynamics as $x$ is lowered., Comment: Thorough revision of manuscript. Added 1 figure. Manuscript submitted to Physics Review Letters

New compounds of the series CaCu3(Mn4−xAlx)O12 have been prepared under high pressure conditions (2 GPa), in the presence of KClO4 as oxidizing agent to stabilize Mn3+,4+ mixed valence. The polycrystalline samples have been characterized by x-ray diffraction, neutron powder diffraction (NPD), magnetic, and magnetotransport measurements. All the samples are cubic, space group Im-3. These oxides adopt a superstructure of ABO3 perovskite given by the long-range 1:3 ordering of Ca2+ and Cu2+ ions at the A sublattice. The NPD study for x = 0.4 shows that Al3+ ions are statistically distributed at the octahedral positions, being the (Mn,Al)O6 octahedra strongly tilted, with superexchange (Mn,Al)-O-(Mn,Al) angles of 142.1°. Also, neutron data clearly show that some Mn3+ ions (0.65(2) per formula) are located together with Cu2+ at the square-planar 6b positions. Regarding the magnetic properties, all the compounds present a spontaneous increase of the magnetization below TC, typical of ferro-or ferrimagnetic materials, with TC decreasing upon Al introduction. The magnetic structure determined from low-temperature NPD data unveils a ferromagnetic coupling between (Cu2+, Mn3+)6b spins and Mn8c spins at octahedral positions; this is in contrast with the ferrimagnetic structure observed for RCu3Mn4O12 and CaCu3Mn4O12, where an AFM coupling is observed between both magnetic sublattices. Interestingly, an enhancement of the magnetoresistance effect is observed for x = 0.2, well beyond that found for the parent compound. This effect, in materials subtly doped with non-magnetic elements at the Mn positions, may be of interest for applications., We thank the financial support of the Spanish Ministry of Economy and Competitiveness to the projects MAT2013-41099-R and RyC-2010-06276.

In this paper we describe some novel members of the RNiO3 perovskites, containing Ni3+ and presenting metal-insulator transitions coupled to a charge disproportionation effect. This phenomenon is observed in the insulating state as a subtle monoclinic distortion, due to the presence of two unequivalent Ni ions with different coordination environments, corresponding to expanded and contracted octahedra alternating in the three-dimensional network. Here we examine how the introduction of mixed rare-earth cations at the R sublattice affects the metal-insulator transition and the structural features, from high resolution neutron powder diffraction studies.

High-pressure synthesis is a powerful technique to stabilize metastable oxides, either containing transition metals in unusual oxidation states, or favouring the formation of dense perovskite-related phases. Happily, many solids synthesized at high pressure-high temperature conditions (where they are fhermodynamically stable) can be "quenched" to ambient conditions, where they are termodynamically metaestable, yet they remain indefinitely kinetically stable. In this paper we illustrate the example of a new family of oxides derived from the CaCu3Mn4O12 perovskite. We have studied the series of nominal composition NdCu3(Mn3M)O12 (M = Fe, Cr) where Mn is replaced by Fe(Cr) cations in the ferrimagnetic perovskite NdCu3Mn4O12. These materials have been synthesized in poly crystalline form under moderate pressure conditions of 2 GPa, in the presence of KClO4 as oxidizing agent. All the samples have been studied by neutron powder diffraction (NPD) below and above the ferromagnetic Curie temperatures. These oxides crystallize in the cubic space group Im (No. 204). Mn4+/Mn3+ and Fe3+(Cr3+) occupy at random the octahedral B positions of the perovskite structure. The materials have also been characterized by magnetic and magnetotransport measurements. All the samples are ferrimagnetic and show a decrease of TC upon Fe(Cr) introduction since these ions disturb the ferromagnetic interactions within this magnetic sublattice. The introduction of Fe changes the resistivity response from metallic to a semiconductor behavior. However, the magnetoresistance is still considerable at 300 K upon Fe doping, and it is enhanced at 100 K probably due to the decrease in the number of charge carriers from the pure oxide to the Fe-doped compound.

We describe the design of a miniature high-pressure cell built for a Magnetic Property Measurement System based on the Superconducting Quantum Interference Device technology magnetometer. The key feature of the cell is the plug for in situ pressure measurements with feed-through wires connected to a manganin pressure sensor, which has a known pressure dependence of electrical resistivity. By monitoring pressure continuously during magnetisation measurements in the range of operational temperatures the true pressure can always be established.