Large magnetocaloric effect and magnetic properties of polymorphic RCrO4 (R=rare earth) oxides

Trabajo presentado al: "International Symposium on the Reactivity of Solids" celebrado en San Petersburgo (Rusia) del 9 al 13 de junio de 2014.-- et al.RCrO4 oxides, where R=rare earth, crystallize at ambient conditions with the tetragonal zircon-type structure, S.G. I41/amd. Recently, we have succeeded to synthesize the scheelite polymorphs (S.G. I41/a) for most of the R elements, by treating the zircon phases at 40 kbar and 813 K. These scheelite high pressure forms are quencheable after releasing the pressure, and TGA and X-ray diffraction data reveal a reversible transition from scheelite polymorph to zircon at 700 K. The coexistence of two paramagnetic ions in these phases, namely Cr5+ and R3+, constitutes a very interesting scenario to study 3d-4f magnetic interactions. In this sense, most of the zircon-type RCrO4 oxides behave as ferromagnetic, while the scheelite polymorphs are antiferromagnetic. The change in the sign of the magnetic interaction can be explained by considering the changes in both distances and bond angles of Cr-O-R pathway through which the superexchange interactions take place. Neutron diffraction studies have been used to determine the nuclear and magnetic structures for these zircon and scheelite RCrO4 polymorphs. The analysis of the data reveals the onset of new reflections below the estimated Néel temperature from the previous magnetic susceptiblity data corresponding to the scheelite phases. The magnetic structure for these scheelite RCrO4 oxides can be described with a propagation vector κ = [0 0 0], where the moments of R3+ and Cr5+ are aligned along the c-axis or confined in the ab-plane of the tetragonal structure depending on the nature of the R element. This behavior has been confirmed from heat capacity measurements where the onsets of λ-anomalies at the ordering temperatures are almost coincident with those determined from magnetic susceptibility and neutron diffraction analysis. A detailed analysis of the zircon and scheelite structural types is included in this work to explain the different magnetic behavior showed by these two polymorphic phases allowing establishing relationships structure-magnetic properties. Very recently it has been reported that the zircon RCrO4 (R= Ho and Dy) phases show large values of the magnetocaloric parameters that make of these oxides potential refrigerant materials to be used for the liquefaction of hydrogen. It is noting that the field induced magnetization in the case of the scheelite polymorphs yields magnetic moments larger than in the case of the homologous zircon phases and hence the magnetocaloric effect is expected to be larger for the scheelite polymorphs in comparison with the zircon ones. In this work a preliminary study has been done in order to determine the magnetocaloric parameters of the two polymorphic phases of TbCrO4 and HoCrO4. In this sense the isothermal entropy change, ΔST, as a function of temperature has been calculated for magnetic field variations from the measurements of magnetization versus field at different temperatures. Both magnetocaloric parameters, ΔST and the adiabatic temperature change, ΔTad, have also been evaluated from the entropy functions at different fields obtained from the heat capacity data.Peer Reviewe

Effect of Gd polarization on the large magnetocaloric effect of GdCrO4 in a broad temperature range

The ferromagnetic zircon-type phase of GdCrO4 presents high values for the magnetocaloric (MC) parameters. This compound has large isothermal entropy changes ¿ST under the magnetic field action in a wide temperature range, from 5 to 35 K, reaching a maximum |¿ST|=29.0±0.1J/kgK at 22 K, for a field increment ¿B=9 T. It orders ferromagnetically at TC=21.3K via the Cr-Cr exchange interaction and shows a second transition at 4.8 K due to the ordering of the Gd sublattice. The large MC effect is enhanced by the polarization of the Gd3+ ions by the Cr5+ ones via a weaker Gd-Cr interaction. This effect is an interesting feature to be considered in the search for new compounds with a high MC effect in the range of liquid hydrogen or natural gas, regarding the liquefaction of gases by magnetization-demagnetization cycles. This paper contains experimental measurements of magnetization, heat capacity, and direct determinations of the MC effect. The magnetic contribution to the heat capacity Cm has been obtained after subtracting the lattice component. Approximate values for the exchange constants J1 (Cr-Cr) and J3 (Gd-Cr) have been deduced from Cm

Mössbauer and susceptibility studies of FeMoVO<SUB>7</SUB>

The oxidic phase FeMoVO7 of the Fe2O3-V2O5-MoO3 system has been synthesized. Susceptibility measurements from 4.2 K to room temperature display an antiferromagnetic behaviour with an effective magnetic moment of 5.80ώb. This value, and the measured Mossbauer isomer shift of 0.40±0.02 mm/s, are characteristic of high-spin Fe(III). The lowTn value of 15 K suggests a weak interaction among the Fe-O polyhedra. The relative small QS=0.28±0.02 mm/s found in this compound shows that the Fe-O polyhedra symmetry is close to octahedral and almost undistorted.Facultad de Ciencias ExactasCentro de Química Inorgánic

Effect of Gd polarization on the large magnetocaloric effect of GdCrO4 in a broad temperature range

The ferromagnetic zircon-type phase of GdCrO4 presents high values for the magnetocaloric (MC) parameters. This compound has large isothermal entropy changes ΔST under the magnetic field action in a wide temperature range, from 5 to 35 K, reaching a maximum |ΔST|=29.0±0.1J/kgK at 22 K, for a field increment ΔB=9 T. It orders ferromagnetically at TC=21.3K via the Cr-Cr exchange interaction and shows a second transition at 4.8 K due to the ordering of the Gd sublattice. The large MC effect is enhanced by the polarization of the Gd3+ ions by the Cr5+ ones via a weaker Gd-Cr interaction. This effect is an interesting feature to be considered in the search for new compounds with a high MC effect in the range of liquid hydrogen or natural gas, regarding the liquefaction of gases by magnetization-demagnetization cycles. This paper contains experimental measurements of magnetization, heat capacity, and direct determinations of the MC effect. The magnetic contribution to the heat capacity Cm has been obtained after subtracting the lattice component. Approximate values for the exchange constants J1 (Cr-Cr) and J3 (Gd-Cr) have been deduced from Cm.This work has been funded by the Spanish MINECO through Projects No. MAT2013-44063-R and No. MAT2013-44964-R, DGA Consolidated Group E100, and Comunidad de Madrid Project No. S2009/PPQ-1626.Peer Reviewe

Impurities in s=1s=1 Heisenberg Antiferromagnets

The $s=1$ Heisenberg Antiferromagnet is studied in the presence of two kinds of local impurities. First, a perturbed antiferromagnetic bond with $J'\ne J$ at the center of an even-length open chain is considered. Using the density matrix renormalization group method we find that, for sufficiently strong or weak $J'$, a bound state is localized at the impurity site, giving rise to an energy level in the Haldane gap. The energy of the bound state is in agreement with perturbative results, based on $s=1/2$ chain-end excitations, both in the weak and strong coupling limit. In a region around the uniform limit, $J'=J$, no states are found with energy below the Haldane gap. Secondly, a $s=1/2$ impurity at the center of an otherwise even-length open chain is considered. The coupling to the $s=1/2$ impurity is varied. Bound states in the Haldane gap are found {\it only} for sufficiently weak (antiferromagnetic) coupling. For a $s=1/2$ impurity coupled with a strong (antiferromagnetic) bond, {\it no} states are found in the Haldane. Our results are in good qualitative agreement with recent experiments on doped NENP and Y$_2$BaNiO$_5$.Comment: 29 pages, RevTeX 3.0, 12 uuencoded postscript figures include

Shells and humans: molluscs and other coastal resources from the earliest human occupations at the Mesolithic shell midden of El Mazo (Asturias, Northern Spain)

Human populations exploited coastal areas with intensity during the Mesolithic in Atlantic Europe, resulting in the accumulation of large shell middens. Northern Spain is one of the most prolific regions, and especially the so-called Asturian area. Large accumulations of shellfish led some scholars to propose the existence of intensification in the exploitation of coastal resources in the region during the Mesolithic. In this paper, shell remains (molluscs, crustaceans and echinoderms) from stratigraphic units 114 and 115 (dated to the early Mesolithic c. 9 kys cal BP) at El Mazo cave (Asturias, northern Spain) were studied in order to establish resource exploitation patterns and environmental conditions. Species representation showed that limpets, top shells and sea urchins were preferentially exploited. One-millimetre mesh screens were crucial in establishing an accurate minimum number of individuals for sea urchins and to determine their importance in exploitation patterns. Environmental conditions deduced from shell assemblages indicated that temperate conditions prevailed at the time of the occupation and the morphology of the coastline was similar to today (rocky exposed shores). Information recovered relating to species representation, collection areas and shell biometry reflected some evidence of intensification (reduced shell size, collection in lower areas of exposed shores, no size selection in some units and species) in the exploitation of coastal resources through time. However, the results suggested the existence of changes in collection strategies and resource management, and periods of intense shell collection may have alternated with times of shell stock recovery throughout the Mesolithic.This research was performed as part of the project “The human response to the global climatic change in a littoral zone: the case of the transition to the Holocene in the Cantabrian coast (10,000–5000 cal BC) (HAR2010-22115-C02-01)” funded by the Spanish Ministry of Economy and Competitiveness. AGE was funded by the University of Cantabria through a predoctoral grant and IGZ was funded by the Spanish Ministry of Economy and Competitiveness through a Juan de la Cierva grant. We also would like to thank the University of Cantabria and the IIIPC for providing support, David Cuenca-Solana, Alejandro García Moreno and Lucia Agudo Pérez for their help. We also thank Jennifer Jones for correcting the English. Comments from two anonymous reviewers helped to improve the paper