59 research outputs found
A cluster model with random anisotropy for hysteresis jumps in CeNiCu alloys
Some Cerium compounds exhibit hysteresis cycles with sharp macroscopic jumps
in the magnetization at very low temperatures. This effect is attributed to the
formation of clusters in which the anisotropy competes with the applied
magnetic field. Here, we present a simple model where a lattice of
ferromagnetically coupled spins is separated in clusters of random sizes and
with random anisotropy. Within this model, we obtain hysteresis cycles
presenting jumps that behave in a similar way that the experimental ones, and
that disappear when increasing the temperature. The results are in good
agreement with the hysteresis cycles measured at very low temperatures in
CeNiCu and the comparison with these experimental results allows
to discriminate the relative importance of the mechanisms driving the thermal
evolution of the cycles.Comment: Accepted in PR
Size effects in the magnetic behaviour of TbAl_2 milled alloys
The study of the magnetic properties depending upon mechanical milling of the
ferromagnetic polycrystalline TbAl_2 material is reported. The Rietveld
analysis of the X-ray diffraction data reveals a decrease of the grain size
down to 14 nm and -0.15 % of variation of the lattice parameter, after 300
hours of milling time. Irreversibility in the zero field cooled - field cooled
(ZFC-FC) DC-susceptibility and clear peaks in the AC susceptibility between 5
and 300 K show that the long-range ferromagnetic structure is inhibited in
favour of a disordered spin arrangement below 45 K. This glassy behaviour is
also deduced from the variation of the irreversibility transition with the
field (H^{2/3}) and frequency. The magnetization process of the bulk TbAl_2 is
governed by domain wall thermal activation processes. By contrast, in the
milled samples, cluster-glass properties arise as a result of cooperative
interactions due to the substitutional disorder. The interactions are also
influenced by the nanograin structure of the milled alloys, showing a variation
of coercivity with the grain size, below the crossover between the multi- and
single-domain behaviours.Comment: 23 pages, 11 figures, to appear in J. Phys.: Condens. Ma
Magnetic fluctuations in frustrated Laves hydrides R(Mn_{1-x}Al_{x})_{2}H_{y}
By neutron scattering, we have studied the spin correlations and spin
fluctuations in frustrated Laves hydrides, where magnetic disorder sets in the
topologically frustrated Mn lattice. Below the transition towards short range
magnetic order, static spin clusters coexist with fluctuating and alsmost
uncorrelated spins. The magnetic response shows a complexe lineshape, connected
with the presence of the magnetic inhomogeneities. Its analysis shows the
existence of two different processes, relaxation and local excitations, for the
spin fluctuations below the transition. The paramagnetic fluctuations are
discussed in comparison with classical spin glasses, cluster glasses, and non
Fermi liquid itinerant magnets
Decision making under uncertainty in environmental projects using mathematical simulation modeling
The final publication is available at Springer via http://dx.doi.org/10.1007/s12665-016-6135-yIn decision-making processes, reliability and risk aversion play a decisive role. The aim of this study is to perform an uncertainty assessment of the effects of future scenarios of sustainable groundwater pumping strategies on the quantitative and chemical status of an aquifer. The good status of the aquifer is defined according to the terms established by the EU Water Framework Directive (WFD). A decision support systems (DSS) is presented, which makes use of a stochastic inverse model (GC method) and geostatistical approaches to calibrate equally likely realizations of hydraulic conductivity (K) fields for a particular case study. These K fields are conditional to available field data, including hard and soft information. Then, different future scenarios of groundwater pumping strategies are generated, based on historical information and WFD standards, and simulated for each one of the equally likely K fields. The future scenarios lead to different environmental impacts and levels of socioeconomic development of the region and, hence, to a different degree of acceptance among stakeholders. We have identified the different stakeholders implied in the decision-making process, the objectives pursued and the alternative actions that should be considered by stakeholders in a public participation project (PPP). The MonteCarlo simulation provides a highly effective way for uncertainty assessment and allows presenting the results in a simple and understandable way even for non-experts stakeholders. The methodology has been successfully applied to a real case study and lays the foundations to performa PPP and stakeholders' involvement in a decisionmaking process as required by the WFD. The results of the methodology can help the decision-making process to come up with the best policies and regulations for a groundwater system under uncertainty in groundwater parameters and management strategies and involving stakeholders with conflicting interests.Llopis Albert, C.; Palacios Marqués, D.; Merigó -Lindahl, JM. (2016). Decision making under uncertainty in environmental projects using mathematical simulation modeling. Environmental Earth Sciences. 75(19):1-11. doi:10.1007/s12665-016-6135-yS1117519Arhonditsis GB, Perhar G, Zhang W, Massos E, Shi M, Das A (2008) Addressing equifinality and uncertainty in eutrophication models. Water Resour Res 44:W01420. doi: 10.1029/2007WR005862Capilla JE, Llopis-Albert C (2009) Gradual conditioning of non-gaussian transmissivity fields to flow and mass transport data. J Hydrol 371:66–74. doi: 10.1016/j.jhydrol.2009.03.015CHJ (Júcar Water Agency) (2016) Júcar river basin authority. http://www.chj.es/CHS (Segura Water Agency) (2016) Segura river basin authority. http://www.chsegura.es/Custodio E (2002) Aquifer overexploitation: what does it mean? Hydrogeol J 10:254–277EC (2000). Directive 2000/60/EC of the European Parliament and of the Council of October 23 2000, establishing a framework for community action in the field of water policy. Official Journal of the European Communities L327/1eL327/72. 22.12.2000EC (2006) Directive 2006/118/EC of the European Parliament and of the Council of 12 December 2006 on the protection of groundwater against pollution and deteriorationGómez-Hernández JJ, Srivastava RM (1990) ISIM3D: an ANSI-C three dimensional multiple indicator conditional simulation program. Comput Geosci 16(4):395–440Harbaugh AW, Banta ER, Hill MC and McDonald MG (2000) MODFLOW- 2000, The US geological survey modular groundwater model-user guide to modularization concepts and the groundwater flow process. US Geol. Surv. Open-File Rep 00–92, 12Hu LY (2000) Gradual deformation and iterative calibration of Gaussian related stochastic models. Math Geol 32(1):87–108Jagelke J, Barthel R (2005) Conceptualization and implementation of a regional groundwater model for the Neckar catchment in the framework of an integrated regional model. Adv Geosci 5:105–111Llopis-Albert C (2008) Stochastic inverse modeling conditional to flow, mass transport and secondary information. Universitat Politècnica de València, València. ISBN 978-84-691-9796-7Llopis-Albert C, Capilla JE (2009a) Gradual conditioning of non-gaussian transmissivity fields to flow and mass transport data. Demonstration on a synthetic aquifer. J Hydrol 371:53–55. doi: 10.1016/j.jhydrol.2009.03.014Llopis-Albert C, Capilla JE (2009b) Gradual conditioning of non-gaussian transmissivity fields to flow and mass transport data. Application to the macrodispersion experiment (MADE-2) site, on Columbus air force base in Mississippi (USA). J Hydrol 371:75–84. doi: 10.1016/j.jhydrol.2009.03.016Llopis-Albert C, Capilla JE (2010a) Stochastic simulation of non-gaussian 3D conductivity fields in a fractured medium with multiple statistical populations: a case study. J Hydrol Eng 15(7):554–566. doi: 10.1061/(ASCE)HE.1943-5584.0000214Llopis-Albert C, Capilla JE (2010b) Stochastic inverse modeling of hydraulic conductivity fields taking into account independent stochastic structures: a 3D case study. J Hydrol 391:277–288. doi: 10.1016/j.jhydrol.2010.07.028Llopis-Albert C, Pulido-Velazquez D (2014) Discussion about the validity of sharp-interface models to deal with seawater intrusion in coastal aquifers. Hydrol Process 28(10):3642–3654Llopis-Albert C, Pulido-Velazquez D (2015) Using MODFLOW code to approach transient hydraulic head with a sharp-interface solution. Hydrol Process 29(8):2052–2064. doi: 10.1002/hyp.10354Llopis-Albert C, Palacios-Marqués D, Merigó JM (2014) A coupled stochastic inverse-management framework for dealing with nonpoint agriculture pollution under groundwater parameter uncertainty. J Hydrol 511:10–16. doi: 10.1016/j.jhydrol.2014.01.021Llopis-Albert C, Merigó JM, Palacios-Marqués D (2015) Structure adaptation in stochastic inverse methods for integrating information. Water Resour Manage 29(1):95–107. doi: 10.1007/s11269-014-0829-2Llopis-Albert C, Merigó JM, Xu Y (2016) A coupled stochastic inverse/sharp interface seawater intrusion approach for coastal aquifers under groundwater parameter uncertainty. J Hydrol 540:774–783. doi: 10.1016/j.jhydrol.2016.06.065McDonald MG and Harbaugh AW (1988) A modular three-dimensional finite-difference groundwater flow model. US geological survey technical manual of water resources investigation, Book 6, US geological survey, Reston, Virginia, 586Molina JL, Pulido-Velazquez M, Llopis-Albert C, Peña-Haro S (2013) Stochastic hydro-economic model for groundwater quality management using Bayesian networks. Water Sci Technol 67(3):579–586. doi: 10.2166/wst.2012.598Peña-Haro S, Llopis-Albert C, Pulido-Velazquez M (2010) Fertilizer standards for controlling groundwater nitrate pollution from agriculture: El Salobral-Los Llanos case study, Spain. J Hydrol 392:174–187. doi: 10.1016/j.jhydrol.2010.08.006Peña-Haro S, Pulido-Velazquez M, Llopis-Albert C (2011) Stochastic hydro-economic modeling for optimal management of agricultural groundwater nitrate pollution under hydraulic conductivity uncertainty. Environ Model Softw 26(8):999–1008. doi: 10.1016/j.envsoft.2011.02.010Pulido-Velazquez D, Llopis-Albert C, Peña-Haro S, Pulido-Velazquez M (2011) Efficient conceptual model for simulating the effect of aquifer heterogeneity on natural groundwater discharge to rivers. Adv Water Resour 34(11):1377–1389. doi: 10.1016/j.advwatres.2011.07.010Reichert P, Borsuk M, Hostmann M, Schweizer S, Spörri C, Tockner K, Truffer B (2005) Concepts of decision support for river rehabilitation. Environ Model Softw 22:188–201Wright SAL, Fritsch O (2011) Operationalising active involvement in the EU water framework directive: why, when and how? Ecol Econ 70(12):2268–2274Zhou H, Gómez-Hernández JJ, Li L (2014) Inverse methods in hydrogeology: evolution and recent trends. Adv Water Resour 63:22–37. doi: 10.1016/j.advwatres.2013.10.01
Carbonylcobalt(I) complexes. The crystal and molecular structure of [Co(CO)(dppm)2]ClO4
New anionic carbonylcobalt(I) complexes [X2Co(CO)2(PPh3)](PR4) (X=Cl, PR4 = PBzPh3 (I); X = Br, PR4 = PEtPh3 (II)) have been prepared by reduction of the cobalt(II) halides with NaBH4 in the presence of PPh3 and the phosphonium salt PR4X. Cleavage of halide bridges in dimeric or polymeric [XCo(PPh3)2]n and [XCo(PPh3)]n gives the neutral dicarbonyl derivatives XCo(CO)2PPh3)2. Treatment of ClCo(CO)2(PPh3)2 with alkylating agents gives the known σ- and η- organocobalt(I) derivatives, and reactions with TIClO4 in the presence of various amounts of different mono- and bi-dentate phosphines give the cationic tricarbonyl [Co(CO)3(PPh3)2]+, dicarbonyl [Co(CO)2(PMePh2)3]+ and monocarbonyl [Co(CO)L4]+ complexes (L4 = 4P(OMe)3, 2 dppe and 2dppm). The dppm complex crystallizes in the monoclinic space group P21/c with a 17.895(6), b 10.751(2), c 24.687(4) Å, β 98.92(1)°, and Dcalc 1.35 g cm-3 for Z = 4. A final R value of 0.077 ( Rw = 0.061), based on 2656 observed reflections, was obtained. The cobalt atom exhibits a distorted trigonal bipyramidal geometry. The perchlorate anion is severely disordered or freely rotating. © 1986.The authors greatly appreciate the financial support received from Comision Asesora de Investigation Cientifica y Tecnica (Spain).Ref.2001-83
Synthesis and structural characterisation of the mixed-metal cluster cation [Nb(η5-C5H4R)2{AuP(C6H5)3}2]+ with R = H or Si(CH3)3
Triangular "NbAu2" cluster compounds have been prepared by the reaction of [Nb(η5-C5H4R)2H3] (R = H, Si(CH3)3) with gold(I) salts and the structure of [Nb{η5-C5H4Si(CH3)3}2{AuP(C6H5)3}2] PF6 has been determined by X-ray diffraction. © 1986.We are grateful to the C.A.I.C.Y.T. for financial support (ref.2001/83) and the Ministerio de Educación y Ciencia for a postdoctoral grant (to M.F.)
Pentamethylcyclopentadienyltitanium-(III) and -(IV) carboxylates. Crystal structures of [Ti(η-C5Me5)(O2CPh)3] and [{Ti(η-C5Me5)(O2CPh)2}2]
The compound [Ti(η-C5Me5)Me3] reacts with 3 equivalents of carboxylic acids HO2CR (R = Me, Ph, or p-MeOC6H4) to give the corresponding tris(carboxylates)[Ti(η-C5Me5)(O2CR)3]. The X-ray structure of the complex with R = Ph shows the three benzoate ligands acting as chelates, the geometry around Ti is a distorted pentagonal bipyramid and the metal attains the 18-electron configuration. Methyl carboxylate complexes [Ti(η-C5Me5)Me3-n(O2CR)n](n= 1 or 2) could not be isolated, but exposure of their solutions to sunlight gave the dimers [{Ti(η-C5Me5)(O2CR)2}2]. The X-ray structure of the acetato complex reveals four O2CMe groups bridging two Ti(C5Me5) fragments.Peer reviewe
Hydrolytic studies on (η5-C5Me5)TiMe3; X-ray structure of [(η5-C5Me5)TiMe(µ-O)]3 containing a Ti3O3 ring
Successive hydrolysis of (η5-C5Me5)TiMe3(2) gives first [(η5-C5Me5)TiMe2]2(µ-O)(3) and then [(η5-C5Me5)TiMe(µ-O)]3(4); the crystal structure of (4) reveals a Ti3O3 ring.Peer reviewe
Reactions of monocyclopentadienylniobium(III) complexes with α-diketones. Crystal structure of the phosphonium salt [P{C(OH)MeC(O)Me} Me2Ph]Cl
The reactions of Nb(η5-C5H5)Cl2(CO)L2 ( PMe3, PMe2Ph) with α-diketones RCOCOR (R Me, Ph) and 3,5-di-t-butyl-o-quinone have been studied. When L PMe2Ph and R Ph the reaction in a molar ratio 1 1 gives Nb(η5-C5H5)Cl2(O2C2Ph2)(PMe2Ph)2 (I) whereas the same reaction for R Me gives the phosphonium salt [P{C(OH)MeC(O)Me}Me2Ph]Cl (VII). When L PMe3 only the phosphonium salts [P{C(OH)RC(O)R}Me3]Cl are obtained for R Me (V) and Ph (VI), but the complex Nb(η5-C5H5)Cl2(O2C2Ph2)PMe3 (II) can be isolated by previous addition of MeI. Similar reactions with 3,5-di-t-butyl-o-quinone give Nb(η5-C5H5)Cl2(o-O2C6H2But)L, L PMe3 (III), PMe2Ph (IV). All the compounds have been characterized by IR, 1H, 13C, 31P NMR spectroscopy and the crystal structure of VII has been determined by X-ray diffraction. VII crystallizes in the monoclinic space group P21/c with a 7.440(2), b 10.377(3), c 17.644(4) Å, β 96.94(2)° and Dcal 1.039 g cm-3 for Z = 4. A final R value of 0.046 (Rw = 0.049) based on 2309 observed reflections was obtained. The geometry around the P atom is tetrahedral. © 1987.The authors gratefully acknowledge financial support from Comision Asesora de Investigation Cientifica y Técnica. (2001-83) Spain
Synthesis of dicarbonyl and halogeno complexes of (η-pentamethyl-cyclopentadienyl)(nitrosyl)-molybdenum and -tungsten. Crystal structure of [{Mo(η5-C5Me5)(NO)Br(μ-Br)}2]
Under appropriate conditions the reaction of [M(η5-C5Me5)(NO)(CO)2] [M = Mo, (1) or W, (2)] with X2 (X = Cl, Br, or l) gives [{M(η5-C5Me5)(NO)X2} n] [M = Mo, (3a)-(3c); or W, (4a)-(4c); n = 1 or 2]. When M = W and X = Cl the new carbonylnitrosyl compound [W(η5-C5Me5)(NO)-Cl2(CO)] (5) has been isolated and characterized; (5) is proposed to be an intermediate in the preparation of (4a) from (2). The compound (3b) or (4b) reacts with mono- or bi-dentate phosphines giving [M(η5-C5Me5)(NO)Br2L] [L = PPh3, M = Mo, (6b); or W, (7); L = OPPh3, M = Mo, (8); or W, (9)] or [Mo(η5-C5Me5)(NO)Br(L-L)][Mo(η 5-C5Me5)(NO)Br3] [L-L = Ph2PCH2PPh2(dppm), (10); or Ph2PCH2CH2PPh2 (dppe), (11)]. Complex (11) is in equilibrium with an isomer [Mo2(η5-C5Me5) 2(NO)2Br4(dppe)] (12). X-Ray structure analysis of [{Mo(η5-C5Me5)(NO)-Br(μ-Br)} 2] (3b) shows space group C2/c with a = 15.102(4), b = 8.403(6), c = 21.498(10) Ã…, β = 98.69(2)°, and Z = 4; R = 0.063 and R′ = 0.055, based on 2 688 observed reflections. The molecule is dimeric with two bridging bromo atoms, in contrast with the monomeric nature of the recently reported structure of the parent (4c).We gratefully acknowledge financial support from the Comisión Asesora de Investigación CientÃfica y Técnica (ref. PB86/0093)
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