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121.
M. T. Fernández-Díaz J. Rodríguez-Carvajal J. L. Martínez G. Fillion F. Fernández R. Saez-Puche 《Zeitschrift für Physik B Condensed Matter》1991,82(2):275-282
Neutron powder diffraction and magnetization measurements have been performed in stoichiometric Pr2NiO4+(0), which at room temperature (RT) is orthorhombic (Bmab). Ni2+ becomes three dimensionally (3D) antiferromagnetically ordered atT
N
325K, with a propagation vectork=[100], and spins oriented along thea axis (parallel to the propagation vector). The magnetic structure belongs to the
7g
(––+)-representation (g
x
mode, Shubnikov groupB
pmab') ofBmab, and a magnetic moment of 1.5
B
is measured at 155K. This compound undergoes two different structural phase transitions. First, going down in temperature, Pr2NiO4 transforms by a first order phase transition from orthorhombic (Bmab) to tetragonal (P42/ncm) atT
c1115K. At high temperature, we can predict a transformation from orthorhombic (Bmab) to tetragonal (I4/mmm) in a continuous way. The extrapolated temperature for this second structural transition isT
c21500±100K. The low temperature structural transformation allows a change in the magnetic structure which forT<T
c1 is better described in the orthorhombic symmetry. Just belowT
c1 the magnetic structure is described by the:
3g
(–+)-representation ofPccn(gxcyfz mode, Shubnikov groupPccn), this magnetic phase begins to disappear going down in temperature and at the same time a new magnetic phase grows. This new magnetic structure is described by the
1g
ofPccn (c
xgyaz mode, Shubnikov groupPccn). Both magnetic structures coexist in a certain temperature range. At 1.5 K thec
xgyaz mode represents the total of the magnetic ordering. To reproduce the observed magnetic intensities we are forced to consider that Pr3+ is polarized below 40 K, with a magnetic structure which is coupled to the Ni sublattices (i.e.
1g
and
3g
). The magnetic moment at low temperature for Pr is about 1.28
B
. 相似文献
122.
123.
Basallote MG Estevan F Feliz M Jesús Fernández-Trujillo M Hoyos DA Llusar R Uriel S Vicent C 《Dalton transactions (Cambridge, England : 2003)》2004,(4):530-536
The novel incomplete cuboidal cluster [W3Se4H3(dmpe)3](PF6), [1](PF6), has been prepared by reduction of [W3Se4Br3(dmpe)3](PF6) with LiBH4 in THF solution. The trihydroxo complex [W3Se4(OH)3(dmpe)3](PF6), [2](PF6), was obtained by reacting [W3Se4Br3(dmpe)3](PF6) with NaOH in MeCN-H2O solution. The complexes [1](PF6) and [2](PF6) were converted to their BPh4- salts by treatment with NaBPh4. Recrystallisation of [1](BPh4) in the presence of traces of water affords the mixed dihydride hydroxo complex [W3Se4H2(OH)(dmpe)3](BPh4). The crystal structures of [1](BPh4), [2](BPh4) and [W3Se4H2(OH)(dmpe)3](BPh4) have been resolved. Although the [1]+ trihydride does not react with an excess of halide salts, reaction with HX leads to [W3Se4X3(dmpe)3]+ (X = Cl, Br). The kinetics of this reaction has been studied at 25 degrees C in MeCN-H2O solution (1:1, v/v) and found to occur with two consecutive kinetic steps. The first step is independent of the nature and concentration of the X(-) anion but shows a first order dependence on the concentration of acid (k1 = 12.0 mol(-1) dm(3) s(-1)), whereas the second one is independent of the nature and concentration of both the acid and added salts (k2 = 0.024 s(-1)). In contrast, the reaction of [2]+ with acids occurs in a single step with kobs = 0.63 s(-1)(HCl) and 0.17 s(-1)(HBr). These kinetic results are discussed on the basis of the mechanism previously proposed for the reactions of the analogous [W3S4H3(dmpe)3]+ cluster, with special emphasis on the effects caused by the change of S by Se on the rate constants for the different processes involved. 相似文献
124.
H. Seco-Lago J. Pérez-Iglesias J. M. Fernández-Solís J. M. Castro-Romero V. González-Rodríguez 《Analytical and bioanalytical chemistry》1997,357(4):464-466
The application of derivative spectrophotometry to the simultaneous determination of chromium (III) and copper (II) with
MEDTA is described. The procedure is suitable for concentrations of 0.40–2.60 mg ml–1 of chromium (III) and 0.15–0.60 mg ml–1 of copper (II). The main interferences, both anionic and cationic, are easily eliminated. The method was applied to different
aqueous matrices. It was compared with an atomic absorption method and good results were obtained.
Received: 12 June 1996 / Revised: 30 July 1996 / Accepted: 2 August 1996 相似文献
125.
Solid-phase spectrophotometric microdetermination of iron with ascorbic acid and ferrozine 总被引:1,自引:0,他引:1
M. L. Fernández-de Córdova A. Ruíz-Medina A. Molina-Díaz 《Analytical and bioanalytical chemistry》1997,357(1):44-49
A very sensitive and selective method for the determination of trace amounts of iron has been developed, based on the reduction
of Fe(III) to Fe(II) by ascorbic acid, followed by chromogenic chelation of Fe(II) with ferrozine. The complex Fe(II)-ferrozine
is easily sorbed on a dextran-type anion-exchange gel packed in a 1 mm cell, and the absorbance of the gel is measured directly
at 569 and 800 nm. The extended linear range of the determination is 0.5–10 ng ml-1 of iron (apparent molar absorptivity=4.4×107 l mol-1 cm-1) and the precision (RSD) 1.3% for a concentration of 5 ng ml-1 of iron (n=10). The detection limit for a sample volume of 1000 ml, using 0.040 g of anion-exchanger, corresponds to 0.12 ng ml-1. The method has been successfully applied to the determination of iron in natural and waste waters, wine, soil extract and
previously digested vegetal tissues, drugs and human hair.
Received: 20 November 1995/Revised: 23 January 1996/Accepted: 26 January 1996 相似文献
126.
J. Santamaria M. Fernández J. Mendez M. T. Bomboi 《Fresenius' Journal of Analytical Chemistry》1990,337(4):362-365
Summary The size distribution of six metals (Cd, Cu, Fe, Mn, Ni and Pb) present in the atmosphere of Madrid was determined in the airborne particulate matter. Samples were collected in an area located in the University Campus in Madrid. Twenty-one samplings were carried out in two different periods: summer 1986 and autumn — winter 1986–87. AAS was employed in the analysis. The particle size distribution of the elements was bimodal. The most toxic metal studied (Cd, Ni and Pb) have extremely high proportions in the smallest particle size range, which can easily enter the respiratory tract lodging in the alveoli. For the metals the enrichment factors based on soil proportions are calculated on each stage. The values are higher for lead, cadmium and nickel, tending to vary from stage to stage, suggesting significant pollution sources. 相似文献
127.
The ability of several water models to predict the properties of ices is discussed. The emphasis is put on the results for the densities and the coexistence curves between the different ice forms. It is concluded that none of the most commonly used rigid models is satisfactory. A new model specifically designed to cope with solid-phase properties is proposed. The parameters have been obtained by fitting the equation of state and selected points of the melting lines and of the coexistence lines involving different ice forms. The phase diagram is then calculated for the new potential. The predicted melting temperature of hexagonal ice (Ih) at 1 bar is 272.2 K. This excellent value does not imply a deterioration of the rest of the properties. In fact, the predictions for both the densities and the coexistence curves are better than for TIP4P, which previously yielded the best estimations of the ice properties. 相似文献
128.
The hydrolysis of p-F, p-Cl, and m-Cl phenyl trifluoracetates was studied in the presence of beta-cyclodextrin (beta-CD). The reactions are inhibited by beta-CD at pH 6 while they are catalyzed in alkaline solution. MM3 calculations reproduce some of the experimental results. The substrates form inclusion complexes with beta-CD which are of similar stability as those of the corresponding acetates; however, the association of the transition state is less favorable in these reactions than in those of the acetates, and consequently less stronger catalysis is observed. 相似文献
129.
Summary 4(5)-D-Arabinotetrahydroxybutylimidazoline-2-thione, THBIT, is proposed as a new reagent for the spectrophotometric determination of Pd(II). Pd(II) forms 11, 12 and 14 complexes with THBIT. The system conforms to Beer's law up to 5g/ml palladium concentration in aqueous medium (molar absorptivity, 1.99×104 l· mole–1·cm–1 at 338 nm). The most serious interference is from Hg(II), Os(VIII), Ru(IV), Cr(VI), V(V) and S2O3
2–. The method has been used successfully for the determination of palladium in catalysts and synthetic samples. 相似文献
130.