Belén F. Alfonso

Double magnetic phase transition inND4Fe(DPO4)2andNH4Fe(HPO4)2

B. F. Alfonso,1 C. Pique,1 C. Trobajo,2 J. R. Garcia,2 E. Kampert,3 U. Zeitler,3 J. Rodriguez Fernandez,4 M. T. Fernandez-Diaz,5 and J. A. Blanco6 1Departamento de Fisica, Universidad de Oviedo, E-33204 Gijon, Spain 2Departamento de Quimica Organica e Inorganica, Universidad de Oviedo, E-33006 Oviedo, Spain 3Institute for Molecules and Materials, High Field Magnet Laboratory, Radboud University Nijmegen, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands 4CITIMAC, Facultad de Ciencias, Universidad de Cantabria, E-39005 Santander, Spain 5Institute Laue-Langevin, BP 156X, F-38042 Grenoble, France 6Departamento de Fisica, Universidad de Oviedo, E-33007 Oviedo, Spain Received 3 June 2010; revised manuscript received 14 September 2010; published 20 October 2010

A flow-through reaction cell for in situ X-ray diffraction and absorption studies of heterogeneous powder–liquid reactions and phase transformations

A portable powder-liquid high-corrosion-resistant reaction cell has been designed to follow in situ reactions by X-ray powder diffraction (XRD) and X-ray absorption spectroscopy (XAS) techniques. The cell has been conceived to be mounted on the experimental stations for diffraction and absorption of the Spanish CRG SpLine-BM25 beamline at the European Synchrotron Radiation Facility. Powder reactants and/or products are kept at a fixed position in a vertical geometry in the X-ray pathway by a porous membrane, under forced liquid reflux circulation. Owing to the short pathway of the X-ray beam through the cell, XRD and XAS measurements can be carried out in transmission configuration/mode. In the case of the diffraction technique, data can be collected with either a point detector or a two-dimensional CCD detector, depending on specific experimental requirements in terms of space or time resolution. Crystallization processes, heterogeneous catalytic processes and several varieties of experiments can be followed by these techniques with this cell. Two experiments were carried out to demonstrate the cell feasibility: the phase transformations of layered titanium phosphates in boiling aqueous solutions of phosphoric acid, and the reaction of copper carbonate and L-isoleucine amino acid powders in boiling aqueous solution. In this last case the shrinking of the solid reactants and the formation of Cu(isoleucine)(2) is observed. The crystallization processes and several phase transitions have been observed during the experiments, as well as an unexpected reaction pathway.

Structural and magnetic phases of Fe(ND3)2PO4

Polycrystalline samples of Fe(ND3)2PO4 were prepared by means of a hydrothermal route and characterized using powder x-ray diffraction, scanning electron microscopy, chemical and thermal analysis, magnetic measurements, specific heat and neutron diffraction experiments. This material orders within an orthorhombic (Pnma space group) crystal structure at room temperature, but below 220 K the crystal structure changes towards a monoclinic P 21/n structure through a phase transition with almost no latent heat. Furthermore, the magnetic cell at 2 K is twice the size of the crystallographic cell observed at 80 K. The magnetic structure associated with the Fe(III) ions is thus commensurate with the crystal lattice having a propagation vector , the magnetic moments lying in the yz plane of the crystallographic cell.

Powder neutron diffraction investigation of the crystal and magnetic structures of NH4Fe(HPO4)2 and its deuterated form

The NH4Fe(HPO4)2 and its deuterated form, ND4Fe(DPO4)2, were investigated in detail from powder neutron diffraction data allowing a precise determination of the hydrogen positions and low temperature magnetic structures. Below TC = 17.82 K they order with a ferrimagnetic structure and the magnetic moments lying in the crystallographic plane ac. As the temperature is lowered to Tt = 3.52 K the system undergoes a magnetic phase transition to an equal moment antiphase structure characterized by the propagation vector close to AF ? (1/16,0,1/16) and a magnetic moment for the Fe3+ ions of 4.8 ?B at 1.89 K. The low symmetry of this crystal structure and the complex pattern of competing superexchange pathways are responsible for the existence of this double magnetic phase transition.

Neutron powder diffraction investigation in ammonium iron(III) bis (hydrogenphosphate)

The deuterated form of ammonium iron(III) bis (hydrogenphosphate), ND4Fe(DPO4)2, was investigated in detail from neutron powder diffraction data with a wavelength λ = 4.724 A. The material undergoes two successive magnetic phase transitions which are associated with the Fe3+ magnetic moments. One at TC = 17.82 ± 0.05 K is attributed to the ferrimagnetic order with the magnetic moments, μFI = 4.19 ± 0.02 μB at 4 K, lying on the crystallographic plane ac. The other transition is found to be at Tt = 3.52 ± 0.05 K due to an antiferromagnetic arrangement, with an equal moment antiphase structure that is characterized by a long-period propagation vector close to AF ≈ (1/16,0,1/16) and a magnetic moment for the Fe3+ ions of μAF = 4.41 ± 0.03 μB at 1.5 K. The low symmetry of its triclinic crystal structure and the complex pattern of competing superexchange pathways seem to be responsible for the existence of this double magnetic phase transition.

Thermal behavior of layered α-titanium phosphates: from the titanium(IV) bis(hydrogenphosphate) monohydrate to an europium(III)-phase via propylamine intercalation

Polycrystalline layered α-titanium phosphate, α-Ti(HPO4)2·H2O, has been obtained under H3PO4(aq) reflux conditions, and its intercalation compound with propylamine, α-Ti(HPO4)2·2C3H7NH2·H2O, was used for trapping luminescent Eu-centers in two-dimensional confined space. All materials were characterized by X-ray powder diffraction, scanning electron microscopy, solid-state nuclear magnetic resonance (SS-NMR, 31P MAS and 13C CPMAS) and thermogravimetric analysis coupled with mass spectrometry. Moreover, the activation energy of thermal decomposition has been calculated as a function of the extent of conversion, applying both a modified Friedman method developed in our laboratory and the advanced nonlinear method proposed by Vyazovkin.

Modeling Chemical Kinetics in Solid State Reactions

This work deals with the kinetics of thermally stimulated processes which take place in the solid state phases. The activation energy of the solid is calculated using several methods of different families of isoconversional methods (differential, integral and incremental). A model of the kinetics is obtained by a method independent from the procedure used to compute the activation energy and it is analysed in three theoretical simulations as well as the thermal degradation of FeNH4(HPO4)2. The reconstructed α − T curves of the simulations and the experimental case indicates that the model works properly.

Ammonium–cobalt–nickel phosphates, NH{sub 4}[Co{sub 1−x}Ni{sub x}PO{sub 4}]·H{sub 2}O

The authors are grateful to the FEDER and the Spanish MINECO for financial support under projects MAT2010-15094 and MAT2011-27573-C04.