Gwenaël Corbel

Luminescence analysis and subsequent revision of the crystal structure of triclinic L-EuBO3

Abstract The structure of L-EuBO 3 is revised by using the centric P-1 space group with two crystallographic sites for Eu 3+ , in perfect agreement with the luminescence results (site A and site B). The structure, refined from single crystal X-ray diffraction data, is triclinic, space group P-1, Z =4, with a =6.468(7) A, b =6.477(7) A, c =6.220(7) A, α =107.78(6)°, β =108.00(7)°, γ =93.07(4)°, R 1 =0.025, wR 2 =0.062. The three-dimensional network is built up from corrugated sheets of EuO 8 polyhedra, approximately parallel to the (111) planes. Luminescence analysis indicates that site to site energy transfer is efficient at 77 K and is phonon assisted from site B to site A. At 12 K, this site B to site A phonon coupling disappears.

Bidimensional sodium ionic conduction of oxyborates Na2M2(BO3)2O (M=Al,Ga) and Na2−2xCaxGa2(BO3)2O (x=0.25,0.50)

Crystalline samples of Na2M2(BO3)2O (M = Al, Ga), Na1.5Ca0.25Ga2(BO3)2O and NaCa0.5Ga2(BO3)2O are prepared by conventional high temperature solid state synthesis technique. Temperature dependent impedance spectroscopy measurements are performed. For Na2Al2(BO3)2O and Na2Ga2(BO3)2O, high and low temperature regimes have to be distinguished. Only one regime is observed for the calcium-substituted samples Na1.5Ca0.25Ga2(BO3)2O and NaCa0.5Ga2(BO3)2O.

Synthesis from solutions and properties of various metal fluorides and fluoride salts

Abstract Parts of the recent advances in the search of new fluorides or fluoride salts, at the Laboratoire des Fluorures in Le Mans, are presented. Hydrothermal synthesis in sub-critical or super-critical conditions is mainly used to obtain rare earth (RE), Al, Ga, In or 3d transition fluorides or fluoride salts (borates, carbonates, phosphates or silicates). Open structures are obtained by low temperature crystallisation from solutions (δ-(H 3 O)RE 3 F 10 · x H 2 O) and, probably, by decomposition of hybrid organic–inorganic fluorides D·MF 5 (D=diprotonated aliphatic diamine), prepared by micro-wave heating of the solutions. Non-centrosymmetric phases are evidenced in fluoride carbonates and fluoride borates: K 4 RE 2 (CO 3 ) 3 F 4 and M 2 (BO 3 )(OH) 0.75 F 0.25 ; the magnetic properties of several iron fluoride phosphates are found to be governed by super or super-superexchange interactions.

OH–F disorder in non-centrosymmetric Zn2(BO3)(OH)0.75F0.25: ab initio structure determination and NMR study; comparison with tridymite and fluoride borates

Abstract The crystal structure of Zn 2 (BO 3 )(OH) 0.75 F 0.25 is determined ab initio from X-ray and neutron powder data. The unit cell is monoclinic, space group P2 1 (no. 4), Z =2, with a =6.8738(4) A, b =4.9178(2) A, c =5.7018(3) A, β =98.829(3) °, V =190.46(3) A 3 (neutron powder data). Alternating up and down ZnO 3 (OH,F) tetrahedra and triangular BO 3 3− groups built corrugated layers. Their connection by hydroxyl ions gives a 3D network similar to that of tridymite. The statistical distribution of fluorine and hydroxyl in infinite chains is evidenced by magical angle spinning-NMR spectroscopy.

Excitation energy transfer between two 2i sites in triclinic L–EuBO3 and L–GdBO3:Eu3+

Abstract Analysis of the excitation of Eu3+ ions in site A and site B in L–EuBO3 and L–GdBO3:Eu3+ at several temperatures have shown very efficient energy transfer and reciprocal processes between both sites. At 12 K, the transfer from site A to site B dominates the process and results in the fluorescence quenching of site A in L–EuBO3. At higher temperature, 77 and 300 K, the transfer and back transfer have nearly an equal rate and accordingly the same spectra from both sites are obtained despite of exciting site A or site B.

Reduction Kinetics of La2Mo2O9 and Phase Evolution during Reduction and Reoxidation.

An amorphous reduced form of oxide ion conductor La2Mo2O9 had been proposed as sulfur-tolerant anode material for solid oxide fuel cell, but its oxygen content was not known. In this paper, we investigate the reduction kinetics by diluted hydrogen of La2Mo2O9 to amorphous, and the oxygen range of the amorphous form. The reduction kinetics is studied as a function of the powder specific surface area and of the temperature, on powders synthesized by solid state reaction and by polyol process using two different solvents. The reduction process was carried out by TGA under 10% H2 diluted in argon, and its kinetics is analyzed and modeled. As expected, small particles and high temperature lead to higher reduction rates. Several reduction steps were identified by XRD during the process. At 700 °C La2Mo2O9 is directly reduced into the amorphous phase La2Mo2O7-y, whereas at 760 °C reduction occurs through an intermediate crystallized La7Mo7O30 (≅ La2Mo2O8.57) phase before amorphization. In both cases, further reduction of La2Mo2O6.2 amorphous phase leads to an exsolution of metallic molybdenum and a molybdenum deficiency in the amorphous phase. Reoxidation of amorphous La2Mo2O7-y was studied by TGA, DTA and XRD. At low temperature in air, the reduced compounds are reoxidized while remaining amorphous. The annealing for 60 h at 350 °C in air of reduced La2Mo2O6.66, obtained beforehand by solid state reaction, gives an amorphous phase with composition La2Mo2O8.85. The existence domain of the reduced amorphous phase in terms of oxygen content therefore ranges at least from O6.2 to O8.85, thus including the composition La2Mo2O8.50 of the amorphous surface layer at the origin of a huge increase of ionic conductivity recently reported in nanowires of La2Mo2O9.

Low temperature synthesis of Na2Eu(CO3)F2.5(OH)0.5, luminescence and crystal field study of Eu3+

Abstract The luminescence of Eu 3+ in sodium europium hydroxyfluoride carbonate, Na 2 Eu(CO 3 )F 2.5 (OH) 0.5 , synthesized by hydrothermal growth at low temperature, is presented. The structure is orthorhombic and is built up from EuO 3 (F, OH) 6 polyhedra linked by triangular faces and edges. The emission spectrum consists of groups of sharp lines attributed to 5 D 0 → 7 F 0–4 transitions of one low symmetry site for Eu 3+ in agreement with the crystallographic analysis. 23 from 25 expected electronic levels for 7 F J ( J =0–4) manifolds are fitted by the phenomenological crystal field parameters in the approaching C 2v point symmetry. The simulation is achieved with very good root mean square ( rms ) deviation of 5.3 cm −1 . The crystal field strength parameter, N v , is weak ( N v =1652). Satellite vibronics of carbonate groups coupled to electronic transitions are also observed.

Bidimensional sodium ionic conduction of oxyborates Na2M2(BO3)2O () and Na2−2xCaxGa2(BO3)2O ()

Crystalline samples of Na2M2(BO3)2O (M = Al, Ga), Na1.5Ca0.25Ga2(BO3)2O and NaCa0.5Ga2(BO3)2O are prepared by conventional high temperature solid state synthesis technique. Temperature dependent impedance spectroscopy measurements are performed. For Na2Al2(BO3)2O and Na2Ga2(BO3)2O, high and low temperature regimes have to be distinguished. Only one regime is observed for the calcium-substituted samples Na1.5Ca0.25Ga2(BO3)2O and NaCa0.5Ga2(BO3)2O.

Bidimensional sodium ionic conduction of oxyborates Na2M2(BO3)2O (M=Al,GaM=Al,Ga) and Na2−2xCaxGa2(BO3)2O (x=0.25,0.50x=0.25,0.50)

Crystalline samples of Na2M2(BO3)2O (M = Al, Ga), Na1.5Ca0.25Ga2(BO3)2O and NaCa0.5Ga2(BO3)2O are prepared by conventional high temperature solid state synthesis technique. Temperature dependent impedance spectroscopy measurements are performed. For Na2Al2(BO3)2O and Na2Ga2(BO3)2O, high and low temperature regimes have to be distinguished. Only one regime is observed for the calcium-substituted samples Na1.5Ca0.25Ga2(BO3)2O and NaCa0.5Ga2(BO3)2O.

Bidimensional sodium ionic conduction of oxyborates Na(2)M(2)(BO(3))(2)O (M = Al, Ga) and Na(2-2x)Ca(x)Ga(2)(BO(3))(2)O (x=0.25, 0.50)

Crystalline samples of Na2M2(BO3)2O (M = Al, Ga), Na1.5Ca0.25Ga2(BO3)2O and NaCa0.5Ga2(BO3)2O are prepared by conventional high temperature solid state synthesis technique. Temperature dependent impedance spectroscopy measurements are performed. For Na2Al2(BO3)2O and Na2Ga2(BO3)2O, high and low temperature regimes have to be distinguished. Only one regime is observed for the calcium-substituted samples Na1.5Ca0.25Ga2(BO3)2O and NaCa0.5Ga2(BO3)2O.