Crystal growth,structure, electron paramagnetic resonance and magnetic properties of (NH4)2V3O8 |
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Authors: | Francois R Theobald Jean-Gérard Theobald JC Vedrine R Clad J Renard |
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Institution: | Faculté des Sciences, la Bouloie, 25030 Besancon Cedex, France;Institut de la Catalyse, 2, avenue Einstein, 69 Villeurbanne, France;Laboratoire Pierre Weiss, Université Paris XI, Orsay, France;Institut d''Electronique Fondamentale, Université Paris XI, Orsay, France |
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Abstract: | A method to grow single crystals of ammonium vanadate (IV, V) (NH4)2V3O8 has been devised. The crystal structure is tetragonal P4bm; residual factor is R = 0.030. Cell parameters are and . The V5+ atom lies at the center of a triangular pyramide (VO4 tetrahedron) while the V4+ atom is on A 4-fold rotation axis at the center of a square-based pyramide VO5 whose symmetry point group is almost C4v with the short V = O bond lying along the 4-fold axis parallel to the c edge of the tetragonal cell. Crystals are thin platlets with (001) cleavage planes. The platlets have very often a square or rectangular shape limited by {100} or {110} planes. Each single crystal was not large enough to record a good e.p.r. spectrum, but by sticking on the same quartz plate a score of them it was possible to gather enough crystals so to record correct spectra and by orienting the plate to obtain resonance lines separately for g∥ = 1.9263 et gτ = 1.9755. Measurements at 283 K on powder samples gave times for spin-spin relaxation T2 = 0.4 × 10?7s and for spin-lattice relaxation T1 = 1.6 × 10?7s. The magnetic structure is characterized by an exchange narrowing ωe = 3 × 1010rad/s which corresponds to a transition temperature of about 0.5 K. Static susceptibility measurements at high magnetic field show a paramagnetic behaviour with an antiferromagnetic interaction which is interpreted in the magnetic space group P2c4bm as the interaction between V4+ ions from consecutive planes parallel to (001). |
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