Etude magnetique et par spectroscopie Mössbauer de NH4FeF4

Tantalum Aluminum Alkoxide as a Double-Metal Precursor for Metalorganic Chemical Vapor Deposition of Sr2AlTaO6

“…[4], it is written TN = 2S(S+ 1)(4J1 + 412)/3k; to determine J1, S will be given by the free-ion value. Finally J , is deduced to be around 6.3 K, a value in good agreement with determinations performed on the one-dimensional compound Rb2FeF5 [13] and on NH,FeF, [14]. Note that J1 corresponds to a mean Fe3+-F distance of 1.93 2 0.02 8, and to Fe-F-Fe angles equal to 160.9' or to 151.1' [ l , 21; the Fe3+-Fe3+ distances are 3.8 and 5.4 8, for J 1 and J2 respectively.…”

First Mixed-Metal Fluoride Pyrochlores Obtained by Topotactic Oxidation of Ammonium Fluorides under F2 Gas

“…At 77 K, the corresponding paramagnetic doublets which show similar mean values of isomer shift and quadrupolar splitting suggest also that the magnetic structures order at low temperature, as previously noted for MnFe-MS. At room temperature, the spectrum of (NH 4 )­Ni 2+ Fe 3+ F 6 exhibits a paramagnetic doublet with broad lines and the presence of two shoulders; it is described by a dominant quadrupolar component resulting from a distribution of quadrupolar splitting and a small quadrupolar component quantified at about 5 atom % Fe with a large quadrupole splitting. This last component corresponds to (NH 4 )­Fe 3+ F 4 according to the reference values . At 77 K, the (NH 4 )­Fe 3+ F 4 impurity is no longer directly identifiable on the spectrum, since the magnetic ordering temperature of well-crystallized (NH 4 )­Fe 3+ F 4 occurs at 135 K. Very fine grains of (NH 4 )­Fe 3+ F 4 can order magnetically at lower Néel temperatures; consequently, the Zeeman sextet that should eventually start to form at 77 K is not well-defined .…”

“…This last component corresponds to (NH 4 )­Fe 3+ F 4 according to the reference values . At 77 K, the (NH 4 )­Fe 3+ F 4 impurity is no longer directly identifiable on the spectrum, since the magnetic ordering temperature of well-crystallized (NH 4 )­Fe 3+ F 4 occurs at 135 K. Very fine grains of (NH 4 )­Fe 3+ F 4 can order magnetically at lower Néel temperatures; consequently, the Zeeman sextet that should eventually start to form at 77 K is not well-defined . It is worth observing that the formation of the three allotropic phases of (NH 4 )­Mn 2+ Fe 3+ F 6 seems to be governed by the Mn 2+ /Fe 3+ cationic disorder.…”

1H NMR study of the layered antiferromagnet NH4FeF4

“…The ammonium ions are linked with the [FeF6I3+ octahedra by strong N-H-F hydrogen bonds. The twodimensional ( 2 ~) magnetic properties are manifested in the susceptibility data of polycrystalline NH4FeF, [2]. At about 200 K, the x-' versus T curve shows a flat minimum typical of a 2~ magnetic system.…”

“…At about 200 K, the x-' versus T curve shows a flat minimum typical of a 2~ magnetic system. A calculation [2] of the exchange integral using the high temperature series expansion technique leads to J / k = -26 K. Neutron scattering data are not available for this compound. However, 2D magnetic behaviour is expected because of the similarity between NH,FeF, and RbFeF, for which neutron scattering studies have shown a pronounced 2~ behaviour [3].…”