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Crystal and Magnetic Structures and Magnetic Properties of Selenate Containing Natrochalcite, AIMII2(H3O2)(SeO4)2 Where A = Na or K and M = Mn, Co, or Ni

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posted on 06.02.2012, 00:00 by Wassim Maalej, Serge Vilminot, Gilles André, Zakaria Elaoud, Tahar Mhiri, Mohamedally Kurmoo
The synthesis of a series of selenate containing natrochalcite, AIMII2(H3O2)­(SeO4)2 where A = Na or K and M = Mn, Co, or Ni (here labeled as AMH and AMD for the hydrogenated and deuterated compounds, respectively), the X-ray crystal structure determinations from single crystals (Ni) and powder (Mn), magnetic properties, and magnetic structures of the cobalt analogues are reported. The nuclear crystal structures for NaNiH, KNiH, and KMnH are similar to those reported for the cobalt analogues (NaCoH and KCoH) and consist of chains of edge-sharing octahedra (MO6) which are connected by H3O2 and SeO4 to form layers which are in turn bridged by the alkali, in an octahedral coordination site, to form the 3D-framework. The magnetic properties are characterized by antiferromagnetic interaction at high temperatures and antiferromagnetic ordering at low temperatures (NaCoH, 3.5 K; KCoH, 5.9 K; KNiH, 8.5 K; and KMnH, 16 K), except for KNi2(H3O2)­(SeO4)2 which displays a weak ferromagnetic interaction and no long-range ordering above 2 K. The neutron magnetic structures of the cobalt analogues, studied as a function of temperature, are different for the two cobalt salts and also different from all the known magnetic structures of the natrochalcite family. Whereas the magnetic structure of NaCoD has a k = (0, 0, 0), that of KCoD has one consisting of a doubled nuclear cell, k = (0, 0, 1/2). Both compounds have four magnetic sublattices related to the four cobalt atoms of the nuclear unit cell. In NaCoD the moments are in the bc-plane, My = 2.51(2) μB and Mz = 1.29(4) μB, with the major component along the cobalt chain and the resultant moment, 2.83(3) μB, making an angle of 27° with the b-axis. The sum of the moments within the cell is zero. For KCoD the moment at each cobalt site has a component along each crystallographic axis, Mx = 2.40(3), My = 1.03(3), Mz = 1.59(8) giving a total M = 2.49(3) μB. Within one nuclear cell the moments are fully compensated. The moments corresponding to the cobalt atoms of the second nuclear cell comprising the magnetic unit cell are oriented in opposite directions.

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