Neutron and X-ray diffraction studies on the high temperature phase of Mn3(VO4)2, the new isostructural compound NaMn4(VO4)3 and their mixed crystals Na Mn4.5−/2(VO4)3 (0≤x≤1)

On the soft magnetic properties of the compounds of the series NaxMn4.5−x/2(VO4)3 and the magnetic structure of h.t.-Mn3(VO4)2 (x = 1)

“…3 It crystallizes in the tetragonal space group I4 ˉ2d (Z = 6). Isostructural compounds are Fe 5.33 Ge 2.67 O 12 , 11 Mg 3 (AsO 4 ) 2 , 12 Co 3 (AsO 4 ) 2 , 13,14 ∼LiMg 4 (VO 4 ) 3 , [15][16][17] 19 NaMg 4 (VO 4 ) 3 20 and NaMn 4 (VO 4 ) 3 (first reported by Yahia et al 21 and not by Clemens et al 22 ). 16 / 18 of the Mn 2+ cations are found in octahedral coordination on two different crystallographic sites (8c and 8d).…”

“…The manganese cations on the Hoard site (4b) can be aliovalently substituted by charge equivalents of Na + or Li + . In the case of sodium, a full replacement is possible and gives the previously reported compound NaMn 4 (VO 4 ) 3 21,22 (a partial replacement by any degree x in Na x Mn 4.5−x/2 (VO 4 ) 3 is possible as well). For the smaller sized Li + cation replacement is only possible up to a composition of ∼Li 0.66 Mn 4.22 (VO 4 ) 3 .…”

“…All the samples were prepared as reported in: 22 "For the synthesis, stoichiometric amounts of Na 2 CO 3 (99%+, Alfa Aesar), MnO (99%, Alfa Aesar) and V 2 O 5 (99.6%+, Aldrich) were used. The precursor oxides were ground using a planetary ball mill (agate, Fritsch pulverisette 7, 440 rpm, 2 h) and pressed into tablets.…”

“…3 It was also shown that already small amounts of alkali cations help to facilitate the formation of this structure at temperatures far below (∼800 °C) the phase transition temperature of pure Mn 3 (VO 4 ) 2 (∼946 °C). 3,22 No detailed magnetic characterisations for compounds in this structure (except measurements on Fe 5.33 Ge 2.67 O 12 above its magnetic ordering temperature 11 ) have been reported so far. Such studies are important to assess the magnetic exchange processes in Mn 2+ in such an unusual coordination.…”

“…The Mn-O distances of the outer tetrahedron are much longer than the Mn-O distances of the inner tetrahedron (2.7 Å vs. 2.2 Å). Although Mn 2+ and vacancies are randomly distributed, a recent study 22 by neutron diffraction at ambient temperature has shown that the oxygen atoms are significantly closer to the center if the 4b site is occupied by Mn 2+ compared to a vacancy, illustrating the importance of detailed characterization to elucidate the site coordination in systems with only partially filled sites. Therefore split sites of the anion positions were reported to be a more reliable model to describe the Mn-O distances around this site correctly.…”