Preparation and characterization of KxMIIxMIII1-xF3 (x .apprxeq. 0.4-0.6; MII = manganese, nickel; MIII = vanadium, chromium)

Abstract

The compounds K,M11xM1111-xF3 ( x = 0.4-0.6; M" = Mn, Ni; MI1' = V, Cr) were prepared and studied. The tetragonal tungsten bronze structure formed for M" = Mn and M"' = V, Cr, and the modified pyrochlore structure formed for MI1 = Ni and M111 = V, Cr, except for &.soNio.soCro.MF3, which formed the tetragonal bronze structure. From their magnetic behavior, we conclude that all of the tetragonal bronze compounds are ionically ordered and belong to space group P4gc (Ck). Magnetic ordering in these compounds occurs in two steps. Linear trimers (MI1)) and (MI1')' form first, giving rise to a quasi-paramagnetic region, followed at lower temperatures by long-range magnetic ordering. The 180' M2t-F-M3t magnetic interactions appear to be ferromagnetic. Samples of compositions Na, 2s&.2sM"o,soM"'o,50F3 and Nao,soM~~o,50M~~~o,~oF3 were also prepared and studied.KxMn.rV1-xF3, &.6ONi0.6Ocr0.40F3, and Na0.25&.25M~.50Cr0,50F3The crystal structure of layered, tetragonal VO(H2As04)2 (a = 9.1305 (1) A, c = 8.1318 (2) A) has been refined in space group M/mcm from laboratory X-ray powder diffraction data by the Rietveld method, giving RW4= 10.1% and RF = 5.5%. Disorder of the vanadium atoms over two sites results from infinite .V=O-.V=O-chains orienting In the fc-directions at random, due to the flexibility of the arsenate groups that link neighboring chains. The two modes of ordering the vanadyl chains to give structures with P4/ncc and 14cm symmetry, both of which have been reported, are also discussed. The structure of the lithium derivative Li4VO(As04)2 (a = 9.0293 (2) A, c = 9.0053 (4) A), obtained by ion exchange, has P4/ncc symmetry due to partial ordering between vanadyl chains and has been refined to Rwp = 13.8% and RF = 9.3%. The lithium ions occupy irregular five-coordinate sites between vanadyl arsenate layers. The different behavior of the VO(H2X04)2 compounds (X = P, As) toward ionic exchange reactions is discussed in the light of these results.

Solid-state chemistry and non-linear properties of tetragonal tungsten bronzes materials

“…Ferroelasticity and ferroelectricity are coupled at temperature lower than T c = 490 K. In addition, a spontaneous ferromagnetic component appears below 120 K, in the magnetically ordered phase [13,45,46].…”

Magnetic properties of linear trimers in fluoride analogs of tetragonal tungsten bronze

“…First, the linear trimers would all have nearest-neighboring M 2 + ions along the c direction. Secondly, the fluoride analogs of TTB seldom form over the entire range of x (0.40-0.60) [3]. The composition closest to ideal is K 0:50 M II 0:5 M III 0:5 F 3 ( ¼KM II M III F 6 ) (Fig.…”

“…This M 2 + -M 3 + -M 2 + -M 3 + ordering assures that M 2 + ions have only M 3 + ions for nearest neighbors along the c axis, and vice versa. Hong et al [3] concluded that M 2 + -M 3 + ionic ordering (as illustrated in Fig. 1b) is common in first row transition metal fluorides having the TTB structure.…”

“…The chemical formula for these compounds is K x M II x M III 1Àx F 3 , where x ¼0.4-0.6. Such ionic ordering makes it possible to design materials with interesting magnetic properties [3]. First row transition metal fluorides have innate characteristics favorable for these studies.…”

“…An idealized plot of the inverse of the magnetic susceptibility Because there are ten magnetic species in Region I but only six in Region III, the ratio C u M =C M should equal 0.60 [3,4]. Ideally, y 0 would be zero, implying that magnetic interactions between the ordered trimers and the uncoupled M 2 + ions are insignificant.…”