Magnetization, Mössbauer and isothermal dilatometric behavior of oxidized YBa(Co,Fe)₄O₇₊δ

Abstract

Mössbauer spectroscopy and magnetization studies of YBaCo(4-x)Fe(x)O(7+δ) (x = 0-0.8) oxidized at 0.21 and 100 atm O(2), indicate an increasing role of penta-coordinated Co(3+) states when the oxygen content approaches 8-8.5 atoms per formula unit. Strong magnetic correlations are observed in YBaCo(4-x)Fe(x)O(8.5) from 2 K up to 55-70 K, whilst the average magnetic moment of Co(3+) is lower than that for δ ≤ 0.2, in correlation with the lower (57)Fe(3+) isomer shifts determined from Mössbauer spectra. The hypothesis on dominant five-fold coordination of cobalt cations was validated by molecular dynamics modeling of YBaCo(4)O(8.5). The iron solubility limit in YBaCo(4-x)Fe(x)O(7+δ) corresponds to approximately x ≈ 0.7. The oxygen intercalation processes in YBaCo(4)O(7+δ) at 470-700 K, analyzed by X-ray diffraction, thermogravimetry and controlled-atmosphere dilatometry, lead to unusual volume expansion opposing to the cobalt cation radius variations. This behavior is associated with increasing cobalt coordination numbers and with rising local distortions and disorder in the crystal lattice on oxidation, predicted by the computer simulations. When the oxygen partial pressure increases from 4 × 10(-5) to 1 atm, the linear strain in YBaCo(4)O(7+δ) ceramics at 598 K is as high as 0.33%.

Orthorhombic YBaCo4O8.4 crystals as a result of saturation of hexagonal YBaCo4O7 crystals with oxygen

“…The data on ceramic samples [8][9][10], results of theoretical calculations (triclinic cell of sym-metry P1, ~11600 atoms) [11], and the data on single crystals obtained in this study are indicative of many ways for implementing symmetry and unit-cell metric. Apparently, depending on the degree of saturation with oxygen, compounds may have different structures.…”

“…5c) and Со (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) in the cross section with z ≈ 0.25 (0.23-0.27) (Fig. 5d).…”

“…Valldor [1] was the first to study the physical properties and determine the crystal structure of cobaltates belonging to the R-Ba-Co-O system (R is a rare earth element or Y) with the element ratio Y : Ba : Co : O = 1 : 1 : 4 : (7 + x) [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. RBaCo 4 O 7 + x (R114) compounds continue to attract much attention due to the unusualness and great variety of their physical properties (ferro-and antiferromagnetism, catalytic activity, etc.).…”

“…Despite the significant interest in the problem of oxygen exchange, the data on the structure of the high-oxygen phase RBaCo 4 O 7 + х are limited and contradictory [8][9][10][11]. Apparently, this is caused by different reasons: both the difficulties in growing YBaCo 4 O 7…”

Controlling structural distortion in the geometrically frustrated layered cobaltate YBaCo4O7+δby Fe substitution and its role on magnetic correlations

“…A single crystal Mössbauer study on YBaCo 3 FeO 7 revealed two spin freezing temperatures of T c1 = 590 K and T c2 = 50 K corresponding to the spin freezing at the Kagomé layers and triangular layers, respectively [32]. However, a Mössbauer study on powder compounds [YBaCo 4-x Fe x O 7+δ (x = 0-0.8) ] reported single spin freezing temperature ~ 70 K [33]. The present neutron diffraction study shows the onset of the magnetic ordering around 130 K for all compounds, however, with no observable change in the magnetic diffraction peak around T c2 .…”