Condensation of a tetrahedra rigid-body libration mode in HoBaCo₄O₇: the origin of phase transition at 355 K

Abstract

Rietveld profiles, Mössbauer spectra and x-ray absorption fine structure (XAFS) were analyzed through the structural phase transition at T s = 355 K in HoBaCo 4 O 7 . Excess of the oxygen content over "O 7 " was avoided via annealing the samples in argon flow at 600 • C. Space groups (S.G.) P bn2 1 c and P 6 3 mc were used to refine the structure parameters in the low-and high-temperature phases, respectively. Additionally, the Cmc2 1 symmetry was considered as a concurrent model of structure of the low-temperature phase. In the high-temperature phase, severe anisotropy of thermal motion of the major part of the oxygen atoms was observed. This anisotropic motion turns to be quenched as the sample is cooled below T s . The variation of quadrupole splitting ε near T s is not similar to a steplike anomaly frequently seen at the charge-ordering transition. We observe instead a dip-like anomaly of the average ε near T s . Narrow distribution of the electric field gradient (EFG) over different cobalt sites is observed and explained on the basis of point-charge model. XAFS spectra show no evidence of significant difference between YBaCo 4 O 7 (T > T s ) and HoBaCo 4 O 7 (T < T s ). The origin of the transition at T s is ascribed to the condensation of the libration phonon mode associated with the rigid-body rotational movements of the starlike tetrahedral units, the building blocks of kagomé network. It is shown that the condensation of the libration mode is not compatible with translation symmetry for the hexagonal S.G., but compatible for the orthorhombic S.G. The orthorhombic lattice parameters and EFG components (V xx , V yy , V zz ) vary smoothly with temperature at approaching T s and closely follow each other.

Structural study of Ni-substituted YBaCo4−Ni O7 frustrated cobaltites

“…A remarkable discontinuity is revealed across the T S , which reaches a difference of 0.05 Å in the c lattice constant. This behavior is completely different to the one reported by Rykhov et al [12] for the Lu-114 cobaltite, but instead agrees with the one observed by Maignan et al [21] in Tm-114 at 175 K. It is interesting to note that in the initial stages of formation of one phase in the other, the lattice seems to be distorted to reduce mismatch, but a few degrees later the twophase mixture already shows well-resolved differences in lattice parameters, as shown in Fig. 4 for one set of reflections.…”

“…It is well-established in the literature that the parent compound YBaCo 4 O 7 exhibits a structural transition to the orthorhombic Pbn2 1 space group below 313 K [9], whereas Caignaert et al [10] reported a T S between 300 K and 310 K. Being a first-order transition [12,13,20], it is characterized by a thermal hysteresis which strongly depends on the kinetics of the experimental technique used for its determination [21]. In any case, precisely at room temperature the parent compound is likely to be in a two-phase state where the P31c (T phase) and Pbn2 1 (O phase) are coexisting.…”

“…The origin and driving force for the structural transition at T S remain under debate. It was originally suggested that a charge ordering mechanism would occur at T S [11]; it was also proposed that the transition is driven by the condensation of a phonon mode associated to the rigid-body rotation of the tetrahedra in the Kagomé layers [12]. Finally, Huq et al [13] proposed that it is driven by a severe under-bonding of Ba 2 þ ions.…”

Unprecedented lattice volume expansion on doping stereochemically active Pb2+ into uniaxially strained structure of CaBa1−xPbxZn2Ga2O7

“…Structure strain is usually released through structure symmetry lowering and polyhedral distortion/rotation, which was observed indeed in numerous "114" oxides such as LnBaFe 4 O 7 (Ln = Y, Dy-Lu) 23,37 , LnBaCo 4 O 7 (Ln = Ca, Y, Tb-Lu) [38][39][40] , and MAZn 2 Ga 2 O 7 (M = Ca 2+ , Sr 2+ ; A = Sr 2+ , Ba 2+ ) 18 . Herein, the structure strain of CaBa 1−x Pb x Zn 2 Ga 2 O 7 is released without a significant distortion of the [Zn 2 Ga 2 O 7 ] 4− framework, that is, the stereochemically active LP effect of Pb 2+ helps the release of the structure strain.…”

Relieving geometrical frustration through doping in the Dy1−xCaxBaCo4O7swedenborgites

“…This is comparable to that observed in other members of the R 3+ BaM 4 O 7 family. For instance, the value of D in the Yb [22] , Tm [23] and Ho [24] systems have been calculated to be 0.67%, 0.53% and 0.44% respectively at different temperatures . A tempera- High temperature x-ray diffraction scans in a narrow region for (a) CaBaCo4O7 and (b) Dy0.4Ca0.6BaCo4O7 specimens showing a transition from the high symmetry P31c ,Trigonal (T)→ low symmetry Pbn21,Orthorhombic (O) phase.…”