X-ray and neutron diffraction studies of coupled structural phase transitions in DyBaCo2O5.5

Abstract

A structural transition at T ≈ 322 K from the P mmm to P mma phase is found to coincide with an anomaly of resistivity. Another structural phase transition doubling the lattice parameter c, which has been postulated earlier to accompany a low-temperature magnetic transition in TbBaCo2O5.5, is observed in a single crystal DbBaCo2O5.5 by means of the X-ray and neutron diffraction. The low temperature phase does not belong to the space group P cca that has been chosen earlier as the highest subgroup of the P mma. The transition is of the first order with the temperature hysteresis, between T ≈ 100 and T ≈ 200 K, which probably explains anomalous magnetic properties in this temperature range.

Low-temperature behavior ofYBaCo2O5.5: Coexistence of two spin-state ordered phases

“…This indicates that the single crystal does not undergo phase separation below T L and has Bmmm symmetry, thus confirming the relevance of polycrystallinity and microstructural factors. It should be noted that the weak nuclear reflections ͑h ,0,l +0,k , l; h / k, l =2n +1͒ observed by Chernenkov et al 14 on the twinned DyBaCo 2 O 5.5 single crystal by x-ray diffraction are consistent with the model proposed in this work, since Bmmm space group allows the presence of ͑h ,0,l͒ reflections with h + l =2n. Moreover, since there is no group-subgroup relationship between the Pmma and Bmmm space groups, the corresponding phase transition should be of the first order, again in agreement with the diffraction experiments.…”

“…The conclusion that below T L the high temperature polymorph ͑Pmma with the k = c * / 2 magnetic wave vector͒ coexists with the new phase ͑Bmmm with k =0͒ is in concurrence with recent work by Chernenkov et al 14 The single- FIG. 4.…”

“…However, taking into account the Wyckoff positions splitting and the distribution of nonequivalent cobalt in the unit cell, this space group was found unlikely. 10 The observation of ͑h ,0,l , +0,k , l; h / k, l =2n +1͒ reflections in the recent low-temperature x-ray and neutron diffraction experiments, 14 where the twinned DyBaCo 2 O 5.5 single crystal was studied, also ruled out the Pcca symmetry. In the present neutron powder diffraction experiment, detection of these very weak superstructure reflections overlapping with magnetic peaks was practically impossible.…”

Magnetic ordering inHoBaCo2O5.5

“…The determination of the space group of the magnetically ordered RBaCo 2 O 5.5 compounds is difficult, and it has recently been shown that DyBaCo 2 O 5.5 in contrast to TbBaCo 2 O 5.5 does not transform to a low-temperature phase with Pcca symmetry. 21 The Pmma space group was therefore retained for the chemical structure of HoBaCo 2 O 5.5 for T ഛ 265 K in the present study. The atomic coordinates were fixed at the values obtained at 275 K, and only lattice parameters and thermal displacement parameters were refined for T ഛ 265 K. This approximation to the chemical structure below 265 K was made to reduce the number of refineable structural parameters.…”

Large single crystals of LnBaCo2O5.5: Initial nucleation, growth and study

“…3, [13]). A huge temperature hysteresis of about 100 K is observed for the X-ray intensities of both the superstructure and basic Bragg reflections [18].…”