The monoclinic/orthorhombic phase transition in Ba₂Cu(HCOO)₆: An optical and x-ray powder diffraction study

Monoclinic–orthorhombic phase transition in Ba2Cu(HCOO)6 crystals: a Raman scattering study

“…We now discuss the above results based on the structural transformation observed by x-ray measurements by Polla et al 10 Consider Fig. 5(a) and (b), which shows the projection of the unit cell of Ba 2 Cu(HCOO) 6 on to the ac plane of C 2 and D 2h structures, respectively; the HCOO groups are represented by triangles and when they are perpendicular to the ac plane, they appear as thick lines.…”

“…The last groups rotate around the a axis becoming aligned to the bc plane. Such a mechanism led Polla et al 10 to propose a displacive equitranslational character for this phase transition.…”

“…For T > 312 K, according Polla et al, 10 Ba 2 Cu(HCOO) 6 has an orthorhombic structure with space group Pbnm (D 16 2h ), with four molecules per unit cell, and lattice parameters a D 11.092Å, b D 7.079Å and c D 17.391Å. One can note that the lattice parameters do not change.…”

“…17,18 It is expected that Ba 2 Cu(HCOO) 6 crystals will transform into an orthorhombic phase at temperatures between 310 and 360 K, depending on the crystal quality. 10 In order to study this phase transition, Raman spectra between room temperature and 317 K were taken using the same scattering geometries as in low-temperature study. Figures 3 and 4 show the phonon modes observed for y zz y and y xz y scattering geometries, respectively.…”

“…From the results obtained, we were able to understand how the 24 HCOO forming the unit cell are distributed into four sets of C 1 2 local site symmetries. As a continuation of this previous work, we present here a temperature-dependent Raman scattering study of Ba 2 Cu(HCOO) 6 in order to verify not only the stability of its room temperature phase with decrease in the temperature but also the phase transformation observed by Polla et al 10 …”