Comparative Raman study of isostructural YCrO3and YMnO3: Effects of structural distortions and twinning

Abstract

Polarized Raman spectra of isostructural orthorhombic YCrO 3 and YMnO 3 single crystals were collected at same conditions using different laser excitation in the visible range. The symmetry of the observed lines was determined and they were assigned to definite atomic vibrations based on lattice dynamical calculations. The frequencies of the lines of the same origin in both compounds are close, however, their intensities differ significantly. While the different intensity of the modes, activated by the Jahn-Teller distortion, can be explained by the different magnitude of this distortion in the two compounds, the cause of the reduced intensity of other lines in low frequency region in the Raman spectra of YMnO 3 is likely due to the fine twinning of the crystal. INTRODUCTIONThe interest in the transition-metal perovskite-like oxides ABO 3 continues for decades because of many attractive phenomena observed in these compounds. Among them are structural second-order phase transitions, high-T c superconductivity, colossal magnetoresistance, charge-and orbital-ordering, complex magnetic properties, etc. As a rule the structural, electrical and magnetic properties of these compounds are strongly correlated. They can be tuned through the change of the mismatch of the A-O and B-O bond lengths, expressed by the tolerance factor t = (R A + R O )/ √ 2(R B + R O ), where R O , R A and R B are the ionic radii of the oxygen and atoms in the A-and B-positions, respectively. Depending on the value of t, most of these compounds crystallize in superstructures of the ideal cubic perovskite (space group Pm3m) with R3c or Pnma symmetry. Using the model of Glazer 1 , treating the BO 6 octahedra as rigid ones, the superstructures can be obtained from the ideal cubic perovskite through some tilts (rotations) of the octahedra around the main crystallographic directions. In the Glazer's notations, the rhombohedral R3c structure is of (a − a − a − ) type and the orthorhombic Pnma structure is of (a − b + a − ) type.Raman spectroscopy is a powerful experimental technique for study of perovskite-like oxides. Due to the fact that there are no Raman-active modes in the ideal cubic perovskite, all observed lines in the one-phonon region of the Raman spectra of the real perovskites have to be connected to some deviations of the real structure from the ideal one. For example, as shown in the pioneering work of Scott 2 , one of the Raman-active modes in the rare-earth aluminates of R3c structure consists in a rigid octahedral tilt around [111] direction. This mode can be considered as a "soft mode" and its frequency goes to zero as the temperature approaches the critical temperature of the structural second-order phase transition. Iliev et al. 3 have shown that in the case of rare-earth manganites with orthorhombic Pnma structure, two of the observed lines in the Raman spectra originate from modes with a shape of the two static octahedral tilts, existing in the structure. Although that these compounds do not undergo second order transition to...

Using Raman and dielectric spectroscopy to elucidate the spin phonon and magnetoelectric coupling in DyCrO3 nanoplatelets

“…In order to study these features in such materials, it is essential to study the lattice dynamics. [17][18][19][20][21][22] The phonon behavior of these multiferroic materials shows an anomaly near Nèel transition temperature in Raman modes. There are several reports which deal with the Raman studies of multiferroic-materials.…”

Polarized Raman scattering on single crystals of rare earth orthochromite RCrO3 (R = La, Pr, Nd, and Sm)

“…All these studies were interested in the coupling between phonon modes and the magnetic ordering, which was done without a complete Raman mode assignment. At the beginning of this decade, N. Iliev et al and N. D. Todorov et al have investigated Raman modes of LaCrO 3 and YCrO 3 single crystals, respectively . These different studies were conducted on single crystals with micrometric size, which make impossible any crystallographic orientation and then phonon modes assignments become a nontrivial task.…”

Temperature dependent X‐ray diffraction and Raman spectroscopy studies of polycrystalline YCrO3 ceramics across the TC ~ 460 K

“…Out of these 60 modes, 24 modes (7 A 1g + 5 B 1g + 7 B 2g + 5 B 3g ) are Raman active . In the present study, we have observed 17 Raman active modes among 24 allowed one in the spectral range of 130–800 cm −1 at 300 K, which matches fairly well with the previous studies on YCO . The higher Raman peaks are assigned to the vibration of oxygen atoms (O1 and O2) only whereas the lower Raman peaks are attributed to the vibration of both Y and O atoms .…”