Isotope effect on Raman spectra of polycrystalline La_0.67Ca_0.33MnO₃

Abstract

Micro-Raman spectra were recorded for two samples of polycrystalline La 0.67 Ca 0.33 MnO 3 , which were synthesized from oxides with oxygen isotopes 16 O and 18 O. Isotope Raman shifts were measured and compared with the theoretical prediction. In this way the oxygen-related Raman modes were clearly identified. Temperature-related blue Raman shifts for 16 O ( 18 O) vibration modes at 230 cm −1 (216 cm −1 ) were unexpectedly observed. These blue Raman shifts were compared with the blue shift due to lattice parameter variation as the temperature is lowered. The large blue Raman shift that occurs with decrease in temperature demonstrates that the Jahn-Teller distortion may play an important role. The appearance of new peaks at 440-480 and 410-450 cm −1 for 16 O and 18 O samples, respectively, as the temperature decreases to 140-100 K indicates a possible charge-orbit ordering transition. A continuous low-wavenumber signal was observed which is superimposed on the broad phonon Raman lines. It was found that the scattering intensity of the low-wavenumber signal is dramatically reduced as the temperature is lowered. The nature of this excitation is not clear.

In-Situ Oxygen Isotopic Exchange Vibrational Spectroscopy of Rhenium Oxide Surface Structures on Cerium Oxide

“…No new chemical species were observed; the current species are more pronounced in the spectrum. Once 18 O isotope exchange occurs, new bands begin to appear at lower wavenumbers (between 900 and 950 cm –1 ), shown in Figure b, due to the red shift caused by the increase in mass . Upon repeat heating in an 18 O environment, as seen in Figure a, the 16 O band intensities decrease, and the intensities of the red-shifted bands associated with 18 O begin to increase to levels similar to the heated samples, as seen in Figure e, indicating the 18 O exchange.…”

Structural properties and Raman spectroscopy of rhombohedral La1−xNaxMnO3 (0.075≤x≤0.15)

“…For La 0.925 Na 0.075 MnO 3 sample the 466.4, 619 E g modes present softening of $21 (24), $9 (7) cm À1 for T = 200 and 100 K, respectively and is associated to magnetic ordering experience at lower temperature. The Raman active phonon, magnon, and electronic modes of monovalent Na doped La 1Àx Na x MnO 3 have not earlier been studied in detail, however, a considerable amount of information is available on orthoferrites, chromites and divalent-doped manganites of rare earth series. It is generally seen that the Raman frequencies of the latter compounds in the lower wave number region below 450 cm À1 depend on the type of the rare earth element and greater 450 cm À1 , related to the internal vibrations of the oxygen octahedra dominate the spectrum.…”

“…A clear manifestation of quasi particles is reported in terms of oxygen isotope effect for the vibration modes in A site Ca doped La 0.67 Ca 0.33 MnO 3 . Furthermore, the orbital ordering that occurs far below the PM-FM transition distinguishes the Mn-O vibration with a different bond length in the JT distorted oxygen octahedral [3]. Recently, Raman phonons of both A and B site doped rhombohedral ceramic La 1Ày Sr y Mn 1Àx M x O 3 manganites were investigated and stressed that frequency of A 1g mode depends linearly on the angle of rhombohedral distortion [4].…”

Isotope Identification Mechanisms Enabled by Swept-Wavelength Raman Spectroscopy

“…Almost all the peaks exhibited wavenumber shifts as a result of deuteration. The magnitude of the shifts varied from a few percent for C–C modes to the theoretical limit 18 of or 29% for a hydrogen to deuteron substitution, in the case of the C–H stretching modes in polyethylene. The observed wavenumber shifts were consistent with previously published data for single wavelength Raman measurements of polyethylene 19 and acetic acid, 20 as well as previous measurements of KH 2 PO 4 , KD 2 PO 4 21,22 , and K 2 HPO 4 .…”