Optical studies of the incommensurate charge ordered phase in La1.775Sr0.225NiO4

Analysis of Temperature Dependence of Electrical Conductivity in La2NiO4.14 Single Crystal

“…The falling into the charge stripe phase above T CO had been confirmed by several authors [10,12]. Recently, optical conductivity measurements in La 2 NiO 4+δ with δ = 2/15 showed that the spectra could be decomposed into two 1 components: one attributed to the stripes while the other to thermally excited carriers [13]. These observations are consistent with a strongly fluctuating nature of the stripes in these materials.…”

“…Recently, charge-ordering phenomena in transition-metal oxides has attracted considerable interests in physics community, since there is an assumption for some common phenomenon concerning a charge stripe formation which were observed in the O-doped or Sr-doped nickelates [1,2], in the cuprates [3][4][5], and high-temperature superconducting cuprates, or giant magnetoresistance in manganites [6][7][8]. In the nickelates, the charge stripes have been observed to be static, at least for certain compositions.…”

Tuning the Oxygen Reduction Reactivity of Layered Perovskites Using the Jahn–Teller Effect

“…210 and 450 cm –1 (Figure A). The observation of only two peaks is common for La 2 NiO 4 , where single-crystal studies have assigned both to A 1g modes. − Spectra for the La 1.2 Sr 0.8 CoO 4 phase instead contain prominent peaks at ca . 210, 450, and 690 cm –1 and smaller peaks at 290, 350, and 600 cm –1 (Figure A).…”

“…Previous reports also suggest that the ca . 350 and 600 cm –1 peaks are symmetry forbidden modes that arise due to defects lowering local symmetry. − Related phases such as Sr 2 CoO 4 , LaCoO 3 , and La 2– x Sr x CoO 4 show a peak near 700 cm –1 in Raman spectra, but its assignment remains uncertain. − Raman spectra therefore reveal a structural complexity that is not captured in the XRD results.…”

“…A similar peak was reported in epitaxially grown La 1.5 Sr 0.5 CoO 4 , where polarized Raman spectra were used to assign the peak to an in-plane breathing mode involving the four B–O eq bonds. , This breathing mode is a symmetry-forbidden Raman mode in the I 4/ mmm unit cell that is observed in XRD results on the La 1.2 Sr 0.8 Ni 1– y Co y O 4 series here. Previous analysis of cobaltate and manganate perovskites have shown that local changes in bond lengths, a feature that often affects XRD patterns by broadening Bragg peaks, permits growth of such symmetry-forbidden peaks in Raman spectra by disrupting lattice symmetry. − The observed changes in Raman spectra and unit cell dimensions can be understood as a replacement of Ni(III), a d 7 ion, with Co(III), a d 6 ion. This substitution contracts the c axis by removing electron density from antibonding orbitals, which is known to affect both OER and ORR according to the Sabatier principle by altering B–O bond strength. ,, The substitution also replaces an ion expected to exhibit strong Jahn–Teller expansion of B–O ax bonds with one that would prefer no expansion (Figure A), which will alter the strain distribution in the lattice.…”