La_2−xSr_xCoO₄ (x = 0.4, 0.5, 0.6) crystals with Ruddlesden-Popper K₂NiF₄-type structure have been investigated as potential dielectric application. In this report, La_2−xSr_xCoO_4−δ crystals have been grown by Floating Zone. Rietveld refinement revealed that these compounds are crystallized in K₂NiF₄-type structures with space group I4/mmm. It was found that the lattice parameter c decreases as x increases. The phonon spectra of La_2−xSr_xCoO₄ have been investigated by Raman spectroscopy. Dielectric properties of La2-xSrxCoO4 crystals were studied in a broad frequency and temperature range. The dielectric relaxation should be attributed to the non-adiabatic small polaroic hopping. There was one obvious dielectric relaxation around room temperature. This dielectric relaxation was a thermal-activated process. It should be attributed to the mixed-valence structure (Co²⁺/Co³⁺) since its activation energy was similar to that of small polaronic hopping process.

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