Structural and micro-Raman studies of DyMnO3 with potassium substitution at the Dy site

Abstract

Observation of colossal magnetoresistance at ferromagnetic and metal insulator transitions in manganites is considered to be closely related to a double exchange interaction between trivalent and tetravalent Mn ions combined with Jahn–Teller (JT) distortions produced by Mn³⁺ ions in MnO₆ octahedra. The rare earth manganite, DyMnO₃ has only Mn³⁺ ions which are JT active. This compound is antiferromagnetic involving Dy spins at low temperature (∼3 K) with weak magnetic interactions around 40 K due to Mn spins. When any aliovalent ion is substituted at the Dy site, tetravalent Mn ions are created for charge neutrality. With an aim to induce ferromagnetism by diluting the Dy sublattice to reduce super-exchange interactions and to provide mixed valent Mn ions to favor double exchange interactions in the DMO compound, we have undertaken monovalent potassium substitution at the Dy site. In this paper, for the first time we report the synthesis of a new series of compounds, Dy_1−xK_xMnO₃ (x = 0.1, 0.2 & 0.3) along with their structural and vibrational properties. All the compounds were found to crystallize in an orthorhombic structure. Structural analysis was performed using Rietveld refinement of X-ray powder diffraction data with the GSAS program. Refinement results revealed that MnO octahedra become more regular with potassium substitution as compared to the pristine compound. Raman spectroscopy studies showed that a 30 at% compound with less octahedral distortions exhibited sharp modes compared to other compounds with large distortions. From the vibrational spectra, Raman modes corresponding to stretching, bending and tilting of metal–oxygen polyhedra have been identified. The mixed valence state of the Mn ion for potassium substituted DyMnO₃ is investigated using XPS. The ratios of Mn³⁺/Mn⁴⁺ suggested that the production of holes is increased with potassium concentration.