Phase separation and phase transitions in undoped and Rh3+ doped iron pnictide CaFe2As2: a Raman scattering study

TL;DR

This Raman scattering study reveals phase separation, multiple phase transitions, and complex magnetic structures in CaFe2As2 with and without Rh doping, providing new insights into the material's phase behavior and magnetic properties.

Contribution

First observation of phase separation in high-temperature phase and detailed characterization of multiple phase transitions in Rh-doped CaFe2As2 using Raman spectroscopy.

Findings

Coexistence of low-spin and high-spin Fe2+ phases at ambient conditions.

Multiple phase transitions including cascade and re-entrant magnetic structures.

Phase separation persists across a wide temperature range in doped samples.

Abstract

Iron-pnictides Ca(Fe1-xRhx)2As2 (x = 0, 0.035, and 0.19) were studied across the tetragonal-orthorhombic and uncollapsed to collapsed tetragonal phase transitions using Raman spectroscopy. The effect of the phase separation was observed in the high-temperature phase for the first time. Two phases with the low-spin and high-spin states of the Fe2+ ions coexist in the undoped (x = 0) and doped (x = 0.035) samples at ambient conditions. These two phases are present on a larger length scale and they are not limited to the local distortions. Both phases in the undoped sample successfully undergo tetra-to-ortho phase transitions approximately at the same temperature T0 = 160 K. In doped samples, a cascade of phase transitions is observed at temperatures T* = 100 K and Tc = 72 K with cooling for the first time. A complex type of the re-entrant magnetic structure is realized in the x = 0.035 sample at temperatures below Tc, which is distinct from the usual orthorhombic twofold one. The overdoped sample (x = 0.19) also shows phase separation at temperatures 7 - 295 K in Raman spectra similar to these of the remnant phase.