Evidence for anomalous structural behavior in CaFe2As2

TL;DR

This study reveals anomalous structural behavior and phase coexistence in CaFe2As2, with evidence of incomplete phase transformation and unusual lattice parameter contraction, differing from other similar compounds.

Contribution

It provides detailed experimental evidence of phase coexistence and anomalous lattice behavior in CaFe2As2, challenging previous assumptions about its structural transitions.

Findings

Coexistence of tetragonal and orthorhombic phases below 170 K

Orthorhombic phase has higher unit cell volume than tetragonal

Anomalous lattice contraction with increasing temperature

Abstract

The structural properties of the CaFe2As2 have been investigated by x-ray and neutron powder diffraction techniques as a function of temperature. Unambiguous experimental evidence is shown for coexistence of tetragonal and orthorhombic phases below 170 K in contrast to existing literature. Detailed Rietveld analyses of thermo-diffractograms show that the sample does not transform completely in to the orthorhombic phase at the lowest temperature even though it is the majority phase. We have found that the unit cell volume of the orthorhombic phase is higher compared to that of the tetragonal phase for all the temperatures. X-ray data on CaFe2As2 shows anomalous (at) lattice parameter contraction with increasing temperature and phase co-existence behavior below 170 K unlike other members of the 122 family of compounds like SrFe2As2 and EuFe2As2. Temperature dependent magnetization of polycrystalline CaFe2As2 sample show weak anomalies below 170 K. This behavior of the polycrystalline sample is in contrast to that of a single crystal reported earlier.