Evidence for monoclinic crystal structure and negative thermal expansion below magnetic transition temperature in Pb(Fe_1/2Nb_1/2)O_3

TL;DR

This study confirms that Pb(Fe_1/2Nb_1/2)O_3 has a monoclinic structure at all temperatures down to 12K and exhibits negative thermal expansion below its magnetic transition, highlighting spin-lattice interactions.

Contribution

The paper resolves controversy by establishing the monoclinic structure of Pb(Fe_1/2Nb_1/2)O_3 at low temperatures using synchrotron X-ray diffraction.

Findings

Monoclinic structure persists down to 12K.

Negative thermal expansion observed below T_N.

Lattice anomalies occur at the magnetic transition.

Abstract

The existing controversy about the room temperature structure of multiferroic Pb(Fe_1/2Nb_1/2)O_3 is settled using synchrotron powder x-ray diffraction data. Results of Rietveld refinements in the temperature range 300 to 12K reveal that the structure remains monoclinic in the Cm space group down to 12K, but the lattice parameters show anomalies at the magnetic transition temperature (T_N) due to spin lattice coupling. The lattice volume exhibits negative thermal expansion behaviour, with Alpha = - 4.64 x 10^-6 K^-1, below T_N.