Evidence for charge localization in the ferromagnetic phase of La_(1-x)Ca_(x)MnO_3 from High real-space-resolution x-ray diffraction

TL;DR

This study uses high-resolution x-ray diffraction to reveal charge localization and coexistence of localized and delocalized phases near the metal-insulator transition in La_(1-x)Ca_(x)MnO_3.

Contribution

It provides direct real-space evidence of charge localization in the ferromagnetic phase of La_(1-x)Ca_(x)MnO_3 using advanced x-ray diffraction techniques.

Findings

Evidence of three distinct Mn-O bond lengths in the insulating phase.

Appearance of Mn^{3+}-O long bonds near the metal-insulator transition.

Coexistence of charge localized and delocalized phases near the transition.

Abstract

High real-space-resolution atomic pair distribution functions of La_(1-x)Ca_(x)MnO_3 (x=0.12, 0.25 and 0.33) have been measured using high-energy x-ray powder diffraction to study the size and shape of the MnO_6 octahedron as a function of temperature and doping. In the paramagnetic insulating phase we find evidence for three distinct bond-lengths (~ 1.88, 1.95 and 2.15A) which we ascribe to Mn^{4+}-O, Mn^{3+}-O short and Mn^{3+}-O long bonds respectively. In the ferromagnetic metallic (FM) phase, for x=0.33 and T=20K, we find a single Mn-O bond-length; however, as the metal-insulator transition is approached either by increasing T or decreasing x, intensity progressively appears around r=2.15 and in the region 1.8 - 1.9A suggesting the appearance of Mn^{3+}-O long bonds and short Mn^{4+}-O bonds. This is strong evidence that charge localized and delocalized phases coexist close to the metal-insulator transition in the FM phase.