We combine a wide variety of experimental techniques to analyze two heretofore mysterious phase transitions in multiferroic bismuth ferrite at low temperature. Raman spectroscopy, resonant ultrasound spectroscopy, EPR, X-ray lattice constant measurements, conductivity and dielectric response, specific heat and pyroelectric data have been collected for two different types of samples: single crystals and, in order to maximize surface/volume ratio to enhance surface phase transition effects, BiFeO3 nanotubes were also studied. The transition at T=140.3K is shown to be a surface phase transition, with an associated sharp change in lattice parameter and charge density at the surface. Meanwhile, the 201K anomaly appears to signal the onset of glassy behaviour.
@article{arxiv.1202.3387,
title = {Surface phase transitions in BiFeO3 below room temperature},
author = {R. Jarrier and X. Marti and J. Herrero-Albillos and P. Ferrer and R. Haumont and P. Gemeiner and G. Geneste and P. Berthet and T. Schülli and P. Cvec and R. Blinc and Stanislaus S. Wong and Tae-Jin Park and M. Alexe and M. A. Carpenter and J. F. Scott and G. Catalan and B. Dkhil},
journal= {arXiv preprint arXiv:1202.3387},
year = {2015}
}