  Our detailed temperature dependent synchrotron powder x-ray diffraction
studies along with first-principles density functional perturbation theory
calculations, enable us to shed light on the origin of ferroelectricity in
GdCrO3. The actual lattice symmetry is found to be noncentrosymmetric
orthorhombic Pna21 structure, sup- porting polar nature of the system. Polar
distortion is driven by local symmetry breaking and by local distortions
dominated by Gd off-centering. Our study reveals an intimate analogy between
GdCrO3 and YCrO3. However, a distinctive difference exists that Gd is less
displacive compared to Y, which results in an orthorhombic P na21 structure in
GdCrO3 in contrast to monoclinic structure in YCrO3 and consequently, decreases
its polar property. This is due to the subtle forces involving Gd-4f electrons
either directly or indirectly. A strong magneto-electric coupling is revealed
using Raman measurements based analysis in the system below Cr-ordering
temperature, indicating their relevance to ferroelectric modulation.
