Unambiguous evidence of isosymmetric phase transition in rare earth orthoferrites (RFeO 3) driven by interaction of R cation with O anions in first and second coordination shells

Abstract This study investigates the existence of an Isosymmetric phase transition(ISPT) as a function of A site (R cation) radii in RFeO 3 (having an orthorhombic crystal structure with Pbnm space group). This ISPT is driven by the competing interaction of the first and second coordination shells of O anions with R cation. The orthorhombic supercell has been expressed in terms of an elementary (pseudomonoclinic) cell with lattice parameters (a p =b p , c p , γ, V mono ). We have also calculated the lattice plane spacing d hhh for an elementary cell (since it includes all the lattice parameters with equal weightage to crystallographic axes), as it can act as animportant parameter to explain ISPT. We have observed anomalies in a p , c p , γ, V mono , and d 111 around Gd in RFeO 3 , without any change in the crystallographic symmetry (SG: Pbnm) characteristic of ISPT. This ISPT is clearly evident by the crossover between the scaled radii of the two coordination shells of O atoms(AO 8 & AO 4 ) corresponding to the R cation. In addition, we have clearly observed nonanalytical behavior in the amplitude of various frozen phonon modes viz., R4+(q =1/2,1/2,1/2), M3+ (q=1/2,1/2,0), and X5+ (q=0,1/2,0) corresponding to out of phase octahedral rotation, in-phase octahedral rotation, and the displacement of R cation in an anti-parallel fashion. Our findings support the anomalous behavior of room temperature octahedral site distortion (ρ) in RFeO 3 as observed by Zhou and Goodenough (Phy. Rev. B 77, 132104, 2008).