Unambiguous evidence of an isosymmetric phase transition in Rare earth orthoferrites ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mtext>RFeO</mml:mtext> <mml:mn>3</mml:mn> </mml:msub> </mml:mrow> </mml:math>

Abstract This study investigates the existence of an isosymmetric phase transition (ISPT) as a function of A site (R cation) radii in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mtext>RFeO</mml:mtext> <mml:mn>3</mml:mn> </mml:msub> </mml:mrow> </mml:math> (having an orthorhombic crystal structure with Pbnm space group). The ISPT is driven by the competing interaction of the first and second coordination shells of O anions with the 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>d</mml:mi> <mml:mrow> <mml:mtext>hhh</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> for an elementary cell (since it includes all the lattice parameters with equal weightage to the crystallographic axes), as it can act as an important parameter to explain ISPT. We have observed anomalies in a p , c p , γ , V mono , and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>d</mml:mi> <mml:mrow> <mml:mtext>111</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> around Gd in RFeO 3 , without any change in the crystallographic symmetry (SG: Pbnm) characteristic of ISPT. This ISPT is clearly evident in the crossover between the scaled radii of the two coordination shells of O atoms( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mtext>AO</mml:mtext> <mml:mn>8</mml:mn> </mml:msub> </mml:mrow> </mml:math> &amp; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mtext>AO</mml:mtext> <mml:mn>4</mml:mn> </mml:msub> </mml:mrow> </mml:math> ) corresponding to the R cation. In addition, we have clearly observed non-analytical behavior in the amplitude of various frozen phonon modes viz. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>R</mml:mi> <mml:mn>4</mml:mn> <mml:mo>+</mml:mo> </mml:msubsup> <mml:mo stretchy="false">(</mml:mo> <mml:mi>q</mml:mi> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> <mml:mo>,</mml:mo> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> <mml:mo>,</mml:mo> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>M</mml:mi> <mml:mn>3</mml:mn> <mml:mo>+</mml:mo> </mml:msubsup> <mml:mo stretchy="false">(</mml:mo> <mml:mi>q</mml:mi> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> <mml:mo>,</mml:mo> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> <mml:mo>,</mml:mo> <mml:mn>0</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> , and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>X</mml:mi> <mml:mn>5</mml:mn> <mml:mo>+</mml:mo> </mml:msubsup> <mml:mo stretchy="false">(</mml:mo> <mml:mi>q</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0</mml:mn> <mml:mo>,</mml:mo> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> <mml:mo>,</mml:mo> <mml:mn>0</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> 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 the room temperature octahedral site distortion ( ρ ) in RFeO 3 as observed by Zhou and Goodenough (2008 Phys. Rev. B 77 132104).