Raman study of T'-phase distortion in R2CuO4 compounds (R=Nd,Sm,Eu,Gd).

Abstract

We present a Raman study of the appearance of structural distortions in T'-phase cuprates ${\mathit{R}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ (R=Sm,Eu,Gd) upon R atomic number increase. Besides some anomalies in the frequencies of the allowed modes, the distortion from the T' phase is evidenced in ${\mathrm{Gd}}_{2}$${\mathrm{CuO}}_{4}$ by the appearance of an additional mode with ${\mathit{B}}_{1\mathit{g}}$ symmetry, ${\mathit{B}}_{1\mathit{g}}^{\mathrm{*}}$, that we assign to the motion of the in-plane oxygen atoms, O(1). This mode, which appears incipiently in ${\mathrm{Eu}}_{2}$${\mathrm{CuO}}_{4}$ and ${\mathrm{SmGdCuO}}_{4}$, is formally forbidden by symmetry, and would be activated by O(1) displacement resulting in a distorted ${\mathrm{CuO}}_{2}$ plane. This interpretation is in agreement with previous experimental results either of structural or magnetic type. The appearance of weak ferromagnetism for R=Gd and beyond, for instance, is incompatible with perfect T'-phase structure. We compare the results in pure compounds with those that are Ce doped and show that the ${\mathit{B}}_{1\mathit{g}}^{\mathrm{*}}$ activity decreases with respect to that of the ${\mathit{B}}_{1\mathit{g}}$ mode as increasing amounts of Ce are introduced in the lattice; this is interpreted as a consequence of T'-phase stabilization upon doping. Temperature-dependence spectra are also presented and discussed.