* Corresponding author. Fax: # 886-3-572-3052. E-mail address: d863351@oz.nthu.edu.tw (B.N. Lin) Physica B 281&282 (2000) 922}923 Variation of crystal symmetry, superconductivity and weak ferromagnetism in (R  Gd  )  Ce  CuO  (R"Nd, Eu) cuprates B.N. Lin*, H.C. Chiang, Y.H. Lin, H.B. Yang, Y.Y. Hsu, H.M. Luo, H.C. Ku Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan Abstract Metal}insulator transition in the Gd-doped (R  Gd  )  Ce  CuO  systems occurred around y&0.1 for R"Eu and y&0.65 for R" Nd with a simultaneous structural transition from the tetragonal ¹-phase (space group I4/mmm) to an orthorhombic O-phase (Acam, a  )b  &2a  ). The structural distortion is due to the lattice mismatch between smaller (R,Gd,Ce)  O  layer and CuO  plane which results in an oxygen distortion in the otherwise perfect CuO  square plane. The phase diagram indicates that the superconductivity can only occur in the metallic T-phase cuprates where the metallic state is controlled by a delicate balance between oxygen reduction and structural stability. The Cu weak-ferromagnetic/canted-antiferromagnetic (WF/CAF) order observed in the insulating region is the direct result of non-1803 (Cu}O}Cu) canting angle of the O-phase.  2000 Elsevier Science B.V. All rights reserved. Keywords: Distorted T-phase (O-phase); Weak ferromagnetism; Superconductivity Unlike superconducting, tetragonal T-phase R  Ce  CuO  cuprates (R"Pr, Nd, Sm, Eu), no superconductivity can be detected for Ce-doped cuprate Gd  Ce  CuO  . Instead, a Cu weak-ferromag- netic/canted-antiferromagnetic (WF/CAF) order with nonzero saturation moment m  (Cu) was observed as its parent compound Gd  CuO  . Furthermore, this insu- lating WF compound forms an orthorhombic O-phase (space group Acam, a"0.5519, b"0.5521, and c"1.1841 nm) structure with non-perfect CuO  square plane due to the layer mismatch between smaller (Gd,Ce)  O  layers and CuO  plane [1,2]. The weak ferromagnetism is the direct result of a non-1803 ((Cu}O}Cu)"1713) superexchange coupling with an additional antisymmetric Dzyaloshinsky}Moriya-type exchange term. The canted-antiferromagnetic align- ment will create a net WF saturation moment m  (Cu)&(Cu)sin[(!)/2]&410   if the usual neutron-derived copper moment of (Cu)&0.5   is used. In order to study the correlation among crystal sym- metry, weak ferromagnetism and superconductivity in these Ce-doped cuprates, (R  Gd  )  Ce  CuO  systems (R"Nd, Eu) are chosen for intense studies. All samples under investigation were prepared by standard solid-state reaction, followed by various oxygen-reducing processes. The temperature dependence of magnetized magnetic moment m(¹) per formula unit in 1 G for three typical (Eu  Gd  )  Ce  CuO  cuprates is shown in Fig. 1. A low "eld of 1 G is used to reduce the magnetic contribution from unwanted but unavoidable paramag- netic Gd ions. Although superconductivity with ¹  around 5}12 K was reported for oxygen-reduced T-phase cuprate Eu  Ce  CuO  (tetragonal lat- tice parameters a"0.3909 nm, c"1.1876 nm) and (Eu  Gd  )  Ce  CuO  cuprates with very small Gd doping (y)0.05) [3,4], no ¹  can be detected for these higher Gd-substitution compounds even after vari- ous oxygen-reducing processes. Instead, all compounds show insulating, weak-ferromagnetic order typically 0921-4526/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 7 8 8 - 7