Infrared properties of T'-phase R2CuO4 insulating compounds.

Abstract

PHYSiCAL REVIEW B VOLUME 43, NUMBER 10 APRIL 1991 Infrared ProPerties of T'-Phase R &CuO4 insulating comPounds S. L. Herr Department of Physics, Virginia Commontvealth University, Richmond, Virginia 23284-2000 K. Kamaras Central Research Institute of Physics, H1525 Budapest, Hungary D. B. Tanner, S-W. Cheong, * and G. R. Stewart Department of Physics, University of Florida, Gainesville, Florida 32611 Z. Fisk Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 18 July 1990) Reflectance studies on single crystals of a series of rare-earth (R =Pr, Nd, Sm, Cxd) copper oxides have been made to determine phonon frequencies as a function of rare-earth mass. The increase in interatomic force constants with increasing rare-earth mass, which leads to a reduction in the lattice parameters, increases the vibrational frequencies. In addition, the change from the T-phase La2Cu04 to the T'-phase R2Cu04 crystal structure causes changes in the phonon frequencies that are due both to the changing lattice parameters and to the relocation of the apical oxygen atoms. I. coo=(tc/p)', INTRODUCTION in The recent discovery of n-type superconductors lanthanide copper oxides' has brought renewed interest There to the optical characterization of these materials. is a structural phase change in the lanthanide cuprates when La is replaced by Pr, Nd, Sm, or Gd. ' These structural changes have a direct effect upon the phonon frequencies in the ir spectrum and, knowing the phonon ' the assignments assignments for the La compound, for the substituted compounds can be made by a direct This paper will discuss the effects of the comparison. different rare earths on the crystal structure and how these changes affect the phonon frequencies. Infrared ab plane reAectance will be analyzed for the frequency shifts due to the relocation of the apical oxygen atoms. An esti- mate of the zero-frequency dielectric constant will be made from two separate calculations. Our results are in good agreement with recent studies of Nd2Cu04 (Refs. 6 and 12) and Pr2CuO&. ' These materials form in a tetragonal crystal structure, when R is one of the rare earths Pr through Vd, with Cu02 planes in which the oxygen atoms are square- planar coordinated about copper ions. ' Unlike the or- thorhombic (quasitetragonal) La2Cu04, where the Cu04 are octahedral coordinated with two apical oxygens, the apical oxygen atoms have relocated to form an R-02-R plane between the Cu02 planes. This allows for a more compact structure and the unit-cell volume decreases monotonically with increased R mass (i. e. , decreased R ionic size), as shown in Fig. 1(a). Phonon assignments can be made by using a simple harmonic-oscillator model incorporating the changing structural parameters. The phonon center frequency is where tc is an effective force constant be- tween the atoms and p is the reduced mass of the partici- pating atoms. Both force constant and mass change with rare earth. In Fig. 1(b) nearest-neighbor distances, DNN, for the pure lanthanide compounds' are plotted versus R mass and show decreasing interatomic distances with in- creasing mass. This implies that the force constant will grow larger with increasing mass. At the same time, the apical oxygen atoms are pushed from their octahedral coordination above the copper atoms to positions on the unit-cell faces and are sandwiched in between the rare- earth atoms to form an RzO2 plane (T to T' phase). The consequence of this structural change IFigs. 1(c) and 1(d)] is that the c axis becomes shorter and the a axis broadens out, going from a long and thin unit cell to a short and wide unit cell. Four in-plane infrared-active (E„) modes have been ' It would be ex- identified in the La2CuO& compound. pected that three of these modes (the Cu02 stretch, the Cu02 bending, and the R-lattice modes) would change in accordance with the lattice parameters, while the mode associated with the apical oxygen bending should disap- pear and be replaced by a higher-frequency mode of the R 20& in-plane vibration. II. EXPERIMENT Single crystals and polycrystalline ceramic pellets were made at the University of Florida and at Los Alamos Na- tional Laboratories using techniques which have been de- scribed elsewhere. ' The optical measurements reported in this paper were performed on Aakelike crystals and ceramic pellets without after-synthesis polish. We at- tempted to dope one NdzCu04 ceramic sample and the The American Physical Society