Journal of Solid State Chemistry 159, 1 } 6 (2001) doi:10.1006/jssc.2001.9112, available online at http://www.idealibrary.com on Temperature Dependence of the Hyperfine Interaction at 181 Ta in SrHfO3 Alberto Lo H pez-Garcia, 1 Patricia de la Presa, 2 and Alejandro Ayala 3 Programa TENAES, Departamento de FnH sica, Universidad Nacional de La Plata, C.C.67, 1900 La Plata, Argentina Received November 1, 2000; accepted January 19, 2001; published online May 11, 2001 The temperature dependence of the quadrupole hyper5ne parameters covering the temperature range from 293 to 1173 K has been measured at 181 Ta probes in SrHfO 3 by perturbed angular correlation spectroscopy. A 6uctuating distribution of quadrupole interactions model has been applied to interpret the data. At low temperatures above & 300 K a static, asymmetric, and distributed electric quadrupole interaction was detected. At intermediate temperatures (+ 600 K) a di4erent quadrupole in- teraction appears, characterized by a 6uctuating distribution of axially symmetric electric 5eld gradient tensors. Above 873 K, the unique presence of a nuclear spin relaxation machanism shows a second change in the perturbation acting on probes. These changes in the hyper5ne interaction are consistent with the structural phase transitions detected by di4raction techniques. The probe e4ects were also analyzed, comparing 181 Ta with 111 Cd experiments.  2001 Academic Press INTRODUCTION The AMO  compounds display complex structural in- stabilities and electronic properties determined by A and M cations. Owing to their varied structure these materials have attracted interest in many applied and fundamental areas of advanced materials and solid-state research. These oxides usually have cubic, tetragonal, or orthorhombic structures. Anions surrounding M atoms form regular oc- tahedra in crystals with cubic symmetry or distorted oc- tahedra in crystals with lower symmetries. The A cations complete the structure, sitting at the vertices of the body. Of the many compounds that can be formed, we are particularly interested in the ones in which A is an alkaline earth (Ca, Sr, or Ba) and M is Hf (a tetravalent metal). At room temperature (RT) CaHfO  and BaHfO  have orthor- hombic and cubic structures, respectively. Strontium hafnate has been a well known compound for a long time, but there has been no agreement about its  Member of Carrera de Investigador Cientm H "co, CONICET, Argentina.  Fellowship of DAAD, Germany.  Fellowship of CNPq, Brasil. crystalline structure. Through X-ray di!raction experiments Ho!man (1) determined a cubic structure with a"4.077 A s ; meanwhile Na H ray-Szabo H (2) and Red'ko et al. (3) described it as orthorhombic and Jona and Shirane (4) reported it as having a cubic or pseudocubic lattice. Recently Guevara and coworkers using powder X-ray di!raction re"ned the structure of SrHfO  at di!erent temperatures (5). At room temperature they found an orthorhombic (Pnma) structure that transforms into orthorhombic (Imma) at about 650 K and into cubic (Pm3m) above 950 K. Using powder neutron di!raction analysis, Kennedy et al. reproduced the RT result of Ref. (5) but proposed di!erent high-temperature struc- tures: orthorhombic (Pnma for RT4¹4673 K), orthor- hombic (Cmcm at ¹"873 K), tetragonal (I4/mcm for 10234¹41353 K), and cubic (Pm3m for ¹'1353 K) (6). Using group theoretical analysis Howard and Stokes found that the Pm3m PI4/mcm PCmcm transitions could be continuous and there would not be a continuous path in the Cmcm PPnma transition. The unusual characteristics of these compounds are mainly determined by the electron distribution associated with the oxygen-4}metal covalent bond. One way to study this electron distribution is by means of the hyper"ne inter- actions. These compounds being non-nonmagnetic perov- skites, their electron density is manifested through the components of the electric "eld gradient (EFG) tensor mea- sured at the M site. Detailed electronic structure calcu- lations (7, 8) show that the EFG measured at this site is predominantly originated by O-2p and M-nd hybridization. Perturbed angular correlation spectroscopy (PAC) measures the nuclear spin precession produced by an ex- tranuclear "eld. From this spin precession the EFG can be determined if a suitable probe is localized at the M site. PAC measurements were performed in CaHfO  (9), BaHfO  (10, 11), SrHfO  (12), and BaHfO  using Ta and Cd probes, respectively. The hyper"ne interaction at RT detected by Ta probes in CaHfO  and BaHfO  is characterized by the quadrupole parameters <  "4.2  10 V/cm, "0.67  , "0.05  , and "45  MHz, respectively. Andrade et al. (13) studied 1 0022-4596/01 $35.00 Copyright  2001 by Academic Press All rights of reproduction in any form reserved.