L Journal of Alloys and Compounds 323–324 (2001) 308–311 www.elsevier.com / locate / jallcom 3 1 EPR Identification of coupled Yb ion pairs in optically bistable compound CsCdBr :Yb 3 a,c a a a, b b * ¨ ´ V. Mehta , O. Guillot-Noel , D. Simons , D. Gourier , Ph. Goldner , F. Pelle a ´ ´ Laboratoire de Chimie Appliquee de l’ Etat Solide, Ecole Nationale Superieure de Chimie de Paris ( ENSCP), UMR CNRS 7574, 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France b ´ Laboratoire de Physico-Chimie des Materiaux, CNRS-UPR 211, 1 Place Aristide Briand, 92190 Meudon-Bellevue, France c Kalindi College, University of Delhi, East Patel Nagar, New Delhi 110008, India Abstract Electron Paramagnetic Resonance (EPR) studies on ytterbium ions in weakly doped CsCdBr are reported. Analysis of the 3 31 21 characteristic features of the observed EPR spectra and their angular dependence shows that Yb ions substitute for Cd ions in D 3d 31 31 point site symmetry mainly to form weakly antiferromagnetically coupled symmetric pair complexes of the type Yb –V –Yb , with Cd 21 ˚ interionic separation of around 5.9 A and J exchange coupling value of 20.0016 cm . Several weaker resonance lines are also observed 31 which are assigned to single ion Yb centers in perturbed sites of lower symmetry.  2001 Elsevier Science B.V. All rights reserved. Keywords: Insulators; Crystal growth; Crystal structure and symmetry; Exchange and superexchange; Electron paramagnetic resonance 42 1. Introduction linear chains of face sharing (CdBr ) octahedra parallel 6 1 to the crystallographic c-axis with Cs ions occupying 1 The interesting phenomenon of intrinsic optical bistab- sites between the chains. Point symmetries are D for Cs 3h 21 ility (IOB) originating from rare earth ion pairs has been and D for Cd . A striking feature of CsCdBr is that 3d 3 31 31 observed at low temperature in the dimer isostructural when rare-earth (RE ) ions such as Yb are incorpo- 31 31 21 systems Cs Y Br :10% Yb and Cs Lu Br:10% Yb rated in the Cd lattice position, they enter nearly 3 2 9 3 2 [1,2] and more recently in a quasi one-dimensional com- exclusively as charge compensated ion-pair centres, even 31 pound CsCdBr :1% Yb [3]. The studies on these at low dopant concentration. The main RE center is a 3 31 31 materials suggest that intense bistable pair luminescence symmetric in chain RE –V –RE pair complex iden- Cd can result from co-operative non-linearities mediated by tified by McPherson and Henling by EPR experiments on 31 21 31 local field effects. These non-linearites arise from strong Gd [4,5] where V denotes Cd vacancy. Each RE Cd ion–ion coupling within well-isolated dimer units and ion of the pair has a C site symmetry and the interionic 3v 31 31 1 ˚ energy migration in the Yb pairs has a degrading effect separation, R, is around 6.0 A. A minority RE -(C S on bistable emission. Thus, a precise knowledge of ion– vacancy) complex of lower symmetry (C ) was also S ion coupling mechanisms in such pairs is of fundamental identified by EPR [6]. Subsequent laser-spectroscopic 31 31 interest and could help to better understand and optimise studies on CsCdBr doped with Nd [7], Er [8] and 3 31 bistable luminescence. Electron Paramagnetic Resonance Pr [9] have reported the presence of a minority 31 31 (EPR) spectroscopy is a powerful technique to probe such asymmetric in chain RE –RE –V complex, with R Cd ˚ ion–ion interactions in the ground state of a coupled around 3.4 A. paramagnetic system. In the present work, EPR studies have been performed on IOB compound CsCdBr :1.3% 3 31 Yb . The host CsCdBr crystallises in the hexagonal 2. Experimental details 3 4 structure of space group P6 / mmc (D ) consisting of 3 6h CsCdBr single crystals with 1.3% ytterbium concen- 3 tration were grown by Bridgman method. EPR spectra were recorded on a Bruker ESP 300e spectrometer *Corresponding author. E-mail address: gourierd@ext.jussieu.fr (D. Gourier). equipped with a variable temperature accessory from 0925-8388 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388(01)01072-6