EPR study of the low temperature ferroelectric phase transition in Cu 2þ doped Rb 2 ZnCl 4 single crystals Mariana Stefan a,b, * , Sergiu V. Nistor a,b , Dirk Schoemaker b , Ioan Ursu c a National Institute for Materials Physics, Laboratory 190, P.O. Box MG-7 Magurele, Bucuresti RO-77125, Romania b Physics Department, University of Antwerp (UIA), B-2610 Antwerp, Wilrijk, Belgium c Romanian Academy, Calea Victoriei no.125, Bucuresti, Romania Received 18 June 2003; accepted 18 July 2003 by D.E. Van Dyck Abstract Temperature dependent EPR measurements on copper doped Rb 2 ZnCl 4 single crystals allowed us to evidence and study the P2 1 cn $ C1c1 structural phase transition that takes place in this compound at 74.6 K. From the two types of Cu 2þ centers localized at different anionic sites, called Cu 2þ (I) and Cu 2þ (II), which are formed in this compound, only the Cu 2þ (II) centers exhibit observable changes in their EPR spectra, attributable to the symmetry lowering. The observed changes have been related to the soft-mode responsible for the structural phase transition. q 2003 Elsevier Ltd. All rights reserved. PACS: 76.30.Fc; 77.80.Bh Keywords: A. Ferroelectrics; D. Phase transitions; E. Electron paramagnetic resonance; C. Point defects 1. Introduction Previous electron paramagnetic resonance (EPR) and optical experiments carried out on copper doped Rb 2 ZnCl 4 single crystals revealed the presence of two Cu 2þ centers, differing in concentration, production properties and localization [1]. The Cu 2þ (I) centers, found in higher concentration, consist of Cu 2þ ions localized substitution- ally at Zn 2þ sites. In the case of the lower concentration Cu 2þ (II) centers, the Cu 2þ ions substitute the Rb þ ions, with electric charge compensation at distance. The EPR spectra of both types of centers were observed up to 135 K, in a temperature range corresponding to the two low temperature ferroelectric phases of Rb 2 ZnCl 4 , namely P2 1 cn and C1c1 [2]. The P2 1 cn symmetry phase, occurring between 192 and 74.6 K, is characterized by the tripling of the unit cell along the c axis with respect to the paraelectric Pmcn phase. Below T M ¼ 74:6K; in the monoclinic C1c1 phase, the unit cell is further doubled along the a and b axes. The literature data about this phase are very scarce, and, to our knowledge, only one EPR study of the P2 1 cn $ C1c1 structural phase transition (SPT), using the Tl 2þ and Tl 0 centers as paramagnetic probes [3] has been reported so far. The different localization of the two types of copper centers brings into focus the possibility to obtain comp- lementary information about the SPT mechanism and related lattice dynamics. This work presents an evaluation of the two copper centers’ sensitivities to the local crystal field variation induced by the P2 1 cn $ C1c1 phase transition, as well as the actual information concerning the microscopic phase transition mechanism that could be extracted from their EPR spectra. 2. Experimental The copper doped Rb 2 ZnCl 4 single crystals used in our experiments were grown from the melt by the Czochralski method, as described in Refs. [1,4]. In order to increase the 0038-1098/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0038-1098(03)00649-5 Solid State Communications 127 (2003) 695–698 www.elsevier.com/locate/ssc * Corresponding author. Address: National Institute for Materials Physics, Laboratory 190, P.O. Box MG-7 Magurele, Bucuresti RO- 77125, Romania. Tel.: þ 40-21-493-0047; fax: þ40-21-493-0267. E-mail address: mstefan@alpha1.infim.ro (M. Stefan).