Mechanical characteristics of solution grown potassium zinc chloride crystals doped with lithium ions A. Abu El-Fadl * , A.S. Soltan, M.A. Hefni, N.M. Shaalan Physics Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt Received 12 April 2007; accepted 1 August 2007 Available online 22 August 2007 Abstract Results of indentation-induced hardness testing studies on potassium zinc chloride crystals doped with Li + ions, leading to an under- standing of their mechanical behaviour, are presented. The Vickers hardness of these crystals for (1 0 0), (0 1 0) and (0 0 1) planes in the load range 20–160 g were studied. Load-independent values of hardness are estimated for the three crystallographic planes by applying Hays-Kendall’s and Li-Bradt models. The results showed that: (1) for the three crystallographic planes the load-independent hardness obtained by Li-Bradt model is higher than that predicted by Hays-Kendall’s, approach; (2) the load independent hardness of the (0 0 1) plane is higher than that of both (1 0 0) and (0 1 0) planes, (3) the values of load-independent hardness depend on Li + concentrations in the K 2 ZnCl 4 crystals, (4) the variations of crack length and crack morphology are described for studied crystal planes. Ó 2007 Published by Elsevier B.V. PACS: 42.70.a; 62.20.x Keywords: Optical materials; Mechanical properties 1. Introduction Materials crystallizing in the tetrahedrally co-oriented A 2 BX 4 structure are currently of interest because of their unusual ferroelectric properties. This structure forms a range of essentially isomorphous materials useful for the study of phase transitions and lattice defects associated with pseudosymmetry. Special attention is given to potas- sium tetrachlorozincate K 2 ZnCl 4 , hereafter KZC, due to the presence of a rather large thermal hysteresis and mem- ory effects [1] and extremely slow relaxation of the dielec- tric constant where a time constant of many hours has been observed [2]. In addition, (NH 4 ) 2 ZnCl 4 hereafter AZC exhibits a sequence of interesting structural phase transi- tions [3] extending over a wide range of temperatures. Crystal growth is affected profoundly by impurities/dop- ant present in the system. Most of the impurities suppress growth, some few impurities enhance it, while some others can stop growth completely even at very low level of impurity concentration, say, less than one ppm. Impurities usually act on certain crystallographic faces. Therefore, impurities can be used to change the growth form of crystals as habit mod- ifiers. The impurity effect depends on impurity concentra- tion, supersaturation and temperature. It also depends on the dopant itself and the system concerned. Thus, the impu- rity effect on crystal growth varies widely [4]. Regarding mechanical properties, hardness testing pro- vides useful information on the strength and deformation characteristics of the material [5] and yield stress [6]. hard- ness is a mechanical parameter which is strongly related to the structure and composition of solids and is defined as the resistance it offers to the motion of dislocations, defor- mations or damage under an applied stress [7]. Microhard- ness of crystalline materials is influenced by the following factors [8,9]: 1567-1739/$ - see front matter Ó 2007 Published by Elsevier B.V. doi:10.1016/j.cap.2007.08.003 * Corresponding author. Tel.: +2 088 2412244; fax: +2 088 2333837. E-mail address: abulfadla@yahoo.com (A. Abu El-Fadl). www.elsevier.com/locate/cap www.kps.or.kr Available online at www.sciencedirect.com Current Applied Physics 8 (2008) 167–176