Etching and microhardness studies on lead molybdate single crystals

Abstract

Growth and characterization of lead molybdate (PbMoO4) single crystals have attracted increasing interest due to the variety of applications of these crystals [1–6]. Defect characterization, together with growth-related fundamental studies, were of special importance for many of the device requirements. This material was initially tested for its luminescence properties [7]. Subsequently, the main application has been in the field of acousto-optics because of its high acousto-optic figure of merit, low acoustic loss, low optical loss in the region 420 nm to 3900 nm and good mechanical impedance for acoustic matching [8, 9]. It has also attracted particular interest as one of the possible candidates for use in low temperature scintillator applications [10]. Difficulties associated with the growth of single crystals of lead molybdate have received significant attention due to a greater influence of thermal conditions during and after the growth [1, 11]. Zeng [12] has reported that when an as-grown crystal is held at a temperature close to the melting point for a long time, physical and chemical properties change. In the present investigation, single crystals of lead molybdate were grown by the Czochralski method with high purity starting materials of PbO and MoO3. They were mixed in the stoichiometric ratio and synthesized at around 800 8C with two intermediate grindings before melting. The pre-synthesized material was melted in a platinum crucible at 1063 8C. The detailed growth process has been discussed elsewhere [13]. The crystals for the present work were grown along the c-direction. As soon as the growth was terminated, the crystal was cooled at a rate of 20 8C hy1 to 500 8C and then at a relatively faster rate to room temperature. The crystal was not annealed at any intermediate temperature. The crystal was carefully cut along the aand c-directions using an internal diameter cutting machine so as to obtain a cube of 5 mm3 in size. The cut pieces were finally polished using 0.25 im diamond polishing paste. The cut and polished samples were subjected to chemical and thermal etching studies. Chemical etching was carried out using NaOH 5% solution as etchant at room temperature for 10–20 min. Since the cube was completely immersed in the etching solution, both aand c-faces were etched simultaneously under identical conditions. The etched samples were cleaned and their microstructure was analysed using an optical microscope (Leitz Metallux-II) in the reflection mode. Pits were not seen clearly on the cface whereas the a-face showed imperfection-induced arrow-headed etch pits (Fig. 1). A subgrain boundary was seen as a line of etch pits on the c-face (Fig. 2). This subgrain boundary line was observed on all samples cut perpendicular to the growth direction along the entire length of the crystal. This shows that the general nature of the subgrain boundaries is to run parallel to the pulling direction [14]. The process adopted for thermal etching is as follows. The samples were carefully loaded into a platinum boat and placed inside a resistively heated muffle furnace with temperature controlled by a Eurotherm 818P temperature controller. The furnace was heated to 900 8C at a rate of 30 8C hy1 and held at this temperature for 10 h. The furnace was then cooled at a rate of 10 8C hy1 to 600 8C and then at 25 8C hy1 to room temperature. After the heat treatment, the samples were analysed with an optical microscope. Heat treatment at 900 8C revealed thermal etch