Internal stress in doped NH 4 Cl:Mn 2+ crystals
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L GROWTH
Internal Stress in Doped NH4Cl:Mn2+ Crystals L. A. P’yankovaa, Yu. O. Punina, V. D. Frankea, A. G. Shtukenberga, and A. S. Bakhvalovb a
St. Petersburg State University, St. Petersburg, 199034 Russia email: [email protected] b ZAO Nauchnye Pribory, St. Petersburg, Russia Received February 7, 2008
Abstract—The relationship of the effect of impurity on crystal growth and morphology, along with the inter nal stress and anomalous birefringence arising upon impurity trapping by a growing crystal, is considered. The NH4Cl–MnCl2–H2O–CONH3 model system and the heterostructural NH4Cl:Mn2+ crystals formed in it are experimentally studied. It is found that up to 6.63 wt % Mn2+ impurity is effectively captured by growing NH4Cl crystals at an impurity trapping coefficient only slightly below unity. The effect of Mn impurity stabi lizes the fullface growth of NH4Cl crystals with a rhombododecahedral habit in aqueous solutions and a cubic habit in water–formamide solutions. The trapping of manganese impurity by ammonium chloride crystals causes high internal stress (up to 4 GPa) in them, which manifests itself in the form of anomalous birefringence and leads to splitting, twinning, and cracking in NH4Cl crystals. PACS numbers: 61.72.y, +61.72.sd DOI: 10.1134/S1063774509040191
INTRODUCTION The most interesting features of NH4Cl crystals are the occurrence of piezooptic properties near the phasetransition point (245 K) [1] and the conductiv ity of ammonium halides at pressures below 50 GPa [2]. Doping of NH4Cl crystals with divalent metals and the ways these impurities are incorporated into the structure of ammonium chloride are subjects of intense study [3–6]. It has been found that, when there are high concentrations of impurities in solu tions, their capture by NH4Cl crystals occurs through 2D (anomalous) isomorphism [7–9]. According to previous studies, 2D isomorphous trapping of divalent copper impurity by ammonium chloride crystals causes anomalous birefringence and high internal stress in them [8, 9]. The regularities of the formation of optical anomalies in crystals have been studied well for systems characterized by true iso morphism [10]. At the same time, the situation regarding the study of such regularities for systems with 2D isomorphous impurities is far from satisfac tory. In this paper, we report the results of studying the effect of divalent manganese impurity on the growth, morphology, and optical properties of NH4Cl crystals. We investigated the NH4Cl:Mn2+ crystallization from aqueous and water formamide solutions in order to compare it with the previously studied NH4Cl– CuCl2–H2O–CONH3 system and estimate the impu rity behavior in different media.
EXPERIMENTAL NH4Cl crystals were grown in the presence of MnCl2 impurity from aqueous and water–formamide solutions in petri dishes by decreasing the temperature in a supercooling range of 2–10°С. The MnCl2 con centration in a solution varied from 0 to 69.97 g per 100 g of solvent. The water : formamide mass ratio in water–formamide mixtures w
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