Dept. of Crystallography, Saint Petersburg State University, Universitetskaya nab. 7/9, 199034, St. Petersburg, Russia. E-mail: IGOR@cryst.geol.pu.ru
Keywords: solution growth, mixed crystals.
Reported is the effect of variable temperature on the growth / dissolution of mixed crystals in aqueous solutions and its application for studying local equilibria at the crystal - solution interface.
Microscopic observations were made on the behavior of the K2(SxCr1-x)SO4, (BaxPb1-x)(NO3)2 and (MgxNi1-x)SO47H2O crystals which were growing from supercooled solutions under their continuous heating with different rates. It was found that the crystals in the first two systems began showing the first signs of dissolution (etching pits on the faces) at the temperatures (Td) considerably lower than the equilibrium saturation temperatures (Ts) of corresponding solutions. The difference Ts - Td was proportional to the initial supercooling of the solution in each run and was fairly reproducible. Experiments performed at different heating rates (0.5 - 3.24 oC/min) showed that the dissolution temperature of the mixed crystals fell exponentially and approached a limiting value as the heating rate increased. For each of the first two systems there was found a critical heating rate Rc such that for any R > Rc the dissolution temperature Td remained almost constant. For the K2(SxCr1-x)SO4 crystals the dissolution temperature was not affected considerably by the hydrodynamic conditions in solution, whilst for the (BaxPb1-x)(NO3)2 crystals Td was always equal to Ts under the forced convection.
Difference in patterns of the effect of heating on the crystal dissolution temperature in different systems suggests its different reasons. For the (BaxPb1-x)(NO3)2 crystals the deviation of Td from Ts admits a simple explanation based on the model of diffusion-controlled quasi-equilibrium component segregation which leads to the enrichment of the solution boundary layer with the more soluble component. According to the phase diagram Ts of the solution in the layer is less than that in its bulk. Under rapid rise of temperature the saturation temperature in the boundary layer can be outreached, whilst the bulk of solution can still remain supercooled. A justification of this explanation follows not only from the equality of Td and Ts under the forced convection but also from the shape of the "growth rate vs. supersaturation" curves which suggests diffusion to be the rate-controlling factor. Assuming the solution homogeneity with respect to the solvent concentration the composition of solution at the interface was evaluated from the phase diagram of the system.
For the K2(SxCr1-x)SO4 crystals the effect could not be explained by the same reasons, thus the composition of solution in the boundary layer could not be determined in a similar way. Nonetheless, independently of the nature of the effect, the effective saturation temperatures Teffs (and, consequently, the effective supercooling) of the solutions were evaluated in a series of experiments performed at different heating rates. For the Teffs the dissolution temperatures found at R > Rc were taken.
Under no conditions (neither free nor forced convection) the effect was seen in the third of the mentioned systems (the (MgxNi1-x)SO47H2O crystals for which the value of distribution factor was much closer to unity than for the others and, consequently, the effect of segregation was much weaker) as well as in the binary subsystems of all the studied systems. From this fact the conclusion can be drawn that a change in the ratio of components of a mixed crystal in the boundary layer leads to a stronger shift of local equilibrium than does a rise of solvent concentration.
The work was supported by RFBR (project 96-05-66060).