Solution growths of remarkable phosphate crystals: From lab researches to industrial applications
Résumé
We will present the growth of several families of phosphates crystals, which exhibit remarkable properties such as high nonlinear optical efficiencies and piezoelectricity. As phosphate materials do not melt congruently due generally to partial decompositions at high temperatures, their crystal growth are performed by involving solvents. Thus, by taking several examples of phosphate crystal growths, we will introduce first different types of solution growth methods at low (close to room temperature) and high temperatures, (up to 1000°C), such as slow cooling, temperature gradient and flux methods. Indeed, we will illustrate all these techniques with examples taken from our studies at CNRS-Grenoble which include growth of hybrid organic inorganic salts, potassium dihydrogen phosphate (KDP) and crystals of the potassium titanyl phosphate family (KTP). Their crystal growths are motivated by the efficient properties for nonlinear optics to develop optical frequency conversion devices like optical parametric oscillators based on their non-centrosymmetric structures. In addition to these standard solution growth techniques, some more particular ones will be presented. Indeed, solution crystal growths, carried out under typical conditions at low supersaturations of the solutions (relative supersaturations σ of around 1%), exhibit low growth rates, less than 1mm/day. By applying an overheating and/or an ultrasonic treatment of the solution, it is possible to inhibit the spurious nucleation that take place at higher supersaturations. Thus, through this type of treatment of the solution during the crystal growth process, higher supersaturations can be applied, σ around 10-20%, and thus increase significantly the growth rates, over 1cm/day. These rapid crystal growths will be illustrated in the cases of potassium dihydrogen phosphate and hybrid organic-inorganic salts. Finally, hydrothermal techniques will be presented in the case of aluminum and gallium orthophosphate crystals, which are isomorphous to α-quartz. These crystals, exhibit promising piezoelectric properties for the development of resonators and physical sensors.
Moreover, for all of these phosphate crystal growths, we will give examples of industrial processes, particularly for the growths of KDP, KTP and GaPO4 crystals.