Thermal X-ray analysis
Method allows determination of the phase transition temperature and most trustable interpretation of various phase transition, particularly: solid state transformations (polymorphism, decomposition, new phase formation, amorphisation etc.), melting processes (congruent, eutectic, peritectic etc.), dehydration and others. For the specific tasks experiment can be performed in different atmospheres: air, vacuum, nitrogen, inert gas. Powder substance under study should be placed on a metal sample holder (Cu for low- and Pt for high-temperatures) into a high/low temperature camera, attached to a diffractometer. Heating is carried out using a resistance furnace allowing accurate achievement of high temperatures with relatively low temperature gradients. Cooling in the low temperature camera is performed using liquid nitrogen in vacuum. XRD patterns could be taken with a temperature step or using continuous heating mode.
Method of X-ray diffraction at non-ambient temperatures is traditionally used for phase diagnostics at different temperatures for the substances in solid state. Special investigations involving gas or liquid phase formation can also be performed. The qualitative approach means first of all the registration the state of substance, determining the fact and temperature of phase transformation. Among thermal solid state transformations there are polymorphic transitions as well as processes of phase formation and decomposition, crystallization and amorphisation. Some of these processes undergo the change of chemical composition, while polymorphic transformations occur at constant chemical composition (Figure 1).
For a quantitative estimation of thermal expansion first temperature dependences of unit cell parameters of crystal structure should be analyzed, then coefficients of thermal expansion can be calculated and character of thermal expansion can be compared with the crystal chemical features of crystal structure.
Figure 1. Reversible polymorphic α-β transformation in quartz
- The research facility with low- and high-temperature cameras based on Rigaku “Ultima IV” equipped with a low-temperature chamber Rigaku «R-300» and high-temperature chamber Rigaku «SHT-1500»
- Maria Krzhizhanovskaya The Leading Researcher in non ambient powder X-ray diffraction