Looking at Rubidium, Pyrotechnics and Metallic Salts with XRD

Origin of visible light

The idea that light is actually composed of colors across the visible portion of the electromagnetic spectrum has been known since the time of Seneca and Pliny the Elder in Rome, as observed through a prism and in rainbows. In the 19th century dispersed light was quantitatively measured by wavelengths, which can be assigned to the various colors.

The systematic attribution of spectra to chemical elements began in the 1860s with the work of German physicist Gustav Kirchhoff and chemist Robert Bunsen. From this work, we know that various metals emit colors when heated. An atom or molecule makes a transition from a high energy state to a lower energy state when energized by flame or radiation. There are many possible electron transitions for each atom, and each transition has a specific energy difference. This collection of different transitions, leading to different radiated wavelengths, make up an emission spectrum. Each element’s emission spectrum is unique. This is the foundation of spectroscopy, which identifies elements or molecules by their wavelength number.

Fireworks – pyrotechnic stars

In fireworks, colors are created in “pyrotechnic stars” that produce intense light of a certain color when ignited. Pyrotechnic stars contain a fuel, an oxidizer, a color-producing chemical, chlorine to strength the color of the flame, and a binder that holds the material together.

Metal salts

Metal salts, known as pyrotechnic colorizers, give specific colors to the pyrotechnic discharge. Strontium gives an intense red color, while lithium is medium red. Calcium is orange, sodium yellow, and copper or chlorine give a blue color. Potassium generates a light pinkish violet and rubidium a violet-red color.

In 1861, Kirchhoff and Bunsen, in an experiment to extract lithium from the mineral lepidolite dissolved the ore in acid and then precipitated the potassium it contained which carried down another heavier alkali metal. By carefully washing this precipitate with boiling water they removed the more soluble potassium component and then confirmed that they really had a new element by examining the atomic spectrum of what remained. This showed two intense ruby red lines never seen before, indicating a new element, which they named after this color.

Rubidium pyrotechnics

When used in pyrotechnics, rubidium exhibits a purple color when burned. Recently we performed a dynamic study of the compound rubidium nitrate (RbNO₃) using X-ray diffraction and a specialized sample chamber designed for high temperatures. We wanted to examine the changes to this compound as it is heated from room temperature to 310°C, the melting point. Rubidium nitrate is a white crystalline powder that is highly soluble in water and very slightly soluble in acetone. In a flame test, RbNO₃ gives a mauve/light purple color.

X-ray diffraction of metal salts

X-ray diffraction is commonly used for structural analysis to characterize materials as a function of temperature, environment and other external conditions. Phase transitions take place when materials undergo thermal cycles (at high temperatures or low temperatures) and their crystal structures can vary significantly leading to different properties of materials. We found several changes to the materials when heated.

At room temperature rubidium nitrate has a trigonal structure up to around 160°C, then it changes suddenly to another structure, which is reported as being cubic. A closer look at around 165°C where we see that the small peaks between the higher ones disappear around 170°C. At higher temperature, around 250°C in our experiment, a second phase transition is seen. It is very clear since we switch from cubic to rhombohedral. In less than 3 hours and 10 clicks we identified four structures of rubidium nitrate. This is just one example of how to quickly produce dynamic studies such as temperature changes using a benchtop X-ray diffractometer.

Read about this experiment in our application note “Dynamic studies with ARL EQUINOX 100 and BTS500 chamber”.