Cesium is a very soft (it has the lowest Mohs hardness of all elements), very ductile, silvery-white metal, which develops a silvery-gold hue in the presence of trace amounts of oxygen. It has a melting point of 28.4 °C (83.1 °F), making it one of the few elemental metals which are liquid near room temperature. Mercury is the only metal with a known melting point lower than cesium. In addition, the metal has a rather low boiling point, 641 °C (1,186 °F), the lowest of all metals other than mercury. Its compounds burn with a blue color.
Cesium is used in magneto-hydrodynamic power generation, ion propulsion motors in aerospace applications, opto-electronics like night-vision glasses, in DNA separation and as a catalyst in chemical applications. A new product known as cesium formate is used as a calibrated drilling lubricant for high temperature, high pressure oil wells.
Cesium is produced primarily from the Tanco mine, which holds 82% of the world's reserves. Current world-wide demand is about 500,000 pounds per year and new applications in the chemical and oil industries are expected to further increase the demand many fold in the near future. Prices have remained stable for the past few years at $1000/tonne for 24% cesium oxide concentrate.
Cesium forms alloys with the other alkali metals, as well as with gold, and amalgams with mercury. At temperatures below 650 °C (1,202 °F), it alloys with cobalt, iron, molybdenum, nickel, platinum, tantalum or tungsten. It forms well-defined inter-metallic compounds with antimony, gallium, indium and thorium, which are photosensitive. It mixes with the other alkali metals (except with lithium), and the alloy with a molar distribution of 41% cesium, 47% potassium, and 12% sodium has the lowest melting point of any known metal alloy, at −78 °C (−108 °F). A few amalgams have been studied: CsHg2 is black with a purple metallic lustre, while CsHg is golden-coloured, also with a metallic lustre.
Cesium is a relatively rare element as it is estimated to average approximately 3 parts per million in the Earth’s crust. This makes it the 45th most abundant of all elements and the 36th of all the metals. Nevertheless, it is more abundant than such elements as antimony, cadmium, tin and tungsten, and two orders of magnitude more abundant than mercury or silver, but 30 times less abundant than rubidium—with which it is so closely chemically associated.
The largest current end-use of caesium is in caesium formate-based drilling fluids for the extractive oil industry. Aqueous solutions of caesium formate (HCOO-Cs+)—made by reacting caesium hydroxide with formic acid—were developed in the mid-1990s for use as oil well drilling and completion fluids. The function of caesium formate as a drilling fluid is to lubricate drill bits, to bring rock cuttings to the surface, and to maintain pressure on the formation during drilling of the well; as completion fluid (which refers to the emplacement of control hardware after drilling but prior to production) is to maintain the pressure.
Caesium-based atomic clocks observe electromagnetic transitions in the hyperfine structure of caesium-133 atoms and use it as a reference point. The first accurate caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK. Since then, they have been improved repeatedly over the past half-century, and form the basis for standards-compliant time and frequency measurements. These clocks measure frequency with an accuracy of 2 to 3 parts in 1014, which would correspond to a time measurement accuracy of 2 nanoseconds per day, or one second in 1.4 million years.
Electric power and electronics
Caesium vapor thermionic generators are low-power devices that convert heat energy to electrical energy. In the two-electrode vacuum tube converter, it neutralizes the space charge that builds up near the cathode, and in doing so, it enhances the current flow.
Caesium is also important for its photoemissive properties by which light energy is converted to electron flow. It is used in photoelectric cells because caesium-based cathodes such as the intermetallic compound K2CsSb have low threshold voltage for emission of electrons. The range of photoemissive devices using caesium include optical character recognition devices, photomultiplier tubes, and video camera tubes. Nevertheless, germanium, rubidium, selenium, silicon, tellurium, and several other elements can substitute caesium in photosensitive materials.