Caesium bisulfate or cesium hydrogen sulfate is an

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

with the formula CsHSO₄. The caesium

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...

of bisulfate, it is a colorless solid obtained by combining Cs₂SO₄ and H₂SO₄.

Properties

Above 141 °C, CsHSO₄ is a

superionic conductor In materials science, fast ion conductors are solid conductors with highly mobile ions. These materials are important in the area of solid state ionics, and are also known as solid electrolytes and superionic conductors. These materials are usefu ...

. The rapid ionic conductivity arise especially in the range of these temperatures due to the high activity of

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

s. Based on the results of

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

, the structure consists of tetrahedral sulfate centers that bridge caesium ions. The proton is associated with the oxygen on sulfate. CsHSO₄ goes through three crystalline phases that are referred to as phase III, II, and I. CsHSO₄ is initially existing in phase III at a room temperature of 21 °C. Phase III ranges from 21 °C to 90 °C with a transition temperature of 90 °C to 100 °C between phase III and phase II. Phase II ranges from 90 °C to 140 °C. At 140 °C, CsHSO₄ undergoes a phase shift from phase II to phase I. Phase III (21 °C to 90 °C) and Phase II (90 °C to 140 °C) are referred to as the monoclinic phases, in which CsHSO₄ exhibits its lowest proton conductivity. As the crystalline structure’s temperature is raised, it will show variations in the unit cell volume and the arrangement of its hydrogen bonds, which will alter the ability of a CsHSO₄ crystalline structure to allow the displacement of protons. At 141 °C, the CsHSO₄ crystal structure experiences a structural change from monoclinic phase II to a tetragonal phase, becoming phase I. Phase I has more elevated crystal symmetry and widened lattice dimensions. Phase I is noted as the superprotonic phase (strong conducting phase), which triggers an extreme growth in proton conductivity by four orders of magnitude, reaching 10 mS/cm. This makes the conductivity of CsHSO₄ ten-fold stronger than the conductivity of a

sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...

. In the superprotonic phase, the movement of an SO₄ tetrahedron generates a disruption of the hydrogen bond network, which accelerates proton transfer. The tetragonal anions available in the structure are accountable for the arrangement of the hydrogen bonds with the moving protons.

Potential applications

The maximum conductivity of pure CsHSO₄ is 10 mS/cm, which is too low for practical applications. In composites with SiO₂, TiO₂, and Al₂O₃), the proton conductivity below the phase transition temperature is enhanced by a few orders of magnitude.Hiroki Muroyama, Toshiaki Matsui, Ryuji Kikuchi, and Koichi Eguchi. "Composite Effect on the Structure and Proton Conductivity for CsHSO4 Electrolytes at Intermediate Temperatures." (n.d.): n. pag. Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan, 13 Apr. 2006. Web. Unlike hydrated protonic conductors, the absence of water in CsHSO₄ provides thermal and electrochemical stability.

Electromotive force In electromagnetism and electronics, electromotive force (also electromotance, abbreviated emf, denoted \mathcal or ) is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical ''transd ...

(EMF) measurements in a humidified oxygen concentration cell verified the high ionic nature of CsHSO₄ in its superprotonic phase.Uda, Tetsuya, Dane A. Boysen, and Sossina M. Haile. "Thermodynamic, thermomechanical, and electrochemical evaluation of CsHSO 4." Solid State Ionics 176.1 (2005): 127-133. Based on heat rotation, the voltage stayed the same for over 85 hours during the measurement, particularly at the high temperature. These results, demonstrate the thermal independence from humidity-type environments. Additionally, the crystal structure of CsHSO₄ allows for quick transport of smaller charged ions, resulting in efficient energy transfer in electrochemical devices.

References

{{Sulfates Caesium compounds