CLATHRATES A Presentation by MAINAK SAHA SHUBHRADEEP PAUL AYAN ROY NASKAR AAKASH GOYAL SIMRAT SINGH

What are CLATHRATES? Clathrate compound A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. Traditionally,clathrate compounds are polymeric and completely envelop the guest molecule, but in modern usage clathrates also include host-guest complexes and inclusion compounds.

WHAT ARE CLATHRATES? According to IUPAC, clathrates are "Inclusion compounds in which the guest molecule is in a cage formed by the host molecule or by a lattice of host molecules."

OCCURRENCE OF CLATHRATES Traditionally clathrate compounds refer to polymeric hosts containing molecular guests. More recently, the term refers to many molecular hosts, including calixarenes and cyclodextrins and even some inorganic polymers such as zeolites and clathrasil. The natural silica clathrate mineral, chibaite was recently described from Japan. Many clathrates are derived from an organic hydrogen-bonded frameworks. These frameworks are prepared from molecules that "selfassociate“ by multiple hydrogen-bonding interactions. The most famous clathrates are methane clathrates where the hydrogen- bonded framework is contributed by water and the guest molecules are methane. Large amounts of methane naturally frozen in this form exist both in permafrost formations and under the ocean sea- bed.In case of clathrate hydrates,frozen water’host’molecules form a cage that can hold ‘guest’gas molecule at high pressure and low temperature.

Methane clathrate

OCCURRENCE OF CLATHRATES Metal-organic frameworks(MOFs) are compounds consisting of metal ions or clusters co-ordinated to 1-,2- or 3- dimensional structures that can be porous and form clathrates. Some common forms of clathrates are: Inter-metallic clathrates.(Eu2Ba6[Cu4Si42]) Clathrate hydrates((Cl2)8-x[H20]46)(or Gas hydrate) Methane clathrates

STRUCTURE OF CLATHRATES Clathrate compounds constitute networked cages consisting of nanoscale tetrakaidecahedrons (14-hedrons) and dodecahedrons (12-hedrons), in which the group-1 or -2 elements in the periodic table are encaged as so-called “rattling” guest atoms. It is remarkable that, although these compounds have a crystalline cubic structure, they exhibit glasslike phonon thermal conductivity over the whole temperature range depending on the states of rattling guest atoms in the tetrakaidecahedron. In addition, these compounds show unusual glasslike specific heats and terahertz-frequency phonon Dynamics. Structure of the 3:1 inclusion complex of urea and 1,6-dichlorohexane.

WHY ARE THE CLATHRATES GOOD CANDIDATES FOR THERMOELECTRIC MATERIALS? Clathrates are periodic solids in which tetrahedrally coordinated atoms form cages which surround a metal atom that scatter phonons but not electrons, thus lowering the thermal but not the electric conductivity.Thus,from the formula for merit figure of thermoelectric materials we have low value of K(phonon) and so the figure of merit of thermoelectric material will increase. we have the formulae : κ = κ electron + κ phonon and

TEMPERATURE REGIME FOR CLATHRATES LOW TEMPERATURE PROTON TRANSFER The impedance spectra are measured for protonated and deuterated clathrates, HClO4⋅5.5H2O and DClO4⋅5.5D2O, between 10 and 300 K. The conductance is investigated between 80 K and room temperature and the dielectric constant between 10 and 120 K,it is observed that there is deviation from the Arrhenius behavior of conductance in the low‐temperature regime. A description of proton conductivity is developed on the basis of quantum theory of an elementary act of protontunneling between donor– acceptor sites interacting with environmental fluctuations.

TEMPERATURE REGIME FOR CLATHRATES A common characteristic of all intermetallic clathrates,independent of composition ,is a relatively large,complex unit cell,N(number of atoms per primitive unit cell),is 54.The effect of N on K(Lattice) was analysed by Slack. The effect of N on K(L) at debye temperature T for a crystal is given by: K(L)=(B*M*V^(1/3)*T^(2))/(N^(2/3)*g^(2)) Where B=3.04*10^-8 for many materials. M is mean atomic weight,g is gruneisen parameter and V is average volume per atom in crystal and T is debye temperature. At temperatures near or above T , a significant fraction of acoustic phonons in the crystal are excited and , thus ,phonon scattering in the crystal becomes dominant.

APPLICATIONS AND LIMITATIONS FOR CLATHRATE HYDRATES Carbon dioxide sequestration Storage and transport of natural gas FOR INTERMETALLIC CLATHRATES Magnetic cooling applications Photovoltaic applications Optoelectronics Sequestration: It is the long-term storage of carbon dioxide or other forms of carbon to either mitigate or defer global warming.

APPLICATIONS AND LIMITATIONS Pure hydrogen clathrate hydrates are stable only at high pressure and low temperature. Clathrates can be chemically stabilised only by near ambient conditions by using a promoter such as Tetrahydrofuran(THF). Slow rate of formation of clathrates.