Chapter24 Materials Chem

SOLID STATE AND MATERIALS CHEMISTRY I. The Formation of Bulk Materials - bulk material can be formed by the direct reaction of two or more solids or by linking of polyhedral units from solution and deposition - many complex solids can be obtained by direct reaction of the components at high temperatures - complex oxides can be synthesized by heating many simpler oxides together for extended periods of time at high temperature or by decomposing an oxide that gives the target product - reaction rates can be improved by pelletizing the reaction mixture at high pressure, regrinding the mixture periodically, and by using low melting solids that facilitate the ion diffusion process - the reaction environment affects the oxidation state of the product obtained ; reactions carried out under high pressures favor denser compounds with higher coordination numbers - frameworks formed from polyhedral species can be formed by condensation reactions in solution - hydrothermal techniques (heating the reaction solution above its normal boiling point in a sealed vessel) are important in the synthesis of zeolites (open-structure aluminosilicates), linked oxo-polyhedra structures, and metal organic frameworks (frameworks in which metal ions are linked by coordinating organic species) - solvothermal techniques (heating a species in solution at high temperatures to form a product) are also used II. Chemical Deposition - chemical vapor deposition (CVD) is a technique in which a volatile inorganic compound is decomposed above a substrate III. Defects and Ion Transport - crystallographic shear planes are planar defects formed by removing shared atoms along a diagonal and then sliding slabs of crystals in a shearing motion to create edge-shared structures along a diagonal

- Wadsley defects are crystallographic shear planes randomly distributed in a solid => continuous range of composition IV. Atom and Ion Diffusion - materials that show a high rate of diffusion (as measured by the diffusion coefficient) at moderate temperatures have low energy barriers, low charges and small radii, high concentrations of intrinsic or extrinsic defects, and a significant number of mobile ions

- the lowest energy pathway for ion diffusion usually involves defect sites

V. Solid Electrolytes - solids exhibiting high anion mobility are rarer than cationic conductors and are generally highly conductive only at elevated temperatures because anions are usually larger and thus have higher energy barriers - solid inorganic electrolytes often have a low temperature form in which the ions are ordered on a subset of sites in the structure => low ionic conductivity - at higher temperatures, the ions become disordered over the sites and the ionic conductivity increases (unoccupied sites allow for ion migration)

- framework electrolytes are materials that have planes or channels through which electrolytes can move

- anion migration in solid anionic electrolytes often occurs by an interstitial mechanism in which the anion first migrates from its normal position into an interstitial site and then moves into a vacant anion site => structures with large numbers of vacant sites generally have the highest ionic conductivity - anion mobility is usually much lower than cation mobility and usually requires much higher temperatures - solid materials exhibiting both ionic and electronic conductivity are those that have defect sites and orbitals that provide conduction bands VI. Monoxides of the 3d Metals

- most 3d metal monoxides have a rock salt structure and a MO stoichiometry - mixed oxidation states and defects lead to nonstoichiometry in solids such as TiO, VO, FeO, CoO, and NiO - the nonstoichiometry of Fe(1-x)O arises from the creation of vacancies on the Fe(II) octahedral sites, with each vacancy charge compensated by the conversion of two Fe(II)

ions to two Fe(III) ions - similar defects and clustering of defects occur with all other 3d metal monoxides (except for NiO) - the 3d metal monoxides MnO, FeO, CoO, and NiO are semiconductors in which electron or hole (which tends to associate with local defects) migration occurs by a thermally activated charge hopping mechanism (their d orbitals are too compact to form broad bands for metallic conduction) - TiO and VO are metallic conductors due to the formation of a conduction band by the

overlap of t2g orbitals

- MnO, FeO, CoO, and NiO order antiferromagnetically with Neel temperatures (temperature of paramagnetic/antiferromagnetic transition) that increase from Mn to Ni

- the Neel temperature increases as the M-O orbital overlap increases due to the decrease in the metal ion size and thus the increase in the superexchange spin interaction

VII. Higher Oxides and Complex Oxides - higher oxides are binary metal oxides that do not have a 1:1 M:O ratio - complex/mixed oxides are compounds that contain ions of more than one metal - the corundum structure is adopted by many metal oxides of M2O3 stoichiometry, including Cr-doped aluminum oxide (ruby)

- the rhenium trioxide structure consist of a cubic unit cell with Re atoms at the corners and O atoms at the midpoint of each edge - the Re(VI) oxide structure can also be considered to be derived from ReO6 octahedral

sharing all vertices - the rhenium trioxide structure is relatively rare due to the requirement of the +6 oxidation state - d block higher oxides and many related mixed metal compounds adopt the spinel structure, but many do not conform to this structure due to the effect of the LFSE on the

site preferences of the ions - for metal spinels in which A(II) ions have 6-9 d electrons, the inverse spinel structure is preferred due to the lack of LFSE - normal spinel structures are expected for d metal oxides that have LFSE

- the perovskites have the general formula ABX3 ; solid solutions and nonstoichiometry

are common in this structure - ferroelectricity occurs when electric dipole moments of many unit cells are aligned => high polarity and high relative permittivity - the Curie temperature is the temperature below which the spontaneous polarization of electric dipoles occurs - piezoelectricity is the generation of an electric field when the crystal is under stress or the change in dimensions of the crystal when an electric field is applied - noncentrosymmetric structures are required for both ferroelectric and piezoelectric behavior (although the two do not necessarily have to occur together) - the perovskite barium titanate BaTiO3 exhibits both piezoelectric and ferroelectric behavior that arise from the cooperative displacement of ions - high temperature cuprate superconductors (such as Y123) have structures related to perovskite - superconductors have zero electrical resistance below their critical temperatures and exhibit the Meissner effect (the exclusion of a magnetic field and magnetic levitation) - type I superconductors show an abrupt loss of superconductivity when the applied magnetic field exceeds a characteristic value - type II superconductors (which include high temperature superconductors) show a gradual loss of superconductivity above a critical field

- the ferromagnetic metals (Fe,Co,Ni), alkali metals, and the coinage metals (Ag,Cu,Au) do not display superconductivity - it is believed that the movement of electron pairs known as Cooper pairs is responsible for high temperature superconductivity and superconductivity - manganites (complex oxides with Mn in the +3 and +4 states) exhibit magnetoresistance, a marked decrease in their resistance to the magnetic field near their Curie temperature => these compounds are known as colossal magnetoresistance (CMR) manganites due to the huge decrease in resistance observed

- CMR manganites have perovskite type structures with the A cation sites occupied by Ln(III) ions and the B site occupied by Mn - the redox chemistry associated with the extraction and insertion of metal ions into oxide structures is exploited in rechargeable batteries VIII. Oxide Glasses - ceramic refers to all inorganic, nonmetallic and nonmolecular materials - glass is an amorphous ceramic with a viscosity so high that it is considered rigid - a substance in its glassy form is in its vitreous form - a glass is prepared by cooling a melt more quickly than it can crystallize - silicon dioxide readily forms a glass because the 3D network of strong covalent Si-O bonds in the melt does not readily break and reform on cooling - the Zachariasen rules summarize the properties likely to lead to glass formation - an abrupt change (decrease) in volume occurs during crystallization while rapidly cooling a glass forming material forms a metastable supercooled liquid that becomes a glass when cooled below the glass transition temperature => many complex metal silicates, phosphates, and borates form glasses because their

rates of crystallization are very slow - glasses can also be synthesized by the sol gel process

- modifiers such as sodium oxide or calcium oxide can be added to silicon dioxide to lower its glass transition temperature - the modifier lowers the transition temperature by disrupting some of the Si-O-Si links

- the properties of glasses can be modified by the introduction of functional inorganic compounds into glasses - electrochromism is the ability to change color or light transmission properties in response to the application of a potential difference - photochromism is the ability to change color under certain light conditions IX. Nitrides and Fluorides - the higher charge on the nitride ion results in a greater degree of covalence in its bonding - because of nitrogen's high bond energy, nitrides tend to form compounds in which the metal is in a lower oxidation state - because the ionic radii of fluoride and oxide are very similar, metal fluorides show many stoichiometric ,structural, and synthetic analogies with complex oxides (although with lower change on the metal ion) - metal chlorides are less ionic than metal fluorides and usually have structures with lower coordination numbers X. Layered MS2 Compounds and Intercalation - d metal disulfides are synthesized by the direct reaction of the elements in a sealed tube and purified by using chemical vapor transport with iodine - layered d block disulfides are formed by metals on the left of the d block while compounds containing the disulfide ion are formed by metals in the middle and towards the right of the d block - elements on the left of the d block form sulfides consisting of sandwich like layers of the metal coordinated to 6 S atoms; the bonding layers is very weak

- the electrical conductivity, covalence and other chemical properties of layered structures can be described using band models

- an insertion reaction is a reaction in which the basic structure of the host is not modified

- topotactic reactions are reactions in which the structure of one of the solid starting materials is not radically altered - the insertion of alkali metal ions into host structures can be achieved by direct combination of the alkali metal and the disulfide - insertion can also be achieved by using a highly reducing alkali metal compound or

electrochemically by electrointercalation - insertion compounds can be formed with molecular guests (such as the metallocene) XI. Chevrel Phases and Chalcogenide Thermoelectrics - Chevrel phases have formulas of Mo6X8 or AxMo6S8 (Se or Te can take the place of S) and the intercalated A atom is a metal atom - Chevrel phases are superconductive, have low thermal conductivities and high electrical conductivities

XII. Structures based on Tetrahedral Oxoanions

- the later 3d metals and lighter p block elements can form very stable tetrahedral MO4 species that can link together into framework structures - in zeolites synthesized from solution, Lowenstein's rule states that no O atom is shared between two AlO4 tetrahedra => O atoms are shared between AlO4 and SiO4 tetrahedra - new zeolite frameworks are synthesized by using complex template molecules - zeolites are used in gas absorption, ion exchange, and molecular separation and purification

- the structures and physical properties of aluminophosphates parallel those of zeolites - calcium hydrogen phosphates are inorganic materials used in bone formation XIII. Structures based on Octahedra and Tetrahedra - sheet like structures are found in many metal hydroxides and clays and can be constructed from linked metal oxo tetrahedra and octahedra - enormous structural diversity can be obtained from linked polyhedra and, with the use of templates, can lead to remarkable porous frameworks - metal organic frameworks consist of metals linked by small organic molecules XIV. Metal Hydrides - metal hydrides can be heated to generate hydrogen gas and metal => can be used for hydrogen storage - intermetallic compounds that adsorb hydrogen can also be used XV. Other Inorganic Hydrogen Storage Materials - inorganic compounds with high surface areas and porosities can adsorb high levels of hydrogen gas XVI. Colored Solids - color in inorganic solids arise from d-d transitions (in noncentrosymmetric molecules) and charge transfer (promotion of an electron from the valence band into the conduction

band or between different valence levels) XVII. White and Black Pigments - white materials have no absorption in the visible region => no incomplete d electron shells => no color inducing d-d transitions - black materials absorb completely in the visible region - titanium dioxide is used almost universally as a white pigment - carbon black (an industrially manufactured form of soot) is an important black pigment - special color, light absorbing and interference effects can be induced in inorganic materials used as pigments XVIII. Group 14 Semiconductors - Si can be doped with group 13 elements to give a p-type semiconductor or with group 15 elements to give a n-type semiconductor - both crystalline and amorphous silicon are cheap semiconducting materials that are widely used in electronic devices XIX. Semiconductor Systems Isoelectronic with Silicon - group 13/15 and group 12/16 compounds have semiconducting properties because they are isoelectronic with group 14 elements ; however, the changes in element electronegativity and bonding type lead to changes in band structure and other properties - group 12/16 semiconductors are more ionic than either group 14 or group 13/15 semiconductors and have larger band gaps XX. Fullerides - solid C60 can be considered as a close packed array of fullerene molecules interacting only weakly through van der Waals forces; holes in arrays of C60 molecules may be filled by simple and solvated cations and small inorganic molecules

XXI. Molecular Materials Chemistry

- a 1D metal is a material that has metallic properties along one direction in the crystal and nonmetallic properties orthogonal to that direction => orbital overlap in a single

direction in the crystal - the Peierls theorem states that no 1D solid is a metal at T=0 => in a 1D solid at T=0, there always exists a Peierls distortion, which leads to a lower energy than in the

perfectly regular solid - the Peierls distortion occurs when a line of atoms distorts to one that has long and short alternating bonds, which results in the lowering of energy and the introduction of a band gap in the center of the conduction band => the distortion results in a semiconductor or an insulator - the incorporation of several d metal ions into a single complex produces a molecule that acts as a tiny magnet => single molecule magnets - molecular solids containing individual molecules, clusters or linked chains of molecules can show bulk magnetic effects like ferromagnetism - mesogenic (liquid crystalline) compounds possess properties that lie between those of solids and liquids and include both - the molecules that form liquid crystalline materials are usually calamitic (rod-like) or discotic (disc-like) => these shapes lead to the ordered liquid type structures in which molecules align in a particular direction => many metal containing systems are based around the low coordination geometries of

the later d metals - the presence of d metal centers in liquid crystalline compounds offer redox and magnetic effects | Created : Jul 29, 2012, 1:20 PM | Style : Background14, Font8, Size21 |