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MATERIALS CHEMISTRY II CHEM2007 DR. M. J. MOLOTO OFFICE SC204 PRESCRIBED TEXT: (1) Shriver and Atkins, inorganic Chemistry, 4th Ed (2) D.R. Askeland and P.P. Phule, “The Science and Engineering of Materials” 5th Ed, Thompson TOPICS TO BE COVERED • • • • • • • • • • • • • • Metal oxides/nitrides/fluorides Glasses Sulfur compounds Pigments Chemistry of Si and Al – Group 13 with emphasis on Al Group 14 elements Semiconductors Extended silicas Aluminosilicates Zeolites Ceramics – properties Synthesis and processing Glasses (ceramics) Clays Synthesis of Materials Synthetic reactions generally involves molecular rearrangements or substitution of one group or ligand by another. This requires small activation energies, low temperature (0 – 150 °C) and in solvents that aid diffusing reacting species. New materials can be obtained by two main methods; the breaking up of one or more continuous, or so-called extended lattice, structures followed by the slow diffusion of ions and crystallization of new structures, and the linking of polyhedral building units from solution and deposition of the newly formed solids. Methods of direct synthesis Many complex solids can be obtained by direct reaction of the components at high temperatures. Heating metals in the presence of oxygen gives metal oxides but complex oxides are obtained by heating a mixture of oxides with different metals. Examples ternary oxides, BaTiO3 and quaternary oxides, YBa2Cu3O7, BaCO3(s) + TiO2(s) 1000 °C BaTiO3(s) + CO2(g) ½Y2O3(s) + 2BaCO3(s) + 3CuO(s) 930°C/air and 450°C/O2 + 2CO2(g) YBa2Cu3O7 3 CsCl(s) + 2ScCl3(s) → Cs3Sc2Cl9(s) Complex chlorides NaAlO2(s) + SiO2(s) → NaAlSiO4(s) Aluminosilicates High gas pressure and inert gases may be used to control the composition of the product. N2/600°C 2 TlTaO (s) Tl2O(s) + Ta2O5(s) 3 V2O5(s) H2/1000 °C V O (s) 2 3 For volatiles reactants are normally sealed in glass tube, under vacuum, prior to heating. Sulfur and thallium(III) oxide are volatile at the reaction temperature. Ta(s) + S2(l) 500 °C TaS2(s) Tl2O3(l) + 2 BaO(s) + 3CaO(s) + 4CuO(s) 860 °C Tl2Ba2Ca3Cu4O12(s) High pressures such as 100GPa at high temperatures about 1500 °C are used. • Solution methods Condensation reactions can lead to the frameworks of polyhedral species. Results in crystallization from solution. The following examples are reactions that occur in water; ZrO2(s) + 2H3PO4(l) → Zr(HPO4)2.H2O + H2O(l) 12NaAlO2(s) + 12Na2SiO3.9H2O 90 °C Na12[Si12Al12O48].nH2O(s) (zeolite LTA) + 24NaOH(aq) These are useful for preparing open structure aluminosilicates (zeolites). 2La3+ + Cu2+ OH-(aq) 2La(OH)3.Cu(OH)2(s) – gel 600 °C La2CuO4(s) + 4H2O(g) High temperature, direct combination methods and solvothermal techniques are synthesis methods used in materials chemistry. Temperatures used in solution methods are lower than in direct methods and low temperature may also be used to reduce the size of particles of materials prepared. • Chemical deposition Thermal decomposition of volatile inorganic compounds can be used to deposit electronic materials on substrates. A thin layer of film is deposited using chemical materials over substrate such as silicon. One such technique is chemical vapour deposition (CVD), in which volatile inorganic molecules are decomposed on the substrate. If a metal-organic compound used – Metallo-organic CVD. Metal alkyls may be used to deposit metal or reacted with gases above the substrate to generate Group 12/16 (II/VI) semiconductor metal chalcogenides Zn(CH3)n + H2S → ZnS Group II/VI semiconductor Examples of volatile metal-organic compounds used include – carbonyls, dithiocarbamates, acetoacetonates, cyclopentadienes. Another approach for depositing films is the single-molecular precursor in which more than atom type is involved. Other techniques for depositing films are laser ablation and sputtering. METAL OXIDES, NITRIDES AND FLUORIDES Monoxides of the 3d-metals This include metals such as Ti, V, Cu, Fe, Ni, etc. which may be obtained in mixed stoichiometries and hence with defects Compound Structure Composition, x Electrical character CaOx Rock-salt 1 Insulator TiOx Rock-salt 0.65 – 1.25 Metallic VOx Rock-salt 0.79 – 1.29 Metallic MnOx Rock-salt 1 – 1.15 Semiconductor FeOx Rock-salt 1.04 - 1.17 Semiconductor CoOx Rock-salt 1 - 1.01 Semiconductor NiOx Rock-salt 1 - 1.001 Semiconductor CuOx PtS (linked CuO4 square planes) 1 Semiconductor ZnOx Wurtzite Slight Zn excess Wide band gap ntype semiconductor (a) Defects and Non-stoichiometry The nonstoichiometry of Fe1-xO arises from the creation of vacancies on the Fe2+ octahedral sites, with each vacancy charge –compensated by the conversion of two Fe2+ ions to two Fe3+ ions. Fe1-xO is metastable at room temperature and It is thermodynamically unstable with respect to disproportionation into Fe and Fe3O4 but does not convert for kinetic reasons. The structure of Fe1-xO is derived from rock-salt FeO by the presence of vacancies on the Fe2+ octahedral sites and each vacancy is charge compensated by the conversion of two adjacent Fe2+ ions to two Fe2+ ions. Generally all 3d-metal oxides have similar defects and clustering of defects except for NiO. Chromium(II) oxide disproportionates into Cr and Cr3+ species similar to FeO. 3 Cr(II)O → Cr(III)2O3(s) + Cr(0)(s) (b) Electronic and Magnetic properties The 3d-metal oxides such as MnO, FeO, CoO and NiO are semiconductors and TiO and VO are metallic conductors. 3d-metal oxides - MnO, Fe1-xO, CoO and NiO have low conductivity that increase with temperature or have such large band gaps that become insulators. The electron-hole migration in these oxides is attributed to the hopping mechanism. The electron or hole hops from one localized metal atom site to the other, and causes the surrounding ions to adjust their locations and the electron or hole is trapped temporarily in the potential well produced by the atomic polarization. The electron reside at its new site until it’s thermally activated to migrate. The electron or hole ends to associate with local defects, so the activation energy for charge transport may also include energy of freeing the hole from the position next to a defect. Doping of metallic oxides with other metal substances results in increased conductivity. In addition to the d-orbital electrons interactions, the transition metal oxides have magnetic behaviors that are derived from the cooperative interaction of the individual atomic magnetic moments. (See Box 23.2on Page 606 of Shriver and Atkins) Higher oxides and Complex oxides Binary metal oxides that do not have a 1:1 metal:oxygen ratio are higher oxides and those containing ions with more than one metal are called complex oxides. Rhenium trioxide: takes the structure ReO6 octahedra sharing all the vertices in three dimensions (Fig 23.22 page 607). It is a bright red lustrous solid and its electrical conductivity is similar to that of copper metal. Spinels Spinel – MgAl2O4 and oxide spinels have general formula; AB2O4 has a combination of A2+ and B3+ cations. A spinel structure consists of a ccp array of O2- ions in which the A cations occupy one eighth of the tetrahedral holes and the B cations occupy half of the octahedral holes. Inverse spinel – general formula B[AB]O4 in which the more abundant B cation is distributed over both coordination spheres. The occupation factor, λ of a spinel is the fraction of B atoms in the tetrahedral sites λ = 0, for a normal spinel and λ = 0.5 for inverse spinel, B[AB]O4. A B Al3+ Cr3+ Mn3+ Fe3+ Co3+ d0 d3 d4 d5 d6 Mg2+ d0 0 0 0 0.45 Mn+ d5 0 0 Fe2+ d6 0 0 Co2+ d7 0 0 Ni2+ d8 0.38 0 0.1 0.5 0.5 0.5 0 Cu2+ Zn2+ d9 d10 0 0 0 0 0.5 0 0 Structure of spinel Perovskite Perovskites – CaTiO3 have general formula; ABX3 in which the 12-coordinate hole of a ReO3 –type BX3 structure is occupied by a large A ion. CaTiO3 exhibits ferroelectric and piezoelectric properties. (Perovskite) Structure High-temperature superconductors Superconductors have two features: below critical temperature, TC, perovskites as an example enter the superconducting state and have zero electrical resistance, in this state they also exhibit the Meissner effect (exclusion of magnetic field). Type I superconductors – show abrupt loss of superconductivity when an applied magnetic field exceeds a value characteristic of the material. Type II – include high temperature materials, show gradual loss of superconductivity above the critical field denoted by HC. Element TC/K Compound TC/K Zn Cd Hg Pb Nb Nb3Ge Nb3Sn LiTi2O4 K0.4Na0.6BiO3 YBa2Cu3O10 Tl2Ba3Ca3Cu4O12 MgB2 K3C60 23.2 18.0 13 29.8 93 134 40 39 0.88 0.56 4.15 7.19 9.50 Collosal magnetoresistance Perovskites with Mn o the B sites can show very large changes in resistance on applying magnetic field known as colossal magnetoresistance. These are manganites – Mn(III) and Mn(IV) complex oxides with the formulation Ln1xAxMnO3 (A = Ca, Sr, Pb, Ba, Ln – Pr or Nb) order ferromagnetically upon cooling below room temperature (typically 100 and 250 K) and their resistance occurs near Curie temperature (TC) Glass formation Silicon dioxide readily forms glass because the three dimensional network of strong covalent Si-O bonds in the melts does not readily break and reform upon cooling. A glass is prepared by cooling a melt more quickly than it can crystallize. Cooling molten silica gives vitreous quartz. The lack of strong directional bonds in metals and simple ionic substances makes it much more difficult to form glasses from these materials. Recently techniques with ultrafast cooling have been developed Glass composition Low valence metal oxides such as Na2O and CaO are often added to silica to soften the Si-O framework and referred to as modifiers. Bottles and windows are made of sodalime glass that contains Na2O and CaO as modifiers. Borosilicate glasses contains B2O3 as a modifier and have lower thermal expansion coefficients than sodalime glass and are less likely to crack when heated. Volume change for supercooled liquids, glasses and crystals Sol-Gel process Sol Volume Liquid Hydrolysis Supercooled liquid Gel Dry Glass Porous Ceramic Crystal T g Temperature Tf Heat Heat Dense Ceramic or Glass NITRIDES AND FLUORIDES Nitrides – complex metal nitrides and oxide nitrides are materials containing the N3- anion. Examples – AlN, GaN and Li3N. Many nitrides are sensitive to oxygen and water and hence difficult to synthesize. Li3N is obtained by heating Li in a stream of N2 gas at 400 °C. Sodium azide, NaN3 is used as nitriding agent, 2NaN3(s) + 9Sr(s) + 6Ge(s) 750°C, sealed Nb tube 3Sr3Ge2N2(s) + 2Na(g) Ammonolysis of oxides result in nitrides; 3Ta2O5(s) + 10NH3(l) 700 °C 2Ta2N5(s) + 15H2O(g) Ca2Ta2O5(s) + 2NH3(l) 800 °C 2CaTaO2N(s) + 3H2O(g) The high charge on N3- compared to O2- results in a degree of increased covalence in its bonding. Fluorides and other halides – fluorine solid state chemistry parallels much of oxides because fluorine and oxygen have similar ionic radii (130 and 140 pm for F and O respectively). Page 617 continued Layered MS2 and Intercalation Synthesis and crystal growth: d-metal disulfides can be prepared by direct reaction of elements in a sealed tube and purified by chemical vapour transport with iodine; TaS2(s) + 2 I2(g) → TaI4(g) + S2(g) effectively prepared at 850 °C. Elements on the left of the d-block form sulfides consisting of sandwich-like layers of the metal coordinated to six sulfur ions, the bonding between layers is very weak. See Fig. 23.42 on page 620. Intercalation and insertion: Insertion compounds can formed by from the d-metal disulfides either by direct reaction or electrochemically; they can also be formed with molecular guests. TaS2(s) + x Na(g) → NaxTaS2(s) (0.4 < x < 0.7) Insertion by electrochemical technique called electrointercalation. This involves current passed through during synthesis and hence the amount of alkali metal incorporated (ne- = It/F) CP e- e- Li R MS2 + LixMS2 Li+ Li(s) → Li+(aq) + e- ; MS2(s) + xLi+(aq) + xe- → LixMS2(s) Alkali metals intercalation compounds of chalcogenides Compound Δ/pm Compound Δ/pm K1.0ZrS2 160 K0.4MoS2 214 Na1.0TaS2 K1.0TiS2 Na0.6MoS2 117 192 135 Rb0.3MoS2 Cs0.3MoS2 245 366 Chevrel phases: has a formula such as Mo6X8 or MxMo6S8 where Se or Te may take the place of S and the intercalated M atom may have a variety of metals – Mn, Li, Fe, Cd and Pb. Structure based on tetrahedral oxoanions; are very stable due to their small size that they coordinate strongly to the four O atoms in preference to higher coordination numbers. Examples are SiO4, AlO4, PO4 although GaO4, GeO4, AsO4, BO4, BeO4, LiO4, Co(II)O4 and ZnO4 are well known in their structural types. Aluminophosphates – their structure and physical properties parallel those of zeolites. The general formula is AlO4PO4 (unit). Phosphates and silicates – generally calcium hydrogenphosphates are inorganic materials used in bone formation. Phospate ions, PO43- a tetrahedral geometry. Hydroxyapatite, Ca5(OH)(PO4)3 in which Ca2+ ions are coordinated by PO43- and OH- groups to form a three dimensional structure and it is the main constituent of teeth and bones. Other related minerals – octacalcium hydrogenphosphate, Ca8H2(PO4)6, apatatite, Ca5(OH,F)(PO4)3 Clays, Pillared clays and layered double hydroxides: Sheet-like structures found in many metal hydroxides and clays can be constructed from linked metal oxo tetrahedra and octahedra. Pillaring is stacking and connecting together two-dimensional materials. Done by chemists in attempt to increase the pore diameters of zeolites in order for larger molecules to be absorbed. Examples of clay – hectorite and montmorillonite have layer structures constructed from vertex- and edgesharing octahedra, MO6, and tetrahedra, TO4. Cs/K SiO4 (Al, Mg)O6 SiO4 SiO4 (Al, Mg)O6 SiO4 Sheet-like structure of the clay hectorite, tetrahedra and octahedra centred on Al, Si or Mg and separated by cations Cs+ or K+. Pillaring of clay by ion exchange of a simple monoatomic interlayer cation with a large polynuclear hydroxometallate followed by dehydration and crosslinking of the layers to form cavities. Heat Dehydration Void Colours inorganic pigments: Adoption of the tetrahedral site by Co(II) in the spinel, AlCo2O4 result in the deep blue colour. Many inorganic solids are used as pigments in colouring inks, plastics, glasses and glazes. Intense colours can arise from the d-d transitions, charge transfer or intervalence charge transfer. Material Colour AlCo2O4 CaCuSi4O10 PbCrO4 Blue Blue Yellow-orange Zn2SiO4 (Mn2+ dopant) Y2O3 (Eu3+ dopant) CaMg(SiO3)2 (Ti dopant) Green Red Blue CaSiO3 (Mn dopant) Ca5(PO4)3F,Cl (Mn dopant) Yellow-orange orange White pigments – TiO2 is used as a universal white pigment. Other examples; ZnO, ZnS, Pb(CO3), lithophene (mixture of ZnO and BaSO4). TiO2 either in its rutile or anatase forms (see Fig. 23.64 page 633) is produced fro Ti ores (ilmenite – FeTiO3) by sulfate process. TiO2 dominates the white pigment market and it’s found in paints, coatings, printing ink to provide brightness in the coloured pigments, plastics, fibres, paper and white cement. Black, absorbing and specialist pigments – Carbon black is the most important black pigment. It is obtained by partial combustion or pyrolysis (i.e heating in the absence of air) of hydrocarbons. Copper(II) chromite CuCr2O4 with spinel structure is also used as black pigment. Unlike carbon black it can also absorb outside the visible region including the infrared. SEMICONDUCTOR CHEMISTRY Semiconductor typical band gaps lies in the few electron volts in the 100 – 200 kJ.mol-1 . The valence and conduction bands separation is in the desired range 0.2 – 4 eV. Further influence of the band gap is particle size in semiconductors. Group 14 semiconductor – crystalline and amorphous silicon are cheap semi conducting materials and widely used in electronic devices. Si (pure form) has band gap of 1.1 eV; Ge has smaller band gap of 0.66 eV; C in diamond form has band gap of 5.47 eV. Doping of Si with Group 13 and 15 elements are extrinsic semiconductors. Conductivity of pure Si is around 10-2 S.cm-1 at room temperature. Amorphous Si is obtained by chemical vapour deposition or by heavy ion bombardment of crystalline solid. It is used in silicon solar cells – pocket calculators but production costs diminish, are likely to find much wider applications as renewable energy sources. Systems isoelectronic to silicon – Groups 13/15 and 12/16 elements can have enhanced properties based on changes in the electronic structure and electron motion. Group 13/15 semiconductor – GaAs, GaP, InP, AlAs and GaN. GaAs has better response to electrical signals than Si as important for number of tasks such as amplifying the high frequency (1-10 GHz) signals of satellite TVs, and can also be used with signals up to 100 GHz. Group 12/16 semiconductors – ZnS (3.64 eV for cubic and 3.74 eV for hexagonal phase), CdS (2.41 eV), CdTe (1.475 eV). Fullerides – Solid C60 are considered as close packed array of fullerenes 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. General formula, MxC60, M = alkali metals K, Rb, Cs sometimes Na or Li. Examples – K3C60, which is superconducting on cooling to 18 K and Rb3C60 at 29 K, CsRb2C60 at 33 K. Cs3C60 does not form fcc structure and is not superconducting Molecular magnets – Molecular solid containing individual molecules, clusters, or linked chains of molecules can show three dimensional magnetic effects such as ferromagnetism. Examples of ferromagnetic materials, decamethylferrocene tetracyanoethenide (TCNE), [Fe(-Cp*)2(C2(CN)4)], Mn analogue, MnCu(2-hydroxy-1,3-propylenebisoxamato).3H2O. The incorporation of several d-metal ions into a single complex provides molecule that acts as a tiny magnet – termed single molecule magnets (SMM). Inorganic liquid crystals – inorganic metal complexes with disc- or rod-like geometries can show liquid crystalline properties. Liquid crystalline or mesogenic are compounds with properties that lie between those of solids and liquids and include both. They are widely used in displays. Rod-like – calamitic and disc-like – discotic.