Solid State Synthesis

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Synthesis ReferencesThe material we discussed in class was drawn primarily from the following sources:A.R. West "Solid State Chemistry and its Applications" Chapter 2 Preparative Methods "Solid-State Chemistry Techniques" Chapter 1 Synthesis of Solid-State Materials J.D. Corbett bo

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Solid State Synthesis

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1. Solid State Synthesis Solid State Reactions Film deposition Sol-gel method Crystal Growth

2. Synthesis References The material we discussed in class was drawn primarily from the following sources: A.R. West "Solid State Chemistry and its Applications" Chapter 2 – Preparative Methods "Solid-State Chemistry – Techniques" Chapter 1 – Synthesis of Solid-State Materials J.D. Corbett – book edited by A.K. Cheetham and P. Day More detailed treatment, including practical details such as what sort of containers to use, how to avoid introducing impurities, what reactants to choose, etc., than above references. Corbett’s treatment is less oriented toward oxides, and more focussed on materials such as chalcogenides, halides and metal rich compounds. No discussion of thin films or growth of large crystals. "Preparation of Thin Films" Joy George This book has a nice succinct treatment of the various thin film deposition methods. The following references discuss various aspects or methods in solid state synthesis in greater detail. I have listed them according to synthesis method. Low Temperature & Precursor Techniques "Crystallization of Solid State Materials via Decomplexation of Soluble Complexes" K.M. Doxsee, Chem. Mater. 10, 2610-2618 (1998). "Accelerating the kinetics of low-temperature inorganic syntheses" R.Roy J. Solid State Chem. 111, 11-17 (1994). "Nonhydrolytic sol-gel routes to oxides" A. Vioux, Chem. Mater. 9, 2292-2299 (1997).    

3. Molten Salt Fluxes & Hydrothermal Synthesis "Turning down the heat: Design and mechanism in solid state synthesis" A. Stein, S. W. Keller, T.E. Mallouk, Science 259, 1558-1563 (1993). "Synthesis and characterization of a series of quaternary chalcogenides BaLnMQ3 (Ln = rare earth, M = coinage metal, Q = Se or Te)" Y.T. Yang, J.A. Ibers, J. Solid State Chem. 147, 366-371 (1999). "Hydrothermal Synthesis of Transition metal oxides under mild conditions" M.S. Whittingham, Current opinion in Solid State & Materials Science 1, 227-232   Chimie Douce & Low Temperature Synthesis "Chimie Douce Approaches to the Synthesis of Metastable Oxide Materials" J. Gopalakrishnan, Chem. Mater. 7, 1265-1275 (1995).   High Pressure Synthesis "High pressure synthesis of solids" P.F. McMillan, Current Opinion in Solid State & Materials Science 4, 171-178 (1999) "High-Pressure Synthesis of Homologous Series of High Cricitcal Temperature (Tc) Superconductors" E. Takayama-Muromachi, Chem. Mater. 10, 2686-2698 (1998). "Preparative Methods in Solid State Chemistry" J.B. Goodenough, J.A. Kafalas, J.M. Longo, (edited by P. Hagenmuller) Academic Press, New York (1972).

4. Classification of Solids There are several forms solid state materials can adapt Single Crystal Preferred for characterization of structure and properties. Polycrystalline Powder (Highly crystalline) Used for characterization when single crystal can not be easily obtained, preferred for industrial production and certain applications. Polycrystalline Powder (Large Surface Area) Desirable for further reactivity and certain applications such as catalysis and electrode materials Amorphous (Glass) No long range translational order. Thin Film Widespread use in microelectronics, telecommunications, optical applications, coatings, etc.

5. (1) The area of contact between reacting solids To maximize the contact between reactants we want to use starting reagents with large surface area. Consider the numbers for a 1 cm3 volume of a reactant Edge Length = 1 cm # of Crystallites = 1 Surface Area = 6 cm2 Edge Length = 10 µm # of Crystallites = 109 Surface Area = 6 ´ 103 cm2 Edge Length = 100Ĺ # of Crystallites = 1018 Surface Area = 6 ´ 106 cm2 Pelletize to encourage intimate contact between crystallites.

6. (2)The rate of diffusion Two ways to increase the rate of diffusion are to Increase temperature Introduce defects by starting with reagents that decompose prior to or during reaction, such as carbonates or nitrates.

7. (3)The rate of nucleation of the product phase We can maximize the rate of nucleation by using reactants with crystal structures similar to that of the product (topotactic and epitactic reactions).

8. What are the consequences of high reaction temperatures? It can be difficult to incorporate ions that readily form volatile species (i.e. Ag+), It is not possible to access low temperature, metastable (kinetically stabilized) products. For example the C-H-O phase diagram (organic chemistry) is pretty simple at 1200° C, High (cation) oxidation states are often unstable at high temperature, due to the thermodynamics of the following reaction: 2MOn (s) ? 2MOn-1(s) + O2(g) Due to the presence of a gaseous product (O2), the products are favored by entropy, and the entropy contribution to the free energy become increasingly important as the temperature increases.

9. Steps in Conventional Solid State Synthesis 1. Select appropriate starting materials a) Fine grain powders to maximize surface area b) Reactive starting reagents are better than inert c) Well defined compositions 2. Weigh out starting materials 3. Mix starting materials together a) Agate mortar and pestle (organic solvent optional) b) Ball Mill (Especially for large preps > 20g) 4. Pelletize 5. Select sample container Reactivity, strength, cost, ductility all important a) Ceramic refractories (crucibles and boats) Al2O3 1950° C $30/(20 ml) ZrO2/Y2O3 2000° C $94/(10 ml) b) Precious Metals (crucibles, boats and tubes) Pt 1770° C $500/(10 ml) Au 1063° C $340/(10 ml) c) Sealed Tubes SiO2- Quartz Au, Ag, Pt

10. 6)Heat a) Factors influencing choice of temperature for volatilization b) Initial heating cycle to lower temperature can help to prevent spillage and volatilization c) Atmosphere is also critical Oxides (Oxidizing Conditions) – Air, O2, Low Temps Oxides (Reducing Conditions) – H2/Ar, CO/CO2, High T Nitrides – NH3 or Inert (N2, Ar, etc.) Sulfides – H2S Sealed tube reactions, Vacuum furnaces 7) Grind product and analyze (x-ray powder diffraction) 8) If reaction incomplete return to step 4 and repeat.

11. example the synthesis of Sr2CrTaO6 1) Possible starting reagents Sr Metal – Hard to handle, prone to oxidation SrO - Picks up CO2 & water, mp = 2430° C Sr(NO3)2 – mp = 570° C, may pick up some water SrCO3 – decomposes to SrO at 1370° C Ta Metal – mp = 2996° C Ta2O5 – mp = 1800° C Cr Metal – Hard to handle, prone to oxidation Cr2O3 – mp = 2435° C Cr(NO3)3*nH2O – mp = 60° C, composition inexact

12. To make 5.04 g of Sr2CrTaO6 (FW = 504.2 g/mol; 0.01 mol) to complete the reaction: 4SrCO3 + Ta2O5 + Cr2O3 ? 2Sr2CrTaO6 + 4CO2 you need: SrCO3 2.9526 g (0.02 mol) Ta2O5 2.2095 g (0.005 mol) Cr2O3 0.7600 g (0.005 mol)

13. Applying Tamman’s rule to each of the reagents: SrCO3 ® SrO 1370° C (1643 K) SrO mp = 2700 K ® 2/3 mp = 1527° C Ta2O5 mp = 2070 K ® 2/3 mp = 1107° C Cr2O3 mp = 2710 K ® 2/3 mp = 1532° C Although you may get a complete reaction by heating to 1150° C, in practice there will still be a fair amount of unreacted Cr2O3. Therefore, to obtain a complete reaction it is best to heat to 1500-1600° C.

14. Precursor Routes Approach : Decrease diffusion distances through intimate mixing of cations. Advantages : Lower reaction temps, possibly stabilize metastable phases, eliminate intermediate impurity phases, produce products with small crystallites/high surface area. Disadvantages : Reagents are more difficult to work with, can be hard to control exact stoichiometry in certain cases, sometimes it is not possible to find compatible reagents (for example ions such as Ta5+ and Nb5+ immediately hydrolyze and precipitate in aqueous solution). Methods : With the exception of using mixed cation reactants, all precursor routes involve the following steps: Mixing the starting reagents together in solution. Removal of the solvent, leaving behind an amorphous or nano-crystaline mixture of cations and one or more of the following anions: acetate, citrate, hyroxide, oxalate, alkoxide, etc. Heat the resulting gel or powder to induce reaction to the desired product. The following case studies illustrate some examples of actual syntheses carried out using precursor routes.

15. Coprecipitation Synthesis of ZnFe2O4 Mix the oxalates of zinc and iron together in water in a 1:1 ratio. Heat to evaporate off the water, as the amount of H2O decreases a mixed Zn/Fe acetate (probably hydrated) precipitates out. Fe2 ((COO) 2) 3 + Zn(COO) 2?Fe2Zn((COO) 2) 5*xH2O After most of the water is gone, filter off the precipitate and calcine it (1000° C). Fe2Zn((COO) 2) 5? ZnFe2O4 + 4CO + 4CO2 This method is easy and effective when it works. It is not suitable when Reactants of comparable water solubility cannot be found. The precipitation rates of the reactants is markedly different. These limitations make this route unpractical for many combinations of ions. Furthermore, accurate stoichiometric ratios may not always be maintained.

16. Molten Salt Fluxes Solubilize reactants -> Enhance diffusion -> Reduce reaction temperature Synthesis in a solvent is the common approach to synthesis of organic and organometallic compounds. This approach is not extensively used in solid state syntheses, because many inorganic solids are not soluble in water or organic solvents. However, molten salts turn out to be good solvents for many ionic-covalent extended solids. Often slow cooling of the melt is done to grow crystals, however if the flux is water soluble and the product is not then powders can also be made in this way and separated from the excess flux by washing with water. Synthesis needs to be carried out at a temperature where the flux is a liquid. Purity problems can arise, due to incorporation of the molten salt ions in product. This can be overcome either by using a salt containing cations and/or anions which are also present in the desired product (i.e. synthesis of Sr2AlTaO6 in a SrCl2 flux) , or by using salts where the ions are of a much different size than the ions in the desired product (i.e. synthesis of PbZrO3 in a B2O3 flux).

17. Example 1 4SrCO3 + Al2O3 + Ta2O5 ?Sr2AlTaO6 (SrCl2 flux, 900° C) Powder sample, wash away SrCl2 with weakly acidic H2O Direct synthesis requires T > 1400° C and Sr2Ta2O7 impurities persist even at 1600° C

18. Solid State Metathesis Reactions A metathesis reaction between two salts merely involves an exchange of anions, although in the context we will use there can also be a redox component. If the appropriate starting materials are chosen, a highly exothermic reaction can be devised. MoCl5 + 5/2 Na2S ?MoS2 + 5NaCl + ˝ S The enthalpy of this reaction is ?H = -213 kcal/mol

19. Hydrothermal Synthesis Reaction takes place in superheated water, in a closed reaction vessel called a hydrothermal bomb (150 < T < 500° C; 100 < P < 3000 kbar). Seed crystals and a temperature gradient can be used for growing crystals Particularly common approach to synthesis of zeolites Example : 6CaO + 6SiO2 ? Ca6Si6O17(OH)2 (150-350° C)

20. Intercalation Involves inserting ions into an existing structure, this leads to a reduction (cations inserted) or an oxidation (anions inserted) of the host. Typically carried out on layered materials (strong covalent bonding within layers, weak van der Waals type bonding between layers, i.e. graphite, clays, dicalchogenides,). Performed via electrochemistry or via chemical reagents as in the n-butyl Li technique. Examples : TiS2 + nBu-Li ? LiTiS2 b-ZrNCl + Naph-Li ? b-LixZrNCl

21. Dehydration By removing water and/or hydroxide groups from a compound, you can often perform redox chemistry and maintain a structural framework not accessible using conventional synthesis approaches Examples : Ti4O7(OH)2*nH2O ? TiO2 (B) (500° C) 2KTi4O8(OH)*nH2O ? K2Ti8O17 (500° C)

22. Ion Exchange Exchange charge compensating, ionically bonded cations (easiest for monovalent cations) Examples : LiNbWO6 + H3O + ? HNbWO6 + Li+ KSbO3 + Na + ? NaSbO3 + K +

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