Solid State Synthesis

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 MaterialsJ.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.
  Corbetts 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 100Å 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
  areab) Reactive starting reagents are better
  than inertc) 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 gt 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
• volatilizationb) Initial heating cycle
  to lower temperature can help to
• prevent spillage and volatilizationc)
  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)3nH2O 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 Tammans 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 11 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) 5xH2O
• 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 -gt Enhance diffusion -gt
  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 gt 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 ½ 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 lt T lt 500 C 100 lt P lt 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)2nH2O ? 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