Unconventionality in Solid State Chemistry Douglas A. Vander Griend Department of Chemistry & Biochemistry Calvin College Grand Rapids, Michigan July 7, 2004

Unconventional ŭn΄kën-věn΄shë-nël/ adjective 1.not bound by or in accordance with convention 2.being out of the ordinary 3.existing without precedent

Conventional Solid State Structures

Conventional Compositions

Idealized Subcell for La 3 Cu 2 VO 9 [La] [(Cu/V)O 2+3/3 ] [La] [A] [BO 2+3/3 ]

La 3 Cu 2 VO 9 Superstructure P6 3 /m a = (1) Å c = (1) Å Cu II V O 2- 87% Cu

La 3 Cu 2 VO 9 : Frustrated Antiferromagnetism Inverse Molar Susceptibility (per copper) Temperature (K)  B 1.14  B 1.68  B 54% Paramagnetic 100% Paramagnetic 16%

La x Ln 3-x Cu 2 VO 9 Lattice Parameters

Idealized Subcell of La 4 Cu 3 MoO 12 [La] [(Cu/Mo)O 2+3/3 ] [La] [A] [BO 2+3/3 ]

Electron Diffraction La 4 Cu 3 MoO 12 Ordering of the B-cations leads to a monoclinic supercell (  = 90.03(1)º) which is 4 times larger than the conventional hexagonal subcell. La 3 Cu 2 VO 9 * Ordering of the B-cations leads to a hexagonal supercell which is 13 times larger than the conventional hexagonal subcell. *K. Jansson, I. Bryntse, Y. Teraoka Mater. Res. Bull., 1996, 31, 827.

La 4 Cu 3 MoO 12 : B-cation Ordering

La 4 Cu 3 MoO 12 : Frustrated Antiferromagnetism Temperature (K) Inverse Molar Susceptibility (per copper) 100% Paramagnetic 35% Paramagnetic 1.02  B 1.73  B

Ln 4 Cu 3 MoO 12 Powder X-ray Diffraction

Ln 4 Cu 3 MoO 12 Lattice Parameters

Rare-earth Hexagonal Structure Type Versatility *Prog. Solid St. Chem. 1993, 22, 197. "Many new and novel compositions and structures remain to be discovered by more traditional means." -J.D. Corbett Ln 4 Cu 3 MoO 12 Ln = La, Pr, Nd, Sm - Tm Ln 3 Cu 2 VO 9 Ln = La, Pr, Nd, Sm - Gd Ln 2 CuTiO 6 Ln = Tb – Lu*

Formation of Single Phases The primary goal of state synthesis is to form single phases Single phases form if and only if their  G is less than all possible multiphase mixtures at the reaction temperature. The following examples demonstrate the importance of stoichiometric analysis in the search for novel materials.

An Expected Result

An Unexpected Result

Thermodynamic Hierarchy  G(La 2 MoO 6 + Ho 2 Cu 2 O 5 ) <  G(Ho 2 MoO 6 + La 2 Cu 2 O 5 ) Ln 2 Cu 2 O 5 is more stable for smaller lanthanides, and/or Ln 2 MoO 6 is more stable for larger lanthanides. GG

Ln' 2 Ln" 2 Cu 3 MoO 12 Synthesis Results

Why does La 4 Cu 3 MoO 12 Form? Structure is unconventional. –A-cation coordination is low (6-7). –B-cation coordination is atypical (trigonal bipyramidal). But La 2+2n Cu 4+n O 7+4n (n = 2) is worse! –“It is remarkable that, given the simple ratio of the constituent elements, such complex structures form instead of the structurally simpler Ruddleson-Popper series.” - Cava et al Ln 2 Cu 2 O 5 is not even known for Ce – Gd. 75% copper is sufficient to promote single phase. La 4 Cu 3 MoO 12 forms so that La 2 Cu 2 O 5 doesn’t.

The La 2 Cu 2 O 5 Umbrella Stoichiometry

La 4 Cu 3+x Mo 1-x O 12

Why does Ho 4 Cu 3 MoO 12 Form? Ho 2 Cu 2 O 5 isn’t the problem anymore. Ho 2 MoO 6 + CuO is the problem! Ln 2 MoO 6 changes structure between Nd and Sm. 25% molybdenum is sufficient to promote single phase. Ho 4 Cu 3 MoO 12 forms so that Ho 2 MoO 6 doesn’t.

Ln 2 MoO 6 Structural Shift Nd 2 MoO 6 (I-42m) a = Å c = Å Sm 2 MoO 6 (I2/a) a = Å b = Å c = Å 2  (copper K  )

Ln 4 (Cu/Mo) 4 O 12 Thermodynamic Stability

More Examples La 2 CuSnO 6 vs. La 2 Cu 2 O 5 + La 2 Sn 2 O 7 –La 2 Sn 2 O 7, stable pyrochlore, infamous thermodynamic sink –La 2 CuSnO 6, lone example of a layered double perovskite that forms at ambient pressure. La 2 Ba 2 Cu 2 Ti 2 O 11 vs. La 2 Cu 2 O 5 + 2BaTiO 3 –La 2 Ba 2 Cu 2 Ti 2 O 11, layered quadruple perovskite –BaTiO 3, well known for centuries. All known phases exist because at least one of the phases in every multiphase alternative has a sufficiently high  G. Identifying and applying these Umbrella Stoichiometries is the key to a more rational search for novel matierals.

Conclusions – searching for unconventionality Umbrella stoichiometries promote single phase results by destabilizing multiphase alternatives. Umbrella stoichiometries facilitate substitutions that shift compositions towards them. Example: La 4 Cu 3+x Mo 1-x O 12-2x 0  x  0.12 Undiscovered phases likely exist near umbrella stoichiometries. Phases discovered near umbrella stoichiometries will tend to be unconventional because they can be structurally discontent and still be the thermodynamic product of a solid state reaction.

Acknowledgements Chemistry Department Kenneth R. Poeppelmeier Dr. Kenji Otzschi Dr. Donggeun Ko Dr. Sophie Boudin Dr. Vincent Caignaert Dr. Sylvie Malo Dr. Antoine Maignan Tony Wang Noura Dabbouseh Scott Barry Materials Science Department Prof. Thomas Mason Dr. Yanguo Wang Prof. Vinayak Dravid Kyoto University Prof. Mikio Takano Dr. Masaki Azuma Hiroki Toganoh Argonne National Laboratory Dr. Simine Short Dr. Zhongbo Hu Dr. James Jorgensen Funding Science and Technology Center for Superconductivity Japan Society for the Promotion of Science National Science Foundation Graduate Fellowship