Ferromagnetism in Topochemically Prepared Layered Perovskite Li 0.3 Ni 0.85 La 2 Ti 3 O 10 Doinita Neiner, †,§ Leonard Spinu, §,‡ Vladimir Golub, ⊥ and John B. Wiley* ,†,§ Department of Chemistry, Department of Physics, and AdVanced Materials Research Institute, UniVersity of New Orleans, New Orleans, Louisiana 70148-2820, and Institute of Magnetism, National Academy of Sciences of Ukraine, 36-B Vernadsky Strasse, 0314 KieV, Ukraine ReceiVed July 26, 2005. ReVised Manuscript ReceiVed NoVember 9, 2005 A nickel layer has been inserted between the perovskite blocks of triple-layer Ruddlesden-Popper titanate Li 2 La 2 Ti 3 O 10 by ion exchange. The reaction takes place at low temperature (50-65 °C) in aqueous solution. Rietveld refinement of X-ray powder diffraction data indicates that Li 0.3 Ni 0.85 La 2 Ti 3 O 10 has Ni in a tetrahedral coordination. In terms of thermal stability, this material is a metastable phase, decomposing above 300 °C in both inert and oxidizing atmospheres. The magnetic data for Li 0.3 Ni 0.85 La 2 Ti 3 O 10 show Curie-Weiss behavior at high temperatures, with a magnetic moment in agreement with the presence of Ni 2+ (S ) 1). At lower temperatures, two magnetic transitions take place, one at 23 K and one at 10 K. The transition at 23 K is ferromagnetic, and the one at 10 K is spin-glasslike. The magnetic entropy gained at the ferromagnetic transition (9.15 J/K) has been calculated from specific heat measurements at low temperatures, and is consistent with an S ) 1 system. The observation of ferromagnetism is rare in nickel oxides, and this is discussed in terms of the composition and structure of this compound. Introduction The reactivity of the A′ cations makes Ruddlesden-Popper (RP) compounds A 2 ′[A n-1 B n O 3n+1 ] (A′ is an alkali metal, A is an alkaline earth or rare earth metal, and B is a transition- metal cation) effective precursors to new low-temperature phases via topochemical routes. One of the most effective topotactic synthetic methods is the replacement of the A′ cations via an ion-exchange reaction. These reactions can take place in either aqueous medium, molten salts, or the solid state. In addition, monovalent and divalent ions can be exchanged. Monovalent ion exchange has led to H 2 La 2 - Ti 3 O 10 by an acidic aqueous-solution reaction. 1 Also, Li 2 - La 2 Ti 3 O 10 is a compound that has been obtained only by an ion-exchange reaction in a molten state, using Na 2 La 2 Ti 3 O 10 and a large excess of LiNO 3 . 2 In our group, we have shown that by using a two-step, low-temperature route (divalent ion exchange and then reductive intercalation), we can obtain mixed-valence compounds, such as Na 2-x+y Ca x/2 La 2 Ti 3 O 10 , that exhibit semiconducting behavior. 3 Divalent ion exchange can also be performed, using transition-metal ions or cationic units. Through an aqueous-solution reaction using vanadyl sulfate hydrate and K 2 La 2 Ti 3 O 10 , Gopalakrishnan and co- workers showed that potassium ions could be replaced with vanadyl units. 4 Also, using a solid-state reaction, the RP compound K 2 La 2 Ti 3 O 10 , and BiOCl, the same authors obtained an Aurivillius phase (BiO)La 2 Ti 3 O 10 . 4 In Na 2 La 2 - Ti 3 O 10 , Hyeon and Byeon replaced sodium ions with transi- tion-metal cations (Co, Cu, Zn) using a eutectic mixture of alkali-metal chlorides and transition-metal chlorides in sealed tubes; 5 by using an aqueous-solution reaction between nickel nitrate and K 2 Eu 2 Ti 3 O 10 , Schaak and Mallouk replaced potassium ions with nickel. 6 Herein, we describe an extension of such ion-exchange reactions in the triple-layered Ruddles- den-Popper series to obtain Li 0.3 Ni 0.85 La 2 Ti 3 O 10 by an aqueous-solution reaction. Also included are the synthesis, crystal structure, thermal behavior, and magnetic properties of this new compound. Especially interesting is the observa- tion of ferromagnetism, an uncommon feature in nickel oxides. Experimental Section 1. Synthesis. Na 2 La 2 Ti 3 O 10 was prepared by a solid-state reaction from Na 2 CO 3 , La 2 O 3 , and TiO 2 (all from Alfa Aesar, 99.99% purity). A 30% molar excess of Na 2 CO 3 was used to compensate for the loss attributable to volatilization. The reagents were pressed into pellets, fired at 550 °C for 6 h, and then sintered at 1050 °C for 12 h. 2 After the reaction, we removed unreacted Na 2 CO 3 by washing the solution with warm water; the sample was then rinsed with acetone, and was dried at 150 °C overnight. Phase purity for Na 2 La 2 Ti 3 O 10 was confirmed by X-ray powder diffraction (XRD); the sample was indexed on a tetragonal unit cell with a ) 3.8352- (7) Å and c ) 28.5737(7) Å, which is in agreement with the values reported in the literature. 2 Sodium was replaced with lithium by an ion-exchange reaction between Na 2 La 2 Ti 3 O 10 and LiNO 3 in a 1:15 molar ratio for 2 days at 300 °C. Phase purity for Li 2 La 2 - * To whom correspondence should be addressed. E-mail: jwiley@uno.edu. † Department of Chemistry, University of New Orleans. ‡ Department of Physics, University of New Orleans. § Advanced Materials Research Institute, University of New Orleans. ⊥ National Academy of Sciences of Ukraine. (1) Gopalakrishnan, J.; Uma, S.; Bhat, V. Chem. Mater. 1993, 5, 132. (2) Toda, K.; Watanabe, J.; Sato, M. Mater. Res. Bull. 1996, 31, 1427. (3) Lalena, J. N.; Cushing, B. L.; Falster, A. U.; Simmons, W. B., Jr.; Seip, C. T.; Carpenter, E. E.; O’Connor, C. J.; Wiley, J. B. Inorg. Chem. 1998, 37, 4484. (4) Gopalakrishnan, J.; Sivakumar, T.; Ramesha, K.; Thangadurai, V.; Subanna, G. N. J. Am. Chem. Soc. 2000, 122, 6237. (5) Hyeon, K.-A.; Byeon, S.-H. Chem. Mater. 1999, 11, 352. (6) Schaak, R.; Mallouk, T. E. J. Am. Chem. Soc. 2000, 122, 2798. 518 Chem. Mater. 2006, 18, 518-524 10.1021/cm0516457 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/21/2005