1616 Chem. Mater. 1995, 7, 1616-1624 Investigation of the Alkali-Metal Vanadium Oxide Xerogel Bronzes: &V205mH20 (A = K and Cs) Y.-J. Liu,tl# J. A. Cowen,**#T. A. &plan,$>$D. C. DeGroot,l J. Schindler,l C. R. &nnewurf,l and M. G. Kanatzidis*stT§ Department of Chemistry, Michigan State University, East Lansing, Michigan 48824; Department of Physics, Michigan State University, East Lansing, Michigan 48824; Center for Fundamental Materials Research, Michigan State University, East Lansing, Michigan 48824; and Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208 Received January 18, 1995. Revised Manuscript Received June 13, 1995@ The synthesis of bronze-like AXV205*nHz0xerogels (A = K and Cs, 0.05 x 0.6) and systematic characterization of their chemical, structural, spectroscopic magnetic, and charge- transport properties are reported. These materials were prepared by the reaction of V205-nHzO with various amounts of alkali iodide (KI and CSI) in acetone under Nz atmosphere for 3 days. X-ray diffraction and spectroscopic data indicate that the V2O5 framework in AZVz05*nH20maintains the pristine V205 xerogel structure. The increased V4+ (dl) concentration in the V205 framework causes the disappearance of EPR hyperfine structure and the increase of magnetic susceptibility and electrical conductivity. The optical diffuse reflectance spectra of these compounds show characteristic absorption bands due to inter-valence (V4+N5+)charge-transfer transitions. The magnetic behavior is best described as Curie- Weiss type coupled with temperature-independent paramagnetism (TIP). The Curie constant and EPR peak width of the &VsO~.nH20 materials show unusual behavior consistent with strong antiferromagnetic coupling of neighboring V4+centers. The electrical conductivity slightly increases with V4+ concentration, and its temperature dependence indicates a thermally activated process. The thermoelectric power of the A,Vz05*nH20 materials is negative and becomes less negative with increasing V4+ concentration (i.e., increasing x). Introduction cochemical study of the oxide framework. Of course, numerous studies have been published on crystalline Recently, we showed that the redox intercalation of bronze phases of the type &V2Os2, but in these materi- organic monomers such as aniline, pyrrole, and 2,2’- als the V205 structure is different from that of VzO5 bithiophene yields layered materials containing mono- xerogels and it varies with A and x. Therefore, these layers of conductive polymers in the intralamellar space materials are not representative of the xerogel bronzes. of VzO5 xeroge1.l The vanadium oxide network is To our knowledge, complete and systematic studies of reduced to form V4+ centers while organic monomers compounds with a reduced Vz05 xerogel framework are oxidatively polymerized into electrically conductive have not been rep~rted.~A series of reduced VzO5 polymers. This results in new molecular composites of xerogels was prepared in this work by the reaction of two electrically active but chemically diverse compo- the xerogel with various amounts of alkali iodide (KI nents: organic conductive polymers and inorganic va- and CsI). The intercalants, K+ and Cs+, are closed-shell nadium bronzes. To achieve a better understanding of ions which can be considered as “innocent”species (e.g., the role of the conductive polymer inside the oxide diamagnetic and insulating), thus allowing the study sheets and its influence on the properties of the molec- of the properties of the reduced V205 framework with ular composites, it is important to understand the no interference from the guests. The structure of the nature and properties of the reduced Vz05 framework vanadium oxide framework remains the same in this alone. For this we need a material with the same case regardless of the identity of the alkali ion or its reduced V205 structure but intercalated with simple concentration (at least for x values ‘0.5). In this paper, chemical species that would not interfere with a physi- we report the synthesis of a series of bronze-like V2O5 xerogels, namely AxV205*nH20,and their X-ray diffrac- t Department of Chemistry. tion, optical, infrared, and spin resonance spectroscopic, * Department of Physics. magnetic, and charge-transport characterization as a 8 Center for Fundamental Materials Research. Department of Electrical Engineering and Computer Science. function of x (Le., V4+ concentration). The Curie con- @ Abstract published in Advance ACS Abstracts, July 15, 1995. stant and EPR peak width of the AxV~05-nH20materi- (1) (a) Kanatzidis, M. G.; Wu, C.-G.; Marcy, H. 0.;Kannewurf, C. R. J.Am.Chem. Soc. 1989,111,4139-4141.(b) Wu, C-G.; Kanatzidis, als show unusual behavior, namely, nonlinear variation M. G.; Marcy, H. 0.;DeGroot, D. C.; Kannewurf, C. R. Polym. Muter. with x; in particular there is a maximum value at x - Sci. Eng. 1989,61,969-973. (c) Kanatzidis, M. G.; Wu, C.-G.; Marcy, 0.3, suggesting strong antiferromagnetic coupling of V4’ H. 0.;DeGroot, D. C.; Kamewurf, C. R. Chem. Muter. 1990,2(3), 222- 224. centers. (2) (a) Murphy, D. W.; Christian, P. A.; DiSalvo, F. J.; Waszczak, J. V. Inorg. Chem. 1979,18(10), 2800-2803. (b) Chakraverty, B. K.; (3) (a)Babmeau, F.; Barboux, P.; Josien, F. A,; Livage, J. J. Chim. Sienko, M. J. Phys. Rev. E 1978, 17f10), 3781-3789. (c) Gendell, J.; Phys. 1986, 82, 761-766. (b) Bullot, J.; Cordier, P.; Gallais, 0.; Cotts, R. M.; Sienko, M. J. J. Chem. Phys. 1962,37(2), 220-225. Gauthier, M.; Babonneau, F. J. Non-Cryst. Solids 1994,68, 135-146. 0897-475619512807-1616$09.00/00 1995 American Chemical Society Alkali Metal Vanadium Oxide Xerogel Bronzes Chem. Mater., Vol. 7, No. 9, 1995 1617 Experimental Section factors for WV and CsN ratios were determined from known compounds such as and CsVO3. The alkali-to-vanadium Materials. Sodium metavanadate (NaV03), potassium ratios for each sample were obtained by averaging three iodide (KI), and cesium iodide (CsI) were purchased from measurements. Aldrich Co., Milwaukee, WI, and were used without further The calculation of a one-dimensional (1-D) electron density purification. Elemental analyses were done by Galbraith map was based on the X-ray reflection data using the observed Laboratories, Knoxville, TN, and Oneida Research Services, (001) group of reflections. The data were obtained from a film Inc., Whitesboro, NY. of Cs ~~V~O~TZHZO.Eight reflections were used out to do08 = Measurements. Infrared spectra were recorded from 4000 1.37 1. The intensities were obtained from the integrated peak to 400 cm-l with a resolution of 1cm-' on a Nicolet 740 FT- areas. The structure factors of these reflections were derived IR spectrometer. Samples were recorded in a pressed KBr from their intensities and corrected for Lorentz-polarization matrix under N2 flow. IR diffise reflectance spectra were also effects. The signs of the phases for the structure factor recorded on the same instrument equipped with a reflectance calculation were obtained directly from the scattering contri- attachment purchased from Spectra Tech. Inc. Samples were butions of the VZO~framework alone. The signs of the phases pressed into pellets and directly put on a sample holder. A were also checked by recalculation including the contribution gold mirror was used as the reference. of the Cs+ ions. All but one reflection changed sign which did X-ray diffraction was carried out on a Rigaku rotating anode not significantly change the electron density pattern. X-ray powder difiactometer, Rigaku-DenkilRW400F2 (Ro- VZO~xerogel was prepared taflex), at 45 kV and 100 mA with a scintillation counter Preparation of V205 Xerogel. detector and a graphite monochromator to yield Cu Ka by a reported method.8 Sodium metavanadate (4 g, 32.8 "01) (wavelength 1.541 84 radiation. Data were collected at was dissolved in 250 mL of distilled water. The resulting A) solution was eluted through a column packed with H+ ion- room temperature over the range 2" I20 5 80" in increments of 0.01". exchange resin (Dowex-50x2-100)forming a pale-orange solu- tion of HVO3. Upon standing, this solution polymerized to a Electron paramagnetic resonance (EPR) spectra were ob- red VzO5 gel in several days. The gel was then poured on flat tained using a Varian E-4 spectrometer operating at 9.5 GHz substrates and the excess water was slowly evaporated to yield (X band) and at room temperature. Solid samples were the VzO5 xerogel as a thick dark red film. The definition of a scanned from 2700 to 3700 G at 8 G field modulation and 0.03 zerogel is product remaining after solvent evaporation from a s time constant. The g value was obtained with reference to gel. the standard diphenylpicrylhydrazine (DPPH). Magnetic susceptibility measurements were done on a Preparation of &V205nH20. Powdered VzO5 xerogel(0.5 MPMS Quantum Design SQUID system (superconducting g, 3.38 mmol) was added to a 50 mL of acetone with the quantum interference device) with a magnetic field of 5000 stoichiometric amount of KI or CsI. The KWzO5 and CSWZOS G. A known quantity of sample was placed in a plastic bag molar ratios were varied from 0.1 to 1.0 in increments of 0.1. and purged with Ar gas. Data were collected with an ascend- The reaction was stirred under Nz atmosphere for 3 days. The ing ramp from 5 to 300 K and then corrected for the dark product was filtered, washed with acetonitrile, and then diamagnetic components using values obtained from the dried under vacuum. The compositions of &Vz05.nHzO com- literat~re.~ pounds were determined from elemental analysis and SEW Direct-current electrical conductivity and thermopower EDS. An attempt to produce the AZV205.nHz0 materials with measurements were performed in the usual four-probe geom- x > 0.6 failed, yielding amorphous products. The value of n etry with 60 and 25 mm gold wires used for the current and is -1.1 when 0.05 < x < 0.1 and -0.5-0.6 when x > 0.1.