Ph.D., University of Connecticut
Inorganic Solid State Chemistry
Our research centers on the synthesis of single crystals of transition metal oxides with unusual structural, electrical or magnetic properties. Our primary method is the electrolysis of fused salts (FSE) at high temperatures in which the products are deposited at one or both of the electrodes. We also use flux growth method and chemical vapor transport. A particularly interesting material that we have prepared by FSE is La5Mo4O16. This compound, which grows at the cathode, is a quasi-two dimensional material whose magnetic properties are highly anisotropic. The structure, as shown in Fig. 1, consists of MoO3 layers containing corner sharing MoO6 octahedra where Mo(V) and Mo(IV), ordered on alternate sites (Fig. 1a), are sandwiched between insulating Mo2O10 groups (Fig. 1b). If the magnetism of the crystal is measured parallel to the perovskite layers at 150K a meta-magnetic behavior is observed while if it is measured perpendicular to the layers paramagnetic behavior is observed (Fig. 2). At lower temperatures the magnetism shows a strong hysteresis, typical of strong ferromagnetic interactions. This is the first known example of such magnetic behavior in which the magnetic ion is Mo and only the one known for a 4d oxide (1-2, 13-15).
Fig. 1 (a): Perovskite-like MoO3 layers in La5Mo4O16 (ab) plane. (b) View showing how the perovskite MoO3 layers are connected by Mo2O7 groups along the c axis.
Fig. 2 Magnetization of La5Mo4O16 at 150 K with field applied parallel to the perovskite layers (solid black squares) and filed parallel to the c axis, i.e. perpendicular to the perovskite layers (solid black squares)
We are also interested in the growth of single crystals of doped rare earth oxomanganates and oxocobaltates. Crystals of LaMnO3 doped with Na+ or Sr+2 2 to 3mm on edge have been prepared by anodic oxidation of Cs2MoO4-MoO3 melts containing Mn2O3 and either Na2MoO4 or SrMoO4 as solutes. Crystals of composition Sr0.21La0.76MnO3, prepared in this laboratory, show %MR (magnetoresistivity) values of about 36% at room temperature and 5 Tesla, among the highest observed to date for room temperature magnetoresistivity. These materials are of potential practical importance since magnetoresistive devices are used in read/write devices for data storage and retrieval. Our studies indicate that the introduction of vacancies in the lanthanum sites will enhance the MR values (5-6, 8-12)
- H. Leligny, M. Ledesert, Ph. Labbé, B. Raveau and W. H. McCarroll, "La5Mo4O16, A New Structural Type Related to Perovskite With Extremely Short Mo-Mo Bonds", J .Solid State Chem.105, 143-150 (1993).
- K. V. Ramanujachary, M. Greenblatt, W. H. McCarroll and J. B. Goodenough, “Anomalous Electrical and Magnetic Properties of a New Quasi-Low-Dimensional Mixed-Valent Molybdenum-Cluster Compound”, Mat. Res. Bull. .28, 1257-67 (1993).
- K. V. Ramanujachary, W. H. McCarroll, E.B. Jones* and M.Greenblatt, "Electrical and Magnetic Properties of LaMo8-xO14 Containing Isolated Mo8 Clusters", J. Solid State Chem.117, 261-68 (1995).
- P. Gall, P. Gougeon, M. Greenblatt, E. B. Jones*, W. H. McCarroll and K. V. Ramanujachary, "Crystal Growth and Unusual Transport Properties of Some Reduced Molybdenum Oxides With Bi-Octahedral Mo10 Clusters", Croatica Chemica Acta 68(4), 849-60 (1995).
- W. H. McCarroll, K.V. Ramanujachary and M. Greenblatt, "Synthesis of Doped Rare Earth Manganate Perovskite Crystals Using Fused Salt Electrolysis", J. Solid State Chem.130, 327-29 (1997).
- W. H. McCarroll, K. V. Ramanujachary, M. Greenblatt and F. Cosandey, "Large Magnetoresistive Effects in Undoped La1-xMnO3 Single Crystals", J. Solid State Chem.136, 322-327 (1998).
- W. H. McCarroll, M.Borgia*, K. V. Ramanujachary, M. Greenblatt, Patrick Gougeon and John E. Greedan, "Synthesis and Properties o Sr1-xLaXMo5O8 (0 = x = 1): A New Class of Oxides with Metallic Properties", J. Solid State Chem.138,7-17 (1998).
- W. H. McCarroll, K. V. Ramanujachary, I. D. Fawcett and M. Greenblatt, " Growth and Properties of Strontium Substituted Lantahnum Manganate Perovskite Single Crystals ", J. Solid State Chem. 145, 88-96 (1999).
- S. Chiu* and W. H. McCarroll, “Preparation of Sr substituted LaMnO3 single crystals by Fused salt electrolysis”, ACS National Meeting, Project SEED Symposium, Sept. 1998.
- W. H. McCarroll, I. D. Fawcett , M. Greenblatt and K. V. Ramanujachary " Synthesis and Properties of Lanthanum Sodium Manganate Perovskite Crystals ", J. Solid State Chem.146, 88-95 (1999).
- J. Ding* and W. H. McCarroll, “ Preparation of large NaxLa1-xMnO3 single crystals by fused salt electrolysis”, ACS National Meeting, Project SEED Symposium, Sept. 1999.
- V.Markovitch, E.Rosenberg, G. Gorodetsky, M. Greenblatt and W. H. McCarroll “ Suppression of the Ferromagnetic-Insulating Phase in Self-doped La0.93Mn0.98MnO3” Crystal Under Pressure”,Phys.Rev. B63, 054423 (2001).
- S.E. Lofland, K. V. Ramanujachary and W. H. McCarroll, “ Intrinsic magnetic inhomogeneity in La-deficient manganate Crystals”, J. Magnetism and Magnetic Materials 238, 22-24 (2002).
- K. V. Ramujachary, S. E. Lofland, W. H. McCarroll, T. Emge, M. Greenblatt and M. Croft, “Substitutional Effects of 3d Transition Metals on the Magnetic and Structural Properties of Quasi-Two-Dimensional La5Mo4O16”, J. Solid State Chem. 164, 60-70 (2002).
- S. E. Lofland, T. Scabarozi, K. V. Ramanujachary and W. H. McCarroll, “ Unusual Magnetic Properties of La5Mo4O16”, J. Magnetism and Magnetic Materials, In press. Oct. 2002
* Rider student co-author