A LANTHANIDE LANTHOLOGY (.pdf) - Davidson Physics

PEROVSKITESA very wide range of materials, many containing lanthanides, are known with thebase-composition, ABO 3 , and perovskite-type structures[1]. Trivalent Ln 3+ ions, with radii from≈95pm to ≈ 120pm, readily form the A component while another cation, particularly from the firsttransition series, can be the smaller nominally-trivalent B ion, e.g. chromites, cobaltates andmanganites. The structure contains octahedral BO 6 groupings with the larger A ions surrounded by12 O's.Many changes, including non-stoichiometry, can be rung on the basic structure. The A sitecan be doped with another ion, e.g. M 2+ ; the charge is balanced by a change of the valency of B,resulting in mixed valency for the B ion. The structure can stabilize a valence, e.g. Ni 3+ , usually noteasily obtained. In addition subtle structural changes can produce other crystal symmetries anddiffering behavior. The perovskite block, ABO 3 , also forms sub-units in more complex structures,e.g. A 2 BO 4 = AO / ABO 3 .Interactions between the B ions can give rise to novel magnetic as well as unusual ionicand/or electronic conductivity behavior. Mixing B-ion contents can help optimize these properties.The potential to vary the Ln ion, and thus the A ion size, makes possible fine-tuning ofstructuresensitive properties. The net result is that LnMO 3 (M = transition metal) compounds are ofinterest in many developing technologies, such as electrocatalysis, high temperature electrodes andelectrolytes, membranes for gas separation, and sensors. [2]3][4]Perovskites can be prepared by high temperature solid-state reactions (if needed underreducing conditions) but, for close control of chemistry and particle morphology, co-precipitationor sol-gel type processes[5] are preferred.Ln PerovskitesLnMo 3M ionScTi*VCr*Mn*FeCo*Niproperty & refstabilitymagneticelectro-opticconductivityelectrode, etccatatysis[2]catalysis, etccatalysis[3]*see entries in the Lanthology series[1] F.S.Galasso, book, Structure, Properties and Preparation of Perovskites-Type Compounds, 1969, publ.Pergamon : Stability and Thermodynamic Properties of Rare Earth Perovskites, J.-P. Coutures et al., HighTemp. sc., 1980, 13, 331[2] Electrolytic Oxygen Evolution in Alkaline Medium on La l-xSr xFeO 3-y Perovskite Related Ferrites,A-Wattiaux et al., J.Electrochem.Soc., 1987, 134(7), 1714[3] Structure and Catalytic Activity of Perovskites La-Ni-O Supported on Alumina and Zirconia, A.K.Ladavosand P.J. Pomonis, Appl. Cat. B Envir., 1993, 2, 27[4] Perovskite Electrodes for Sensors, C.B.Alcock et al., Solid State Ionics, 1992, 51, 281[5] Precipitation of LaNiO 3 Powder from Co-precipitated Lanthanum Nickel Oxalates, J. Takahashi et al.,J.Mat.Sci., 1990,25,1557: Modified Resin-Intermediate Processing of Perovskite Powders, L-W. Tai andP.Lessing, J.Mater.Res., 1992, 7(2), 51127