Laurea Magistrale in Scienza dei Materiali Materiali Inorganici Funzionali Electrolytes: New materials Prof. Antonella Glisenti - Dip. Scienze Chimiche - Università degli Studi di Padova
PEROVSKITES AS ELECTROLYTES: LaMO 3 Nomura et al. Solid State Ionics 98 (1997) 229-236
LaSrGaMgO 3 LSGM: WHY? Comparison of the total conductivity in several oxide-ion conductors. Electrolytic domain. LSGM: slightly less performant but much more stable under reducing conditions. Morales et al. J. Europ. Ceram. Soc. 36 (2016) 1-16
1. Ga and Sc: Higher performance 2. Higher conductivity in O 2 : p-type conductivity Arrhenius plots of total electrical conductivity of (La 0.9 Sr 0.1 )M III O 3 IN AIR Arrhenius plots of total electrical conductivity of (La 0.9 Sr 0.1 )M III O 3 IN N 2 Nomura et al. Solid State Ionics 98 (1997) 229-236
PEROVSKITE PROPERTIES AND DOPING DIELECTRIC LaGaO 3 I ST doping with Sr per La: Sr-doped lanthanum gallate = ionic conductor II nd doping with Mg per Ga: Mg doped lanthanumgallate = ionic conductor Double doping = ION CONDUCTOR La (1-x) Sr x Ga (1-y) Mg y O 3- La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3-δ seems to guarantee the best performance among perovskites
EFFECT OF A AND B-SITE SUBSTITUTION 1. Among the alkaline earth ions occupying the A-site, Sr provides the highest ionic conductivity 2. Co: the more effective in increasing oxygenion conductivity; Fe is less efficient 3. Ni, Mn, Cr increase electrons and holes 4. Mg(II) always in low amounts Morales et al. J. Europ. Ceram. Soc. 36 (2016) 1-16
Why? Enthalpy of migration vs ionic radii? Poor agreement Hayashi et al. Solid State Ionics 122 (1999) 1-15
Doping Oxygen vacancies Coordination number vs ionic radii Change of the coordination number of cations Effective ionic radius vs CN a) A-site cations b) B-site cations
Conductivity Tolerance factor Specific Free Volume Oxygen deficiency ABO 3-δ r A and r B = mean ionic radii considering the coordination numbers r O = 0,140 nm oxygen radius Specific Free Volume, V sf = Free Volume/Unit cell volume = (Volume Total volume of the constituent ions)/volume Calculated vs measured = difference only for highly distorted cells Cell volumes of La 0.9 Sr 0.1 MO 2.95 for different B site metals M as a function of tolerance factor t; Open = calculated Filled = from lattice constants
Lattice stability higher for t = 1 Electrical conductivity has a maximum value at t = 0.96 Factor other than lattice stability affects conductivity Specific Free Volume Electrical conductivities at 1000 K as a function of t for BaMO 2.5 related and BaZr 1-x In x O 3-0.5x LaMnO 3 -related NdMO 3 related SrTiO 3 -related La 0.5 Sr 0.5 Ga 1-x Zr x O 3 -related SrSc 1-x Al x O 2.5 -related
Higher amount of oxygen vacancies: Brownmillerites and perovskite wit disordered high oxygen vacancies Two different groups: high and low V sf Specific Free Volume, changes with: 1. Size of the origina lattice without oxygen vacancies 2. Oxygen deficiency Specific Free Volume vs t for BaMO 2.5 related and BaZr 1-x In x O 3-0.5x LaMO 3 -related NdMO 3 related SrTiO 3 -related La 0.5 Sr 0.5 Ga 1-x Zr x O 3 -related SrSc 1-x Al x O 2.5 -related
< t > distortion > V sf La-series = > V sf Large V sf is desirable in order to obtain larger oxyge ion mobility Small deformation from cubic symmetry is desirable in order for oxygen sites to be equivalent otherwise oxygen atoms may be difficult to jump due to local strain of the lattice Optimum t = 0.96 Conductivity vs t Open = La 0.9 O 0.1 MO 2.95 Filled = Nd 0.9 O 0.1 MO 2.95
> Radius = different t The increase of ion size in B is effective to enlarge the unit cell volume The increase of ion size in A is slightly effective Specific Free Volume vs t for LaMO 3 -related NdMO 3 -related The increase of ion size increase slightly the total volume of ions The specific volume is determined by the balance between the increase of unit cell volume and that of the total ionic volume
SIZE EFFECT OF DOPANT IONS Better dopant: Sr for La series; Ca for Nd series Maximum electrical conductivity for r dopand /r host around 1.05 Nearly the same size of dopant ion and host desirable for minimizing local strain around the dopant and the long-range strain in the host lattice
SIZE EFFECT OF DOPANT IONS No cations not TM of the same range of Al Only Mg(II) has a radius similar to Ga(III) The radius ratio = 1.16 The effect of oxygen deficiency on electrical conductivity is larger than that of the lattice strain TM cations = risk to introduce electrical condutivity Specific Free Volume vs t for La 1-x Sr x Ga 1-y Mg y O 3-δ for different δ against the t; Sr and Mg written as (x,y)
LaSrGaMgO3 system Electrical conductivity increase s with the oxygen deficiency The electrical conductivity become larger when t approach that of LaGaO 3 Conductivity vs t for La 1-x Sr x Ga 1-y Mg y O 3-δ for different δ against the t; Sr and Mg written as (x,y)
LaSrGaMgO3 system Good correlation between conductivity and Activation Energy Activation energy and conductivity (1000 K) for La 0.9 Sr 0.1 MO 2.95 (M = Lu, In, Sc, Ga, Al) as a function of t; The fine structure of defect association in the perovskites changes with temperature and a small activation energy does not necessarily bring a high electrical conductivity
High V sf systems Difficult to increase significatly the conductivity of these highly deficient perovskites. Electrical conductivity of perovskite type compounds with larger specific free volume against deficiency at 1000 K. Electrical conductivity of La 1-x Sr x Ga 1-y Mg y O 3-δ for comparison
B-cations A-cations O 2- Free volume and tolerance factor as a function of the idealized cubic lattice parameter, a, at RT Crystal structure of cubic perovskite (a) and orthorhombic perovskite (b) Free volume dependence of total conductivity in air (black) and N 2 (open symbols)
LSGM: Conductivity vs TM doping Ionic radii versus coordination number for the perovskites. For Co, Fe, Mn the radii of the high spin states were used.
Co doped LSGM y = 0.3 y = 0.1 Splitting of the cubic (110) reflection into the (110) and (104) reflections of the hexagonal cell Reduction (10-10 Pa 800 C) and Mg play an important role in the solubility of Co ions in the lattice = cubic structure
Sammels 1990 high oxygen conductivity in mixed oxides characterized by a high free volume of the unit; weak association between mobile ions and the lattice bulk t-factor near to 1: no distorsion of the perovskite Cation and doping cations almost equal radii t = (r A + r O )/ 2(r B + r O ) A III 1-aA II ab III O 3-x r A = (1-a)r AIII + ar AII x = a/2 A III B III 1-bB II bo 3-x r B = (1-b)r BIII + br BII x = b/2 Additional doping with transition metal cations M on B-site which have non-integer oxidation states depending on the oxidation conditions La 0.9 Sr 0.1 (Ga 0.9 (Fe III 1-zFe IV z) 0.1 ) 0.8 Mg 0.2 O 3-x+δ
Real t factors Oxygen ΔO exchanged from airoxidized samples of La 0.9 Sr 0.1 (Ga 1-y Co y ) 0.8 Mg 0.2 O 3-x-δ as a function of p O2 at 800 C. Oxygen stoichiometry range, δ, mean ionic radii and t-factor
Free lattice volume and t factors: which is the effect of the oxidating or reducing conditions? Free volumes and t-factors for the air-oxidized samples as functions of dopant concentrations for La 0.9 Sr 0.1 (Ga 1-y M y ) 0.8 Mg 0.2 O 3-x-δ (M=Co, Fe).
Ionic and Electronic Conductivity In the range up to y = 0.1 Co and Fe no dependence of conductivity on P O2 = ionic domain Y > 0.1 = p-conduction Conductivity of La 0.9 Sr 0.1 (Ga 1- ym y ) 0.8 Mg 0.2 O 3-x-δ (M = Co, Fe) as a function of dopant concentration y at Conductivity of La 800 C 0.9 Sr 0.1 (Ga 1- ym y ) 0.8 Mg 0.2 O 3-x-δ (M = Co, Fe) as a function of dopant concentration y and P O2
Conductivity vs TM doping Und Co Fe Cr Cr Mn Fe Co V f 14.817 14.362 14.436 (16.583) 14.307 (16.102) t 0.9599 0.9608 0.9605 0.9613 δ 0.004 0.0211 0.0344 0.0603 Mn E a 0.748 0.898 0.745 0.657 (0.578) 0.707 (0.694) La 0.9 Sr 0.1 (Ga 0.9 M y ) 0.8 Mg 0.2 O 2x-δ M = Mn, Cr, Co, Fe y = 0.1 y = 0.2 y = 0.3 V f (Fe) 14.436 13.861 13.472 t (Fe) 0.9605 0.9612 0.9621 (Fe) 0.0344 0.0642 0.0963 E a (Fe) 0.657 0.368 0.311 V f (Co) 14.307 13.156 12.682 t (Co) 0.9613 0.9642 0.9653 (Co) 0.0603 0.1324 0.1641 E a (Co) 0.707 0.433 0.310 0.265 (La 0.85 Sr 0.1 (Ga 0.9 M 0.1 ) 0.8 Mg 0.2 O 2x-δ M = Fe, Co) > TM doping > δ; in the row Cr<Mn<Fe<Co. y > 0.1: conductivity increases due to additional p-type conduction. Mn, Cr n-type conductivity Understoichiometry: another possibility