Dielectric Materials Chemistry 754 Solid State Chemistry Lecture #27 June 4, 2002
References • A.R. West – “Solid State Chemistry and it’s Applications”, Wiley (1984) • R.H. Mitchell – “Perovskites: Modern & Ancient ”, Almaz Press, (www.almazpress.com) (2002) • P. Shiv Halasyamani & K.R. Poeppelmeier – “Non-centrosymmetric Oxides”, Chem. Mater. 10, 2753-2769 (1998). • M. Kunz & I.D. Brown – “Out-of-center Distortions around Octahedrally Coordinated d0 Transition Metals”, J. Solid State Chem.115, 395-406 (1995). • A. Safari, R.K. Panda, V.F. Janas (Dept. of Ceramics, Rutgers University) http://www.rci.rutgers.edu/~ecerg/projects/ferroelectric.html
Dielectric Constant • If you apply an electric field, E, across a material the charges in the material will respond in such a way as to reduce (shield) the field experienced within the material, D (electric displacement) • D = eE = e0E + P = e0E + e0ceE = e0(1+ce)E • where e0is the dielctric permitivity of free space (8.85 x 1012 C2/N-m2), P is the polarization of the material, and ce is the electric susceptibility. The relative permitivity or dielectric constant of a material is defined as: • er = e/e0 = 1+ce • When evaluating the dielectric properties of materials it is this quantity we will use to quantify the response of a material to an applied electric field.
Contributions to Polarizability a = ae + ai + ad + as 1. Electronic Polarizability (ae) Polarization of localized electrons 2. Ionic Polarizability (ai) Displacement of ions 3. Dipolar Polarizability (ad) Reorientation of polar molecules 4. Space Charge Polarizability (as) Long range charge migration Polarizability (a) increases Response Time Increases (slower response)
Frequency Dependence • Reorientation of the dipoles in response to an electric field is characterized by a relaxation time, t. The relaxation time varies for each of the various contributions to the polarizability: 1. Electronic Polarizability (ae) Response is fast, t is small 2. Ionic Polarizability (ai) Response is slower 3. Dipolar Polarizability (ad) Response is still slower 4. Space Charge Polarizability (as) Response is quite slow, t is large Audiofrequencies (~ 103 Hz)a = ae+ai+ad+as Radiofrequencies (~ 106 Hz)(as 0)a = ae+ai+ad Microwave frequencies (~ 109 Hz)(as, ad 0)a = ae+ai Visible/UV frequencies (~ 1012 Hz)(as, ad, ai 0)a = ae
Dielectric Loss • When the relaxation time is much faster than the frequency of the applied electric field, polarization occurs instantaneously. • When the relaxation time is much slower than the frequency of the applied electric field, no polarization (of that type) occurs. • When the relaxation time and the frequency of the applied field are similar, a phase lag occurs and energy is absorbed. This is called dielectric loss, it is normally quantified by the relationship • tan d = er”/er’ • where er’ is the real part of the dielectric constant and er” is the imaginary part of the dielectric constant.
Frequency Dependence e(w) ad+ai+ae e0 ai+ae er (Dielectric Const.) ae only Microwaves IR e UV tand (Loss) log(w)
Ionic Polarization and Ferroelectricity • Most dielectric materials are insulating (no conductivity of either electrons or ions) dense solids (no molecules that can reorient). Therefore, the polarizability must come from either ionic and electronic polarizability. Of these two ionic polarizability can make the largest contribution, particularly in a class of solids called ferroelectrics. The ionic polarizability will be large, and a ferroelectric material will result, when the following two conditions are met: • Certain ions in the structure displace in response to the application of an external electric field. Typically this requires the presence of certain types of ions such as d0 or s2p0 cations. • The displacements line up in the same direction (or at least they do not cancel out). This cannot happen if the crystal structure has an inversion center. • The displacements do not disappear when the electric field is removed.
What is a Ferroelectric • A ferroelectric material develops a spontaneous polarization (builds up a charge) in response to an external electric field. • The polarization does not go away when the external field is removed. • The direction of the polarization is reversible. • Applications of Ferroelectric Materials • Multilayer capacitors • Non-volatile FRAM (Ferroelectric Random Access Memory)
2nd Order Jahn-Teller Distortions • Occurs when the HOMO-LUMO gap is small and there is a symmetry allowed distortion which gives rise to mixing between the two. This distortion is favored because it stabilizes the HOMO, while destabilizing the LUMO. Second order Jahn-Teller Distortions are typically observed for two classes of cations. • Octahedrally coordinated high valent d0 cations (i.e. Ti4+, Nb5+, W6+, Mo6+). • BaTiO3, KNbO3, WO3 • Increasingly favored as the HOMO-LUMO splitting decreases (covalency of the M-O bonds increases) • Cations containing filled valence s shells (Sn2+, Sb3+, Pb2+, Bi3+) • Red PbO, TlI, SnO, Bi4Ti3O12, Ba3Bi2TeO9 • SOJT Distortion leads to development of a stereoactive electron-lone pair.
Octahedral d0 Cation G point (kx=ky=0) non-bonding In the cubic perovskite structure the bottom of the conduction band is non-bonding M t2g, and the top of the valence band is non-bonding O 2p. If the symmetry is lowered the two states can mix, lowering the energy of the occupied VB states and raising the energy of the empty CB states. This is a 2nd order JT dist.
M t2g(p*) M t2g(p*) EF EF DOS DOS 2nd Order JT Distortion Band Picture Overlap at G is slightly antibonding in the CB & slightly bonding in the VB. Overlap at G is non-bonding by symmetry O 2p The 2nd order JT distortion reduces the symmetry and widens the band gap. It is the driving force for stabilizing ionic shifts. The stabilization disappears by the time you get to a d1 TM ion. Hence, ReO3 is cubic. See Wheeler et al.J. Amer. Chem. Soc. 108, 2222 (1986), and/or T. Hughbanks, J. Am. Chem. Soc.107, 6851-6859 (1985).
What Determines the Orientation of the Cation Displacements? d=1.83Å s = 0.96 d=2.33Å s = 0.32 d=1.67Å s = 1.90 d=2.21Å s = 0.34 d=1.95Å s = 0.90 • The 2nd Order JT effect at the metal only dictates that a distortion should occur. It doesn’t tell how the displacements will order. That depends upon: • the valence requirements at the anion (i.e. 2 short or 2 long bonds to same anion is unfavorable), • cation-cation repulsions (high oxidation state cations prefer to move away from each other) MoO3 Tetragonal BaTiO3 See Kunz & Brown J. Solid State Chem.115, 395-406 (1995).
Why is BaTiO3 Ferroelectric • Ba2+ is larger than the vacancy in the octahedral network tolerance factor > 1. • This expands the octahedron, which leads to a shift of Ti4+ toward one of the corners of the octahedron. • The direction of the shift can be altered through application of an electric field.
BaTiO3 Phase Transitions Cubic (Pm3m) T > 393 K Ti-O Distances (Å) 62.00 In the cubic structure BaTiO3 is paraelectric. That is to say that the orientations of the ionic displacements are not ordered and dynamic. Below 393 K BaTiO3 becomes ferroelectric and the displacement of the Ti4+ ions progressively displace upon cooling. Tetragonal (P4mm) 273 K < T < 393 K Ti-O Distances (Å) 1.83, 42.00, 2.21 Toward a corner Orthorhombic (Amm2) 183 K < T < 273 K Ti-O Distances (Å) 21.87, 22.00, 22.17 Toward an edge Rhombohedral (R3m) 183 K < T < 273 K Ti-O Distances (Å) 31.88, 32.13 Toward a face See Kwei et al.J. Phys. Chem. 97, 2368 (1993),
Structure, Bonding and Properties • Given what you know about 2nd order JT distortions and ferroelectric distortions can you explain the following physical properties. • BaTiO3 : Ferroelectric (TC ~ 130°C, er > 1000) • SrTiO3 : Insulator, Normal dielectric (er ~ x) • PbTiO3: Ferroelectric (TC ~ 490°C) • BaSnO3 :Insulator, Normal dielectric (er ~ x) • KNbO3 : Ferroelectric (TC ~ x) • KTaO3 : Insulator, Normal dielectric (er ~ x)
Structure, Bonding and Properties • BaTiO3 : Ferroelectric (TC ~ 130°C, er > 1000) • Ba2+ ion stretches the octahedra (Ti-O dist. ~ 2.00Å), this lowers energy of CB (LUMO) and stabilizes SOJT dist. • SrTiO3 : Insulator, Normal dielectric (er ~ x) • Sr2+ ion is a good fit (Ti-O dist. ~ 1.95Å), this compound is close to a ferroelectric instability and is called a quantum paraelectric. • PbTiO3: Ferroelectric (TC ~ 490°C) • Displacements of both Ti4+ and Pb2+ (6s26p0 cation) stabilize ferroelectricity • BaSnO3 :Insulator, Normal dielectric (er ~ x) • Main group Sn4+ has no low lying t2g orbitals and no tendency toward SOJT dist. • KNbO3 : Ferroelectric (TC ~ x) • Behavior is very similar to BaTiO3 • KTaO3 : Insulator, Normal dielectric (er ~ x) • Ta 5d orbitals are more electropositive and have a larger spatial extent than Nb 4d orbitals (greater spatial overlap with O 2p), both effects raise the energy of the t2g LUMO, diminishing the driving force for a SOJT dist.
2nd Order Jahn-Teller Distortions with s2p0 Main Group Cations Fact: Main group cations that retain 2 valence electrons (i.e. Tl+, Pb2+, Bi3+, Sn2+, Sb3+, Te4+, Ge2+, As3+, Se4+, ect.) tend to prefer distorted environments. M-X Bonding:The occupied cation s orbitals have an antibonding interaction with the surrounding ligands. Symmetric Coordination: The occupied M s and empty M p orbitals are not allowed by symmetry to mix. Distorted Coordination: The lower symmetry allows mixing of s and at least one p orbital on the metal. This lowers the energy of the occupied orbital, which now forms an orbital which is largely non-bonding and has strong mixed sp character. It is generally referred to as a stereoactive electron lone pair (for example as seen in NH3). Tetrahedral Coordination (Td): s-orbital = a1, p-orbitals = t2 Trigonal Pyramidal Coord. (C3v): s-orbital = a1, p-orbitals = e,a1 Octahedral Coordination (Oh): s-orbital = a1g, p-orbitals = t1u Square Pyramidal Coord. (C4v): s-orbital = a1, p-orbitals = e,a1
Some Examples s2p0 SOJT Red PbO Distorted CsCl CsGeBr3 Distorted Perovskite SbCl3 Trig. Pyramidal Sb3+
Cooperative SOJT Distortions Tetragonal BaTiO3 TC = 120°C Ti displacement = 0.125 Å Ti-O short = 1.83 Å Ti-O long = 2.21 Å Ba2+ displacement = 0.067 Å Tetragonal PbTiO3 TC = 490°C Ti displacement = 0.323 Å Ti-O short = 1.77 Å Ti-O long = 2.39 Å Pb2+ displacement = 0.48 Å
Related Dielectric Phenomena • Pyroelectricity – Similar to ferroelectricity, but the ionic shifts which give rise to spontaneous polarization cannot be reversed by an external field (i.e. ZnO). Called a pyroelectric because the polarization changes gradually as you increase the temperature. • Antiferroelectricty – Each ion which shifts in a given direction is accompanied by a shift of an ion of the same type in the opposite direction (i.e. PbZrO3) • Piezoelectricity – A spontaneous polarization develops under the application of a mechanical stress, and vice-versa (i.e. quartz)
PZT Phase Diagram • Pb(Zr1-xTix)O3 (PZT) is probably the most important piezoelectric material. The piezoelectric properties are optimal near x = 0.5, This composition is near the morphotropic phase boundary, which separates the tetragonal and rhombohedral phases.
Hysteresis Loops in PbZr1-xTixO3 PbTiO3 Ferroelectric Tetragonal PbZr1-xTixO3 x ~ 0.3 Ferroelectric Rhombohedral PbZr1-xTixO3 Paraelectric Cubic PbZrO3 Antiferroelectric Monoclinic An antiferroelectic material does not polarize much for low applied fields, but higher applied fields can lead to a polarization loop reminiscent of a ferroelectric. The combination gives split hysteresis loops as shown above.
What is Piezoelectricity • A piezoelectric material converts mechanical (strain) energy to electrical energy and vice-versa. Voltage In Mechanical Signal Out i.e. Speaker Mechanical Signal In Voltage Out i.e. Microphone
Applications of Piezoelectrics • Piezo-ignition systems • Pressure gauges and transducers • Ultrasonic imaging Ceramic phonographic cartridge • Small, sensitive microphones • Piezoelectric actuators for precisely controlling movements (as in an AFM) • Powerful sonar
Symmetry Constraints and Dielectric Properties • Dielectric properties can only be found with certain crystal symmetries • Piezoelectric • Do not posses an inversion center (noncentrosymmetric) • Ferroelectric/Pyroelectric • Do not posses an inversion center (noncentrosymmetric) • Posses a Unique Polar Axis • The 32 point groups can be divided up in the following manner (color coded according to crystal system: triclinic, monoclinic, etc.). • Piezoelectric • 1, 2, m,222, mm2, 4, -4, 422, 4mm, 42m, 3, 32, 3m, • 6, -6, 622, 6mm, 6m2, 23, 43m • Ferroelectric/Pyroelectric • 1, 2, m,mm2, 4, 4mm, 3, 3m,6, 6mm • Centrosymmetric (Neither) • -1, 2/m, mmm, 4/m, 4/mmm, -3, 3/m, 6/m, 6/mmm, m3, m3m
E - q + q Electronic Polarizability • Let’s limit our discussion to insulating extended solids. In the absence of charge carriers (ions or electrons) or molecules, we only need to consider the electronic and ionic polarizabilities. without field with field x The presence of an electric field polarizes the electron distribution about an atom creating a dipole moment, m=qx The dipole moment per unit volume, P, is then given by P = nmm where nm is the number of atoms per unit volume.
Microwave Dielectrics • Were not talking microwave ovens here, rather communication systems which operate in the microwave region: • Ultra high frequency TV (470-870 MHz) • Satellite TV (4 GHz) • Mobile (Cellular) Phones (900-1800 MHz) • All such systems depend upon a bandpass filter that selects a narrow frequency range and blocks all others. These filters are constructed from ceramics with desirable dielectric properties.
Microwave Dielectrics-Properties • The following dielectric properties are intimately related to it’s performance • Dielectric Constant (Permitivity) • A high dielectric constant allows components to be miniaturized • Dielectric Loss • A low dielectric loss is needed to prevent energy dissipation and minimize the bandpass of the filter • Temperature Coefficient • For device stability the dielectric properties should be relatively insensitive to temperature
Microwave DielectricsMaterials by Design • The the required properties it is possible to apply some concepts of rational design to the search for materials. • High Dielectric Constant • High electron density (dense structure type, polarizable cations, i.e. Ta5+). • Low Dielectric Loss • Ionic polarizability comes with large losses in the microwave region. Therefore, one needs to avoid ferroelectrics, disorder and impurities. Ions should not be able to rattle around too much. • Temperature Coefficient • Very sensitive to rotations of polyhedra, vibrations of atoms, as well as thermal expansion. In perovskites the temperature coefficient is linked to octahedral tilting distortions. Tolerance factors just below 1 tend to have very low temperature coefficients.
Commercial Microwave Dielectrics • See Dr. Rick Ubic’s (University of Sheffield) site for a more detailed treatment of microwave dielectrics. • http://www.qmul.ac.uk/~ugez644/index.html#microwave