BUILDING THE STRUCTURES OF HIGH TEMPERATURE SUPERCONDUCTORS USING PEROVSKITE STRUCTURE UNITS. As discussed in Section 3.2 there are many

Perovskite and Related Structure Systems

CHAPTER 3 CHAPTER 3

3.4.4.3. BUILDING THE STRUCTURES OF HIGH TEMPERATURE SUPERCONDUCTORS USING PEROVSKITE STRUCTURE UNITS. As discussed in Section 3.2 there are many

types of oxygen-deficient perovskite-like structure units (Fig. 3.10), and there are, of

course, many ways to stack the regular perovskite unit cell with the oxygen-deficient perovskite-like units for creating different 2-D systems which have high Tc super- conductivity. The oxygen-deficient perovskite-like units can be unified into a simple formula AB03 ^-x' which has positive charge,

+

2x. Thus, the basic stacking layers of (A0_3)4- should be replaced by (A0_{3_x})(4-2x)-. As illustrated in Fig. 3.9, the oxygen vacancies can be introduced in the 'Y and/or ~ oxygen triangles, resulting in the modification of the B cation coordination number to 5 or 4. The disproportionation of the B cations may satisfy the five- or four-coordinated anion environment, but the AB03 _ x

unit may still retain some unbalanced charges. Therefore, a negatively charged layer must be introduced, for which the layer with oxygens and A-type cations is the best choice.

The alternate stacking of (A1_xO)2x- with (AB03_x)2x+ layers can form a series of compounds with formula (A1-xO)m(AB03-x)m' The structure of high Tc superconductors can be understood with this mechanism.

(a) Three-dimensional framwork

_

^....

_.

(b) Two-dimen ional framework

_-

...

.

. . .

(c) One-dimensional framwork

:::: r-

.

' . ' "

.

Figure 3.21. Dimensionality in the formation of compounds. (a) Three-dimensional framework has strong bonds, (b) two-dimensional framework has strong bonds within the plane but weak in the third dimension, and (c) one-dimensional chain is only strong along the chain direction.

125

PEROVSKITE AND RELATED STRUCTURE SYSTEMS

126

CHAPTER 3

(a) Module (1) (b) Rip (a) vertically and horizotalJy

@ @)

@) @

Figure 3.22. Evolution from the oxygen-deficient perovskite modules into the crystal structures of the Y-Ba..:...Cu-O system. (a) The module given in Fig. 3.101, (b) the module is flipped over vertically and horizontally. (c) The modules in (a) and (b) are combined with superimposition of the B04 units to form a new module, which is the structural building block for YB2CU307. (d) The structure ofYB2Cu307' (e) A new module created by combining the two modules in (a) and (b) to share the edges of the B04 units. This new module is the building block of the structure of YBa2Cll40g. (f) The structure of YBa2C~Og. (g) The structure of

Y2B~CU7015 is a combination of the modules in (c) and (e).

In high Tc superconductors, the B cation is Cu, which has 1 +, 2+, and 3+ valence states with coordination numbers 2, 3, and 4 for Cu(I), 4 and 6 for Cu(II) and Cu(III). If the A and B cations in AB03 _x have valences 3+ and 2+, respectively, the (A03)4- layers must be replaced by (A03_J3-2x)-. Since the Cu cation (the B cation) may have valence 1 +, 2+, or 3+, the perovskite-type cell containing oxygen vacancies gives Cu cations the possibility to disproportionate its valence from 2+ into 1 + and 3+. La2Cu04 is generally believed to have the K2NiF4 type of structure, which, in fact, is a perovskite- related structure. In the literature (Goodenough, 1971; Rao and Raveau, 1995) the structures of high Tc superconductors are expressed as the result of the intergrowth of defective perovskite layers of ACU03_x with AO-type rock salt layers. Our explanation here is based on evolution from the perovskite structure. Using our perovskite unit cells containing oxygen deficiency, atomic structures of the high Tc superconductors can be adequately built.

Figure 3.22a gives the module of Fig. 3.101. The B cation in this module has two types of coordinations: B05 and B04. We can flip this module (Fig. 3.22b), and combine

'.- - - -

~

- - i

cu-;'~

I Oxygen aruons ~ . I

115'.::. in A Band C Y cauons

I ^{W"I '.} I

1_~:C~~~_~~a~~...!

Figure 3.22. (continued)

127

PEROVSKITE AND RELATED STRUCTURE SYSTEMS

128

CHAPTER 3

the coordinations by superimposing the B04 units of the two to obtain the new module as given in Fig. 3.22c. This new module is the building block for the Y -Ba-Cu-O system of high Tc superconductors with an introduction of Ba cations between the modules. The structures ofYBa2Cu307 is given in Fig. 3.22d. If the combination is to share the edges of the B04 units in the modules, a new module (Fig. 3.22e) is obtained, which is the building block of the structure of YBa2Cu40S (Fig. 3.22f). When these two new modules combine by a cation, then the structure of Y 2Ba4Cu7015 is constructed (Fig. 3.22g). This explanation may be useful for synthesizing new compounds with these oxygen-deficient perovskite units, if they can exist in some precursor molecules. The module idea based on the perovskite unit cells may be an appropriate way to elucidate the perovskite-related compounds, providing a clear and simple understanding of complex structural systems.

3.4.5. MAGNETOSTRICTIVE, ELECTROSTRICTIVE, AND PIEZOELECTRIC ACTUATOR MATERIALS

Actuator materials are an important class of smart materials. These materials are smart because they can convert an electric/magnetic signal into a mechanical displacement, or vice versa. The most widely used actuator materials are piezoelectric Pb(Zr,Ti)03' electrostrictive Pb(Mg,Nb)03 (PMN), magnetostrictive (Tb,Dy)Fe2, and the shape memory alloy NiTi. PZT and PMN have perovskite-type structure. Electrostrictivel magnetostrictive effect means that the strain created in the film is a linear or close-to- linear function of the externally applied electric/magnetic field and stress. Electrostrictive and magnetostrictive materials are electric and magnetic shape memory materials, and they can also be high-energy-density transducers. (DYoc3 Teo.7)Fe2 (Terfenol-D), Ni2MnCa (Chernenko et al., 1995), multilayered Tb--Te, Tb-Dy-Fe (Quandt et al., 1994), and TbCo-FeCo have very strong magnetostrictive effect.

PZT and magnetostrictive materials can be integrated into an alternate-layered actuator system in which a magnetic field parallel to the layer strains the magnetostrictive materials, which in tum stresses the piezoelectric layers and displaces electric charge.

Pb(Mg,Nb )03 and NiTi are cubic at high temperatures and, on annealing, transform to a partially ordered state. On further cooling, Pb(Mg,Nb)03 passes through a diffuse phase transformation at RT where it exhibits very large dielectric and electrostrictive coefficients. Just below RT, it transforms to a ferroelectric rhombohedral phase. The partially ordered shape memory alloy NiTi undergoes an austenitic (cubic) to martensitic (monoclinic) phase change just above RT. It is easily deformed in the martensitic state but recovers its original shape when reheated to austenite. This is the smart shape memory property of these materials.

PZT is a perovskite-related compound with piezoelectric property produced by the alignment of the ferroelectric domains under an applied electric field. Strain is linearly proportional to the electric field in piezoelectric materials, implying that the piezoelectric coefficient is constant and cannot be electrically tuned with a bias field. Thus, it may not be used alone in tunable electromechanical coupling.

PMN is not piezoelectric at room temperature since its Curie temperature is below DoC, but exhibits very large electrostrictive effect. The piezoelectric d33 coefficient is the slope of the strain-electric field curve when strain is measured in the same direction as the applied field. Its value for Pb(Mgo.3NbO.6TiO.l)03 is zero at zero field, but it increases

to a maximum of 1300pC/N, three times larger than that of PZT, under a bias field of 3.7kV/cm. This means that the electromechanical coupling coefficient can be tuned over a wide range to change the transducer from inactive to extremely active. The dielectric constant of PMN depends on the bias field. The polarization saturates at a high field to cause a decrease in the capacitance. Therefore, the electrical impedance can also be controlled. This tunable property is based on compositional microdomains. Because the ionic radii of Sc2+, Ta6+, Mg2+, and Nb6+ in perovskite are close to each other, the long-range order is not favorable but the shorter-range order is preferable. The ordered phase has a double size in each axis of the perovskite unit cell with face-centered cubic structure. They are reexpressed as the Pb2MgNb06 order domain and the PbNb03 disorder domain if the original compound is Pb(Mgo.33Nbo.67 )03, in which the valence states of Nb' Mg, and Pb are 5+,2+, and 2+, respectively. HRTEM revealed that the domains are about 3 nm in diameter and the ordered regions are small islands separated by narrow walls of niobium-rich PMN (Kang et al., 1990; Boulesteix et al., 1994). The ordered regions, Pb2MgNb06, have negative charge, but the disordered areas, PbNb03, have positive charge. The ordered regions have to be very small to minimize the Coulomb energy, but the distortion of perovskite is so small that the interface energy is negligible. The compositions of the ordered and disordered regions are different.

Therefore, the domains may be called compositional domains. The ordered islands with negative charge are enclosed by the disordered layers with positive charge to make the system neutral on a local scale and minimize the electrostatic energy. The very fine particles of PbTi03 demonstrate a very interesting phenomenon in which the polar tetragonal phase becomes unstable if the particle size is smaller than '" 20 nm. This is called a ferroelectric-superparaelectric transformation. Relaxor ferroelectric PMN exhibits many of the characteristics of a superparaelectric solid, one of them being that the dipole moments are strongly coupled to one another but not to a crystallographic axis.

The coupling electric dipoles oscillate in orientation and respond readily to an applied field. Therefore, it gives a large dielectric constant and massive electrostrictive coefficient. The order-disorder transition occurs in a wide intermediate temperature range and the process is controlled by atom diffusion. Relaxor ferroelectric materials sometime have an abnormally low thermal expansion coefficient near the diffuse phase transformation where the dielectric constant and the electrostrictive strain are usually large. The structural evolution in the order-disorder transition is the basis of the tunable transducer. The nonlinear behavior is the origin of functionality. Nanosize particles, microdomain, nanoscale fluctuations, the instabilities associated with phase transforma- tion, superparaelectric, and superparamagnetic, etc., can have nonlinear behavior.

Pbo.ssLao.os(Zro.6s Tio.3s)03 is a ferroelectric material whose composition is near the morphotropic phase boundary between the rhombohedral and tetragonal unit cells formed by the distortion of perovskite structure due to A and B cation substitution. The rhombohedral and tetragonal phases have different permanent dipoles and different dimensions. In a high electric field they have hysteresis loops. The microdomains of the rhombohedral and tetragonal phases determine the overall property of the ferroelectric hysteresis loop. Because of the dimensional difference in the c and a axes, the strain along the c axis is different from those along the a and b axes if the microdomains are aligned by an external electric field. This anisotropic strain is switchable by changing the direction of the applied field. Orthorhombic Ta20s has also been found to have the piezoelectric property (Nakagawa and Gomi, 1985).

129

PEROVSKITE AND RELATED STRUCTURE SYSTEMS

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CHAPTER 3

3.4.6. OPTICALLY SWITCHABLE COMPOUNDS

Optically switchable behavior means the optical properties (transmittance and reflectance, color, etc.) are switchable by temperature, electric field, stress, or other environmental parameters. Optical properties may also be tunable. Reflectance and transmittance are related to the refraction index as determined by the dielectric constant.

Thus, the reflectance and transmittance switchable behaviors are closely related to phase transformation. Color switching is related to the metal-metal electron-charge transfer transitions that involve mixed valence and valence mixture elements. All are linked with structural evolution.

Any electrolyte cell has a hysteresis loop of the cyclic voltammetric curve. But if during electrochemical reaction cycle the electrolyte compound has electron-charge transfer between the metal cations, the color ofthe electrolyte compound is switchable or tunable. This phenomenon is called the electrochromatic effect. A typical example is tungsten oxide, W03. The basic reaction is

W03 has light color. When Li ions are inserted in the structural tunnels it switches to dark blue due to the electron-charge transfer from Li to W to make W valence mixture (from W6+ to W5+ and W6+). This reaction is reversible. The switch process can be driven by an external electric field. W03 has the perovskite structure in which the A cations are replaced by vacancies, and it is an important component in a variety of smart systems, including electrochromatic devices and conductivity, acoustic wave, and fiber optic mlcrosensors.

Many compounds with mixed valence cations, such as NiOx, MoOr W0^3, H2Ti307,

HNb W06, Mn02, and Ce02, can have electrochromatic behavior and are candidates for making smart windows. Structures with large structural tunnels and mixed valences are the foundation of switchable behavior.

V02 has the rutile structure under 68°C and it is an insulator with a narrow band gap (or say a semiconductor), but it transforms to a metallic phase above 68°C. This insulator-metallic phase transition causes a jump in conductivity by a factor as large as 10^5. At the same time the transmittance of light with wavelength of 0.4-30 !lm is reduced from more than 50% to less than 10%. This process is called the thermochromatic effect.

3.5.

^{DOPING AND OXYGEN VACANCIES}

As discussed previously, doping of a third element is a key technique for changing the structures of perovskite-related compounds, possibly leading to the discovery of new properties. A typical example is substituting Sr for La in Lal_xSrxC003_y, as observed experimentally (Section 7.2). First, both La³+ and Sr2+ cations have almost the same ionic radii, so a site exchange will not introduce any significant lattice distortion (Yakel, 1955). Second, the site distribution of 0- 2 anions around either La³+ or Sr²+ is equivalent, permitting lattice substitution between La3+ and Sr2+. Third, substituting S~+ for La³+ introduces a local charge compensation, but the local p-type charge carrier is balanced by the partial conversion of C0³+ into C0⁴+ . Finally, the loss of local charge

due to replacing La³+ by Sr²+ is balanced by creating oxygen vacancies as well as converting Co3+ to C04+. The ionic structure of Lal_xSrxC003_y is

L al-x 3+ S 2+ C rx 01-x+2y 3+ C 0x-2y 4+ 0 3-y 2- VO y (for Y

:s

x/2) (3.2) where V~ stands for oxygen vacancies (Yakel, 1955; lonker and van Santen, 1953;

Cillessen et al., 1993). This formula has been applied to interpreting the electric conductivity of this material as a function of Sr content (Yakel, 1955; lonker and Van Santen, 1950). If there is no oxygen vacancy (y

=

0), the ionic state Lal_xSrxCo03 is

La 3+ Sr 2+ Co 3+ Co 4+ 0 2-_{I-x x I-x x 3} (3.3) For x=0.5, the ionic state of L"'~ "'U.5 0.5 Sr CoO is [La3+C03+] 3 0.5 [Sr2+C04+] 0.5 0 2-3 . For x

=

0.67, the ionic structure of La0.33SrO.67Co03 is Lao.333+ SrO.672+ C00.333+ COO.67⁴+ 0 32-. Therefore, the lattice substitution between La and Sr is possible because of the mixed valences of Co and the creation of oxygen vacancies.

To limit the density of oxygen vacancies, an examination of the (A03_yi4-2Yl- layer may be helpful. As shown in Fig. 3.8d, the maximum content of anion vacancies is Ymax

=

1 to maintain the basic framework of AB03 _ y' On the other hand, with the consideration of the B cation stacking layer and the presence of cation dopants, Eq. (3.2) indicates the maximum vacancy content is x - 2Ymax

=

0, or Ymax

=

^x/2. Substituting into Eq. (3.2) yields

L al-x 3+ S 2+ C 3+ 0 rx 0 3-x/2 2- VO x/2 (3.4) No C04+ is present. Thus, the material loses its functionality. Therefore, the maximum allowed content of vacancies is less than half of the content of the dopants unless C03+ is converted to C0²+ (Section 8.10).

If the substitution of La by Sr is ordered and the content of the Sr dopants in the unit cell is not an integer, a superlattice structure is expected. The electron diffraction technique (Section 7.3) would be very useful for determining the 3-D unit cells of the superstructure. This analysis can be performed analogously for Lal_xCa.xMn03'

The foregoing analysis is based on the assumption that the oxygen deficiency remains

y:s

x/2. Thus, the valence conversion from C03+ to Co 4+ is logical. In this situation, oxygen vacancies may not be needed to balance the charge, so the oxygen content can remain 3 and the structure is most stable. On the other hand, the anion deficiency can be high enough, leading to a conversion of C0³+ to C0²+ if the anion deficiency is high:

L al-x 3+ S 2+ C rx 01-2y+x 3+ C 02y-x 2+ 0 3-y 2- VO . y for x/2

:s

Y

:s

1) (3.5) If the doping x is fixed, oxygen vacancies must be created to balance the charge. But the content of the vacancies needs to be less than one

(y:s

1) to maintain structure stability. In practice, spectroscopy measurements of Co²+, Co3+, and C04+ are needed to determine the oxygen vacancies.

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PEROVSKITE AND RELATED STRUCTURE SYSTEMS

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The analysis clearly indicates that the conversion of valence states is an important feature of oxide functional materials. In practice, the ratio of the two valence states can be determined by EELS at a high spatial resolution (Sections 8.5 and 8.10). It also indicates that oxygen vacancies are tentatively created for partially balancing the local charge. The analysis of short-range order of oxygen point defects will be given in Section 7.4.

3.6.

GIANT MAGNETORESISTANCE (GMR) AND COLOSSAL