**3. Perovskite-type structures**

Many transition metal oxides show the very versatile perovskite structure. The rich variety of physical properties such as high-temperature superconductivity and colossal magnetoresistance observed in these compounds makes them very attractive from both fundamental and applied perspectives.

The general chemical formula for perovskite compounds is *AB*X3, where *A* and *B* are two cations of very different sizes, and X is an anion that bonds to both. The *A* atoms are larger than the *B* atoms, and besides, its ionic radii close to that on the anion X, thus they can form together a cubic close packing. The smaller *B* cation is usually a 3d-transition metal ion which occupies the octahedral interstices of the close packing. X is oxygen or a halide ion. The ideal cubic-symmetry structure has the *B* cation in 6-fold coordination, surrounded by an octahedron of anions, and the *A* cation in 12-fold cuboctahedral coordination. In short, in can be also described as a network of edge sharing octahedra *B*X6. The relative ion size requirements for stability of the cubic structure are quite stringent, so slight buckling and distortion can produce several lower-symmetry distorted versions, in which the coordination numbers of *A* cations, *B* cations or both are reduced. The orthorhombic and tetragonal phases are the most common non-cubic variants.

For the stoichiometric oxide perovskites, the sum of the oxidation states of *A* and *B* cations should be equal to six. The occupancy of *A* and *B* positions of different ions with appropriate ionic radii as well as for mixed occupancy of both cation positions leaded to the preparation of numerous compounds with wide spectrum of physical and chemical properties. Among the most famous representatives of perovskite class are the dielectric BaTiO3, high-temperature superconductor YBa2Cu3O7-x, materials exhibiting colossal magnetoresistance *R*1-x*A*xMnO3, where R = La3+, Pr3+ or other rare earth ion, A = Ca2+, Sr2+, Ba2+, multiferroic materials, etc.

The structure of an ideal cubic perovskite is shown in Figure 4, where the *A* cations are shown at the corners of the cube, and the *B* cation in the centre with oxygen ions in the facecentred positions. The space group for cubic perovskites is Pm3m (221); the equivalent positions of the atoms are detailed in Table 1.

Structural Characterization of New Perovskites 113

Fig. 5. Perovskite structure (*AB*X3). The small spheres are X atoms (usually oxygens), the medium spheres are *B*-atoms (a smaller metal cation, such as Ti(IV)), and the big spheres are

The synthetic methodology and the characterization of the perovskite often go hand in hand in the sense that not one but a series of reaction mixtures are prepared and subjected to heat treatment. The stoichiometry is typically varied in a systematic way to find which ones will lead to new solid compounds or to solid solutions between known ones. A prime method to characterize the reaction products is X-ray powder diffraction (XRD), because many solid state reactions will produce polycristalline powders. Thus, powder diffraction will facilitate the identification of known phases in the mixture. If a pattern is found that is not known in the diffraction data libraries an attempt can be made to index the pattern, i.e. to identify the symmetry and the size of the unit cell. Obviously, if the product is not crystalline enough

Once the unit cell of a new phase is known, the next step is to establish the stoichiometry of the phase. This can be done in a number of ways. Sometimes the composition of the original mixture will give a clue, if one finds only one product -a single powder pattern- or if one was trying to make a phase of a certain composition by analogy to known materials but this is rare. Often considerable effort in refining the synthetic methodology is required to obtain a pure sample of the new material. If it is possible to separate the product from the rest of the reaction mixture elemental analysis can be used. Another ways involves SEM and the

The easiest way to solve the structure is by using single crystal X-ray diffraction. The latter often requires revisiting and refining the preparative procedures and that is linked to the question which phases are stable at what composition and what stoichiometry. In other words what does the phase diagram looks like. An important tool in establishing this is thermal analysis techniques like DSC or DTA and, increasingly also, the synchrotron temperaturedependent powder diffraction. Increased knowledge of the phase relations often leads to further refinement in synthetic procedures in an iterative way. New phases are thus characterized by their melting points and their stoichiometric domains. The latter is important for the many solids that are non-stoichiometric compounds. The cell parameters obtained from

XRD are particularly helpful to characterize the homogeneity ranges of the latter.

the *A*-atoms (a larger metal cation, such as Ca(II)).

**4. Structural characterization of perovskites** 

the characterization is typically much more difficult.

generation of characteristic X-rays in the electron beam.

Fig. 4. Structure of the ideal cubic perovskite, *AB*O3 (*A*: big stripped circle, *B*: small grey circles, O: empty circles).

Perovskite materials exhibit many interesting and intriguing properties from both the theoretical and the application point of view. Colossal magnetoresistance, ferroelectricity, superconductivity, charge ordering, spin dependent transport, high thermopower and the interplay of structural, magnetic and transport properties are commonly observed features in this family. These compounds are used as sensors and catalyst electrodes in certain types of fuel cells and are candidates for memory devices and spintronics applications.


Table 1. Atomic positions in cubic perovskites

Many superconducting ceramic materials (the high temperature superconductors) have perovskite-like structures, often with 3 or more metals including copper, and some oxygen positions left vacant. One prime example is yttrium barium copper oxide which can be insulating or superconducting depending on the oxygen content.

Chemical engineers are considering this material as a replacement for platinum in catalytic converters in diesel vehicles.

Figures 4 and 5 show different ways to represent the perovskite structure. Figure 5 represents the undistorted cubic structure; the symmetry is lowered to orthorhombic, tetragonal or trigonal in many perovskites.

Fig. 4. Structure of the ideal cubic perovskite, *AB*O3 (*A*: big stripped circle, *B*: small grey

of fuel cells and are candidates for memory devices and spintronics applications.

Site Location Coordinates *A* cation 2*a* (0,0,0) *B* cation 2*a* (1/2,1/2,1/2)

O anion 6*b* (1/2,1/2,0)(1/2,0,1/2)(0,1/2,1/2)

Many superconducting ceramic materials (the high temperature superconductors) have perovskite-like structures, often with 3 or more metals including copper, and some oxygen positions left vacant. One prime example is yttrium barium copper oxide which can be

Chemical engineers are considering this material as a replacement for platinum in catalytic

Figures 4 and 5 show different ways to represent the perovskite structure. Figure 5 represents the undistorted cubic structure; the symmetry is lowered to orthorhombic,

Perovskite materials exhibit many interesting and intriguing properties from both the theoretical and the application point of view. Colossal magnetoresistance, ferroelectricity, superconductivity, charge ordering, spin dependent transport, high thermopower and the interplay of structural, magnetic and transport properties are commonly observed features in this family. These compounds are used as sensors and catalyst electrodes in certain types

circles, O: empty circles).

Table 1. Atomic positions in cubic perovskites

tetragonal or trigonal in many perovskites.

converters in diesel vehicles.

insulating or superconducting depending on the oxygen content.

Fig. 5. Perovskite structure (*AB*X3). The small spheres are X atoms (usually oxygens), the medium spheres are *B*-atoms (a smaller metal cation, such as Ti(IV)), and the big spheres are the *A*-atoms (a larger metal cation, such as Ca(II)).
