Physicochemical properties of ordered perovskite-type oxides

Chapter 4. Physicochemical properties of perovskite-type oxides 50 the connection of B’X6 octahedra (or A’X12 polyhedra) in one dimension. Consequently, layered-type of arrangement allows B’X6 octahedra (or A’X12 polyhedra) to connect in two dimensions, and therefore it is called 2D (layered) ordering. Another notation specifies the plane containing the same atoms and determines the arrangement of the type of rock salt, columnar and layered, as respectively: (111), (110) and (001) [145].

Assuming that one of the cations (A’ or B’) is of higher charge than the other one present in the same sublattice (A or B), electrostatic interactions can be considered important for determination of the type of the adopted structure. From this point of view, the most preferred ordering is the rock salt one, since it provides a maximum distance between the cations having higher oxidation state. Column ordering, in which each B’ (A’) cation has four B (A) and two B’ (A’) neighbors is the next preferred arrangement.

Layered-type ordering is the least likely to be adopted, as in this arrangement B’ (A’) cation is surrounded by four B’ (A’) and two B (A) neighbors [145]. It should be noted that in general, ordering in the A-sublattice occurs less common than in the B-sublattice, and number of materials adopting this structure is rather small [118], however, the considered in this work BaLnMn2O5+δ oxides are A-site cation-ordered.

Explanation why B-site ordering is more common can be related to a fact that difference of the oxidation state of cations occupying B-sublattice can be as high as 6, as for Sr2LiReO6, while for A-sublattice normally it does not exceed two. However, this does not explain why A-site ordered materials typically exhibit layered ordering, while for the B-site ordered materials the preferred arrangement is of a rock salt-type. The explanation of this phenomenon can be given as follows [118]: for a single ABX3 perovskite each anion is surrounded by two cations from B-sublattice and four from A-sublattice, and when B cations adopt rock salt-type structure, all positions of anions remain chemically and crystallographically equivalent with each anion surrounded by one B cation and one B’

cation. This enables the anion to move in a direction of the smaller of B-site cations, in order to optimize length of the chemical bond. In contrast, layered-type of arrangement in B-sublattice creates three chemically different crystallographic environments. One sixth of anions is in the layers, where they are surrounded by two B’ cations having higher oxidation state, the next one sixth of the anions is surrounded by cations of a lower oxidation state. The remaining two thirds of the anions are coordinated with one B’

and one B cation, as in the rock salt-type system. This arrangement, however, does not comply with the fifth Pauling principle, that, if possible, the same ions should have the same chemical environment [118].

Similar reasoning can be carried out for perovskites characterized by A-sublattice arrangement: rock salt-type system provides the same environment for all anions, but the occupied positions are centrally-symmetric, and therefore, there is no possibility to change the bond lengths of A’-X or A-X, to accommodate the difference in size of A’ and A cations. The columnar arrangement creates three different environments for anions, as it was in the case for the B-sublattice. The most common type of ordering for AA’B2X6

materials is the layered arrangement, as the structure is stabilized by difference in ionic radii of A and A’ cations (see chapter 4.4.1), as well as by characteristic for these materials anionic (e.g. oxygen) vacancies [118].

In addition, AA’BB’X6 materials exhibit a mixed arrangement: layered for the A-sublattice and rock salt in the B-sublattice. Also, there are known examples of materials in which ratio of A and A’ (B and B ') cations differ from perfect 1:1, and yet they exhibit the ordered structure [118, 145].

It is worth mentioning that simple ABO3-type perovskites having different cations present at the A- or/and B-site (e.g. A0.5A’0.5BO3, AB0.5B’0.5O3) can be treated as completely disordered end members of a series starting from perfectly ordered double perovskites and comprising also materials exhibiting partial order. This fact is worth emphasizing, since in some of the cases, depending on the preparation method, it is possible to obtain compound with the same chemical composition, which does or does not possess cation ordering. Quantitative analysis of the ordering effect was established by Sleight [146] in a form of degree of order parameter , where is defined as a fraction of cations present at their proper site (e.g. A’ cations at the A’-site). In two extreme cases, when than and the structure is fully ordered, while for the order parameter , and the structure is fully disordered.

From crystallographic point of view presence of cation ordering in AA’B2X6 or A2BB’X6 always results in a decrease of the crystal symmetry, in relation to the aristotype Pm-3m one of ideal perovskite. Furthermore, reduction of the symmetry is also observed as resulting from presence of various structural distortions.

4.4.1. Perovskites with layered-type of A-site ordering

Apart from the mentioned in previous chapter layered AA’B2X6 materials having 1:1 ratio of A and A’ cations, also compounds with 1:2 (AA’2B3O9, with famous, oxygen

Chapter 4. Physicochemical properties of perovskite-type oxides 52 known. The 1:1 layered ordering of A-site cations creates three different environments for the anions. As can be seen in Fig. 4.9, the first position X(1) is coordinated by four A (bigger) cations and two B-site cations. On the other hand, X(3) position is surrounded by four A’ (smaller) cations and two B cations, while X(2) site is coordinated by two A and two A’ cations arranged in cis configuration, as well as by two B-site cations. Since the X anions are in different chemical surrounding, in order to relieve the bonding instability, additional effects may appear, with three possibilities mentioned in work [145]:

 presence of simultaneous, layered ordering of anion vacancies in the A’-related layer (X(3) position), as in AA’B2X6-δ perovskites,

 presence of layered ordering of A-site cation vacancies coupled with second order JTE distortions of cations on the B-site,

 presence of rock salt-type ordering of B and B’ cations coupled with second order JTE distortions, as in AA’BB’X6 perovskites.

Fig. 4.9. P4/mmm aristotype unit cell for AA’B2X6 perovskites with A-site layered ordering of cations. Three different X positions can be distinguished in the structure. Radii of ions not to scale.

The structure presented in Fig. 4.9 is the aristotype one for AA’B2X6 compounds having A-site layered arrangement of cation, which corresponds to tetragonal P4/mmm (no.

123) space group. The unit cell in this case is created by a doubling of the simple perovskite lattice constant along c-axis, which is often noted as , or simply as [147]. It can be also stated that since the ionic radius ratio of the oxidant (usually oxygen) and metal present in 12-fold coordination in ABX3 perovskites is

approximately equal to one, similarly, the average ionic radius of A and A’ cations in AA’B2X6 should be equal to the radius of the oxidant as well, despite that one of the cations is substantially larger than the other one.

With advanced mathematical and crystallographic considerations it was possible to derive relationship between possible lower symmetry structures, which are related to the basic P4/mmm one [147]. Graph showing this relationship is given in Fig. 4.10.

Fig. 4.10. Schematic drawing showing different possible structures generated by the layered-type of ordering of cations in the A-site, which is followed by the corner-linked tilting of BX6 octahedra in AA’B2X6

perovskites. The approximate cell dimensions in comparison to the cell edge of the Pm-3m aristotype structure are included in the diagram. Glazer notation is used for the octahedra tilts. Solid lines indicate second order phase transition, while dashed lines specify first order phase transformations. Figure based on [147].

4.4.2. Perovskites with rock salt-type of B-site ordering

Double perovskites exhibiting ordering of cations in the B-sublattice constitute a majority of the ordered perovskites group in general. While the most common ratio of the B and B’ cations is 1:1 (A2BB’O6) or 1:2 (A3BB2’O9), there are also known examples of materials having 1:3 ratio (A4BB3’O12) [118].

Ordering mechanism of cations in such double perovskites is complex and depends on many factors. Degree of order depends not only on the size, charge and polarization

Chapter 4. Physicochemical properties of perovskite-type oxides 54 temperature and time, ). An example for such behavior is found for Ba2NiMoO6

compound with rock salt-type ordering of Ni²⁺ and Mo⁶⁺ cations. It is generally accepted that the higher temperature of the synthesis (but within certain limit), the higher the degree of order. Materials in which both B-site cations are transition metals exhibit usually no or a low degree of order, however this is not a general rule. When one of the B cations is a non-transition element, tendency for ordering was found to increase significantly [118].

Interestingly, charge-ordering effects of the same type of cation present in the B-sublattice can also give rise to formation of the rock salt ordering, as documented in Mn-containing BaLnMn2O5+δ (see chapter 4.7).

Double perovskites with the perfect rock salt-type arrangement in B-sublattice possess regular structure with Fm-3m (no. 225) space group, the same as for the materials characterized by the rock salt order of cations in the A-sublattice [148]. This structure can be considered as created by doubling of the simple perovskite unit cell in all directions, and can be written as [114, 143, 144]. Similarly as for the A-site layered AA’B2X6, using mathematical and crystallographic reasoning it was possible to derive relationship between the aristotype Fm-3m structure and possible distorted structures. For the respective diagram see work [148].

4.4.3. Anion vacancy-ordered brownmillerite-type structure

Apart from ordering effects of cations in perovskite-type oxides, presence of a specific arrangement in the X-sublattice is also possible in nonstoichiometric perovskite-type materials. It can be considered that brownmillerite-perovskite-type structure (Fig. 4.11) represents such type of ordering.

Originally the designation “brownmillerite” concerned a mineral with Ca2(Al,Fe)2O5

formula. Its discovery is somewhat unclear, but the mineral was named after Lorrin Thomas Brownmiller. Soon, other oxides were determined to exhibit similar crystal structure [118]. General chemical formula of these materials can be written as A2B2O5

(ABO2.5), and the basic structure possesses orthorhombic unit cell. Corner-sharing BO6

octahedra and BO4 (B’O4) tetrahedra create alternating layers along c-axis (Fig. 4.11).

Presence of ordering of the oxygen vacancies along [101] direction is a characteristic feature of this structure. In general, vacancy ordering in such type of compounds causes lowering of the oxygen conductivity despite comparably large number of oxygen-vacant sites [149-151]. Good example of this effect was documented for Ba2In2O5, for which in

the oxygen vacancy-ordered phase (below ∼ 900 °C) ionic conductivity is much lower, comparing to the high temperature disordered phase [152]. Brownmillerite-type phase has been also observed in properly reduced La1-xSrxFe1-yCoyO3-δ and La-rich La1-xSrxMnO3-δ

materials [151].

Fig. 4.11. Orthorhombic structure of brownmillerite with oxygen vacancy ordering present along [101]

direction. Radii of ions not to scale.