**Use of Synchrotron X-rays to Investigate the Structure of Commensurate
and Incommensurate Composite Oxides Related to the 2H-Perovskite
Structure Type**

H.-C. zur Loye (University of South Carolina.)

Abstract Number: **zur Loye 93**

Beamline: **X7A**

**Introduction**: Our group is one of several that have been
investigating the structures and magnetic properties of a family of low
dimensional oxides, A~3n+3m~A'~n~B~3m+n~O~9m+6n~, (*n*, *m* = integers,
A = alkaline earth; A', B = large assortment of metals including alkali,
alkaline earth, transition, main group and rare earth metals) that are
structurally closely related to the 2H hexagonal perovskites. These
complex oxides contain one-dimensional chains of transition metals
consisting of octahedra and trigonal prisms sharing faces along the
c-axis of the hexagonal unit cell. As a group, these oxides can be
considered to represent a structural continuum between the 2H-perovskite
structure of BaNiO~3~ and the K~4~CdCl~6~ structure of Sr~4~PtO~6~. An
early general structural classification of these materials based on the
filling of interstitial sites generated by the stacking of
*m*\[A~3~O~9~\] and *n*\[A~3~A'O~6~\] layers was developed by Darriet
and Subramanian. This approach easily describes the structural
composition of all the commensurate members of this family of structures
and can be extended to encompass members that form incommensurately
modulated (aperiodic)
structures.![](media/image1.wmf){width="2.795138888888889in"
height="1.9833333333333334in"}

A general formula for this oxide family has been expressed in an
equivalent way as A~1+x~(A'~x~B~1-x~)O~3~ where the composition variable
*x* can be envisaged to be any number between 0 and 1/2. The ratio of
prisms and octahedra in the chains is determined by the ratio of A'
atoms to B atoms, that is N~prisms~/N~octahedra~ = *x*/(1-*x*). A more
unifying picture is obtained if the compounds are described as modulated
composite structures. For this family of oxides, the structure is best
depicted as two mutually interacting subsystems, modulated along z but
periodic on the xy plane. The first subsystem (1) consists of \[(A',
B)O~3~\] chains with an average c-lattice parameter **c~1~** close to
**c~perov~**/2 (**c~perov~** = c-parameter of 2H hexagonal perovskite).
The other subsystem (2) is formed by the A cations and has a c-lattice
parameter **c~2~** = **c~perov~**. Therefore, the composition of a given
compound is directly related to the γ-value = **c~2~\* / c~1~\*** =
**c~1~ /c~2~** by the relation : γ = (1+*x*)/2. The γ parameter can vary
between 0.5 and 0.75 corresponding to the range of values of *x* (0 ≤
*x* ≤ 0.5).

**Methods and Materials**: Powder samples are synthesized by high
temperature solid state reactions while crystals are grown out of molten
carbonate fluxes. Several of these phases can be oxidized under
controlled temperature and oxygen partial pressure conditions, which is
thought to result in the interconversion of one trigonal prism into two
octahedra (oxidation) one of which would be vacant. Since the careful
measurements of the c~1~ and c~2~ lattice parameter can be used to
determine the composition (A~1+x~(A'~x~B~1-x~)O~3~) and subsequently the
polyhedral sequence of this family of materials, a controlled study
using synchrotron radiation to follow changes in the lattice parameters
and the concomitant structural evolution as a function of oxidation
state is of interest.

**Results**: Several compositions have been prepared and their
structures analyzed based on the value of the c~1~ and c~2~ lattice
parameters. A typical powder pattern collected at beam line X7A is shown
in Figure 1.

**Conclusions**: As very precise lattice parameters must be obtained to
carry out the structural analysis, high resolution synchrotron X-ray
diffraction is the method of choice.

**Acknowledgments**: H-C. zur Loye acknowledges funding from the
National Science Foundation. Research carried out in part at the
National Synchrotron Light Source, Brookhaven National Laboratory, which
is supported by the US Department of Energy, Division of Materials
Science and Division of Chemical Sciences.

**References**: M. Zakhour-Nakhl, J. B. Claridge, J. Darriet, F. Weill,
H.-C. zur Loye, J.-M. Perez-Mato, "Structure Determination of Two New
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