Ordering of A-cations in lead-free oxides with a filled tetragonal tungsten bronze structure

By X-ray powder diffraction, the structure of polycrystalline lead-free oxides K4Bi2Nbl0O30 and Na2Sr4Nb10O30 was refined with an emphasis on ordering in the A-sites of the tetragonal tungsten bronze crystal lattice. It was shown that the studied oxides have different types of cationic ordering, which does not change at ferroelectric phase transition. The results are consistent with the piezoresponse force microscopy data.


Introduction
The oxygen framework of the tetragonal tungsten bronze (TTB) structure forms three different coriented channel positions (A1, A2 and C). The resulting favorable structural modifications are one of the reasons why TTB-based materials are considered the second most important structural class of ferroelectrics after perovskites. The coexistence of different cations favorable in both the octahedral framework as well as in the A sites and the amount of disorder are closely related to the complex dielectric properties of these materials [1][2][3][4][5]. The classification of TTB oxides [3,6,7] by completeness of filling the A positions in square and pentagonal channels include stuffed, filled, and unfilled groups. As shown in [8,9], the presence of a structural vacancy in the unfilled phases is accompanied by the formation of polar nanoregions (PNR) responsible for relaxor properties. However, the formation of such inhomogeneities in TTB oxides is not necessarily associated with the presence of structural vacancies, and can occur not only in an unfilled structure. Such regions can also be found in the filled compounds as a consequence of the specific distribution of A-cations [10].

Filled TTB structure
To reveal the general regularities of the A-cation distribution in the TTB structure, the most suitable are widespread filled oxides represented by the structural formula , where M1 and M2 are two different cations. The cation distribution in these oxides can be described by the parameter s -the probability for the M1 cation to occupy an A1 position in the square channel; 0 ≤ s ≤ 1. Accordingly, the structural formula of The structural ordering in filled TTB oxides is supposed to be the key for understanding the variety of TTB-type materials properties.
When refining the complex TTB structure, it is desirable to reduce the number of refined parameters; therefore, it is advisable to choose a cation distribution model (i.e., parameter s) by an independent method. As usual, a larger ion is placed in a pentagonal channel A2, but sometimes an acceptable convergence is achieved under other assumptions. In [11], it was experimentally shown that for tetragonal lead-free tantalates and niobates  [3,16]. Structure refinement of the nanostructured powder was conducted in [16] with the assumption of a partially ordered structure with A1 sites filled by strontium, and A2 sites statistically occupied by equal amounts of sodium and strontium:

Experiment
The dense ceramic Powder X-ray diffraction (PXRD) data for the sample phase qualification and precise PXRD data for profile analysis were collected using the DRON-7 diffractometer, filtered Co-Kα radiation, standard attachment. Temperature studies were carried out with the Anton Paar 1000 temperature attachment with the temperature stability not worse than ± 0.5 degrees. Rietveld data were collected within the broad angular interval with the scanning step 0.04 deg and counting time 10 sec. Data processing was performed with the PowderCell package [18]. Surface piezoelectric response data

O Nb
Sr Na samples at room temperature and in the paraelectric state show the formation of pure TTB structure without any impurity phases (Figures 1,2). Both samples show tetragonal cell symmetry because additional splitting testifying lowering the lattice symmetry, was not found as at 18 C o and at paraelectric state. The possible ordering of A-cations in the TTB structure, unlike in perovskites, is difficult to reveal by the appearance of additional superstructural reflections.
is an exception, because due to the large difference in the A-cation scattering factors, the ordered type of their distribution is accompanied by the appearance of reflections (110), (200), (210), and (220), which are intense enough to be detected by PXRD.
The main goal of this study is determination of the cation distribution over positions in the square and pentagonal channels. For the TTB structure, it is preferable to start solving the problem from the paraelectric phase with a minimal set of variable atomic parameters. For the purpose, XRD powder data for Nb ions occupy positions in two nonequivalent octahedra B1(2c) and B2(8j).
In the 5 4h D space group, there are three atomic parameters of cations: the x coordinate of the A atom in the 4g position and the x, y coordinates of niobium in the B2 (8j) position. These parameters with the corresponding occupation factors were refined. Also at Rietveld refinement lattice constants, background polynom, and the overall temperature factor B were minimized. The reflections were fit to a pseudo-Voigt function for the peak shape with the refining coefficients U, V, W of the Caglioti function:   . The atomic and profile data obtained for the paraelectric phase were used as an initial approximation.   Table 2 and Figure 2. The use of a partially ordered (s = 0) model according to [16] resulted in a significant decrease in data convergence.

Summary
Based on the XRD powder diffraction data, the type of structural ordering for oxides and was established. The results are consistent with the [11] criterion for the distribution of A-cations in the filled TTB structure. For the ferroelectric phase of both compounds, the tetragonal cell symmetry is proved. At cooling from paraelectric to ferroelectric state, the ordering degree s does not change, consequently this phase transition is not accompanied by the order-disorder phenomena.
The A cations in 30 10 2 4 O Nb Bi K are distributed over the A1, A2 positions in tetragonal and pentagonal channels according to the ordered configuration with the occupancy parameter s= 0.899 (0.90). According to the refinement results, the sample has two types of defects: 1.
The refined composition In violation of the main ordered motive, some cations appear not in "their own" positions. The refined structural formula indicates that the vacancies associated with the deviation of stoichiometry are concentrated in the square channels A1.
A similar deviation from the ordered structure was described earlier for The assumption, the structural order can influence the various charged subsystems formation is confirmed by the piezoresponse force microscopy (PFM) data.
The possibility of such electrically active nano/meso-sized regions for