# Fileset

[BiFeO3-KWO3-comment-281223.doc](https://mdr.nims.go.jp/filesets/d63c009c-4892-4f3e-8a3b-83cec1b3b872/download)

## Creator

[Alexei A. Belik](https://orcid.org/0000-0001-9031-2355)

## Rights

[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

## Other metadata

[Comments on the paper “Fabrication and physical characteristics of K/W double doped BiFeO3 complex electro-ceramic; (Bi1/2K1/2)(Fe1/2W1/2)O3”](https://mdr.nims.go.jp/datasets/fb71f2fd-3c93-4dcc-8d59-70e92756a5e1)

## Fulltext

Comments on the paper “Influence of Ba-doping on the structural and physical properties of Sr2−xBaxFeVO6 double perovskites”Comments on the paper “Fabrication and physical characteristics of K/W double doped BiFeO3 complex electro-ceramic; (Bi1/2K1/2)(Fe1/2W1/2)O3”Alexei A. BelikResearch Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, JapanE-mail: Alexei.Belik@nims.go.jpAbstractBy this commentary, we showed that the claimed compound, (Bi1/2K1/2)(Fe1/2W1/2)O3, does not exist. We tried to reproduce the claimed compound through the synthesis in air at 1053 K (similar to the synthesis conditions used in the commented paper), and we obtained a mixture of Bi2WO6, K2WO4, and Fe2O3. The X-ray diffraction pattern of this mixture partly matches with the X-ray diffraction pattern of the claimed compound reported in the commented paper.Keywords: perovskites; doped BiFeO3; X-ray diffraction; phase analysis; lattice parameters; errorsThe BiFeO3 perovskite formed the basis for thousands of publications and continues to attract a lot of attention. S. S. Hota et al. have recently reported a conventional solid-state synthesis of a new doped BiFeO3-related compound with the chemical composition of (Bi1/2K1/2)(Fe1/2W1/2)O3 (BKFWO) [1]. BKFWO was prepared from a stoichiometric mixture of K2CO3 + 2FeCO3 + 2WO3 + Bi2O3 by annealing in air at 1053 K for 5 h and at 1023 K for an unspecified period of time. The authors of Ref. [1] could index an X-ray powder diffraction pattern of BKFWO with lattice parameters of a = 12.1967 Å, b = 9.0468 Å, c = 6.4971 Å, and ( = 105.02( and, therefore, concluded that a new single-phase compound was prepared. The P21 space group was assigned to BKFWO based on measured properties.However, a number of serious problems can be immediately identified based on the reported results in Ref. [1]. 1) All perovskite-type oxides have characteristic X-ray diffraction patterns with the strongest reflection near 31–34( (for the CuK( radiation) originating from a cubic subcell with the lattice parameter of about 3.8–4.0 Å, while BKFWO has the strongest reflection at 28.3(. 2) The reported lattice parameters of BKFWO do not match with any perovskite-related supercells. 3) The formation of Fe2+, needed for the charge balance in BKFWO, is highly unlikely near 1000 K in air. 4) The Rietveld analysis was claimed to be performed (Figure 1b in Ref. [1]), but a structural model, used for the fitting, was not described, and obtained structural, refined parameters were not reported. 5) The X-ray diffraction pattern of BKFWO was reported between 20( and 80(, while important reflections could be missed below 20( – this is very critical for correct (and successful) indexing, in general; and for the phase analysis. 6) A Bi2WO6 phase (Powder Diffraction File (PDF) 39-0256 and 73-2020) can be easily found as the main phase in BKFWO using the International Center for Diffraction Data database and the strongest reflection at 28.3( (d = 3.15 Å).No other phases, except Bi2WO6, could be clearly found on the reported X-ray diffraction pattern of BKFWO in Ref. [1], while phases containing K and Fe (and remaining W) should be present (Figure 1a of this commentary). Therefore, we tried to synthesize BKFWO. We annealed a mixture of Fe2O3, Bi2O3, K2CO3, and 2WO3 in three steps at 1023 K for 4 h + 1053 K for 13 h + 1053 K for 20 h as powder in an Al2O3 crucible (mixing was performed in an agate mortar under acetone). The sample was reground after each annealing step marked by “+”, and X-ray powder diffraction data were measured. The starting chemicals were dried before their use. X-ray powder diffraction data were collected at room temperature with a RIGAKU MiniFlex600 diffractometer [CuK( radiation; a 2( range of 5(80(; a step of 0.02(, and a scan speed of 3 (/min]. Phase identification was performed with the International Center for Diffraction Data database. After the first annealing step, the sample was a mixture of Bi2WO6 (PDF 39-0256 and 73-2020), K2WO4 (PDF 24-0905), Fe2O3 (PDF 33-0664), and a small amount of K5Bi(WO4)4 (PDF 52-1697). With further annealing, the amount of the K5Bi(WO4)4 phase gradually decreased, but the final sample still had traces of K5Bi(WO4)4. Therefore, the following reaction takes place (after reaching the phase equilibrium at 1053 K; and ignoring traces of the K5Bi(WO4)4 phase): Bi2O3 + Fe2O3 + K2CO3 + 2WO3 = Bi2WO6 + K2WO4 + Fe2O3 (+CO2 ()To support our phase identification, we performed the Rietveld fittings of our X-ray diffraction data (see Figure 1b of this commentary) using reported structural parameters for the phases found (Inorganic Crystal Structure Database (ICSD) codes 23584 for Bi2WO6, 26181 for K2WO4, and 15840 for Fe2O3). The Rietveld analysis was performed using the RIETAN-2000 program [2]. Contributions of K2WO4 and Fe2O3 to the total X-ray diffraction pattern are also highlighted in Figure 1b of this commentary. From this presentation, it is clear that the sample in Ref. [1] also contained the Fe2O3 phase in approximately the same amount as our sample. On the other hand, K-containing phases were missed on the reported X-ray diffraction pattern of Ref. [1]. We note that the X-ray diffraction pattern in Ref. [1] contained some additional reflections (for example, a reflection at about 21.1(, indexed as (120)), which were not observed on our X-ray diffraction pattern. However, we failed to identify those additional reflections.Therefore, a mixture of (partly) known phases – not related to perovskites and BiFeO3 – was investigated in Ref. [1] instead of a new claimed perovskite-related compound, (Bi1/2K1/2)(Fe1/2W1/2)O3. The absence of any standard phase-analysis procedures appears to be the main reason for erroneous claims of Ref. [1].References[1] S. S. Hota, D. Panda, R. N. P. Choudhary, Fabrication and physical characteristics of K/W double doped BiFeO3 complex electro-ceramic; (Bi1/2K1/2)(Fe1/2W1/2)O3. J. Alloys Compd. 976 (2024) 172900. doi: 10.1016/j.jallcom.2023.172900[2] F. Izumi, T. Ikeda, A Rietveld-analysis program RIETAN-98 and its applications to zeolites. Mater. Sci. Forum 321–324 (2000) 198–205.   -20-100102030401020304050607080Fe2O3 K2WO4 diff. 2q  (deg): Cu Ka Intensity (counts/103) (a) (b) Bragg peaks: Bi2WO6 K2WO4 Fe2O3 * * Figure 1. (a) An experimental X-ray powder diffraction pattern of a “(Bi1/2K1/2)(Fe1/2W1/2)O3” sample from Figure 1a of Ref. [1]. (b) Experimental (black crosses), calculated (red line), and difference (diff.: blue line) X-ray diffraction patterns of our sample after the final annealing at 1053 K for 20 h. The sample was a mixture of Bi2WO6 (the first row of brown tick marks for possible Bragg reflection positions), K2WO4 (the second row of black tick marks), and Fe2O3 (the third row of green tick marks). Contributions of K2WO4 and Fe2O3 phases to the total X-ray diffraction pattern are also highlighted at the bottom. Stars show two strongest reflections from the K5Bi(WO4)4 phase. No experimental reflections were observed between 5( and 10(. Figure 1a is reproduced with the permission from Elsevier.3