I. INTRODUCTION
Columbite compounds possess interesting properties and applications (Pullar, Reference Pullar2009). In the area of wireless communication technologies operating at microwave frequencies, there is an increasing demand for low-cost, high-performance dielectric ceramics. Columbite niobates such as ZnNb2O6 and MgNb2O6 have potential applications as microwave materials. Photo-induced splitting of water to produce H2 is very important because of environmental, energy, and sustainability concerns; the catalytic properties of columbites are demonstrated by the photoelectrochemical behavior of reduced NiNb2O6 single-crystal electrodes (Guochang et al., Reference Guochang, Peeraldo Bicelli and Razzini1991). Columbites also have unusual optical and magnetic properties (Senegas & Galy Reference Senegas and Galy1972). Two kinds of luminescence have been investigated so far, rare-earth activated emission (laser applications) and self-activated emission in pure columbites. Rare-earth-doped luminescence materials such as Er3+: CdNb2O6 are attractive because of their thermal and chemical stabilities (Erdem et al., Reference Erdem, Ghafouri, Ekmekçi, Mergen, Özen and Di Bartolo2014; Wong-Ng et al., Reference Wong-Ng, McMurdie, Paretzkin, Zhang, Davis, Hubbard, Dragoo and Stewart1987). CaNb2O6 is a self-activated blue phosphor. Columbite niobates such as FeNb2O6, NiNb2O6, and MnNb2O6 exhibit magnetic ordering at rather low temperature, with a Néel temperature (TN) of 3–6 K (Hanawa et al., Reference Hanawa, Shinkawa, Ishikawa, Miyatani, Saito and Kohn1994; Huang et al., Reference Huang, Zhou, Li, Huang, Xu, Li and Cui2014).
Another possible area of application for columbites is energy conversion, for example, waste heat conversion into electricity. Despite significant efforts in searching for efficient oxides (stability at high temperatures) for energy conversion, the material performance at high temperature still needs a significant improvement. As cobaltate compounds, including NaCoO x (Terasaki, et al., Reference Terasaki, Sasago and Uchinokura1997; Ca2Co3O6 (Mikami et al., Reference Mikami, Funahashi, Yoshimura, Mori and Sasaki2003; Mikami & Funahashi, Reference Mikami and Funahashi2005), Ca3Co4O9 (Masset et al., Reference Masset, Michel, Maignan, Hervieu, Toulemonde, Studer, Raveau and Hejtmanck2000; Minami, et al., Reference Minami, Itaka, Kawaji, Wang, Koinuma and Lippmaa2002; Grebille et al., Reference Grebille, Lambert, Bouree and Petricek2004; Wong-Ng et al., Reference Wong-Ng, Hu, Vaudin, He, Otani, Lowhorn and Li2007), and Bi2Sr2Co2O x (Wang et al., Reference Wang, Venimadhav, Guo, Chen, Li, Soukiassian, Schlom, Katz, Pan, Wong-Ng, Vaudin and Xi2009) have shown promising thermoelectric properties, the search for low-dimensional compounds such as two-dimensional (2D)-layered and 1D-chain cobaltate compounds with improved thermoelectric properties continues in our laboratory, including phase diagram studies of ternary oxide systems, including CaO and Co3O4 as two of the end members (Wong-Ng et al., Reference Wong-Ng, Liu, Martin, Thomas, Lowhorn and Kaduk2010, Reference Wong-Ng, Luo, Xie, Tang, Kaduk, Huang, Yan, Tang and Tritt2011, Reference Wong-Ng, Laws and Yan2013, Reference Wong-Ng, Laws, Talley, Huang, Yan, Martin and Kaduk2014).
A detailed study of the effects of non-stoichiometry of (Zn, Co)Nb2O6 and (Ni, Zn)Nb2O6 have been reported by Belous et al. (Reference Belous, Ovchar, Jancar and Bezjak2007) and Butee et al. (Reference Butee, Kulkarni, Prakash, Aiyar, George and Sebatian2009), respectively. In this paper, we are interested in the solid solution formation of the (Co,Zn,Ni)Nb4O12 columbite compounds. The first goal of this paper is to investigate the structure of the Co(Ni x Zn1−x )Nb4O12 (x = 0.2, 0.4, 0.6, 0.8) series, in particular, the effect of the larger size of Zn2+ (Shannon, Reference Shannon1976) on substituting on the Ni site. In these compositions, the concentration of Co2+ was kept constant. Since X-ray diffraction (XRD) is a non-destructive technique for phase identification, XRD patterns are especially important for phase characterization; therefore the second goal of this paper is to determine the experimental patterns for Co(Ni x Zn1−x )Nb4O12 (x = 0.2, 0.4, 0.6, 0.8), and to make them available through submission to the Powder Diffraction File (PDF) (2016).
II. EXPERIMENTAL
A. Sample preparation
Samples were prepared from stoichiometric amounts of NiO, ZnO, Co3O4, and Nb2O5 using solid-state high-temperature techniques. The starting samples were mixed, pelletized, and heat treated in air at 800 °C for 12 h and subsequently annealed at 1100 °C for 12 h, 1300 °C for 24 h with intermediate grindings. During each heat treatment in air, the samples were furnace cooled. The heat treatment process was repeated until no further changes were detected in the powder XRD patterns.
B. X-ray Rietveld refinements and powder reference patterns
The Co(Ni x Zn1 −x )Nb4O12 series (x = 0.2, 0.4, 0.6, 0.8) powders were mounted as ethanol slurries on zero-background cells. The X-ray powder patterns were measured (5° to 130° 2θ, 0.0202144° steps, 0.5 s step−1, CuK α radiation) on a Bruker D2 Phaser diffractometer equipped with a LynxEye detector. (The purpose of identifying the equipment in this paper is to specify the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology.) To reduce effects of fluorescence, the lower limit of the multichannel analyzer was changed from its default value of 0.11–0.19 V.
The Rietveld refinement technique (Rietveld, Reference Rietveld1969) with the software suite GSAS (Larson and Von Dreele, Reference Larson and Von Dreele2004) was used to determine the structure of the Co(Ni x Zn1–x )Nb4O12 series (x = 0.2, 0.4, 0.6, 0.8) compounds. The structure of Nb2Fe0.4Co0.6O6 (Sarvezuk et al., Reference Sarvezuk, Kinast, Colin, Cusmäo, da Cunha and Isnard2011) was used as the starting model for the refinements. Reference patterns were obtained using a Rietveld pattern decomposition technique. Using this technique, the reported peak intensities were derived from the extracted integrated intensities, and positions calculated from the lattice parameters. The pseudo-Voigt function (profile function #4) was used for the refinement of both series of compounds (Thompson et al., Reference Thompson, Cox and Hastings1987; Finger et al., Reference Finger, Cox and Jephcoat1994; Stephens, Reference Stephens1999). When peaks are not resolved at the resolution function, the intensities are summed, and an intensity-weighted d-spacing is reported. In summary, these patterns represent ideal specimen patterns.
C. Bond-valence sum (BVS) calculations
The BVS values for Ni, Zn, Nb, and Co sites were calculated using the Brown–Altermatt empirical expression (Brown and Altermatt, Reference Brown and Altermatt1985; Brese and O'Keeffe, Reference Brese and O'Keeffe1991). The BVS of an atom i is defined as the sum of the bond valences v ij of all the bonds from atoms i to j. The most commonly adopted empirical expression for the bond valence v ij as a function of the interatomic distance d ij is v ij = exp[(R 0 –d ij )/B]. The parameter, B, is commonly taken to be a “universal” constant equal to 0.37 Å. The values for the reference distance R 0 for Ni2+–O, Zn2+–O, Co2+–O, and Nb5+–O, are 1.654, 1.704, 1.692, and 1.911, respectively (Brown and Altermatt, Reference Brown and Altermatt1985; Brese and O'Keeffe, Reference Brese and O'Keeffe1991). In the sites where there are more than two different types of atoms, the BVS value is the weighted sum of the fraction of occupancy.
III. RESULTS AND DISCUSSION
Table 1 gives the Rietveld refinement results for the Co(Ni x Zn1−x )Nb4O12 compounds. The pseudo-Voight function profile function #2 with 18 terms was used for the refinements (Howard, Reference Howard1982; Thompson et al., Reference Thompson, Cox and Hastings1987). Figure 1 provides the Rietveld refinement results for Co(Ni0.4Zn0.6)Nb4O12 as an example. The observed (crosses), calculated (solid line), and difference XRD patterns (bottom) are shown. The difference pattern is plotted at the same scale as the other patterns up to 70° 2θ. At higher angles, the scale has been magnified five times. An excellent agreement between observed and calculated profiles is demonstrated.
Figure 1. (Colour online) Rietveld pattern for Co(Ni0.4Zn0.6)Nb4O12. The observed (crosses), calculated (solid line), and difference XRD patterns (bottom) are shown. The difference pattern is plotted at the same scale as the other patterns up to 70° 2θ. At higher angles, the scale has been magnified five times.
Table I. Rietveld refinement residuals (R wp, R p, and χ 2) (Larson and Von Dreele, Reference Larson and Von Dreele2004) for Co(Ni x Zn1−x )Nb4O12.
Table 2 provides the crystallographic data for Co(Ni0.4Zn0.6)Nb4O12 (the number inside the bracket refers to standard deviation). The Co(Ni x Zn1−x )Nb4O12 series crystallizes in the orthorhombic space group Pbcn, and its structure is essentially of a disordered columbite-type structure (α-PbO2) (Filatov et al., Reference Filatov, Bendeliani, Albert, Kopf, Dyuzeva and Lityagina2005). The lattice parameters of the series range from a = 14.11190(13) to 14.1569(3) Å, b = 5.69965(6) to 5.71209(13) Å, c = 5.03332(5) to 5.03673 (11) Å, and V = 404.844 (8) to 407.30 (2) Å3 for x = 0.8 to 0.2. As the ionic radius of Zn2+ is smaller than that of Ni2+ (Shannon, Reference Shannon1976), a monotonic decreasing trend of unit-cell volume (as a function of x) is observed in Figure 2. The atomic coordinates and displacement parameters are listed in Table 3.
Figure 2. Plot of unit-cell volume, V, of Co(Ni x Zn1−x )Nb4O12 as a function of x. A monotonic trend of decrease of V as a function of x is observed. The line is a guide for the eyes.
Table II. Experimental cell parameters for Co(Ni x Zn1−x )Nb4O12 [Pbcn (No. 60), Z = 2]. Numbers inside the brackets represent standard deviation, which resides in the last digit.
Table III. Atomic coordinates and displacement factors for compounds for Co(Ni x Zn1−x )Nb4O12 [Pbcn (No. 60), Z = 2]; M represents site symmetry multiplicity; WS represents Wyckoff symbol. The coordinate values pertaining to the oxygen atoms were not refined during the refinement process. Numbers inside the brackets represent standard deviation, which resides in the last digit.
The structure of the Co(Ni0.4Zn0.6)Nb4O12 solid solution was confirmed to be of the distorted columbite type (Sturdivant, Reference Sturdivant1930; Bordet et al., Reference Bordet, McHale, Santoro and Roth1986; Pagola and Carbonio, Reference Pagola and Carbonio1997; Pullar, Reference Pullar2009). Figures 3–5 give the crystal structure with views projected along a-, b-, and c-axes, respectively. The structure, in general, contains double zig-zag chains of NbO6 octahedra and edge-sharing (Co,Ni,Zn)O6 octahedra running parallel to the bc-plane (Fig. 3). Within the same chain the NbO6 octahedra share edges via O5 and O7, while the adjacent NbO6 chains are joined to each other through common corners, O7 (Fig. 4). These double NbO6 chains are further linked together along the [100]-direction through another (Co,Ni,Zn)O6 units, via O5 and O6 in common corners (Fig. 5).
Figure 3. (Colour online) View of the crystal structure of Co(Ni x Zn1−x )Nb4O12 along the a-axis. The structure consists of zig-zag chains of NbO6 and (Ni,Zn,Co)O6 octahedral units along the c-axis.
Figure 4. (Colour online) View of the crystal structure of Co(Ni x Zn1−x )Nb4O12 along the b-axis. Double chains of NbO6 and single chain of (Ni,Zn,Co)O6 octahedra run parallel along the c-axis.
Figure 5. (Colour online) View of the crystal structure of Co(Ni x Zn1−x )Nb4O12 along the c-axis. Double chains of NbO6 and single chain of (Ni,Zn,Co)O6 octahedra run parallel to the bc-plane.
Both the NbO6 and (Co,Ni,Zn)O6 octahedra are distorted. The distorted Nb and (Co,Ni,Zn) sites are evidenced from the different octahedral Nb–O distances and the (Co,Ni,Zn)–O distances. For example, the Nb–O distances range from 1.977(6) to 2.140(5) Å, and the (Co,Ni,Zn)–O distances range from 1.960(5) to 2.008(5) Å, respectively. Similarly, the octahedral bond angles were also found to be deviated from the ideal 90°. The most distorted angles in these four members range from 67.2(4)° to 75.4(5)° in NbO6 and 76.5(3)° to 77.9(3)° in (Co,Ni,Zn)O6.
The BVS values in Co(Ni x Zn1 −x )Nb4O12 show a large compressive stress at the (Ni/Zn/Co) sites and a large tensile stress for the Nb sites for all four compositions. From x = 0.2 to 0.8, the “ideal BVS” values are 2.0 and 5.0 for the (Ni/Zn/Co) and Nb sites, respectively. The experimental BVS were found to range from 3.91 to 4.06 along the Nb-site from x = 0.2 to 0.8, indicating tensile stress, as these values are much smaller than “5” (or the cage at which Nb resides is too large). On the other hand, the BVS value for the mixed (Ni/Zn/Co) site ranges from 2.75 down to 2.66 as the x-value decreases, representing substantial compressive stress, even though the value decreases somewhat as the amount of the larger Ni2+ increases (Table 4).
Table IV. Bond distances and BVS values for Co(Ni x Zn1−x )Nb4O12 [Pbcn (No. 60), Z = 2]. In the sites where there are more than two different types of atoms, the BVS is the weighted sum of the fraction of occupancy. The ideal BVS values for (Ni1/Zn2/Co3) and Nb sites are 2.0 and 5.0 respectively for all four compositions. Numbers inside the brackets represent standard deviation, which resides in the last digit.
A. Reference XRD patterns
An example of reference patterns of, Co(Ni0.4Zn0.6)Nb4O12, is given in Table 5. In these patterns, the symbol “M” refers to peaks containing contributions of two reflections. The particular peak that has the strongest intensity in the entire pattern is assigned an intensity of 999 and other lines are scaled relative to this value. In general, the d-spacing values are calculated values from refined lattice parameters. The intensity values reported are integrated intensities (rather than peak heights) based on the corresponding profile parameters as reported in Table 5. For resolved overlapped peaks, intensity-weighted calculated d-spacing, along with the observed integrated intensity and the hkl indices of both peaks (for “M”) are used. For peaks that are not resolved at the instrumental resolution, the intensity-weighted average d-spacing and the summed integrated intensity value are used. In the case of a cluster, unconstrained profile fits often reveal the presence of multiple peaks, even when they are closer than the instrumental resolution. In this situation, both d-spacing and intensity values are reported independently. All patterns of Co(Ni x Zn1−x )Nb4O12 (x = 0.2, 0.4, 0.6, 0.8) have been submitted for inclusion in the PDF.
Table V. X-ray powder pattern for Co(Ni0.4Zn0.6)Nb4O12 [Pbcn (No. 60), Z = 2]. a = 14.1437 (3) Å, b = 5.70857(11) Å, c = 5.03537(11) Å, V = 406.56(2) Å3, and Z = 2. The symbols “M” refers to peaks containing contributions from two reflections. The particular peak that has the strongest intensity in the entire pattern is assigned an intensity of 999 and other lines are scaled relative to this value. The d-spacing values are calculated values from refined lattice parameters, and “I obs” represents integrated intensity values.
B. Summary
The end members (Co,Ni)Nb2O6 and (Co,Zn)Nb2O6 form a complete solid solution Co(Ni x Zn1 −x )Nb4O12. The X-ray patterns and structure of Co(Ni x Zn1−x )Nb4O12 (x = 0.2, 0.4, 0.6, 0.8) have been determined. Co(Ni x Zn1−x )Nb4O12 adopts the columbite structure, which consists of mixed distorted octahedral (Co, Ni, Zn)O6 and distorted NbO6 sites. The (Co, Ni, Zn)O6 sites are under compressive stress, whereas the NbO6 sites are under tensile stress. Powder X-ray patterns of these four solid solution members have been submitted for inclusion in the PDF.
ACKNOWLEDGEMENT
Partial financial support from ICDD is acknowledged.
SUPPLEMENTARY MATERIAL
The supplementary material for this article can be found at http://dx.doi.org/10.1017/S0885715616000531.
