The columbite structure ZnNb₂O₆ is a wide-bandgap semiconductor (3.84 eV)^[1] and a typical good performance microwave dielectric material.^[2,3] In the lattice, Zn²⁺ and Nb⁵⁺ are encircled by six oxygen atoms, thereby forming ZnO₆ and NbO₆ octahedrons. The ZnO₆ and NbO₆ octahedrons share oxygen atoms through edge, and an independent zigzag chain is formed along the c-axis. Along the a-axis direction, ZnO₆ and NbO₆ octahedrons are connected by corner-sharing oxygen atoms, forming ZnO₆–NbO₆–NbO₆–ZnO₆ arrangement. This arrangement generally appears as a layered structure of ZnNbNbZnNbNb.^[4,5] Significantly, this arrangement allows the columbite structure materials to exhibit excellent optical, magnetic, and electric properties.^[2,6–8]

Studies on diluted magnetic semiconductors have mainly focused on the doping of transition metals into an oxide semiconductor with a wide bandgap.^[9–11] For the oxide based dilute magnetic semiconductor, the doping of transition metal ions is an important method to control the magnetic performance and may significantly influence the electrical conductivity, dielectric properties, and other physical performances.^[12–14] The ZnNb₂O₆ doped with transition metal ions may present different performances and thus should be investigated. However, there are not very many studies concerning the magnetic performance of the ZnNb₂O₆ compound doped with cobalt ions.

Meanwhile, the single crystal samples exhibit integrated structures and explicit growth direction and are free of defects and impurities.^[15] They are the most appropriate samples for understanding the intrinsic characteristics of the materials. Thus far, no study on the single-crystal growth of cobalt-doped ZnNb₂O₆ has been reported besides our previous work.^[16] Therefore, the growth of Zn_0.8Co_0.2Nb₂O₆ single crystals must be of great value. Moreover, the intrinsic magnetism of the materials should be understood. In this work, Zn_0.8Co_0.2Nb₂O₆ single crystals were grown successfully via optical floating zone methods for the first time. The optical and magnetic properties were characterized through a series of experiments, aiming at the optical and magnetic applications.

The powder was prepared by the traditional solid-state reaction technique to mix stoichiometry amount ZnO, Nb₂O₅, and CoO from Alfa Aesar company with 4N purity in air at 1300 °C for 20 h with intermediate grindings. The nominal content Zn_0.8Co_0.2Nb₂O₆ single crystals were grown by the Crystal Systems Inc. infrared-heating floating-zone furnace (model: FZ-T-10000-H-VP-VI) in the air atmosphere.

The x-ray diffraction (XRD) spectra were acquired by an x-ray diffractometer from Rigaku (model: RU-200b) with a Cu Kα radiation source. A Rigaku Micro Max-007HF two-dimensional x-ray diffractometer (XRD²) was also employed to tested the growth directions of the crystals. The Raman spectra were recorded by a Jobin-Yvon LABRAM-HR 800 high resolution Raman spectroscope with 514.5 nm exciting laser from Spectrum Physics (model: Stable 2017). The Quantum Design physical property measurement system (model: PPMS-16) was used to obtain the magnetization M versus magnetic field H and magnetic susceptibility χ versus temperature T curves.

The grown Zn_0.8Co_0.2Nb₂O₆ single crystal pieces were ground into powder in a mortar. Then, the XRD spectra of the Zn_0.8Co_0.2Nb₂O₆ powder samples were obtained through XRD tests. The XRD spectrum of the powder sample was refined via Rietveld method by using MAUD software with Sig. = 1.41. Figure 1(a) shows the refined XRD spectrum. The Zn_0.8Co_0.2Nb₂O₆ has the columbite structure and belongs to the Pbcn space group. The lattice parameters of Zn_0.8Co_0.2Nb₂O₆ are a = 14.190 Å, b = 5.724 Å, c = 5.037 Å, which are slightly smaller than those of the pure ZnNb₂O₆ samples (a = 14.208 Å, b = 5.726 Å, c = 5.04 Å). The radius of Co²⁺ is 0.650 Å at low spin state and 0.745 Å at high spin state. In Zn_0.8Co_0.2Nb₂O₆, the low and high spin states should co-exist at the same time. Then the average radius of Co²⁺ is slightly smaller than that of Zn²⁺ (0.740 Å). So Co²⁺ can replace Zn²⁺ without causing abundant mismatching of the crystal lattice. No impurity peak is observed in the entire XRD spectrum, indicating that the growing crystal is columbite structure Zn_0.8Co_0.2Nb₂O₆ without impurity. Moreover, the crystal sample matches well with the original columbite structure and the dopant reduces the crystal lattice slightly. The XRD² result, as shown in Fig. 1(b), was tested on the wafer cut vertical the growth direction of the Zn_0.8Co_0.2Nb₂O₆ crystal. There is only one peak located at 64.2°. It indicates that the crystal grows along the 〈191〉 direction.

Fig. 1.

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	Fig. 1. (a) Rietveld method refined XRD spectrum of Zn0.8Co0.2Nb2O6 powder samples, and (b) XRD2 spectrum of the crystal wafer.

Room temperature Raman spectrum of Zn_0.8Co_0.2Nb₂O₆ crystal has been measured. The result is shown in Fig. 2. It can be found that all Raman peaks in the spectrum can be indexed to columbite structure ZnNb₂O₆.^[17] The peak position shifts to the lower wavenumber direction. The strongest Raman peak, corresponding to the symmetrical stretching vibration of Nb–O bond, is fitted by a Gaussian function. Its peak location is 890 cm⁻¹ and its full width of half-maximum (FWHM) is 15 cm⁻¹. The peak center and the FWHM of the strongest symmetrical stretching vibration of Nb–O bond in the undoped columbite structure ZnNb₂O₆ are 894 cm⁻¹ and 7 cm⁻¹, respectively. The changes are obviously. When the Co²⁺ ions are doped into the columbite structure ZnNb₂O₆, the lattice parameters change as shown in the XRD results and the microscope structure is also affected by the Co²⁺. The bond energy, electronegativity, and the uniforms of the bonds become lower.

Fig. 2.

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	Fig. 2. Room temperature Raman spectrum.

The temperature-dependent Raman tests of the as-grown Zn_0.8Co_0.2Nb₂O₆ crystal were conducted to understand the optical phonon behaviors of Zn_0.8Co_0.2Nb₂O₆ at different temperatures. The Raman spectra were recorded at every 40 °C from −180^C to 500 °C. Thus, 18 temperature-dependent Raman spectra were recorded, as shown in Fig. 3(a). A total of 19 recognizable Raman bands could be observed at −180 ^C and all of the Raman bands can be indexed well with those of the undoped columbite ZnNb₂O₆ samples. With the increase in temperature, the Raman peak moves toward the low wave number, accompanied with a continuous linewidth growth and gradual intensity decrease. As a result, weak Raman peaks disappear or overlap. Only 10 Raman peaks can be distinguished well at 500 °C. And no new Raman peak was observed throughout the process. In the whole temperature range, we fit four typical Raman bonds, the stretching vibration of terminal bond at 888 cm⁻¹, the stretching vibration of Nb–O bridged bond at 527 cm⁻¹, the bending vibration of O–Nb–O bond at 280 cm⁻¹, and the stretching vibration of Nb–Nb bond between NbO₆ octahedron links at 127 cm⁻¹, with Gaussian, and give the curves of Raman wavenumber and FWHM varying with temperature in Figs. 3(b) and 3(c), respectively. In the whole range, all of the temperature dependent Raman wavenumber and three FWHW curves can be fitted linearly. The bond at 127 cm⁻¹ shows an abnormal phenomenon at low temperature (from −180 °C to −60 °C). The Nb–Nb bond is a weak interaction and the uniform of the Nb–Nb bond is improved from −180 °C to −60 °C. Hence, the optical phonon modes of Zn_0.8Co_0.2Nb₂O₆ are stable in this temperature interval. This result reflects the good thermostability of Zn_0.8Co_0.2Nb₂O₆ in this temperature range.

Fig. 3.

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	Fig. 3. (a) Temperature-dependent Raman spectra of Zn0.8Co0.2Nb2O6; the curves of Raman wavenumber (b) and FWHM (c) varying with temperature.

The FC curves of the samples in the temperature range of 300–3 K under the external field of 500 Oe were measured, thereby obtaining the relation between the magnetic susceptibility and temperature (Fig. 4). The FC curves increase with the reduction of temperature. No splitting occurs at the lowest temperature (3 K). This result conforms to the observation results of CoNb₂O₆ in Refs. [18]–[20]. While, as the magnetic susceptibility is relatively smooth in the high-temperature region, some parts must be unrelated with temperature. Hence, the magnetism is composed of two parts, namely, the Curie–Weiss paramagnetism related with temperature and the Pauli paramagnetism unrelated with temperature^[21]

Fig. 4.

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	Fig. 4. Temperature dependent magnetic susceptibility of Zn0.8Co0.2Nb2O6 sample from 300 K to 3 K.

The M–H curve of the Zn_0.8Co_0.2Nb₂O₆ powder was also tested by using the PPMS. The temperatures were set at 300 K, 20 K, and 5 K as shown in Fig. 5. The externally applied magnetic field ranged from −9 T to 9 T. At 300 K and 20 K, Zn_0.8Co_0.2Nb₂O₆ exhibits paramagnetism. When the temperature is 5 K, a weak magnetic hysteresis occurs, indicating the existence of ferromagnetism. It shows a similar behave with our previous work.^[16] As illustrated in Fig. 5, the area surrounding the 5 K zero field is amplified, and the coercivity is approximately 0.0018 T, which is lower than the Zn_0.9Co_0.1Nb₂O₆ ones.^[16] The ferromagnetism becomes saturated when the external field is increased to 9 T. The columbite ANb₂O₆ can be viewed as a layered structure of A–Nb–Nb–A–Nb–Nb. The Zn²⁺ and Co²⁺ ions occupy the A site randomly in Zn_0.8Co_0.2Nb₂O₆. The doped ZnNb₂O₆, compared with CoNb₂O₆, does not destroy the 1D ferromagnetic chain of the original system, but it influences the antiferromagnetism of the 2D isosceles triangular lattices. The CoNb₂O₆ is reported to be antiferromagnetism. When nonmagnetic ion Mg²⁺ is doped into CoNb₂O₆ to replace Co²⁺, the original antiferromagnetism disappears. The Zn_0.8Co_0.2Nb₂O₆ can also be viewed as Zn²⁺ doped into CoNb₂O₆. In Zn_0.8Co_0.2Nb₂O₆, no antiferromagnetism is observed, which conforms to the previously reported results of Mg²⁺ doped CoNb₂O₆.^[23,24]

Fig. 5.

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	Fig. 5. The M–H curve of Zn0.8Co0.2Nb2O6.

The columbite Zn_0.8Co_0.2Nb₂O₆ crystals were grown via infrared heating optical floating zone methods. The XRD results show that the Zn_0.8Co_0.2Nb₂O₆ crystals grow along 〈191〉 direction and have the columbite structure with Pbcn space group, and the lattices parameters are a = 14.190 Å, b = 5.724 Å, c = 5.037 Å. The room temperature Raman spectrum shows the bond energy and the uniforms of the bonds become lower when Co²⁺ ions are doped. The temperature dependent Raman spectra indicated that Zn_0.8Co_0.2Nb₂O₆ is stable in the whole temperature range. The Curie–Weiss temperature is 1 K and when the temperature is down to 5 K, Zn_0.8Co_0.2Nb₂O₆ exhibits a weak ferromagnetism.