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Mengyao Wang, Bin Lu, Bo You, Ruijie Pei, Zhigang Sun, [Ji‐Guang Li](https://orcid.org/0000-0002-5625-7361), [Yoshio Sakka](https://orcid.org/0000-0001-8357-5843), Naifeng Zhuang

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This is the peer reviewed version of the following article: Nickel element doping impacts on structure features and Faraday effects of magneto-optical transparent holmium oxide ceramics, which has been published in final form at https://doi.org/10.1111/ijac.14677. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Nickel element doping impacts on structure features and Faraday effects of magneto‐optical transparent holmium oxide ceramics](https://mdr.nims.go.jp/datasets/f55c19fb-4560-4887-8e19-5f0092dc9c5d)

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Nickel element doping impacts on structure features and Faraday effects of magneto-optical transparent holmium oxide ceramics  Mengyao Wang a,b, Bin Lu a,c,*, Bo You a, Ruijie Pei a, Zhigang Sun a, Ji-Guang Li d, Yoshio Sakka d, Naifeng Zhuang e,*   a School of Materials Science and Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, China b China Railway 20th Bureau Group Co., Ltd., Xi'an, Shaanxi 710016, China c Key Laboratory of Photoelectric Detection Materials and Devices of Zhejiang Province, Ningbo, Zhejiang 315211, China d Research Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan  e College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, China       *Corresponding authors Bin Lu (Ningbo University); Naifeng Zhuang (Fuzhou University) Emails: lvbin@nbu.edu.cn (Bin Lu); nfzhuang@fzu.edu.cn (Naifeng Zhuang)  Abstract The nickel element doped holmium oxide (Ho2O3:Ni) transparent magneto-optical ceramics were fabricated by vacuum sintering and the dopant impacts on structure features and Faraday effects were investigated. The starting oxide powders were synthesized by pyrolyzing the resulting layered holmium-based hydroxide nanosheets prepared from a chemical precipitation route using the sodium hydroxide as precipitant at the freezing temperature. Upon high-temperature sintering, the Ni𝑖∙∙  defect is introduced by Ni2+ substitution for Ho3+ to form the interstitial solid solution. The 1 at.% Ni2+ doped Ho2O3 ceramic sample exhibits an in-line transmittance of ~70.04% at 1550 nm with a relative density of ~99.88%, while more Ni2+ incorporation (e.g., 2‒5 at.%) even leads to a completely opaque state. The magneto-optical transparent Ho2O3:1%Ni ceramic developed in this work has Verdet constants of ~-195, -65, and -29 rad/(T·m) at 635, 1064, and 1550 nm, respectively, which are ~1.8-fold higher than the commercial terbium gallium garnet crystal or ~1.4-fold higher than the pure Ho2O3 ceramic. This material also possesses relatively large figure of merit of ~14.6 °/T at 1064 nm and relatively high thermal conductivity of ~7.5 W/(m∙K) at room temperature.        Keywords: magneto optics, transparent ceramics, rare earths, defects, Faraday effect   1. INTRODUCTION The Faraday effect refers to a magneto-optical phenomenon caused by alterations in the magnetic field, which induces the change of the transmission characteristics of the incident light beam inside a transparent material.1-3 Based on it, a magnetically-activated material can be employed as a magneto-optical element (MOE) in systems such as Faraday isolators, Faraday modulators, and magneto-optical switches.4-6 It can also indirectly monitor the strength of the magnetic field.6 Common magneto-optical materials can be classified as films, glass, crystals, and ceramics. At present, the most widely used magneto-optical medium is the terbium gallium garnet (TGG) single crystal with a Verdet constant of -134 rad/(T∙m) at 633 nm.7 According to the Faraday effect, the Faraday rotation angle is positively related to the size and magnetic induction. Therefore, a large-size TGG crystal is required to enhance the Faraday effect so as to obtain a large Faraday rotation angle, which may generate more light loss and accumulated heat to possibly induce depolarization effect and thermal lens effect.8,9 In order to overcome this problem, more and more researchers recently devoted to developing new-type magneto-optical materials with a higher Verdet constant. The most studied magneto-optical ceramics primarily include lanthanide sesquioxides (Ln2O3), garnets, pyrochlores, and fluorites,10-16 among which the sesquioxides generally possess the largest Verdet constant due to the highest Ln3+ concentration. In addition, a regular strategy for enhanced Verdet constant in ceramic system is to being doped with another Ln3+ cation such as Pr3+, Ce3+, Nd3+, Ho3+, Dy3+, or Tm3+ via supper-exchange interaction.3,17-20 However, these Ln3+ cations contain abundant energy levels, frequently causing more optical self-absorption to limit the practical application. According to the Hund rule and Pauli exclusion principal, electrons are always arranged in pairs with opposite spins. The magnetic moments produced by two  electrons with opposite spin directions would cancel each other out. Thus, unpaired electrons are desired in the outermost layer of the nucleus for achieving magnetic moments. The main elements that meet this condition are Fe, Co, and Ni, among which the Fe element has variable valence while the abundance of the Co element is much less than that of the Ni element. Furthermore, the Ni element has good resistance corrosion and chemical stability. NiO is a typical natural antiferromagnet,21,22 however, Ho2O3 is paramagnetic. Their combined influence on Faraday magneto-optical effect has scarce study so far. In this work, we fabricated transparent Ni2+ doped Ho2O3 ceramics and studied dopant impacts on structure features and Verdet constants.  In order to obtain a sinterable starting powder, we herein prepared exchangeable layered rare-earth hydroxide (LRH) as the precipitation precursor using a freezing temperature technique, since the obtained LRH nanosheet down to ~7 nm has proved its superiority for the production of high-optical-quality oxide ceramic.23-26 During the liquid-phase synthesis, the ammonium hydroxide solution is frequently employed as the precipitant. However, such a precipitant is unable to precipitate nickel cation rather than preference for the formation of soluble coordination compound, especially under a high pH level. Considering the sodium hydroxide solution also could provide adequate OH- anions for LRH synthesis and the soluble Na+ byproduct could be removed by subsequent washing. Hence, we alternatively utilized the sodium hydroxide solution as the precipitant in the present work, finally leading to transparent Ho2O3:Ni ceramics through vacuum sintering. More importantly, unlike the Ln3+ cation, the Ni2+ dopant never generates additional optical self-absorption in the near-infrared region, which avoids imposing restriction on the application of magneto-optical Ho2O3 ceramics. The research findings of this work could provide a reference for the development of high-performance magneto-optical materials.  2. EXPERIMENTAL PROCEDURES The Ho2O3 raw material (99.95% purity, Shanghai Diyang Chemical Co., Ltd., Shanghai, China) was dissolved by excessive amount of nitric acid upon heating. The superfluous nitric acid was fully removed by evaporating the salt solution until dryness. The dried nitrate was dissolved into distilled water to prepare a 0.075 mol/L Ho(NO3)3 mother liquor. The stoichiometric NiCl2∙6H2O (99.9% purity, Aladdin Biochemical Technology Co., Ltd., Shanghai, China) was dissolved into the mother liquor according to 1‒5 at.% cationic concentration. Meanwhile, a 1.0 mol/L NaOH solution was prepared by dissolving its commercial solid powder (98% purity, Shanghai Macklin Biochemical Co., Ltd.) into distilled water as the precipitant. The alkaline precipitant was dropwise dropped into the mother liquor at a rate of 2‒3 mL/min under magnetic stirring at a freezing temperature of ~4 ºC till the pH value reached 8.5. The resulting light green suspension, after aging for 1.5 h, was filtered and repeatedly washed with distilled water to remove byproducts. The precipitation was added into (NH4)2SO4 solution and the molar ratio of SO42- to Ho3+ was selected to be 0.03. After ion exchange for 1.5 h, the product was repetitively rinsed with distilled water and absolute ethanol. After drying at 90 °C for 12 h, the precipitation precursor was calcined in a tube furnace under flowing oxygen (~150 mL/min) at 1050 °C for 4 h to yield the oxide particle. The powder was filled into a stainless-steel die for precompression, and then cold isostatically pressed under 300 MPa. The green body was sintered in a tungsten-wire furnace at 1850 °C for 6 h under 10-4‒10-5 Pa vacuum. The sintered body was finally polished on double faces to improve the surface smooth finish. Phase identification was performed on X-ray diffractometer (XRD; Model D8 Advance Davinci, Bruker, Karlsruhe, Germany) using nickel filtered Cu Kα radiation. The particle size distribution was determined by laser diffraction particle size analyzer  (Model ZS90, Malvern Instruments, Malvern, UK). The microscopic morphologies of the particle and ceramic samples were both observed on a field emission scanning electron microscope (FE-SEM; Model Nova NanoSEM 450, FEI Company, Hillsboro, USA) at 15‒30 kV, which is equipped with an Apollo silicon drift detector series for energy-dispersive X-ray spectrometry (EDS). The in-line transmittances of sintered bodies were recorded on a UV/VIS/NIR spectrophotometer (Model Lambda 950, PerkinElmer, Shelton, USA) from 200 to 1700 nm. The Faraday rotation angles were measured using the extinction method on a self-assembled device equipped with three independent laser light sources (635, 1064, and 1550 nm) under adjustable magnetic field from 0 to 0.8 T. Thermal diffusivity and specific heat value were simultaneously obtained from a laser flash diffusivity apparatus (Model LFA467, Netzsch, Selb, Germany) using xenon lamp as the laser heating source from room temperature to 400 °C. 3. RESULTS AND DISCUSSION Figure 1 shows XRD patterns of the precipitation precursor, calcination product, and sintered bodies with 1−5 at.% Ni2+ doping. The precursor exhibits typical LRH structure with characteristic sharp (220) diffraction and a series of (00l) (l = 2, 4···) diffractions.27 The lattice constant (c) of LRH is calculated to be ~1.822 nm using Bragg’s equation from the (002) diffraction. The interlayer distance for LRH is half of the c constant,28 and thus its value is ~0.911 nm. After calcining at 1050 °C for 4 h, the diffraction peaks of the product match well with those of the Ho2O3 standard card (JCPDS No. 83-0932) and the sharp diffraction peaks indicate the high crystallinity. Owing to the close similarities between (001) direction for LRH and (111) direction for oxide, the phase transformation substantially belongs to quasi-topotactic evolution.29 The crystallite size (DXRD) can be calculated from the full width at half maximum  (FWHM) of the (222) diffraction band using Scherrer’s equation: DXRD = Kλ/(βCosθ), where K is the shape factor (K = 0.89), λ is the wavelength of X-ray (λ = 0.15406 nm), β is the FWHM, and θ is the Bragg angle.30,31 The determined crystallite size of the Ho2O3:1%Ni oxide powder is ~39 nm. After high-temperature sintering, the three Ho2O3 bulk specimens with 1−5 at.% Ni2+ doping still retain the cubic crystalline phase and no trace of impurities can be observed. The main (222) diffraction peaks gradually shift towards the low angle side with more Ni2+ addition. The lattice constants calculated by Bragg’s law are ~10.572, 10.586, and 10.592 Å for 1, 2, and 5 at.% Ni2+ doped Ho2O3 samples, respectively. These phenomena indicate that homogeneous solid solutions have formed. Ho2O3 belongs to cubic C-type sesquioxide, which has 80 atoms per unit cell containing 32 Ho atoms and 48 O atoms. Its X-ray density (dth) could be determined from Eq. (1): 332 [(1 ) 1.5 ]Ho Ni OthAm M mM Mda N − + +=                                    (1) where Mi stands for atomic weight of element i (i = Ho, Ni, and O), a refers to the lattice constant, NA represents the Avogadro constant, and m denotes the percentage content of Ni atom. Their corresponding theoretical densities are ~8.46, 8.42, and 8.40 g/cm3. The slightly decreasing theoretical density is because Ni element is lighter relative to Ho element while the increasing lattice volume with more Ni2+ doping is another one reason. The Ni2+ doping mechanism is discussed as follows: Since the oxidation states of Ni and Ho elements differ (Ni being 2+ and Ho being 3+), the introduction of low-valent Ni2+ may induce one of two possible point defect forms, as described in Eqs. (2) and (3): '2 32 2 2Ho O oHo ONiO Ni V O⎯⎯⎯⎯→ + +                                       (2)  '2 33 2 3Ho i oHo ONiO Ni Ni O⎯⎯⎯⎯→ + +                                      (3) In the first case, every two divalent Ni acceptors create one VO∙∙ (Eq. 2), while in the second case, the interstitial Ni𝑖∙∙ cation is formed (Eq. 3). The Ni2+ ionic radius [0.690 Å for coordination number (CN) = 6] is smaller than that of Ho3+ (0.901 Å for CN = 6). The formation of oxygen vacancies, as described in Eq. (2), results in the contraction of the unit cell. However, this was inconsistent with the XRD results. Sometimes, an exaggerated mismatch in ionic radii could lead to the formation of an interstitial solid solution according to Eq. (3). Thus, the smaller Ni2+ penetrates the Ho2O3 lattice, expending the cell dimensions. There are two inequivalent Ho³⁺ sites in Ho2O3 unit cell. Either Ho³⁺ is bonded to six equivalent O²⁻ atoms to form a mixture of distorted edge and corner-sharing HoO₆ octahedra or to form a mixture of edge and corner-sharing HoO₆ octahedra. Overall, the Ni2+ dopant cannot form substitutional solid solution, and thus Ni2+ site is impossible at the octahedral center. The formation of the interstitial Ni𝑖∙∙ defect in the Ho2O3 lattice was confirmed by aforementioned analysis. Figure 2 exhibits the morphologies of the precipitation precursor synthesized at the freezing temperature and the Ho2O3:1%Ni oxide powder calcined at 1050 °C for 4 h. The LRH precursor presents typical two-dimensional nanosheet shape and self-assembles into collective three-dimensional petal-like pattern. After thermal decomposition, the nanosheets collapse into rounded powders with ultrafine size. The average particle size measured by laser diffraction particle size analyzer is ~263 nm [inset in Fig. 2(b)], `while the statistic FE-SEM size is ~83 nm. The former is normally larger than the latter for the non-monodispersed powder due to the self-limitativeness of laser diffraction measurement method.32 The result of the laser diffraction particle size analyzer could partly reflect the agglomerated extent. Our determined particle size test result is relatively small in fact while the particle size curve in differential volume  distribution exhibits a unimodal particle size distribution without observed hard agglomerates, implying the good particle sinterability for dense ceramic preparation. Figure 3(a) shows the microstructure of the Ho2O3:5%Ni bulk sample, especially focusing on the triple junction region. The grain boundary can be clearly observed after thermal etching treatment and the element mapping results reveal that the elemental distributions of holmium, oxygen, and nickel are relatively uniform without detected secondary phases [Figs. 3(b)-(e)]. That is, the Ni2+ dopant has well dissolved into the Ho2O3 matrix. The elemental distribution analysis is fully consistent with the XRD result [Fig. (1)]. The qualitative EDS analysis also detects Ho, Ni, and O elements, whereas the additional Pt element ascribes to the surface sputtering for electrical conduction [Fig. 3(f)]. By excluding the Pt-coating effect, the semiquantitative EDS analysis reveals that the elemental contents of Ho, Ni, and O respectively are 74.09, 5.19, and 20.72 at.% in close proximity to our practical stoichiometric proportion, which further verifies that our wet chemical route is feasible for Ni2+ and Ho3+ precipitation completeness. Figure 4(a) exhibits appearances of the Ho2O3:xNi2+ (x = 1−5 at.%) ceramic bodies fabricated by vacuum sintering. The letters covered under the Ho2O3:1%Ni bulk can be well read-through. This transparent body presents brown hue similar to the pure Ho2O3 ceramic,13 since the intra-4f10 transitions for Ho3+ greatly absorb the purple, blue, and green lights in the visible region [Fig. 4(b)]. Additionally, the absorption bands arising from 3d−3d transitions of Ni2+ in the visible region almost overlap with the abundant intra-4f10 transitions of Ho3+ to be difficult to separately distinguish. Based on Tanabe–Sugano diagram, only 3A2→3T2 transition of Ni2+ may generate self-absorption effect in the infrared region, but it has been swallowed in the Ho3+ 5I8→5I6 transition induced absorption band. That is to say, the Ni2+ introduction never generates additional  individual optical absorption in the infrared region, indicating its merit for application in this spectrum scope. The overall transparency has not been reduced by 1 at.% Ni2+ doping in comparison with our previously reported pure Ho2O3 ceramic.13 Nevertheless, the color of the samples gradually deepens as the increase in Ni2+ contents, even leading to completely opaque black close to NiO own color. The theoretical transmittance of the defect-free Ho2O3 single crystal has been reported to be ~81.6% at 1550 nm.25 In this work, the Ho2O3:1%Ni sample has a transparency of ~70.0% at the same wavelength, which is ~86% of the theoretical value. Unlike the case in the visible region, the near-infrared range has much less absorption from electronic transitions, and thus this material exhibits good potential to apply to the high-power near-infrared laser system such as Yb3+ or Nd3+ doped Y3Al5O12 laser device. The density of the Ho2O3:1%Ni ceramic has been measured by the Archimedes method and the result shows that this specimen has an experimental density of ~8.45 g/cm3. Considering its theoretical density is ~8.46 g/cm3 (Eq. 1), the relative density is as high as ~99.88% in accordance with the good optical quality. A self-assembled magneto-optical test equipment is used to measure the Faraday effect of the transparent Ho2O3:1%Ni ceramic sample via the extinction method. In this system, three lasers with wavelengths of 635, 1064, and 1550 nm are used as light sources, and the magnetic field intensity is adjusted from 0 to 0.8 T through current-controlled magnet coil. The relationship between Verdet constant (V) and Faraday rotation angle (θ) can be expressed as: V = θ / BL, where B is the applied magnetic field strength and L is the thickness of the transparent material.33 Thus, a plot of B against θ/L has a linear correlation, where the slope (s) of the fitting is equal to the V parameter. The measured Faraday rotation angle exhibits a good linear relationship with the applied magnetic field [Fig. 5(a)]. The Verdet constants are determined to be ~-195, - 65, and -29 rad/(T∙m) at 635, 1064, and 1550 nm, respectively. Table 1 compares the Verdet constants among our developed Ho2O3:Ni ceramics and several typical magneto-optical materials.13,25,34-39 At 1064 nm, the Verdet constant of our sample is ~1.8-fold higher than that of the commercial TGG crystal or ~1.4-fold higher than that of the pure Ho2O3 ceramic. Our results demonstrate that the nickel element is a quite effective dopant for enhanced Verdet constant via the supper-exchange interaction. When the temperature remains constant, the V value and the test wavelength (λ) satisfy the following formula: 2 20( )EV =−                                               (4) In Eq. (4), λ0 is relative with the effective migration wavelength of electrons and E is a constant.34 A plot of V-1 versus λ2 yields a good linear relation with a high degree of fitting (R2 = 99.94%) as shown in Fig. 5(b). The E and λ0 parameters are determined to be ~-6.8×107 and 232 nm from the slope (E-1) and the intercept (-λ02/E), respectively. Figure of merit (FoM), defined by the ratio of the Verdet constant to the light absorption coefficient (α), was used to roughly access the quality of a magneto-optical material as shown from Eqs. (5) and (6): FoMV=                                                           (5) 1 1lgL T =                                                                (6) where T is the transmittance and L is the material thickness.35,40,41 The calculated FoM values are ~9.8, 14.6, and 7.3 °/T at 635, 1064, and 1550 nm, respectively. The lower FoM values at 635 and 1550 nm ascribe to the strong absorption from f-f transitions of Ho3+ and longer wavelength based on Eq. (4), respectively. At 1064 nm, the FoM value of our sample is much higher than those of the previously reported undoped Ho2O3  ceramic and the commercial TGG single crystal (Table 1). The measured thermal diffusivity of the Ho2O3:1%Ni sample is shown in Fig. 6. The thermal diffusion (α) values are ∼2.94, 2.46, 2.07, 1.80, and 1.60 mm2/s at 25, 100, 200, 300, and 400 °C, respectively. The corresponding specific heats (Cp) are ∼0.302, 0.319, 0.347, 0.347, and 0.366 J/(g∙K). The thermal conductivity (k) can be deduced from the equation: k = α∙Cp∙ρ, where ρ is the material density (~8.45 g/cm3 for Ho2O3:1%Ni). As a result, the k values are ∼7.5, 6.6, 6.1, 5.3, and 5.0 W/(m∙K) at 25, 100, 200, 300, and 400 °C, respectively. As the ambient temperature rises, the thermal diffusion together with the thermal conductivity goes a declining trend. This is because the accelerated phonon vibration frequency and the increasing collision probability caused by elevated temperature shorten phonon mean free path. At room temperature, the thermal conductivity of our sample is much higher than those of previously reported Ho2O3:5%La ceramic [∼4.8 W/(m∙K)] and the commercial TGG crystal [∼4.9 W/(m∙K)],42,43 further suggesting the good application prospect as Faraday devices. 4. CONCLUSION Layered holmium-based hydroxide nanosheets were prepared by wet-chemical method upon themolysis into rounded Ho2O3:Ni oxide powders with an average particle size of ~83 nm. The optimum Ni2+ concentration is 1 at.%, at which the sintered body exhibits an in-line transmittance of ~70.04% at 1550 nm (~86% of the theoretical transmittance) and a thermal conductivity of ~7.5 W/(m∙K) at room temperature.  The nickel dopant in Ho2O3 has been proved to be effective to enhance the magneto-optical effect, by which the Ho2O3:1%Ni ceramic has ~1.4-fold higher Verdet constant than the undoped Ho2O3 ceramic or ~1.8-fold higher Verdet constant than the commercial TGG crystal. Our developed ceramic materials exhibit good potential to apply to the high-power laser system, especially for use in the near-infrared region.   ACKNOWLEDGMENT The research was supported by Zhejiang Provincial Natural Science Foundation of China under Grant No. LY23F050007.  CONFLICT OF INTEREST STATEMENENT The authors declare they have no conflicts of interest.                    REFERENCES 1. Vojna D, Slezak O, Lucianetti A, Mocek T. Verdet constant of magneto-active materials developed for high-power Faraday devices. Appl Sci. 2019, 9(15):3160. 2. Feng WX, Hanke J-P, Zhou XD, Guo G-Y, Blugel S, Mokrousov Y, Yao Y. Topological magneto-optical effects and their quantization in noncoplanar antiferromagnets. Nat Commun. 2020, 11(1):118.  3. Wang MY, Lu B, Li HX. Fantastic valence impacts of Pr on microstructures and Faraday magneto-optical effects of transparent (Ho,Pr)2O3 ceramics. J Eur Ceram Soc. 2021, 41(10):5258–5263. 4. Willamson LA, Chen YH, Longdell JJ. 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FE-SEM micrographs of the LRH precursor intended for Ho2O3:1%Ni (a) and its calcined powder at 1050 °C for 4 h (b). Fig. 3. FE-SEM micrograph of the Ho2O3:5%Ni ceramic sample (a), elemental mappings for Ho (b), O (c), Ni (d) and their comprehensive distributions (e), and EDS elemental analysis (f). Fig. 4. Appearances (a) and in-line transmittances (b) of the Ho2O3:xNi2+ (x = 1−5 at.%) ceramics fabricated by vacuum sintering. Fig. 5. A plot of θ/L versus B for the transparent magneto-optical Ho2O3 ceramic doped with 1 at.% Ni2+ (a) and a plot of 1/V versus λ2 for the Ho2O3:1%Ni bulk sample (b). Fig. 6. Thermal diffusivity and thermal conductivity of the transparent Ho2O3:1%Ni ceramic.