ynthesis and characterization of M–Cl (M5Fe, Co, Ni) boracites

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Journal of Alloys and Compounds 351 (2003) 235–240

www.elsevier.com / locate / jallcom

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ynthesis and characterization of M–Cl (M5Fe, Co, Ni) boracites

a ,

*

a a a a a b b

D. Li , Z.J. Xu , Z.H. Wang , D.Y. Geng , J.S. Zhang , Z.D. Zhang , G.L. Yuan , J.-M. Liu

aShenyang National Laboratory for Materials Science and International Centre for Materials Physics, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China

bLaboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China Received 2 May 2002; received in revised form 9 September 2002; accepted 9 September 2002

Abstract

The M–Cl (M5Fe, Co, Ni) boracites are prepared by a procedure consisting of solution mixing, evaporating to dryness, grinding and heating reaction in N₂ atmosphere. The lattice parameter and the phase transition temperatures of the boracites Co B O Cl and₃ ₇ ₁₃ Ni B O Cl are investigated by means of X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. There₃ ₇ ₁₃ are three reversible phase transitions in the Co–Cl boracite and one phase transition in the Ni–Cl boracite above room temperature.

Temperature dependences of dielectric loss of M–Cl (M5Fe, Co, Ni) boracites are studied by the rectangular cavity perturbation method at a microwave frequency of 2.45 GHz. The ferroelectric property at room temperature of the Co B O Cl is demonstrated.₃ ₇ ₁₃

 2002 Published by Elsevier Science B.V.

Keywords: Transition metal compounds; Chemical synthesis; Dielectric response; Phase transition; X-ray diffraction; TEM

1 . Introduction netic susceptibility for all compounds show the existence of antiferromagnetism, accompanied with a weak fer- Boracites M B O X (M;Mg, Cr, Mn, Fe, Co, Ni, Cu,₃ ₇ ₁₃ romagnetic component [10,16].

Zn, or Cd; X;F, Cl, Br, or I) form a large crystal family The chemical vapor transport method is currently the with more than 20 isomorphous compounds [1–6]. In most widely used technique for the synthesis of boracites some cases, X can be OH, S, Se, or Te, and M monovalent in single crystal form [20] and permits the preparation of a Li [1–5]. Nearly all the halogen boracites exhibit transi- large number of boracites [3–5,8–11,16,20,21]. Recently, tions from a high temperature cubic paraelectric phase to a process was developed for preparing Fe–Cl boracite, one or more lower symmetry ferroelectric phases at low which consists of solution mixing, heating, grinding and temperatures [7–11]. The transition temperature of the H₂ reduction [22]. However, the step of H₂ reduction boracites varies widely (60–800 K), but for any given made the product mix witha-Fe. In this work, the method metal it falls in the sequence X5Cl→Br→I [1,12]. The has been improved by using N₂ protecting atmosphere existence of ferroelectricity in the low temperature phase instead of H₂ to prepare M B O Cl (M5Fe, Co, Ni)₃ ₇ ₁₃ of cobalt and nickel boracites was demonstrated in the boracites. The motivation of the present work is to develop literature [13,14]. The temperature dependence of sponta- a new and simple method for preparing boracites with neous birefringence indicated that three types of phases higher purity. The present process consists of solution sequentially occur for the ferroelectric phases of various mixing, evaporating to dryness, grinding and heating boracites [15]. Linear and quadratic magnetoelectric ef- reaction in N atmosphere. The powders obtained by this₂ fects in Ni–Cl boracite were studied [16,17]. The dielectric method are almost pure M–Cl (M5Fe, Co, Ni) boracites behavior of boracites was investigated as a model sub- with small surplus H BO that can be separated by using₃ ₃ stance of improper ferroelectrics [13,18,19]. Many bora- hot de-ionized (DI) water. The structures and phase cites become antiferromagnetic and weakly ferromagnetic transitions of M–Cl (M5Co, Ni) boracites are identified.

at temperatures below 61.5 K, e.g. for Ni–I boracite. The ferroelectric properties at room temperature of Temperature dependence of the magnetization and mag- Co B O Cl and Ni B O Cl are studied. The temperature₃ ₇ ₁₃ ₃ ₇ ₁₃ dependences of dielectric loss of M B O Cl (M5Fe, Co,₃ ₇ ₁₃ Ni) are investigated by the rectangular cavity perturbation

*Corresponding author.

E-mail address: [email protected](D. Li). method [23,24].

0925-8388 / 02 / $ – see front matter  2002 Published by Elsevier Science B.V.

P I I : S 0 9 2 5 - 8 3 8 8 ( 0 2 ) 0 1 0 4 0 - X

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2 . Experiments |200 nm in thickness, which were fabricated using a sputtering technique. In the measurement, five voltage M–Cl (M5Fe, Co, Ni) boracite crystals were prepared pulses, one for pre-polarization, two for negative poling by solution mixing, evaporating to dryness, grinding and and two for positive poling, were generated and a charge- heating reaction in N atmosphere at (1) 800₂ 8C for Fe–Cl integrator was coupled to count the as-generated charge.

boracite, (2) from 700 to 10008C for Co–Cl boracite, and The total tie interval for the five pulses was 108.80 ms (3) from 600 to 9008C for Ni–Cl boracite. The starting using the HVS6000RT standard ferroelectric testing unit.

constituents are boric acid (H BO ) ($99.5%), iron₃ ₃ chloride hydrous (FeCl₂?4H O) ($99.7%), nickel chloride₂

hydrous (NiCl₂?6H O) ($98.0%) and cobaltous chloride₂ 3 . Results and discussion hydrous (CoCl₂?6H O) ($99.0%). Synthesizing the M–Cl₂

(M5Fe, Co, Ni) boracite powders consisted of the follow- In order to identify the products, XRD was performed ing steps. The mixtures of the starting raw materials of on the powders synthesized by each step of the process.

H BO and MCl₃ ₃ ₂?6H O (M5Co, Ni) or FeCl₂ ₂?4H O with₂ The difficulty for preparation of single crystals is mainly weight ratio of 3:5 were dissolved into appropriate DI due to the fact that the precursors are mixtures of chlorides water in a beaker. Then the solutions were evaporated on and boron acid and the reaction is very fast. When the an electric stove to obtain dry mixtures. The dry mixtures mixtures react at several hundreds of degrees Centigrade, as the precursors were ground into fine powders in a there are a lot of crystal nuclei growing at the same time. It mortar with different colors of yellow–green, blue and is very difficult to obtain large single crystals, also due to yellow–brown for H BO –FeCl₃ ₃ ₂?H O, H BO –CoCl₂ ₃ ₃ ₂? the fact that the present method includes a crushing step.

H O and H BO –NiCl₂ ₃ ₃ ₂?H O, respectively. The precursors₂ XRD patterns are shown in Fig. 1 for each step to prepare in an alumina crucible were heated in nitrogen atmosphere M B O Cl (M5Ni, Co). The dried precursors are the₃ ₇ ₁₃ in a quartz tube for half an hour at 8008C to prepare mixtures of H BO –NiCl₃ ₃ ₂?H O, and H BO –CoCl₂ ₃ ₃ ₂?H O,₂ Fe–Cl boracite, at 600, 700, 800, 9008C to prepare Ni–Cl respectively. Ni B O Cl can be obtained in the tempera-₃ ₇ ₁₃ boracite, and at 800, 900, 10008C to prepare Co–Cl ture range from 600 to 9008C. When temperature rises, boracite, respectively. The reacted compacts were crushed Ni B O₃ ₂ ₆ appears from the thermal decomposition of in a mortar protected by petroleum ether. Then the

mixtures of H BO and M B O Cl (M5Fe, Co, Ni) were₃ ₃ ₃ ₇ ₁₃ obtained and dried in air. Almost pure M–Cl (M5Fe, Co, Ni) boracites were obtained through washing by hot DI water.

The phases of the powders synthesized by each step were identified using Cu Ka X-ray diffraction (XRD) at room temperature in a D/ max-gA diffractometer with a pyrolytic monochromator. The grain size and the lattice parameters of the samples were determined using Philips EM 420 transmission electron microscopy (TEM) with an accelerating voltage of 100 kV. The phase transition temperatures were determined using a Perkin Elmer-DSC- 7 differential scanning calorimeter (DSC) with heating rate of 408C min21. The temperature dependences of the dielectric loss of M B O Cl (M5Fe, Co, Ni) were₃ ₇ ₁₃ investigated by means of the rectangular cavity perturbation method and HP8510 network analyzer at a microwave frequency of 2.45 GHz and the drift rate of frequency is less than 1%.

The samples for ferroelectric measurements were simply

4 22

pressed under a pressure of 1.5310 kg cm . The resistivity measured for the pressed sample of the Co–Cl boracite was not measurable. However, the sample of Ni–Cl boracite cannot be imposed at a high voltage due to a high leakage current. The ferroelectric properties of the Co–Cl boracite were measured using a HVS6000RT

standard ferroelectric testing unit (Radiant, NM, USA) Fig. 1. X-ray diffraction patterns of the dried precursors with MCl₂?H O₂

under a virtual ground mode at room temperature. The and H BO as the starting materials and the powders obtained at different₃ ₃

bottom and top electrodes were (111)-oriented Pt layers of temperatures in N atmosphere.₂

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Ni B O Cl and at 900₃ ₇ ₁₃ 8C for Co B O Cl through wash-₃ ₇ ₁₃ ing by hot DI water. X-ray diffraction patterns of the M B O Cl (M5Fe, Ni, Co) after the separation process₃ ₇ ₁₃ are shown in Fig. 2. Compared with the results in our previous work [22], there is no a-Fe mixed with Fe B O Cl in the present samples.₃ ₇ ₁₃

The microstructures of the M B O Cl (M5Ni, Co)₃ ₇ ₁₃ powders separated above were studied by TEM observa- tions. The bright field TEM photographs shown in Figs. 3a and 4a indicate that the mean size of the powders is of the order of microns. Figs. 3b and 4b give the corresponding selected area diffraction patterns of a grain of Ni B O Cl₃ ₇ ₁₃ and Co B O Cl, respectively. The interplanar spacing of₃ ₇ ₁₃

˚ ˚

Ni B O Cl is 6.03 A, and that for Co B O Cl is 6.08 A.3 7 13 3 7 13

The value corresponds to the lattice parameters of a5

˚ ˚ ˚

8.528 A, b58.528 A, c512.06 A for Ni B O Cl, and₃ ₇ ₁₃

˚ ˚ ˚

a58.60 A, b58.60 A, c512.16 A for Co B O Cl,₃ ₇ ₁₃ respectively. The lattice parameters of M–Cl (M5Ni, Co) boracites determined by XRD and TEM in this work are

Fig. 2. X-ray diffraction patterns of the M B O Cl (M5Fe, Ni, Co)₃ ₇ ₁₃

very close to those of reference data [25,26]. Moreover, the

after the separation process.

lattice parameters of Fe–Cl boracite determined by XRD are in good agreement with our previous work [22]. The Ni B O Cl. Co B O Cl can be obtained in the tempera-3 7 13 3 7 13 data are compared in Table 1.

ture range from 800 to 10008C without any impurity DSC measurements were performed on the Ni B O Cl₃ ₇ ₁₃ phase. According to our previous results [22], we selected and Co B O Cl powders. The DSC curves shown in Fig.₃ ₇ ₁₃ 8008C for heating reaction to prepare pure Fe B O Cl.3 7 13 5 reveal that the phase transitions of Ni–Cl and Co–Cl We can infer the possible reaction: boracites occur upon both heating and cooling. The phase transition temperature of Ni–Cl boracite is consistent with 3MCl₂?H O₂ 17H BO₃ ₃→M B O Cl₃ ₇ ₁₃ 111H O↑₂ 1

the data studied by spontaneous birefringence (Dn ) [15]._i 5HCl↑(M5Fe, Ni, Co) However, the Co–Cl boracite shows two phase transition temperatures at 193 and 3468C during heating, and 230 H BO3 3 that does not react is separated from the and 3508C during cooling, respectively. The phase transi- products obtained at 8008C for Fe B O Cl, at 7003 7 13 8C for tion (mm2→m) occurs at 2308C during cooling, which is

Fig. 3. (a) Bright field TEM graph and (b) corresponding selected area diffraction pattern of Ni–Cl boracite synthesized in N at 700₂ 8C.

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Fig. 4. (a) Bright field TEM graph and (b) corresponding selected area diffraction pattern of Co–Cl boracite synthesized in N at 900₂ 8C.

lower than the datum studied by spontaneous birefringence only the data for the dielectric loss´0are shown in Fig. 6.

(Dn ) [15] as compared in Table 2. The differences_i Dielectric losses of these boracites do not change below between the data are within the error bars for the different some temperature. With increasing temperature, the dielec- sample preparations and detection techniques. The first and tric losses of these materials vary strongly at microwave second phase transitions of Co–Cl boracite at low tempera- frequency. Compared with DSC curves in Fig. 5 and Ref.

ture are not sensitive to heat and cannot be distinguished [22], the dielectric losses of M–Cl (M5Fe, Ni, Co) clearly by DSC measurement, similar to the case in the boracites change severely in the vicinity of phase transition Fe–Cl boracite [22]. points. At other temperatures, the changes of dielectric loss Through dielectric spectra, the important information of can be ascribed to the motion of ferroelectric domains. The ferroelectric phase transition can be obtained. The tem- dielectric losses of the M–Cl (M5Fe, Ni, Co) boracites on perature dependences of the dielectric loss ´0 of M–Cl

(M5Fe, Ni, Co) boracites are shown in Fig. 6. The dielectric loss ´0 can reflect more information about materials, for instance, the characterizations of second- order transition (or multiple-order transition) of the measured materials. Under a high frequency electric field, the dielectric loss ´0 of ferroelectric materials can be in- fluenced by both ferroelectric phase transition [27] and the motion of ferroelectric domains [28]. However, the dielectric constant ´9 cannot reflect this information and thus,

Table 1

Lattice parameters a, b, and c (A) of Fe–Cl, Ni–Cl and Co–Cl boracites˚

a b c Techniques Refs.

Fe–Cl 8.60 8.60 12.17 XRD [20]

boracite 8.65 8.65 12.24 TEM [20]

8.60 8.60 12.16 XRD This work

Ni–Cl 8.517 8.517 12.037 XRD [23]

boracite 8.507 8.507 12.038 XRD This work

8.528 8.528 12.060 TEM This work

Co–Cl 8.56 8.56 12.07 XRD [24]

boracite 8.564 8.564 12.125 XRD This work

Fig. 5. DSC graphs of (a) Co–Cl and (b) Ni–Cl boracites with increasing

8.60 8.60 12.16 TEM This work

and decreasing temperature.

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Table 2

Phase transition temperatures (8C) of Fe–Cl, Ni–Cl and Co–Cl boracites

]43m↔mm2 mm2↔m m↔3m Techniques Refs.

Fe–Cl 336 270 255 Dn_i [14]

boracite 332 274 252 DSC heating [20]

Ni–Cl 337 Dn heatingi [14]

boracite 335 Dn coolingi [14]

327 DSC heating This work

332 DSC cooling This work

Co–Cl 195 265 350 Dn_i [14]

boracite 193 346 DSC heating This work

230 350 DSC cooling This work

the whole increase in a certain temperature range. When saturated under the maximum applied electric field of 1.5 some temperatures are reached, the dielectric losses of MV m21. The remnant polarization as evaluated from the these boracites are nearly unchanged with increasing largest loop is about 0.17mC cm22, whilst the coercivity is temperature, indicating the end of the phase transitions and about 0.37 MV m21. The measured coercivity (0.37 MV of the motion of ferroelectric domains. m21) is not really the static coercivity, compared with the As mentioned above, we fail in measuring the ferroelec- extremely high quasi-static ferroelectric coercivity field of tricity of the Ni–Cl boracite, because of its high leakage highly insulating single crystals of Co–Cl boracite (i.e.

current. However, the ferroelectricity of the Co–Cl bora- beginning of switching occurs at about 240 MV m21 and cite has been substantially demonstrated. Fig. 7 presents saturation at 600 MV m21) [29]. The as-measured hyster- several ferroelectric hysteresis loops at room temperature esis in this work does not show a fully saturated electrical of the Co–Cl boracite as measured at different maximum polarization because the static coercivity as reported earlier applied electric fields. It is seen that the polarization is not is quite high [29] and the HVS6000RT test unit cannot apply a voltage higher than 4.0 kV to the present thick sample. This is also due to the fact that the polycrystalline

Fig. 7. Ferroelectric hysteresis loops at room temperature of Co–Cl boracite. The loops denoted by P1–P6 correspond to those measured at different maximum applied electric fields. The standard ferroelectric RT66A tester was used and the pulsed signals were applied to the sample.

Fig. 6. Temperature dependence of the dielectric loss of (a) Fe–Cl, (b) For each cycle, five pulses (two positive and two negative and then one Co–Cl and (c) Ni–Cl boracites measured at a microwave frequency of positive). The ‘periodicity’ for the cycle is 108.80 ms, i.e. the ‘frequency’

2.45 GHz. is 10 Hz.

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¨

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¨

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