BaMF4 (M = Co, Ni and Zn) samples having orthorhombic structure have been synthesized by a mild hydrothermal method and characterized by x-ray diffraction (XRD), magnetic and electrical measurements. Single phase formation of these compounds has been found to be dependent on various factors like reaction time, pH, temperature etc. All the samples showed ferroelectricity, which decreases with increase in temperature. BaMF4 (M = Co, Ni) samples show 10-15 times more leakage current compared to that of BaZnF4 at higher applied field. Absence of multivalent ions in BaZnF4 can be the reason for its minimum leakage current. All the samples except, BaZnF4, exhibit weak room temperature ferromagnetism also.

Multiferroic materials with coexistence of two or more ferroic orderings like ferroelectric, ferromagnetic or ferroelastic ordering have been drawing increasing interest due to their potential for applications in memory devices, read-write devices1 etc. Due to the combination of magnetic and dielectric properties and with mutual coupling between these properties, multiferroics have enormous potential for technological applications and at the same time they pose very interesting and rich fundamental problems. Recently, several perovskite compounds such as BiFeO3, BiMnO3, HoMnO3, YMnO3 and TbMnO3 have been reported as multiferroic materials.2–5 While it is worth to mention that most of these materials exhibit either ferroelectric or antiferromagnetic ordering or a combination of both properties, few systems like BiMnO3, Ni3B7O13I which are ferroelectric at room temperature and exhibit ferromagnetic ordering well below room temperature.6,7 Multiferroic materials with ordering temperatures close to room temperature have potential of technological applications.

In general fluoride based compounds are highly symmetric. Perhaps for this reason fluoride based ferroelectric compounds are scarcely reported in literature. Moreover, very rare reports on fluoride based materials with the coexistence of room temperature ferroelectricity and ferromagnetism are available.

The preparation of complex fluorides by solid-state reactions at high temperature has been reported,8 but requirements of complex synthetic apparatus, has limited such studies on solid-state synthesis. Reports on the synthesis of some binary fluorides by high-temperature hydrothermal methods above 400°C are reported, but were also technically challenging.

In this study an environmental friendly and low cost mild hydrothermal synthesis has been employed to synthesize fluoride based materials. BaMF4 (M = Co, Ni, Zn) are known to be ferroelectric materials. The BaMF4 compounds crystallize with the polar space group Cmc21. In this base-centered orthorhombic structure, the M cations are octahedrally surrounded by fluorine anions. The special connectivity of the fluorine octahedra in these systems, which are arranged in quasi-two-dimensional sheets, gives rise to one unstable phonon mode that involves alternating octahedral rotations together with an overall shift of the Ba cations relative to the other ions.9 

In the present study, it has been shown that BaMF4 samples can be successfully synthesized by mild hydrothermal route. Their ferroelectric properties, variation of polarization with temperature, leakage current densities and magnetic properties have been presented.

In present work BaMF4 (M = Co, Ni and Zn) samples have been synthesized by facile hydrothermal route. The starting materials for the synthesis of BaMF4 samples were analytical grade Ba(NO3)2.4H2O (99.99%), Co(NO3)2.6H2O (99.99%), Ni(NO3)2.6H2O (99.99%), Zn(NO3)2.6H2O (99.99%) and NH4HF2. In order to determine the phase purity, powder x-ray diffraction data were recorded on a PANalytical X’pert Pro X-Ray Diffraction (XRD) unit with monochromatized Cu-Kα radiation. The diffraction patterns were recorded in a step scan mode with step width 0.02o and step time 3.30 second. DC magnetization measurements, as a function of field were carried out using an E.G. and G P.A.R vibrating sample magnetometer (model 4500) and the electrical polarization was measured on the thin pellets by a home-made LCR circuit. The pellets of the samples were made using an iso-static press and sintered at 500°C for 4h in argon atmosphere. The pellets were coated with Pt paste and the field dependent polarization and temperature dependent polarization were measured on a FE test system (aixACCT: TF-2000). The same instrument was used to investigate the leakage current density of the samples.

The ratios of starting materials, pH, temperature, time, phase information are summarized in Table I. It has been found that the ratio of the reactants and duration of autoclaving were found to be very crucial for the formation and crystallization of mono phasic products in this system. In the synthesis of these compounds, NH4HF2 has several functions, as it acts as fluorinating agent, it assists to keep the pH of the solution and F- ions are efficient mineralizers. The excess of F- ions help in decreasing the reaction time and crystallization temperature.10,11 The M/Ba ratio is an very important factor for the formation of phase pure compound. When the concentration of M(NO3)2 is less than 1.5 times of that of Ba(NO3)2, biphasic mixture of BaMF4 and BaF2 has always been obtained, irrespective of reaction temperature and time. As mentioned earlier, pH also plays an important role in the formation of pure BaMF4.

Table I.

Physical parameters for synthesis of phase pure BaMF4 (M = Co, Ni, Zn).

Ba(NO3)2.4H2OM(NO3)2NH4HF2 TempTime 
(mol %)(mol %)(mol %)pH(°C)(h)Product
120 36 Biphasic 
1.3 120 60 Biphasic 
1.4 120 60 Biphasic 
1.5 10 150 60 Phase pure 
Ba(NO3)2.4H2OM(NO3)2NH4HF2 TempTime 
(mol %)(mol %)(mol %)pH(°C)(h)Product
120 36 Biphasic 
1.3 120 60 Biphasic 
1.4 120 60 Biphasic 
1.5 10 150 60 Phase pure 

The XRD patterns of all the products having the chemical formula BaMF4 are shown in Fig. 1. The XRD pattern shows that all the compounds are in single phasic in nature. All these compounds present the same crystalline structure; they belong to the orthorhombic system, with space group Cmc21. The obtained cell parameters for different compositions were calculated by using POWDERX software. The cell parameters of all the compounds have been summarized in the Table II.

FIG. 1.

Powder XRD patterns of BaMF4 (M = Co, Ni, Zn).

FIG. 1.

Powder XRD patterns of BaMF4 (M = Co, Ni, Zn).

Close modal
Table II.

Different physical parameters of BaMF4 (M = Co, Ni, Zn) compounds.

 BaCoF4BaNiF4BaZnF4
Cell parameters (Å) a = 5.851(3) a = 5.775 (1) a = 5.825 (2) 
  b = 14.62 (3) b = 14.351 (3) b = 14.477(1) 
  c = 4.218 (2) c = 4.133 (2) c = 4.188(2) 
Saturation polarization (Ps) (μC/cm20.13 0.10 0.09 
Remanance polarization (Pr) (μC/cm20.11 0.08 0.015 
Loop Squarness (Pr/Ps)% 84 80 63 
 BaCoF4BaNiF4BaZnF4
Cell parameters (Å) a = 5.851(3) a = 5.775 (1) a = 5.825 (2) 
  b = 14.62 (3) b = 14.351 (3) b = 14.477(1) 
  c = 4.218 (2) c = 4.133 (2) c = 4.188(2) 
Saturation polarization (Ps) (μC/cm20.13 0.10 0.09 
Remanance polarization (Pr) (μC/cm20.11 0.08 0.015 
Loop Squarness (Pr/Ps)% 84 80 63 

The ferroelectric (PE) hysteresis loops showed well defined but lossy hysteresis loops (Fig. 2) and thereby indicating the presence of ferroelectricity in these samples. It is very important to mention here that several authors have written articles emphasizing the tendency in obtaining P-E (polarization-electric field) data which may be artifact of ferroelectric.12–18 

FIG. 2.

Hysteresis loops of (a) BaCoF4 (b) BaNiF4 (c) BaZnF4.

FIG. 2.

Hysteresis loops of (a) BaCoF4 (b) BaNiF4 (c) BaZnF4.

Close modal

In a lucid article, Scott has suggested that the materials which show saturation in polarization and have a concave region in P-E plot are true ferroelectrics. These compounds also show saturated loop and concave region in P-E plot.

The saturation polarization of BaCoF4, BaNiF4, BaZnF4 are found to be lower than reported values.19,20 The reason could be that previous authors have used single crystal whereas in this work, powder samples have been used.

The rounded corner of the loops indicates that the samples show leakage current. For practical purpose ferroelectric materials should have low leakage current. The leakage current for different materials are shown in Fig. 3.

FIG. 3.

Leakage current densities of BaMF4 (M = Co, Ni, Zn) at different filed.

FIG. 3.

Leakage current densities of BaMF4 (M = Co, Ni, Zn) at different filed.

Close modal

Amongst the samples, BaZnF4 shows minimum leakage behavior and all other shows 10-15 times more leakage current densities. It can be noted that only in BaZnF4, all the ions are at their stable oxidation states which leads to low leakage current, whereas other M ions in BaMF4 compounds may exist in variable oxidation state.12 This may be the reason for leaky behavior in BaMF4 where M ion shows variable oxidation states. XPS have been performed on BaCoF4 and BaNiF4 to determine the oxidation sates of Co and Ni. The peak corresponding to Co (2p3/2) and Ni (2p3/2) in BaCoF4 and BaNiF4 are at 779 eV and 854.5 eV respectively.21 The broad and asymmetric nature of the XPS peaks indicates the presence of mixed valence of the transition metal ion. Since the intensity of the peak corresponding to the metal ion is weak in nature therefore, the fitting was also not meaningful.

The variation of polarization with temperature is also measured for all the samples. As expected the polarization decreases with increase in temperature as shown in Fig. 4(a)–4(c).

FIG. 4.

Temperature dependent polarization on (a) BaCoF4 (b) BaNiF4 (c) BaZnF4.

FIG. 4.

Temperature dependent polarization on (a) BaCoF4 (b) BaNiF4 (c) BaZnF4.

Close modal

It can be mentioned here that none of the compounds contain any ions which are generally considered to be “ferroelectrically active” like d0 cations (like Ti4+, Nb5+ etc.) or lone pair-active cations such as Bi3+ or Pb2+ which are responsible for ferroelectricity. In BiFeO3 or BiMnO3 systems, charge transfer from oxide ion to cation take place to stabilize the non centrosymmetric phase. However, in these compounds charge transfer from anions to cation is also very much unlikely because the compounds are highly ionic in nature. Perhaps, the ferroelectricity in BaMF4 is purely due to distortion in structure. Ederer et al.9 reported that in these types of compounds (BaMF4) the ferroelectricity originates due to the softening of a single polar phonon mode, which involves both rotational motions of the MF6 octahedra and displacements of the Ba cations caused by relative ionic size effects and geometrical constraints. It can be mentioned here that the crystal structure of the BaMF4 consists of MF6 octahedra connected together by the corner fluoride ions. Such linkages of the octahedra lead to a distorted structure due to the inter-cation interaction in M-F-M chain. Therefore, M-F-M bond angle in BaMF4 deviates significantly from the linear configuration (180°). It has been suggested by Ederer et al.9 that the M-F-M distance in the structure is too small to accommodate two M-F bonds, resulting structural distortion in the BaMF4 systems. Therefore, with increase in size of M, the distortion decreases. In our samples also we found that polarization decreases from BaCoF4 to BaNiF4 to BaZnF4. There was typographical mistake during manuscript preparation. This again indicates that the ferroelectricity in these systems is of different origin than in the conventional oxide perovskite ferroelectrics (BaTiO3 or PbTiO3) where charge transfer between the transition-metal d and the oxygen's p orbital is crucial for stabilizing the non-centrosymmetric ferroelectric state.9 It is unlikely that charge transfer from highly electronegative ion like fluoride to transition metal takes place in this system to stabilize the non-centrosymmetric ferroelectric state.

Magnetic hysteresis loops for the BaMF4 samples with different M ions have been measured at room temperature, as presented in Fig. 5. These samples prepared by other routes have been reported as antiferromagnetic in nature.9 However, in this study weak ferromagnetic behavior is observed. The ferromagnetic data has been extracted by subtracting the paramagnetic component from the measured M-H data (Fig. 5). Since the materials were prepared by soft chemical route, the obtained particles size was very small (∼100 nm). Similar effect of particle size on magnetism was also reported in nanosized BiFeO3,22,23 La1/3Sr2/3FeO324 and NiO particles.25 

FIG. 5.

Magnetic field dependent magnetization of BaMF4 (M = Co, Ni, Zn).

FIG. 5.

Magnetic field dependent magnetization of BaMF4 (M = Co, Ni, Zn).

Close modal

In fact, weak surface ferromagnetic component is a common feature for nano sized antiferromagnetic materials, which is due to the deviation of the antiferromagnetic arrangement to the disordered surface spins due to the lattice strain or the anion vacancies or under cordination.26 Based on above discussion, the magnetic structure of these systems can be considered as a core/shell system, where the core of the particle is antiferromagnetic phase and the surface is ferromagnetic component. The straight line with negative slope in the M-H plot for BaZnF4 indicates its diamagnetic nature.

BaMF4 (M = Co, Ni, Zn) could be prepared by mild hydrothermal process. Temperature, pH, reaction time are found to have great influence for obtaining phase pure BaMF4 compounds. All the compounds are found to exhibit ferroelectric properties and the polarization decreases with increase in temperature. BaZnF4 does not contain any ion having variable oxidation states, therefore, shows minimum leakage current. On the other hand, other compositions i.e. BaMF4 (M = Co, Ni) shows 10–15 times more leakage current at higher applied field. Very weak ferromagnetic behavior found in BaMF4 (M = Co, Ni) could be attributed to smaller particle size of the compounds.

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Supplementary Material