1. Introduction
Cu
2CrBO
5 is a new compound in the ludwigite family. It was obtained and studied as a polycrystal sample for the first time several years ago [
1]. This ludwigite is the first and the only one known to demonstrateamagnetoelectric effect, since data on similar studies of other ludwigites are absent. The presence of magnetoelectric polarization in Cu
2CrBO
5 is related to the copper subsystem. It is necessary to note that Cu
2CrBO
5 possesses structural cationic ordering. This is not typical for almost all heterometallic ludwigites due to the presence of four nonequivalent cation positions in the unit cell (
Figure 1) [
2,
3,
4,
5]. The temperature of magnetic ordering in the copper-chromium ludwigite is quite high for the representatives of this family, with
TN = 120K [
1]. Moreover, the thermal dependence of the magnetic properties of Cu
2CrBO
5 behaves non-monotonically in the low-temperature phase. It possesses additional peculiarities such as the
M(
T) curve inflection and the temperature range in which the spin-flop transition occurs [
6]. The magnetic structure of Cu
2CrBO
5 was studied usingpowder neutron diffraction [
1]. An incommensurate antiferromagnetic phase was found below
TN, which was characterized by geometrical frustrations of the exchange interaction.
For a detailed study of the unusual properties of the Cu
2CrBO
5 ludwigite, especially that of the magnetoelectric effect, it is necessary to conduct orientational experiments on single crystal samples. In this paper, we expand the research in this direction using the flux growth technique. This is the most widelyused method for obtaining single crystal samples of the oxyborates with a ludwigite structure due to the presence of growth anisotropy. Natural faceting of single crystals and the possibility of using different solvent types for individual synthesis of different compounds are undoubtedly some merits of this technique [
7].
The research stepsinclude the selection of the initial solvent, establishment of the Cu2CrBO5 single crystal phase, study of the competition between high-temperature crystallizing phases in the system and theiroptimization. In this paper, we study the possibility of growing a copper-chromium ludwigite viathe flux method using two solvents, Li2WO4-B2O3 and Bi2O3-MoO3-B2O3-Na2O.
Despite the presence of growth anisotropy, copper ludwigite crystals achieve significant dimensions in comparison with other ludwigites, as was shown on the example of Cu
2FeBO
5 [
8] and Cu
2MnBO
5 [
9]. The size of these compounds allows one to carry out different orientational studies of the physical properties. However, Cu
2CrBO
5 behaves differently atthe early growth stage, which is caused by the infusibility of Cr
2O
3 oxide and its low solubility in many solvents [
6]. The starting point for the search of a flux system appropriate for growing Cu
2CrBO
5isthe systems used earlier for other copper ludwigites [
8,
9]. Also, one of the main problems is studying the sequence of high-temperature crystallizing phases in the chosen fluxes, and the emphasis placed on the valence state transformation of chromium cations similarly to manganese-containing ludwigites.
In this research, we present the possibility of growing Cu2CrBO5 crystals viathe flux technique using two different solvents based on Li2WO4-Li2O-B2O3 and Bi2O3-MoO3-B2O3-Na2O, and through structural analysis of the obtained phases, study of the thermal and field anomalies of magnetization, and the analyses of the polarized (at room temperature, T = 295K) and thermal (in the vicinity of magnetic phase transition temperature) Raman spectra of Cu2CrBO5.
4. Discussion
The study of the possibility of obtaining Cu2CrBO5 crystals usingthe flux technique consists of several stages as the crystallization research in the systems is based on two types of solvents, Li2WO4-Li2O-B2O3 and Bi2O3-MoO3-B2O3-Na2O. The phase composition of the obtained samples was controlled using powder and single crystal X-ray diffraction, and the chemical composition of some samples was investigated viathe EDX technique.
Four flux systems were studied, one based on Bi2O3-MoO3-B2O3-Na2O and three based on Li2WO4-Li2O-B2O3 with different starting points. The ludwigite phase (Cu2CrBO5) was obtained in each system. However, a “pure” ludwigite phase was obtained in two of them: (2) and (4). Co-crystallization with the phases CuO and CuCr2O4was obtained in systems (3) and (5), respectively. Thus, for further study of the crystallization processes aimed at growing a Cu2CrBO5sample of sufficient size, systems (2) and (4) was used. Thus, both flux systems, based on Li2WO4-Li2O-B2O3 and on Bi2O3-MoO3-B2O3-Na2O, are appropriate for obtaining the crystallization of the Cu2CrBO5 ludwigite phase. Hence, after taking into account the study of three systems with different component ratios, it can be concluded that there is a wide area of the ludwigite phase on the phase diagram of the Li2WO4-Li2O-B2O3-based multicomponent flux system. The main restricting factor for obtaining the sufficient dimensions of the Cu2CrBO5 single crystals is the solubility of chromium oxide. This problem needs additional study.
During the research, the possibility of the formation of copper-chromium compounds with other valence composition was established. Delafossite Cu+CrO2 and tungstate (Cu, Cr)2+WO4 were obtained as secondary phases. This possibility arises owing to the growth technique applied, in particular, due to the presence of the solvent.
The phase (Cu, Cr)WO
4 was obtained as an intermediate one in the system based on lithium tungstate. This compound contains bivalent cations of transition metals including Cr
2+. As in the case of the manganese systems, in the Bi-Mo-O fluxes [
8], no crystallization of the phase with trivalent cations is observed in the absence of free alkaline metal oxides (Na
2O and Li
2O) due to the formation of MnMoO
4 or (Cu, Cr)WO
4-like phases. In this experiment, two samples of tungstates from system (3) with concentrations
n1 = 2.2% and
n2 = 3.87% were obtained. The space groups and lattice parameters of these compounds are given in
Table 1. For comparison, the analogous data for pure CuWO
4 [
19] and CrWO
4 [
20] are also presented in
Table 1.
As one can see from
Table 1, pure tungstates are characterized by the space group different from the mixed ones. Copper tungstate is triclinic, while that of chromium is monoclinic. Apparently, the combination of two cations Cu
2+ and Cr
2+ leads to the tungstate structure with the space group
P2/
c. The lattice parameters of the second mixed sample increase relative to analogous data for the first one. This correlates with an increase in the chromium content, which is in agreement with the ratio of the cationic radii
R(Cu
2+) = 0.75Å and
R(Cr
2+) = 0.80Å. Thus, despite the same nominal Cu/Cr ratio in the flux, chromium contentin the crystal increases upon sequential sampling. This demonstrates a significant difference between the partition coefficients of CuO and Cr
2O
3 and thepoor solubility of chromium.
In this research, Cu
2CrBO
5 was obtained using several flux systems. In some cases, co-crystallization with the phases CuO and CuCr
2O
4 was observed.
Table 2 presents the structure parameters of two samples of Cu
2CrBO
5 obtained from systems (2) and (4), corresponding to the bismuth-molybdenum and lithium-tungstate flux systems.
Table 2 shows the data of the other sample obtained viaa solid state reaction [
1].
As mentioned earlier [
6], the lattice parameters of Cu
2CrBO
5 obtained usingthe flux technique are different from the sample obtained viathe solid state reaction. In particular, the differences are clearly seen in the monoclinic angle. Significant differences were also revealed upon the analysis of the bond lengths of metal–oxygen octahedra [
6]. The lattice parameters of samples (2) and (4) obtained viathe flux technique correlate quite well witheach other. Some small differences can be caused due to thedifferent X-ray experimental techniques used: the structural data for sample (2) were obtained withpowder X-ray diffraction, while the data for sample (4) were obtained using single crystal X-ray diffraction.
The actual composition of sample (2) is Cu1.89Cr1.11BO5, as found withthe EDX analysis. A possible cause of the structural data discrepancy is the different cation composition, in particular, the presence of Cr2+ in the compounds grown from the fluxes.
The ludwigite Cu
2CrBO
5 has a number of peculiarities in the thermal and field behavior of magnetization. Below the temperature of the antiferromagnetic phase transition (
TN ≈ 120K), there is a non-monotonic thermal dependence of magnetization, that is, a sharp peak in the vicinity of the phase transition temperature, then the “plateau” and the inflection point in the temperature range of
T = 40–50K (
Figure 3, [
1,
6]), which can also be seen in the thermal dependence of the dielectric constant [
1].
To study this anomaly at
T = 40–50K, the dependences ∂(χ·
T)/∂
T(
T), corresponding to the thermal behavior of specific heat for antiferromagnets, were analyzed (
Figure 8) [
24]. The analogous dependence of S2 obtained at
H = 0.1T is also presented in
Figure 8. The most significant anomalies, as was expected, were observed in the vicinity of the antiferromagnetic phase transition at
TN = 119 K. The green curve, corresponding to sample S1, demonstrates the maximum of the antiferromagnetic phase transition. Two other curves, corresponding to sample S2 with the spinel admixture, demonstrate the sharp minimum corresponding to the ferromagnetic type of the phase transition (the magnetic ordering temperature of spinel is
TC = 120K) together with the maximum of the antiferromagnetic transition. At
T2 = 42K, the black curve (S2, 0.5T) demonstrates the second distinct positive maximum of a lower value. Thus, this anomaly can have the antiferromagnetic origin related to the reordering of some subsystem.
It is mentioned in [
1] that the ludwigite Cu
2CrBO
5 demonstrates metamagnetic transition. The field dependences of magnetization demonstrate reversible inflections in the temperature range
T = 60–100K, corresponding to the spin-flop transition. The distinct peaks of the thermal dependences of ∂
M/∂
H at 100K and 80K are observed in
Figure 9. The range of the magnetic field up to 9T was not enough to fully register the peak of the curve at 60K. The type of the field dependences
M(
H) changes at 40K: magnetization decreases and there is a linear law of the dependence. Below 20 K, the temperature dependence is absent (
Figure 9). However, at
T = 40K the spin-flop transition can still occur, but at much higher magnetic fields.
The Raman spectrum of Cu
2CrBO
5 consists of rather narrow separate lines in the vibration range of the metal–oxygen octahedra. This confirms a high degree of the structural cationic order in this compound. It is greater than in the Cu
2MnBO
5 ludwigite, which can easily be distinguished among the previously studied ludwigites in terms of Raman spectroscopy [
17].
The dependence of the intensity of Raman lines on the rotation angle of Cu
2CrBO
5 (
Figure 5) can be compared with other copper ludwigites. In analyzing other copper-containing ludwigites which are also monoclinic, phase shifts were detected in broad spectral ranges in the area of the Me-O octahedra and lattice vibrations [
17]. The phase shift was different from 45° and 90° in the HV and HH modes, respectively. The shift angle of the maxima of the low-frequency region in the Cu
2GaBO
5-like phase was about 70° and 120° for HV and HH, respectively, and in the Cu
2MnBO
5-like phase it was 50° for the HH mode. For the solid solutions of the Cu
2GaBO
5-Cu
2MnBO
5 phase boundary, the lines in the spectral range of 670–700cm
−1 demonstrate another shift, which is 60° for the HH mode. This implies a change in the angles of the Me-O bonds in the octahedra, i.e., the octahedra turn due to the monoclinic distortions in the Cu
2GaBO
5-Cu
2MnBO
5 phases. Another situation occurs in Cu
2CrBO
5, namely the angular behavior of the intensity is close to orthorhombic ludwigites, which can indicate the minimal distortions of the structure in this compound.
5. Conclusions
Ludwigites are a complex compound family demonstrating multidirectional and diverse properties and high sensitivity to the composition, with a difficult process of preparing single crystal samples. Cu2CrBO5studied in this paperis representative of the family, in which the magnetoelectric effect and a number of magnetic features were detected for the first time. This paper presents the results ofastudy on the possibility of crystallization of this compound in flux systems using two different solvents based on Li2WO4-Li2O-B2O3 and Bi2O3-MoO3-B2O3-Na2O. The phase diagrams of these multicomponent systems were studied. The possibility and area of occurrence of the ludwigite phase were determined. The most perspective components ratios for growing samples of sufficient dimensions and without secondary phases were also determined. Toanalyzethe growth process, the solubility of Cr2O3 in the fluxes of different composition and changes in the valence state of copper and chromium were studied, depending on the working temperature range and flux content. The phase and chemical composition of the obtained samples were controlled using X-ray diffraction and the EDX technique.
The magnetic properties of the obtained Cu2CrBO5 samples were analyzed. In addition to the antiferromagnetic phase transition at TN = 119K, the anomaly at T = 42K was studied. The antiferromagnetic nature of the studied anomaly was shown, which can be related to the process of reordering a part of the magnetic subsystems. The spin-flop transition found in the field dependences of magnetization was also analyzed.
Polarized Raman spectra of Cu2CrBO5 were obtained for the first time. The angular intensity distribution was shown to be close to the pattern typical for orthorhombic ludwigites. The metal–oxygen octahedra had a small distortion degree, unlike other monoclinic ludwigites. Raman spectra of Cu2CrBO5 in the vicinity of the antiferromagnetic phase transition were also obtained and analyzed for the first time. Small shifts inthe lines were found, which can be associated with the magnetic ordering in the crystal.
Future research will be focused on the growth of Cu2CrBO5 single crystals with sufficientsize for the orientational study of magnetic and magnetoelectric properties. As it was shown in this paper, the main limiting factor hampering the possibility of producing larger crystals is the solubility of Cr2O3. To overcome this limitation, it is planned to start the production process with system (4) which allows for the stable single phase crystallization of Cu2CrBO5. Attention will be paid to increase the flux convection via an attempt to increase the saturation temperature, and to study the influence of mechanical mixing on the growth process.