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An investigation of the magnetic properties of elemental boron modifications
161
Journal of the Le~~-~o~~on Metals, 67 (1979) 161- 165 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
AN INVESTIGATION OF THE MAGNETIC ELEMENTAL BORON MODIFICATIONS*
N. E. SOLOV’EV, Voronezh
V. S. MAKAROV,
State University,
University
PROPERTIES
OF
Ya. A. UGAI and S. I. REMBEZA Square 1, Voronezh
{U.S.S.R.)
Summary The temperature dependence of the magnetic susceptibility of a! rhombohedral boron in the temperature range 20 - 800 “C was investigated, together with the electron spin resonance (ESR) spectra at 77 K. The presence of FeB in some samples was revealed and the nature of its formation was determined. ESR spectra are characterized by single lines with g factors 4.21 and 2.003. In the spectrum with g = 2.003 a narrow line (2.7 7.2 Oe) was found in addition to a wide line (about 30 Oe), the appearance of which is possibly connected with the presence of FeB.
Magnetic susceptibility and electron spin resonance (ESR) in elemental boron have been studied in the amorphous and 0 rhombohedral modifications [ 1 - 31. In our paper we report the investigation of the temperature dependence of the magnetic susceptibility and the ESR spectra in (Yrhombohedral boron (we shall call this a-B). Measurements of the magnetic properties of amorphous and p rhombohedral boron (we shall call this 0-B) were carried out for comparison. In this investigation two batches of powdery amorphous boron were obtained by decomposition of diborane. From a chemical analysis the boron content in the samples of both batches constituted 99.6 wt.%. e-B and 0-B samples were obtained by amorphous boron annealing in an argon atmosphere at 1270 and 1400 “C respectively [4, 53. The boron content in the crystalline modifications was 99.8 wt.%. The magnetic susceptibility was measured by the Gouy method in the temperature range 25 - 800 “C at a magnetic field intensity of 2.5 - 12.2 kOe. The samples were placed in quartz tubes in a vacuum of residual pressure 1 X lo‘-* Ton.
*Paper presented at the 6th Internation~ Varna, Bulgaria, October 9 - 12, 1978.
Symposium on Boron and Borides,
162
The ESR spectra were investigated using an RE 1301 spectrometer at 77 K. a,a-diphenyl-/3-picrylhydrazyl (DPPG) samples were used as standards of g factor measurement. X-ray diffraction analysis and differential thermal analysis (DTA) of the samples were carried out using a URS-50-IM X-ray diffractometer (Fe K, radiation) and an FPK-59 pyrometer respectively. The results of the specific magnetic susceptibility measurements obtained for the first batch of samples are shown in Fig. 1; the results of Kubler et al. [ 21 are also shown in Fig. 1. As can be seen from the measurements, the susceptibility of wB, as well as those of amorphous boron and P-B, have negative values, i.e. these materials are diamagnetic. It should be noted that the values obtained for the magnetic susceptibility of /3-B agree very well with the data of Kubler et al. [2] ; however, the susceptibility values obtained for amorphous boron are somewhat different from those of Kubler et al. [ 21. This can be explained because Kubler et al. used amorphous boron of greater purity (99.99 wt.%). The magnetic susceptibility of CC-Bdoes not depend on the temperature and is close to the value for 8-B.
Fig. 1. The temperature dependence of the specific magnetic susceptibilities of elemental boron modifications: curve 1, our data for amorphous boron; curve 2, data of Kubler et al. [ 21 for amorphous boron; curve 3, -, data of Kubler et al. [ 21 for P-B; curve 3, A, our results for 0-B; curve 4, our data for o-B. Fig. 2. The temperature dependence of the volumetric magnetic susceptibilities of the amorphous samples and o-B samples in the second batch: curves 1 and l’, curves on heating amorphous boron ; curves 2 and 2’, curves on cooling amorphous boron; curves 3 and 3’, curves on heating o-B; curves 1 - 3 were obtained at H = 9.7 kOe; curves 1’ - 3’ were obtained at H = 12.2 kOe.
The results of the magnetic susceptibility measurements on amorphous and a-B obtained for the second batch of samples are shown in Fig. 2. In spite of an identical boron content in both batches, as shown by chemical analysis, the thermal magnetic curves are different. The reason for this difference is the presence of impurities in the second batch of samples which have peculiarities in the magnetic properties over the temperature range studied. The temperature-susceptibility curves of amorphous boron obtained on heating (Fig. 2, curves 1 and 1’) and on cooling (Fig. 2, curves 2 and 2’) are different, indicating that phase transformations took place during the heating process.
163
ar-B samples from the second batch apparently have the same impurities as amorphous boron has when heated. The impurities in cw-Bhave ferromagnetic properties at room temperature and at slightly higher temperatures, as the dependence of susceptibility on the temperature and on the magnetic field intensity shows. The point of inflection of the thermal magnetic curve at 320 f 5 “C corresponds to the Curie temperature of a ferromagnetic impurity. The Curie temperature is a parameter which is characteristic of a particular substance. Thus we can identify an impurity by its Curie temperature. We only need to consider ferromagnetic compounds because a ferromagnetic phase was observed to form on heating amorphous boron. The point of inflection found on the thermal magnetic curve almost coincides with the Curie temperature of FeB (325 f 2 “C [6] ). Thus we can infer that FeB is possibly present in our second batch of samples; however, this has not yet been proved conclusively. X-ray diffraction analysis did not show the presence of FeB in amorphous boron and a-B samples. It is possible that the impurity content is lower than can be detected within the limits of the X-ray method. Thus the phase impurity composition can be determined by comparing the thermal magnetic properties of the second batch with the thermal magnetic properties of the first batch when mixed with iron and with a-Fe20s. The fact that the peculiarities in the thermal magnetic curves coincide confirms the identity of the phase transformations. The temperature dependences of the magnetic susceptibilities of amorphous boron mixed with 21.4 wt.% Fe and with 3 wt.% a-FesOs are shown in Fig. 3; it is clear that the temperature dependence (Fig. 3, curve 1) of the susceptibility of the B-Fe mixture has a different character from that of amorphous boron (Fig. 2, curves 1 and l’), whereas the temperature variation (Fig. 3, curve 2) of the susceptibility obtained by heating a mixture of amorphous boron and a-FezOa is similar to the curve obtained for amorphous boron from the second batch of samples. The curves obtained on cooling (Fig. 2, curves 3 and 3’, and Fig. 3, curve 3) are also similar. Thus the temperature variation of the magnetic susceptibility shows that in some amorphous boron samples there is a-FezOs which reacts with boron on heating. If we assume that the interaction of boron with cu-Fe,Oa leads to the formation of FeB [ 71, the appearance of FeB impurities in the second batch of boron samples cannot be doubted. An X-ray diffraction analysis of the B-a-Fe,Oa mixtures which had been annealed according to the reaction 10B + 2FeaOs
-+ 4FeB + 3B202f
confirmed the formation of FeB. DTA results of this mixture shown in Fig. 3, curve 4, confirm that the formation of FeB is accompanied by an exothermal effect, the starting temperature of which is 556 + 2 “C. As well as the temperature increase caused by the exothermal effect due to the formation of FeB, a temperature increase is also observed on the differential curve at approximately 390 “C. A sharp
164 1
Al
Fig. 3. The temperature dependences of the volumetric magnetic susceptibilities and the DTA results: curve 1, amorphous boron with iron; curve 2, amorphous boron with (YFe203 (heating); curve 3, amorphous boron with &Fe203 (cooling); curve 4, DTA of amorphous boron with a-Fen03 (heating).
increase in magnetic susceptibility at the same temperature is observed in the thermal magnetic curves (Fig. 2) of amorphous boron containing a-Fe,03. This increase cannot be due to the formation of FeB because FeB does not possess ferromagnetic properties at 390 “C. The change in the differential and thermal magnetic curves at 390 “C is apparently caused by the formation of Fe,B which has ferromagnetic properties up to 740 “C [ 81. The disappearance of the ferromagnetic properties (Fig. 2) in the second batch of boron samples confirms this. However, we could not find any FesB using X-ray diffraction analysis. cr-B from other batches was also studied. In the majority of the batches the samples contained FeB. The thermal magnetic curves of these samples had shapes similar to those in Fig. 2, and the specific susceptibilities at temperatures higher than 500 “C usually exceeded the susceptibility for a-B from the first batch. However, a value for the specific magnetic susceptibility of cu-B at 600 “C equal to -1.47 lop6 cm3 g-l was obtained in one of these batches; this is probably due to the presence of diamagnetic impurities. We found from the ESR investigations that a spectrum with g = 2.003 and a linewidth AH,, = 2.5 - 7.2 Oe between maximum declination points was obtained for all cr-B samples (Fig. 4, curves a, b and c). ESR spectra with similar parameters have been observed previously by Geist [ 31 for 0-B and were explained as due to the presence of silicon and carbon impurities. A spectrum with g = 4.21 and AHpp = 40 Oe (Fig. 4, curve a’) was also found for some (Y-Bsamples of both batches. The nature of this spectrum has
165
Fig. 4. ESR spectra of powdery o-B at 77 K: curves a and a’, sample in first batch; curve b, sample in second batch; curve c, sample with the higher FeB content.
not yet been finally determined. A spectrum with a similar value for the g factor has been found previously for glasses with transition element impurities [9] . In the second batch of OL-B,apart from the narrow line AH,, = 2.3,7.2 Oe with g = 2.003, a second line with approximately the same value of the g factor but of width 30 Oe was observed. A comparison of the ESR results with the magnetic susceptibility measurements in the same samples shows that the intensity of the wide spectrum line (AH,, = 30 Oe) correlates to the impurity (FeB) content in the samples (Fig. 4, curves a, b and c). Thus in the first batch of Q-B, where the FeB phase was not found, a wide line with g = 2 was not observed in the ESR spectrum. The spectrum with g = 2.003, observed for samples in the second batch containing FeB, has wider wings possibly because the narrow and the wide lines are superimposed. Finally, for the sample which has an FeB content higher than that in the samples of the second batch, both wide and narrow lines are present. Thus the wide line of the ESR spectrum with g = 2 may be due to the presence of the FeB impurity in the cu-B samples studied. As shown above, the anomalies in the thermal magnetic properties of a-B are caused by the impurity (FeB) content.
References 1 L. Kubler, G. Gewinner, J. J. Koulmann and A. Jaegle, Phys. Status Solidi B, 60 (1973) 117. 2 L. Kubler, G. Gewinner, J. J. Koulmann and A. Jaeglb, Phys. Status Solidi B, 69 (1975) 323. 3 D. Geist, in V. I. Matkovich, P. Hagenmuller, G. V. Samsonov and T. Lundstrom (eds.), Boron and Refractory Borides, Springer, Berlin, 1977, p. 65. 4 E. M. Averbakh, N. E. Soloviev and J. A. Ugai, Author’s Certificate 331661, cl. CO1 b, 35/00, claimed 1970, published January 16, 1973. 5 P. M. Pirtshalashvili, I. A. Bairamashvili, T. V. Samkurashvili, J. P. Lomidze, S. A. Loladze and N. Y. Gudushauri, in F. N. Tavadze (ed.), Boron: Preparation, Structure and Properties, Nauka, Moscow, 1974, p. 23. 6 N. Lundquist, N. P. Myers and H. Westin, Philos. &Zag., 7 (1962) 1187. 7 I. P. Borovskaya, N. P. Novikov and A. G. Merzhanov, in G. V. Samsonov (ed.), Refractory Borides and Silicides, Naukova Dumka, Kiev, 1977, p. 29. 8 K. H. J. Buschov, in V. I. Matkovich, P. Hagenmuller, G. V. Samsonov and T. Lundstrom (eds.), Boron and Refractory Borides, Springer, Berlin, 1977, p. 494. 9 A. Abragam and B. Bleaney, in S. A. Altshuller and G. V. Skrotsky (eds.), Electron Paramagnetic Resonance of Transition Zons, Vol. 1, Mir, Moscow, 1972, p. 227.
Journal of the Le~~-~o~~on Metals, 67 (1979) 161- 165 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
AN INVESTIGATION OF THE MAGNETIC ELEMENTAL BORON MODIFICATIONS*
N. E. SOLOV’EV, Voronezh
V. S. MAKAROV,
State University,
University
PROPERTIES
OF
Ya. A. UGAI and S. I. REMBEZA Square 1, Voronezh
{U.S.S.R.)
Summary The temperature dependence of the magnetic susceptibility of a! rhombohedral boron in the temperature range 20 - 800 “C was investigated, together with the electron spin resonance (ESR) spectra at 77 K. The presence of FeB in some samples was revealed and the nature of its formation was determined. ESR spectra are characterized by single lines with g factors 4.21 and 2.003. In the spectrum with g = 2.003 a narrow line (2.7 7.2 Oe) was found in addition to a wide line (about 30 Oe), the appearance of which is possibly connected with the presence of FeB.
Magnetic susceptibility and electron spin resonance (ESR) in elemental boron have been studied in the amorphous and 0 rhombohedral modifications [ 1 - 31. In our paper we report the investigation of the temperature dependence of the magnetic susceptibility and the ESR spectra in (Yrhombohedral boron (we shall call this a-B). Measurements of the magnetic properties of amorphous and p rhombohedral boron (we shall call this 0-B) were carried out for comparison. In this investigation two batches of powdery amorphous boron were obtained by decomposition of diborane. From a chemical analysis the boron content in the samples of both batches constituted 99.6 wt.%. e-B and 0-B samples were obtained by amorphous boron annealing in an argon atmosphere at 1270 and 1400 “C respectively [4, 53. The boron content in the crystalline modifications was 99.8 wt.%. The magnetic susceptibility was measured by the Gouy method in the temperature range 25 - 800 “C at a magnetic field intensity of 2.5 - 12.2 kOe. The samples were placed in quartz tubes in a vacuum of residual pressure 1 X lo‘-* Ton.
*Paper presented at the 6th Internation~ Varna, Bulgaria, October 9 - 12, 1978.
Symposium on Boron and Borides,
162
The ESR spectra were investigated using an RE 1301 spectrometer at 77 K. a,a-diphenyl-/3-picrylhydrazyl (DPPG) samples were used as standards of g factor measurement. X-ray diffraction analysis and differential thermal analysis (DTA) of the samples were carried out using a URS-50-IM X-ray diffractometer (Fe K, radiation) and an FPK-59 pyrometer respectively. The results of the specific magnetic susceptibility measurements obtained for the first batch of samples are shown in Fig. 1; the results of Kubler et al. [ 21 are also shown in Fig. 1. As can be seen from the measurements, the susceptibility of wB, as well as those of amorphous boron and P-B, have negative values, i.e. these materials are diamagnetic. It should be noted that the values obtained for the magnetic susceptibility of /3-B agree very well with the data of Kubler et al. [2] ; however, the susceptibility values obtained for amorphous boron are somewhat different from those of Kubler et al. [ 21. This can be explained because Kubler et al. used amorphous boron of greater purity (99.99 wt.%). The magnetic susceptibility of CC-Bdoes not depend on the temperature and is close to the value for 8-B.
Fig. 1. The temperature dependence of the specific magnetic susceptibilities of elemental boron modifications: curve 1, our data for amorphous boron; curve 2, data of Kubler et al. [ 21 for amorphous boron; curve 3, -, data of Kubler et al. [ 21 for P-B; curve 3, A, our results for 0-B; curve 4, our data for o-B. Fig. 2. The temperature dependence of the volumetric magnetic susceptibilities of the amorphous samples and o-B samples in the second batch: curves 1 and l’, curves on heating amorphous boron ; curves 2 and 2’, curves on cooling amorphous boron; curves 3 and 3’, curves on heating o-B; curves 1 - 3 were obtained at H = 9.7 kOe; curves 1’ - 3’ were obtained at H = 12.2 kOe.
The results of the magnetic susceptibility measurements on amorphous and a-B obtained for the second batch of samples are shown in Fig. 2. In spite of an identical boron content in both batches, as shown by chemical analysis, the thermal magnetic curves are different. The reason for this difference is the presence of impurities in the second batch of samples which have peculiarities in the magnetic properties over the temperature range studied. The temperature-susceptibility curves of amorphous boron obtained on heating (Fig. 2, curves 1 and 1’) and on cooling (Fig. 2, curves 2 and 2’) are different, indicating that phase transformations took place during the heating process.
163
ar-B samples from the second batch apparently have the same impurities as amorphous boron has when heated. The impurities in cw-Bhave ferromagnetic properties at room temperature and at slightly higher temperatures, as the dependence of susceptibility on the temperature and on the magnetic field intensity shows. The point of inflection of the thermal magnetic curve at 320 f 5 “C corresponds to the Curie temperature of a ferromagnetic impurity. The Curie temperature is a parameter which is characteristic of a particular substance. Thus we can identify an impurity by its Curie temperature. We only need to consider ferromagnetic compounds because a ferromagnetic phase was observed to form on heating amorphous boron. The point of inflection found on the thermal magnetic curve almost coincides with the Curie temperature of FeB (325 f 2 “C [6] ). Thus we can infer that FeB is possibly present in our second batch of samples; however, this has not yet been proved conclusively. X-ray diffraction analysis did not show the presence of FeB in amorphous boron and a-B samples. It is possible that the impurity content is lower than can be detected within the limits of the X-ray method. Thus the phase impurity composition can be determined by comparing the thermal magnetic properties of the second batch with the thermal magnetic properties of the first batch when mixed with iron and with a-Fe20s. The fact that the peculiarities in the thermal magnetic curves coincide confirms the identity of the phase transformations. The temperature dependences of the magnetic susceptibilities of amorphous boron mixed with 21.4 wt.% Fe and with 3 wt.% a-FesOs are shown in Fig. 3; it is clear that the temperature dependence (Fig. 3, curve 1) of the susceptibility of the B-Fe mixture has a different character from that of amorphous boron (Fig. 2, curves 1 and l’), whereas the temperature variation (Fig. 3, curve 2) of the susceptibility obtained by heating a mixture of amorphous boron and a-FezOa is similar to the curve obtained for amorphous boron from the second batch of samples. The curves obtained on cooling (Fig. 2, curves 3 and 3’, and Fig. 3, curve 3) are also similar. Thus the temperature variation of the magnetic susceptibility shows that in some amorphous boron samples there is a-FezOs which reacts with boron on heating. If we assume that the interaction of boron with cu-Fe,Oa leads to the formation of FeB [ 71, the appearance of FeB impurities in the second batch of boron samples cannot be doubted. An X-ray diffraction analysis of the B-a-Fe,Oa mixtures which had been annealed according to the reaction 10B + 2FeaOs
-+ 4FeB + 3B202f
confirmed the formation of FeB. DTA results of this mixture shown in Fig. 3, curve 4, confirm that the formation of FeB is accompanied by an exothermal effect, the starting temperature of which is 556 + 2 “C. As well as the temperature increase caused by the exothermal effect due to the formation of FeB, a temperature increase is also observed on the differential curve at approximately 390 “C. A sharp
164 1
Al
Fig. 3. The temperature dependences of the volumetric magnetic susceptibilities and the DTA results: curve 1, amorphous boron with iron; curve 2, amorphous boron with (YFe203 (heating); curve 3, amorphous boron with &Fe203 (cooling); curve 4, DTA of amorphous boron with a-Fen03 (heating).
increase in magnetic susceptibility at the same temperature is observed in the thermal magnetic curves (Fig. 2) of amorphous boron containing a-Fe,03. This increase cannot be due to the formation of FeB because FeB does not possess ferromagnetic properties at 390 “C. The change in the differential and thermal magnetic curves at 390 “C is apparently caused by the formation of Fe,B which has ferromagnetic properties up to 740 “C [ 81. The disappearance of the ferromagnetic properties (Fig. 2) in the second batch of boron samples confirms this. However, we could not find any FesB using X-ray diffraction analysis. cr-B from other batches was also studied. In the majority of the batches the samples contained FeB. The thermal magnetic curves of these samples had shapes similar to those in Fig. 2, and the specific susceptibilities at temperatures higher than 500 “C usually exceeded the susceptibility for a-B from the first batch. However, a value for the specific magnetic susceptibility of cu-B at 600 “C equal to -1.47 lop6 cm3 g-l was obtained in one of these batches; this is probably due to the presence of diamagnetic impurities. We found from the ESR investigations that a spectrum with g = 2.003 and a linewidth AH,, = 2.5 - 7.2 Oe between maximum declination points was obtained for all cr-B samples (Fig. 4, curves a, b and c). ESR spectra with similar parameters have been observed previously by Geist [ 31 for 0-B and were explained as due to the presence of silicon and carbon impurities. A spectrum with g = 4.21 and AHpp = 40 Oe (Fig. 4, curve a’) was also found for some (Y-Bsamples of both batches. The nature of this spectrum has
165
Fig. 4. ESR spectra of powdery o-B at 77 K: curves a and a’, sample in first batch; curve b, sample in second batch; curve c, sample with the higher FeB content.
not yet been finally determined. A spectrum with a similar value for the g factor has been found previously for glasses with transition element impurities [9] . In the second batch of OL-B,apart from the narrow line AH,, = 2.3,7.2 Oe with g = 2.003, a second line with approximately the same value of the g factor but of width 30 Oe was observed. A comparison of the ESR results with the magnetic susceptibility measurements in the same samples shows that the intensity of the wide spectrum line (AH,, = 30 Oe) correlates to the impurity (FeB) content in the samples (Fig. 4, curves a, b and c). Thus in the first batch of Q-B, where the FeB phase was not found, a wide line with g = 2 was not observed in the ESR spectrum. The spectrum with g = 2.003, observed for samples in the second batch containing FeB, has wider wings possibly because the narrow and the wide lines are superimposed. Finally, for the sample which has an FeB content higher than that in the samples of the second batch, both wide and narrow lines are present. Thus the wide line of the ESR spectrum with g = 2 may be due to the presence of the FeB impurity in the cu-B samples studied. As shown above, the anomalies in the thermal magnetic properties of a-B are caused by the impurity (FeB) content.
References 1 L. Kubler, G. Gewinner, J. J. Koulmann and A. Jaegle, Phys. Status Solidi B, 60 (1973) 117. 2 L. Kubler, G. Gewinner, J. J. Koulmann and A. Jaeglb, Phys. Status Solidi B, 69 (1975) 323. 3 D. Geist, in V. I. Matkovich, P. Hagenmuller, G. V. Samsonov and T. Lundstrom (eds.), Boron and Refractory Borides, Springer, Berlin, 1977, p. 65. 4 E. M. Averbakh, N. E. Soloviev and J. A. Ugai, Author’s Certificate 331661, cl. CO1 b, 35/00, claimed 1970, published January 16, 1973. 5 P. M. Pirtshalashvili, I. A. Bairamashvili, T. V. Samkurashvili, J. P. Lomidze, S. A. Loladze and N. Y. Gudushauri, in F. N. Tavadze (ed.), Boron: Preparation, Structure and Properties, Nauka, Moscow, 1974, p. 23. 6 N. Lundquist, N. P. Myers and H. Westin, Philos. &Zag., 7 (1962) 1187. 7 I. P. Borovskaya, N. P. Novikov and A. G. Merzhanov, in G. V. Samsonov (ed.), Refractory Borides and Silicides, Naukova Dumka, Kiev, 1977, p. 29. 8 K. H. J. Buschov, in V. I. Matkovich, P. Hagenmuller, G. V. Samsonov and T. Lundstrom (eds.), Boron and Refractory Borides, Springer, Berlin, 1977, p. 494. 9 A. Abragam and B. Bleaney, in S. A. Altshuller and G. V. Skrotsky (eds.), Electron Paramagnetic Resonance of Transition Zons, Vol. 1, Mir, Moscow, 1972, p. 227.