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Crystal structure, magnetic and electrical transport properties of SrKFeWO6 double perovskite
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Materials Letters 62 (2008) 2033 – 2035 www.elsevier.com/locate/matlet
Crystal structure, magnetic and electrical transport properties of SrKFeWO6 double perovskite Guoyan Huo ⁎, Xiaoyu Zhang, Meixiu Zang, Yongxuan Cai School of Chemistry and Environmental Science, Hebei University, Baoding 071002, PR China Received 16 August 2007; accepted 5 November 2007 Available online 12 November 2007
Abstract The crystal structure, magnetic and electrical transport properties of SrKFeWO6 double perovskite have been investigated. The structure of this double perovskite compound is assigned to monoclinic system with space group P21/n and structural analysis suggests that cationic Fe and W alternately occupy B′- and B′′-sites. Thermal magnetization demonstrates that the magnetic transition temperature is higher than 500 K. Isothermal magnetization shows that the net magnetic moment per unit formula in the direction of the magnetization is 0.72 μB (300 K). The electrical transport process can be described by non-adiabatic small polaron hopping model at low temperature, while the electronic transport behavior at high temperature is described by adiabatic small polaron hopping model. Large magnetoresistance, −0.89, can be observed under low magnetic field, 0.5 T, around room temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: Ceramics; Magnetic materials; Electrical properties; Crystal structure
1. Introduction Ordered double perovskite oxides (DP) A2B′B′′O6 (A: alkaline-earth or rare-earth elements and B′, B′′: transition metal elements) consist of alternative arrangement of AB′O3 and AB′′O3 perovskite units. The characteristics of some DP oxides are half metallicity and magnetism with Curie temperature (TC) higher than room temperature (RT) [1]. A large low-field magnetoresistance, due to intergrain tunneling, even at RT, makes DP oxides as promising materials for spintronics [2]. The electronic structure near the Fermi level in half-metallic A2FeMoO6, where A is a divalent cation such as Sr2+and Ba2+, is characterized by localized Fe3+ spins in the majority-spin state (3d5, S = 5/2) and negative spin-polarized conduction 1 electrons (t2g , S = 1/2) in the hybridized Fe and Mo orbitals [3]. It is considered that the conduction electron mediate the ferromagnetic correlation between the Fe spins. This is inferred from
⁎ Corresponding author. Tel.: +86 3125079359; fax: +86 3125079525. E-mail address: [email protected] (G. Huo). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.11.007
the increase of TC with electron doping by substituting trivalent cations such as La3+ [4] and Nd3+ [5] for the A-site element. Sr2FeWO6 is known as an anti-ferromagnetic insulator with TN of 16–37 K, where Fe2+ ion is in the high-spin state (3d6, S = 2) and W6+ion (5d0) is in the nonmagnetic state [6]. The large Fe–W charge–transfer gap gives rise to a complete localization of valence electrons, explaining the increase in resistivity. To our knowledge, very few references to the hole or electron doped in DP Sr2FeWO6 have been found. A study on the hole doped in this compound may throw some light on the A2FeWO6 compound application as electronic materials. In this paper we investigate the crystal structure, magnetic and transport of DP SrKFeWO6 polycrystalline sample above room temperature. 2. Experimental The polycrystalline sample SrKFeWO6 has been synthesized by standard solid state reaction technique. The raw materials, SrCO3, K2CO3 (more than stoichiometric amount of potassium carbonate was added in order to compensate loss in sintering process), Fe2O3 and WO3, of high purity (more than 99.99%) were mixed by hand in an agate mortar for at least 40 min. Then
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G. Huo et al. / Materials Letters 62 (2008) 2033–2035
Fig. 1. XRD pattern of SrKFeWO6 sample.
it was pressed into pellets under 10 Mpa pressure for 1 min. following preheating in air at 800 °C and 1000 °C for 8 h, respectively. The calcined mixture was pulverized and pressed into pellets. The pellets were sintered at 1150 °C for 16 h in a stream of 7% H2/Ar with intermediate grindings. Phase analysis and characterization were carried out by X-ray diffraction (XRD) using CuKα radiation on X'TRA model X-ray diffractometer. The estimation of cations in the sample was determined by atomic adsorption spectroscopic analysis. The oxygen content was determined by redox titration method. Temperature dependence of magnetization curves was measured by a vibrating-sample magnetometer (VSM) in field of 0.5 T over the temperature range 80–500 K. Transport properties were determined by a standard four-probe DC method in the temperature range 285–500 K. 3. Results and discussion The XRD data of powdered sample were collected at room temperature and the patterns are shown in Fig. 1 which shows that the sample is single phase. No impurities were detected based on XRD pattern. The diffraction peaks could be indexed in the monoclinic system with space group P21/n. The lattice parameters are measured by pirum program and found to be: a = 0.5619 nm, b = 0.5558 nm, c = 0.7955 nm and β = 89.990. The monoclinic distortion can be described in the P21/n space group (Glazer tilt system a+b−b−), corresponding to a “classic” Pnma structure of simple perovskites without cation ordering. However, the presence of cation ordering in SrKFeWO6 can clearly be seen from substantial intensity of the supperlattice diffraction indicated by ⁎ in Fig. 1 [7]. According to report [7] these supperlattice diffraction peaks are explained as due to the antiphase tilting of the octahedral BO6. The cation amount of specimen and oxygen was determined by atomic adsorption spectroscopic analysis and redox titration. The contents of constituted elements in the compound SrKFeWO6 are: Sr 10.24(3) at.%, K 9.67(8) at.%, Fe 10.01(2) at.%, W 9.98(4) at.% and O 59.88(6) at.%. It can be seen from results of element analysis that about 3.2% Fe2+appears in the sample and composition of this sample consists with nominal content. Magnetization measurement made as a function of temperature M (T) on warming the SrKFeWO6 sample from 300 to 500 K in a magnetic field
Fig. 2. Temperature dependent magnetization registered on warming in 0.5 T magnetic field. Inset: plot of M–H at 300 K.
of 0.5 T is shown in Fig. 2. It is evident to see from the M–T curve that no TC appears in this temperature range (we also measured the thermal magnetization curve in the temperature range from 80 to 300 K and no magnetic transition temperature was also observed in the M–T curve. The M–T curve is not presented here.) This indicates that the Curie temperature of SrKFeWO6 sample is higher than 500 K. The isothermal magnetization M (T, H) at 300 K with magnetic field up to 1.3 T was measured and is shown in the inset of Fig. 2. The magnetic moment is nearly saturated in magnetic field higher than 1.2 T, and M (300 K, H) changes linearly with reciprocal of the magnetic field. A linear extrapolation at 1/H = 0 allows us to obtain the saturation magnetization, M (300 K) and thus, the net magnetic moment per unit formula in the direction of the magnetization, 0.72 μB (300 K). The spontaneous moment M (300 K) = 0.56 μB, can be obtained by linear extrapolation high field slope to H = 0.0 T in M–H curve. The ρ (T) curves registered upon warming in zero and 0.5 T magnetic fields are shown in Fig. 3. One can see from Fig. 3 that SrKFeWO6 compound demonstrates semiconducting behavior under zero and 0.5 T magnetic fields over the temperature range, from 285 to 500 K, and shows a significant magnetoresistance (MR) effect in 0.5 T applied magnetic field. We define MR = [ρ (H) −ρ (0)]/ρ (0), where
Fig. 3. Temperature dependent resistivity registered upon warming in zero and 0.5 T magnetic fields (referred left vertical axis) and magnetoresistance in 0.5 T magnetic field (referred right vertical axis).
G. Huo et al. / Materials Letters 62 (2008) 2033–2035
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can be seen from inset of Fig. 4 that the electronic transport mechanism below the shoulder temperature can also be described by non-adiabatic small polaron hopping model. The activation energy above and below shoulder temperature can be obtained from the slopes of lines presented in the inset of Fig. 4 and are 0.220 and 0.177 eV, respectively.
4. Conclusions
Fig. 4. Dependence of resistivity (ρ) on the temperature (T) in a ln(ρ/T) against 1/T for high temperature. Inset: variation of ln(ρ/T3/2) as a function of inverse temperature. Solid lines are the best fit to model described by Eqs. (1) and (2).
ρ (H) and ρ (0) are the resistivity in a magnetic field and without a magnetic field, respectively. Fig. 3 shows the temperature dependence of the –MR in 0.5 T magnetic field. The –MR of this sample under 0.5 T magnetic field increase from 0.15 to 0.89 with decreasing temperature from 500 to 309 K. Such large –MR above room temperature indicates that the specimen can be used as a potential candidate for magnetoelectronic devices and magnetic storage application. There is a shoulder around 313 K in the curve of temperaturedependent resistivity. This phenomenon also exists in the Sr2FeMoO6 [8] polycrystalline materials. The authors did not explain this behavior in their paper. The similar features were seen in the simple perovskite manganites [9,10]. They assign this behavior to the result of competition between ferromagnetic and anti-ferromagnetic interactions [10]. In order to understand the electronic transport processes in the DP, the resistivity data have been fitted for various models and following models are the best for our sample: adiabatic model [11] q ¼ q0 T expðEA =kB T Þ
ð1Þ
and non-adiabatic model [12] q ¼ q0 T 3=2 expðEA =kB T Þ
ð2Þ
where ρ0 is the residual electrical resistivity, EA is the activation energy and kB is the Boltzmann constant. Results show in Fig. 4 and inset of Fig. 4. It can be seen from Fig. 4 that the true nature of electrical transport at low temperature is ascertained by non-adiabatic small polaron hopping model, while the electronic transport behavior at high temperature is described by adiabatic small polaron hopping model. It
We have investigated the structure, magnetoresistance, magnetic and electrical transport properties of double perovskite SrKFeWO6. The X-ray diffraction pattern shows that cations of Fe and W orderly occupy on B′- and B′′-sites, respectively. Thermal magnetization indicates that magnetic transition temperature of this compound is above 500 K. The net magnetic moment and spontaneous magnetization obtained from isothermal magnetization are 0.72 and 0.56 μB, respectively. The magnetoresistance measured show large values under 0.5 T magnetic field and decreases with temperature from 285 to 500 K. The detailing analysis of electrical transport process demonstrates that the resistivity is controlled by nonadiabatic small polaron hopping model at low temperature and that the resistivity can be described by adiabatic small polaron hopping model at high temperature. Acknowledgment This work is supported by the Natural Science Foundation of Hebei Province (No B2004000095). References [1] K.I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677–680. [2] Y. Tomioka, T. Okuda, Y. Okimoto, R. Kumai, K.I. Kobayashi, Y. Tokura, Phys. Rev. B 61 (2000) 422–427. [3] J.S. Kang, H. Han, B.W. Lee, C.G. Olson, S.W. Han, K.H. Kim, J.I. Jeong, J.H. Park, B.I. Min, Phys. Rev. B 64 (2001) 24429 (1–6). [4] J. Navarro, J. Fontcuberta, M. Izquierdo, J. Avila, M.C. Asensio, Phys. Rev. B 69 (2004) 115101 (1–6). [5] D. Rubi, C. Frontera, J. Fontcuberta, M. Wojcik, E. Jedryka, C. Ritter, Phys. Rev. B 70 (2004) 94405 (1–8). [6] I.V. Solovyev, Phys. Rev. B 65 (2002) 144446 (1–17). [7] W.T. Fu, D.J.W. Ijdo, Solid State Commun. 134 (2005) 177–181. [8] X.M. Feng, G.Y. Liu, Q.Z. Huang, G.H. Rao, Trans. Nonferr. Met. Soc. China 16 (2006) 122–126. [9] B.T. Cong, S.C. Yu, N.D. Tho, N. Chau, T.N. Huynh, T.L. Phan, J. Magn. Magn. Mater. 304 (2006) e448–e450. [10] V. Sen, N. Panwar, G.L. Bhalla, S.K. Agarwal, J. Phys. Chem. Solid (in press). [11] D. Emin, T. Holstein, Ann. Phys. 53 (1969) 439–520. [12] N.F. Mott, E.A. Davis, Electronic Process in Non-Crystalline Materials, Clarendon, Oxford, 1971.
Materials Letters 62 (2008) 2033 – 2035 www.elsevier.com/locate/matlet
Crystal structure, magnetic and electrical transport properties of SrKFeWO6 double perovskite Guoyan Huo ⁎, Xiaoyu Zhang, Meixiu Zang, Yongxuan Cai School of Chemistry and Environmental Science, Hebei University, Baoding 071002, PR China Received 16 August 2007; accepted 5 November 2007 Available online 12 November 2007
Abstract The crystal structure, magnetic and electrical transport properties of SrKFeWO6 double perovskite have been investigated. The structure of this double perovskite compound is assigned to monoclinic system with space group P21/n and structural analysis suggests that cationic Fe and W alternately occupy B′- and B′′-sites. Thermal magnetization demonstrates that the magnetic transition temperature is higher than 500 K. Isothermal magnetization shows that the net magnetic moment per unit formula in the direction of the magnetization is 0.72 μB (300 K). The electrical transport process can be described by non-adiabatic small polaron hopping model at low temperature, while the electronic transport behavior at high temperature is described by adiabatic small polaron hopping model. Large magnetoresistance, −0.89, can be observed under low magnetic field, 0.5 T, around room temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: Ceramics; Magnetic materials; Electrical properties; Crystal structure
1. Introduction Ordered double perovskite oxides (DP) A2B′B′′O6 (A: alkaline-earth or rare-earth elements and B′, B′′: transition metal elements) consist of alternative arrangement of AB′O3 and AB′′O3 perovskite units. The characteristics of some DP oxides are half metallicity and magnetism with Curie temperature (TC) higher than room temperature (RT) [1]. A large low-field magnetoresistance, due to intergrain tunneling, even at RT, makes DP oxides as promising materials for spintronics [2]. The electronic structure near the Fermi level in half-metallic A2FeMoO6, where A is a divalent cation such as Sr2+and Ba2+, is characterized by localized Fe3+ spins in the majority-spin state (3d5, S = 5/2) and negative spin-polarized conduction 1 electrons (t2g , S = 1/2) in the hybridized Fe and Mo orbitals [3]. It is considered that the conduction electron mediate the ferromagnetic correlation between the Fe spins. This is inferred from
⁎ Corresponding author. Tel.: +86 3125079359; fax: +86 3125079525. E-mail address: [email protected] (G. Huo). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.11.007
the increase of TC with electron doping by substituting trivalent cations such as La3+ [4] and Nd3+ [5] for the A-site element. Sr2FeWO6 is known as an anti-ferromagnetic insulator with TN of 16–37 K, where Fe2+ ion is in the high-spin state (3d6, S = 2) and W6+ion (5d0) is in the nonmagnetic state [6]. The large Fe–W charge–transfer gap gives rise to a complete localization of valence electrons, explaining the increase in resistivity. To our knowledge, very few references to the hole or electron doped in DP Sr2FeWO6 have been found. A study on the hole doped in this compound may throw some light on the A2FeWO6 compound application as electronic materials. In this paper we investigate the crystal structure, magnetic and transport of DP SrKFeWO6 polycrystalline sample above room temperature. 2. Experimental The polycrystalline sample SrKFeWO6 has been synthesized by standard solid state reaction technique. The raw materials, SrCO3, K2CO3 (more than stoichiometric amount of potassium carbonate was added in order to compensate loss in sintering process), Fe2O3 and WO3, of high purity (more than 99.99%) were mixed by hand in an agate mortar for at least 40 min. Then
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Fig. 1. XRD pattern of SrKFeWO6 sample.
it was pressed into pellets under 10 Mpa pressure for 1 min. following preheating in air at 800 °C and 1000 °C for 8 h, respectively. The calcined mixture was pulverized and pressed into pellets. The pellets were sintered at 1150 °C for 16 h in a stream of 7% H2/Ar with intermediate grindings. Phase analysis and characterization were carried out by X-ray diffraction (XRD) using CuKα radiation on X'TRA model X-ray diffractometer. The estimation of cations in the sample was determined by atomic adsorption spectroscopic analysis. The oxygen content was determined by redox titration method. Temperature dependence of magnetization curves was measured by a vibrating-sample magnetometer (VSM) in field of 0.5 T over the temperature range 80–500 K. Transport properties were determined by a standard four-probe DC method in the temperature range 285–500 K. 3. Results and discussion The XRD data of powdered sample were collected at room temperature and the patterns are shown in Fig. 1 which shows that the sample is single phase. No impurities were detected based on XRD pattern. The diffraction peaks could be indexed in the monoclinic system with space group P21/n. The lattice parameters are measured by pirum program and found to be: a = 0.5619 nm, b = 0.5558 nm, c = 0.7955 nm and β = 89.990. The monoclinic distortion can be described in the P21/n space group (Glazer tilt system a+b−b−), corresponding to a “classic” Pnma structure of simple perovskites without cation ordering. However, the presence of cation ordering in SrKFeWO6 can clearly be seen from substantial intensity of the supperlattice diffraction indicated by ⁎ in Fig. 1 [7]. According to report [7] these supperlattice diffraction peaks are explained as due to the antiphase tilting of the octahedral BO6. The cation amount of specimen and oxygen was determined by atomic adsorption spectroscopic analysis and redox titration. The contents of constituted elements in the compound SrKFeWO6 are: Sr 10.24(3) at.%, K 9.67(8) at.%, Fe 10.01(2) at.%, W 9.98(4) at.% and O 59.88(6) at.%. It can be seen from results of element analysis that about 3.2% Fe2+appears in the sample and composition of this sample consists with nominal content. Magnetization measurement made as a function of temperature M (T) on warming the SrKFeWO6 sample from 300 to 500 K in a magnetic field
Fig. 2. Temperature dependent magnetization registered on warming in 0.5 T magnetic field. Inset: plot of M–H at 300 K.
of 0.5 T is shown in Fig. 2. It is evident to see from the M–T curve that no TC appears in this temperature range (we also measured the thermal magnetization curve in the temperature range from 80 to 300 K and no magnetic transition temperature was also observed in the M–T curve. The M–T curve is not presented here.) This indicates that the Curie temperature of SrKFeWO6 sample is higher than 500 K. The isothermal magnetization M (T, H) at 300 K with magnetic field up to 1.3 T was measured and is shown in the inset of Fig. 2. The magnetic moment is nearly saturated in magnetic field higher than 1.2 T, and M (300 K, H) changes linearly with reciprocal of the magnetic field. A linear extrapolation at 1/H = 0 allows us to obtain the saturation magnetization, M (300 K) and thus, the net magnetic moment per unit formula in the direction of the magnetization, 0.72 μB (300 K). The spontaneous moment M (300 K) = 0.56 μB, can be obtained by linear extrapolation high field slope to H = 0.0 T in M–H curve. The ρ (T) curves registered upon warming in zero and 0.5 T magnetic fields are shown in Fig. 3. One can see from Fig. 3 that SrKFeWO6 compound demonstrates semiconducting behavior under zero and 0.5 T magnetic fields over the temperature range, from 285 to 500 K, and shows a significant magnetoresistance (MR) effect in 0.5 T applied magnetic field. We define MR = [ρ (H) −ρ (0)]/ρ (0), where
Fig. 3. Temperature dependent resistivity registered upon warming in zero and 0.5 T magnetic fields (referred left vertical axis) and magnetoresistance in 0.5 T magnetic field (referred right vertical axis).
G. Huo et al. / Materials Letters 62 (2008) 2033–2035
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can be seen from inset of Fig. 4 that the electronic transport mechanism below the shoulder temperature can also be described by non-adiabatic small polaron hopping model. The activation energy above and below shoulder temperature can be obtained from the slopes of lines presented in the inset of Fig. 4 and are 0.220 and 0.177 eV, respectively.
4. Conclusions
Fig. 4. Dependence of resistivity (ρ) on the temperature (T) in a ln(ρ/T) against 1/T for high temperature. Inset: variation of ln(ρ/T3/2) as a function of inverse temperature. Solid lines are the best fit to model described by Eqs. (1) and (2).
ρ (H) and ρ (0) are the resistivity in a magnetic field and without a magnetic field, respectively. Fig. 3 shows the temperature dependence of the –MR in 0.5 T magnetic field. The –MR of this sample under 0.5 T magnetic field increase from 0.15 to 0.89 with decreasing temperature from 500 to 309 K. Such large –MR above room temperature indicates that the specimen can be used as a potential candidate for magnetoelectronic devices and magnetic storage application. There is a shoulder around 313 K in the curve of temperaturedependent resistivity. This phenomenon also exists in the Sr2FeMoO6 [8] polycrystalline materials. The authors did not explain this behavior in their paper. The similar features were seen in the simple perovskite manganites [9,10]. They assign this behavior to the result of competition between ferromagnetic and anti-ferromagnetic interactions [10]. In order to understand the electronic transport processes in the DP, the resistivity data have been fitted for various models and following models are the best for our sample: adiabatic model [11] q ¼ q0 T expðEA =kB T Þ
ð1Þ
and non-adiabatic model [12] q ¼ q0 T 3=2 expðEA =kB T Þ
ð2Þ
where ρ0 is the residual electrical resistivity, EA is the activation energy and kB is the Boltzmann constant. Results show in Fig. 4 and inset of Fig. 4. It can be seen from Fig. 4 that the true nature of electrical transport at low temperature is ascertained by non-adiabatic small polaron hopping model, while the electronic transport behavior at high temperature is described by adiabatic small polaron hopping model. It
We have investigated the structure, magnetoresistance, magnetic and electrical transport properties of double perovskite SrKFeWO6. The X-ray diffraction pattern shows that cations of Fe and W orderly occupy on B′- and B′′-sites, respectively. Thermal magnetization indicates that magnetic transition temperature of this compound is above 500 K. The net magnetic moment and spontaneous magnetization obtained from isothermal magnetization are 0.72 and 0.56 μB, respectively. The magnetoresistance measured show large values under 0.5 T magnetic field and decreases with temperature from 285 to 500 K. The detailing analysis of electrical transport process demonstrates that the resistivity is controlled by nonadiabatic small polaron hopping model at low temperature and that the resistivity can be described by adiabatic small polaron hopping model at high temperature. Acknowledgment This work is supported by the Natural Science Foundation of Hebei Province (No B2004000095). References [1] K.I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677–680. [2] Y. Tomioka, T. Okuda, Y. Okimoto, R. Kumai, K.I. Kobayashi, Y. Tokura, Phys. Rev. B 61 (2000) 422–427. [3] J.S. Kang, H. Han, B.W. Lee, C.G. Olson, S.W. Han, K.H. Kim, J.I. Jeong, J.H. Park, B.I. Min, Phys. Rev. B 64 (2001) 24429 (1–6). [4] J. Navarro, J. Fontcuberta, M. Izquierdo, J. Avila, M.C. Asensio, Phys. Rev. B 69 (2004) 115101 (1–6). [5] D. Rubi, C. Frontera, J. Fontcuberta, M. Wojcik, E. Jedryka, C. Ritter, Phys. Rev. B 70 (2004) 94405 (1–8). [6] I.V. Solovyev, Phys. Rev. B 65 (2002) 144446 (1–17). [7] W.T. Fu, D.J.W. Ijdo, Solid State Commun. 134 (2005) 177–181. [8] X.M. Feng, G.Y. Liu, Q.Z. Huang, G.H. Rao, Trans. Nonferr. Met. Soc. China 16 (2006) 122–126. [9] B.T. Cong, S.C. Yu, N.D. Tho, N. Chau, T.N. Huynh, T.L. Phan, J. Magn. Magn. Mater. 304 (2006) e448–e450. [10] V. Sen, N. Panwar, G.L. Bhalla, S.K. Agarwal, J. Phys. Chem. Solid (in press). [11] D. Emin, T. Holstein, Ann. Phys. 53 (1969) 439–520. [12] N.F. Mott, E.A. Davis, Electronic Process in Non-Crystalline Materials, Clarendon, Oxford, 1971.