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Low-temperature study of magnetic ordering in gadolinium orthophosphate
Solid State Communications 134 (2005) 409–412 www.elsevier.com/locate/ssc
Low-temperature study of magnetic ordering in gadolinium orthophosphate C. Thirieta,*, P. Javorsky´a,b, R.J.M. Koningsa a
European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germany Charles University, Faculty of Mathematics and Physics, Department of Electronic Structures, Ke Karlovu 5, 12116 Prague 2, Czech Republic
b
Received 3 December 2004; received in revised form 3 February 2005; accepted 3 February 2005 by H. von Lo¨hneysen
Abstract The zero-field heat capacity shows an antiferromagnetic ordering of Gd3C in gadolinium orthophosphate at 0.8 K. The application of the external magnetic field leads to the splitting of the Gd3C ground-state multiplet. The antiferromagnetic ordering becomes gradually suppressed with increasing field, and the loss of the long-range magnetic ordering with a threshold field between 0.2 and 0.5 T is indicated by heat-capacity data. Estimated entropy of the anomaly due to magnetic ordering or the Schottky-type anomaly (above 0.5 T) is close to Rln8 as expected for Gd3C ground-state multiplet. Magnetization measurements above 2 K corroborate this magnetic behaviour. q 2005 Elsevier Ltd. All rights reserved. PACS: 65.40.Cg; 75.40.Cx; 75.50.Ee Keywords: A. Magnetically ordered materials; D. Heat capacity; D. Order–disorder effects
1. Introduction The specific properties of the lanthanide orthophosphate series such as their refractory nature make them useful for several interesting applications. We are studying the monoclinic monazite-type compounds of the lighter lanthanide elements, (La to Gd) as a potential nuclear waste form. Synthetic monazite shows very promising behaviour according to various criteria for a conditioning matrix such as a large incorporate amount of actinide (plutonium and americium) in solid solution [1,2]. The stability and compatibility of these compounds can be predicted by the variation of their Gibbs
* Corresponding author. Tel.: C49 7247 951 173; fax: C49 7247 951 99173. E-mail addresses: [email protected] (C. Thiriet), [email protected] (P. Javorsky´ ), [email protected] (R.J.M. Konings). 0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.02.005
free energy. This thermodynamic function is derived from the heat capacity indirectly. In this context the heat capacity of GdPO4 is measured from 0.5 to 301 K [3]. A heat capacity peak occurs below about 1 K suggesting a magnetic transition as already observed in other Gd compounds (i.e. GdF3 [4]). No magnetic studies for GdPO4 were published in the past to characterize the origin of the heat capacity peak except the work of Wang et al. [5]. We here report further studies using magnetization experiments.
2. Experimental GdPO4 was prepared by the neutral reaction of gadolinium nitrate with diammonium hydrogen phosphate in stoichiometric amounts at room temperature. Diluted NH4OH (5 M) was added until pH 4 after mixing. The gel formed was filtered, washed with water and finally dried in air at around 373 K. Next the dried powder was calcined
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C. Thiriet et al. / Solid State Communications 134 (2005) 409–412
Fig. 1. Temperature dependence of heat-capacity of GdPO4 under different external magnetic fields.
(2 h in air at 1073 K). Finally the sample was prepared in a form of a pressed and sintered pellet (5 h in air at 1673 K). X-ray diffraction was carried out at room temperature using a Bruker D8 diffractometer (Cu Ka1) with an integration time of 13 h. A single monoclinic phase was observed which is a monazite phase (P21/n space group) with lattice parameters aZ0.66503(2) nm, bZ0.68466(2) nm, cZ 0.63333(2) nm and bZ1048003(2) in accordance with the literature [6,7]. The specific heat (Cp) was measured by the relaxation method on the quantum design PPMS-9 system [8] between 0.5 and 301 K in magnetic fields up to 9 T. Several samples with different mass (0.1, 1 and 42 mg) were used in order to measure properly all the features. The DC magnetization (M) was measured on the PPMS-14 system on a 68 mg sample between 2 and 300 K. All the samples measured were pieces of the sintered pellet.
Fig. 2. Temperature dependence of Cexs/T for 0.2 T. , are experimental points and B the assumptions based on a T3 dependence of Cexs below TN. In inset, temperature dependence of the excess entropy for 0.2, 1 and 3 T, the dashed line represents SexsZRln8.
Fig. 3. Simulation of Cexs/T versus T for 3 T by Schottky anomaly involving 8 energy levels. In inset, magnetic field linear dependence of the Zeeman splitting with BZm0H.
3. Results and discussion The heat capacity data are plotted versus temperature T in Fig. 1. A l-type anomaly occurs at the Ne´el temperature TNZ0.8, 0.7 and 0.6 K under 0, 0.1 and 0.2 T respectively (inset of Fig. 1). The expected suppression of TN when increasing magnetic field suggests an antiferromagnetic ordering of Gd moments. A comparable magnetic behaviour was found for monoclinic B-type GdO1.5 [9] and GdF3 [4] for example. When increasing the field above certain limit Hlim, being between 0.2 and 0.5 T, the anomaly peak shape changes to Schottky-type anomaly and shifts gradually towards the higher temperature as shown in Fig. 1. These changes reveal the loss of the magnetic order in GdPO4 beyond Hlim as a consequence of increasing splitting of the ground-state level. Detailed analysis of the excess heat capacity (Cexs) requires the substraction of the lattice term Clat from the total heat capacity. We assumed that Clat is not influenced by magnetic field so that it is equal to the relations found from the zero-field measurement. As explained in a previous article [3] Clat is given by a polynomial function of order 6 above 25 K and by 2.169!10K4T 3 between 0 and 25 K. The detailed calculation of the Debye temperature (qD) is reported in [3]. qD is equal to 208 K. The temperature dependence of Cexs/T where CexsZ CpKClat is represented for 0.2 T in Fig. 2. Between 0 and 0.5 K the heat capacity was approximated assuming a 3D antiferromagnetic spin wave model: CpZCexsZaT 3. The value of a is determined as a single point fit to Cp at 0.5 K. Its value is 53.5, 59.4 and 77.0 J molK1 KK4 under 0, 0.1 and 0.2 T respectively. The area under the curve Cexs/T gives the excess entropy Sexs. Sexs is similar for data under all applied fields, inferior and superior to Hlim. So there is no latent heat that could be separable from the magnetic entropy. Sexs is derived as 16.6
C. Thiriet et al. / Solid State Communications 134 (2005) 409–412
411
Fig. 4. Magnetization as function of applied magnetic field at 2 and 5 K with BZm0H and as solid line its fit with Brillouin function.
close to the theoretical value for Gd3C ions, Rln8Z17.3, R being the gas constant (inset of Fig. 2). It confirms that the complete excitation from the ground state is covered in the temperature range considered here. Above Hlim, the heat capacity reveals a Schottky anomaly associated with Zeeman splitting the ground state level of Gd3C. An example of the simulation of Cexs/T versus T from the heat capacity measurement under 3 T is given in Fig. 3. The Zeeman splitting versus the magnetic field applied was estimated roughly. As expected the magnetic field dependence of the Zeeman splitting is linear as shown in inset of Fig. 3. The magnetic susceptibility measured between 2 and 300 K in 3 T follows the simple Curie-Weiss law. The fit of experimental data in the whole range yielded an effective magnetic moment meff of 8.37 mB, close to the Gd3C free-ion pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi value ðg JðJ C 1Þ Z 7:94 mB Þ, and a paramagnetic CurieWeiss temperature qp of K3.7 K. This negative value is another indication of antiferromagnetic interactions in GdPO4. The magnetization was carried out as function of applied magnetic field at 2 and 5 K as shown in Fig. 4. The shape of magnetization curves suggests a paramagnetic behaviour for GdPO4 in these conditions, in agreement with the heatcapacity data. There is a weak hysteresis on the 2 K curve that can be due to small amount of magnetic impurity. As expected in the paramagnetic region, the measured curves for both temperatures follow well the Brillouin function, except for the fact that the observed saturation value of the Gd moment reaches 7.3 mB which exceeds slightly the Gd3C free ion value (gJZ7.0 mB). Possible reason could be again small amount of impurity that was not detected by X-ray diffraction. Magnetization measurements down to around 0.5 K are necessary to understand more deeply the physics of this material, e.g. to decide about the nature of the phase
transition as its sharpness could indicate first-order type transition.
4. Conclusion An antiferromagnetic ordering of Gd3C in GdPO4 was shown below 0.8 K under an external magnetic field inferior to 0.5 T. The application of a higher magnetic field leads to the splitting of the Gd3C ground-state multiplet and the loss of the long-range magnetic ordering as indicated by heatcapacity and magnetization data. The estimated excess entropy is close to Rln8 as expected.
Acknowledgements The work done in Prague is a part of the research program MSM 0021620834 financed by the Ministry of Education of the Czech Republic. C.T. and P.J. acknowledge the European Commission for support given in the frame of the program ‘Training and Mobility of Researchers’.
References [1] G.J. McCarthy, W.B. White, D.E. Pfoertsh, Mater. Res. Bull. 13 (1978) 1239–1245. [2] L.A. Boatner, M.M. Abraham, B.C. Sales, Inorg. Chim. Acta 94 (1984) 146–148. [3] C. Thiriet, R.J.M. Konings, P. Javorsky´, N. Magnani, F. Wastin, J. Chem. Thermodyn. 37 (2005) 129. [4] H.E. Flotow, P.A.G. OHare, J. Chem. Phys. 74 (1981) 3046. [5] F.F.Y. Wang, R.S. Feigelson, Proc. Rare Earth Res. Conf. 375 (1964) 77–91.
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C. Thiriet et al. / Solid State Communications 134 (2005) 409–412
[6] J.G. Pepin, E.R. Vance, J. Inorg. Nucl. Chem. 43 (1981) 2807– 2809. [7] Y. Ni, J.M. Hughes, A.N. Mariano, Am. Mineral. 80 (1995) 21– 26.
[8] PPMS: Physical Property Measurement System, Quantum Design, San Diego, 1999. [9] A.E. Miller, F.J. Jelinek, K.A. Gschneidner, B.C. Gerstein, J. Chem. Phys. 55 (1971) 2647.
Low-temperature study of magnetic ordering in gadolinium orthophosphate C. Thirieta,*, P. Javorsky´a,b, R.J.M. Koningsa a
European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germany Charles University, Faculty of Mathematics and Physics, Department of Electronic Structures, Ke Karlovu 5, 12116 Prague 2, Czech Republic
b
Received 3 December 2004; received in revised form 3 February 2005; accepted 3 February 2005 by H. von Lo¨hneysen
Abstract The zero-field heat capacity shows an antiferromagnetic ordering of Gd3C in gadolinium orthophosphate at 0.8 K. The application of the external magnetic field leads to the splitting of the Gd3C ground-state multiplet. The antiferromagnetic ordering becomes gradually suppressed with increasing field, and the loss of the long-range magnetic ordering with a threshold field between 0.2 and 0.5 T is indicated by heat-capacity data. Estimated entropy of the anomaly due to magnetic ordering or the Schottky-type anomaly (above 0.5 T) is close to Rln8 as expected for Gd3C ground-state multiplet. Magnetization measurements above 2 K corroborate this magnetic behaviour. q 2005 Elsevier Ltd. All rights reserved. PACS: 65.40.Cg; 75.40.Cx; 75.50.Ee Keywords: A. Magnetically ordered materials; D. Heat capacity; D. Order–disorder effects
1. Introduction The specific properties of the lanthanide orthophosphate series such as their refractory nature make them useful for several interesting applications. We are studying the monoclinic monazite-type compounds of the lighter lanthanide elements, (La to Gd) as a potential nuclear waste form. Synthetic monazite shows very promising behaviour according to various criteria for a conditioning matrix such as a large incorporate amount of actinide (plutonium and americium) in solid solution [1,2]. The stability and compatibility of these compounds can be predicted by the variation of their Gibbs
* Corresponding author. Tel.: C49 7247 951 173; fax: C49 7247 951 99173. E-mail addresses: [email protected] (C. Thiriet), [email protected] (P. Javorsky´ ), [email protected] (R.J.M. Konings). 0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.02.005
free energy. This thermodynamic function is derived from the heat capacity indirectly. In this context the heat capacity of GdPO4 is measured from 0.5 to 301 K [3]. A heat capacity peak occurs below about 1 K suggesting a magnetic transition as already observed in other Gd compounds (i.e. GdF3 [4]). No magnetic studies for GdPO4 were published in the past to characterize the origin of the heat capacity peak except the work of Wang et al. [5]. We here report further studies using magnetization experiments.
2. Experimental GdPO4 was prepared by the neutral reaction of gadolinium nitrate with diammonium hydrogen phosphate in stoichiometric amounts at room temperature. Diluted NH4OH (5 M) was added until pH 4 after mixing. The gel formed was filtered, washed with water and finally dried in air at around 373 K. Next the dried powder was calcined
410
C. Thiriet et al. / Solid State Communications 134 (2005) 409–412
Fig. 1. Temperature dependence of heat-capacity of GdPO4 under different external magnetic fields.
(2 h in air at 1073 K). Finally the sample was prepared in a form of a pressed and sintered pellet (5 h in air at 1673 K). X-ray diffraction was carried out at room temperature using a Bruker D8 diffractometer (Cu Ka1) with an integration time of 13 h. A single monoclinic phase was observed which is a monazite phase (P21/n space group) with lattice parameters aZ0.66503(2) nm, bZ0.68466(2) nm, cZ 0.63333(2) nm and bZ1048003(2) in accordance with the literature [6,7]. The specific heat (Cp) was measured by the relaxation method on the quantum design PPMS-9 system [8] between 0.5 and 301 K in magnetic fields up to 9 T. Several samples with different mass (0.1, 1 and 42 mg) were used in order to measure properly all the features. The DC magnetization (M) was measured on the PPMS-14 system on a 68 mg sample between 2 and 300 K. All the samples measured were pieces of the sintered pellet.
Fig. 2. Temperature dependence of Cexs/T for 0.2 T. , are experimental points and B the assumptions based on a T3 dependence of Cexs below TN. In inset, temperature dependence of the excess entropy for 0.2, 1 and 3 T, the dashed line represents SexsZRln8.
Fig. 3. Simulation of Cexs/T versus T for 3 T by Schottky anomaly involving 8 energy levels. In inset, magnetic field linear dependence of the Zeeman splitting with BZm0H.
3. Results and discussion The heat capacity data are plotted versus temperature T in Fig. 1. A l-type anomaly occurs at the Ne´el temperature TNZ0.8, 0.7 and 0.6 K under 0, 0.1 and 0.2 T respectively (inset of Fig. 1). The expected suppression of TN when increasing magnetic field suggests an antiferromagnetic ordering of Gd moments. A comparable magnetic behaviour was found for monoclinic B-type GdO1.5 [9] and GdF3 [4] for example. When increasing the field above certain limit Hlim, being between 0.2 and 0.5 T, the anomaly peak shape changes to Schottky-type anomaly and shifts gradually towards the higher temperature as shown in Fig. 1. These changes reveal the loss of the magnetic order in GdPO4 beyond Hlim as a consequence of increasing splitting of the ground-state level. Detailed analysis of the excess heat capacity (Cexs) requires the substraction of the lattice term Clat from the total heat capacity. We assumed that Clat is not influenced by magnetic field so that it is equal to the relations found from the zero-field measurement. As explained in a previous article [3] Clat is given by a polynomial function of order 6 above 25 K and by 2.169!10K4T 3 between 0 and 25 K. The detailed calculation of the Debye temperature (qD) is reported in [3]. qD is equal to 208 K. The temperature dependence of Cexs/T where CexsZ CpKClat is represented for 0.2 T in Fig. 2. Between 0 and 0.5 K the heat capacity was approximated assuming a 3D antiferromagnetic spin wave model: CpZCexsZaT 3. The value of a is determined as a single point fit to Cp at 0.5 K. Its value is 53.5, 59.4 and 77.0 J molK1 KK4 under 0, 0.1 and 0.2 T respectively. The area under the curve Cexs/T gives the excess entropy Sexs. Sexs is similar for data under all applied fields, inferior and superior to Hlim. So there is no latent heat that could be separable from the magnetic entropy. Sexs is derived as 16.6
C. Thiriet et al. / Solid State Communications 134 (2005) 409–412
411
Fig. 4. Magnetization as function of applied magnetic field at 2 and 5 K with BZm0H and as solid line its fit with Brillouin function.
close to the theoretical value for Gd3C ions, Rln8Z17.3, R being the gas constant (inset of Fig. 2). It confirms that the complete excitation from the ground state is covered in the temperature range considered here. Above Hlim, the heat capacity reveals a Schottky anomaly associated with Zeeman splitting the ground state level of Gd3C. An example of the simulation of Cexs/T versus T from the heat capacity measurement under 3 T is given in Fig. 3. The Zeeman splitting versus the magnetic field applied was estimated roughly. As expected the magnetic field dependence of the Zeeman splitting is linear as shown in inset of Fig. 3. The magnetic susceptibility measured between 2 and 300 K in 3 T follows the simple Curie-Weiss law. The fit of experimental data in the whole range yielded an effective magnetic moment meff of 8.37 mB, close to the Gd3C free-ion pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi value ðg JðJ C 1Þ Z 7:94 mB Þ, and a paramagnetic CurieWeiss temperature qp of K3.7 K. This negative value is another indication of antiferromagnetic interactions in GdPO4. The magnetization was carried out as function of applied magnetic field at 2 and 5 K as shown in Fig. 4. The shape of magnetization curves suggests a paramagnetic behaviour for GdPO4 in these conditions, in agreement with the heatcapacity data. There is a weak hysteresis on the 2 K curve that can be due to small amount of magnetic impurity. As expected in the paramagnetic region, the measured curves for both temperatures follow well the Brillouin function, except for the fact that the observed saturation value of the Gd moment reaches 7.3 mB which exceeds slightly the Gd3C free ion value (gJZ7.0 mB). Possible reason could be again small amount of impurity that was not detected by X-ray diffraction. Magnetization measurements down to around 0.5 K are necessary to understand more deeply the physics of this material, e.g. to decide about the nature of the phase
transition as its sharpness could indicate first-order type transition.
4. Conclusion An antiferromagnetic ordering of Gd3C in GdPO4 was shown below 0.8 K under an external magnetic field inferior to 0.5 T. The application of a higher magnetic field leads to the splitting of the Gd3C ground-state multiplet and the loss of the long-range magnetic ordering as indicated by heatcapacity and magnetization data. The estimated excess entropy is close to Rln8 as expected.
Acknowledgements The work done in Prague is a part of the research program MSM 0021620834 financed by the Ministry of Education of the Czech Republic. C.T. and P.J. acknowledge the European Commission for support given in the frame of the program ‘Training and Mobility of Researchers’.
References [1] G.J. McCarthy, W.B. White, D.E. Pfoertsh, Mater. Res. Bull. 13 (1978) 1239–1245. [2] L.A. Boatner, M.M. Abraham, B.C. Sales, Inorg. Chim. Acta 94 (1984) 146–148. [3] C. Thiriet, R.J.M. Konings, P. Javorsky´, N. Magnani, F. Wastin, J. Chem. Thermodyn. 37 (2005) 129. [4] H.E. Flotow, P.A.G. OHare, J. Chem. Phys. 74 (1981) 3046. [5] F.F.Y. Wang, R.S. Feigelson, Proc. Rare Earth Res. Conf. 375 (1964) 77–91.
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C. Thiriet et al. / Solid State Communications 134 (2005) 409–412
[6] J.G. Pepin, E.R. Vance, J. Inorg. Nucl. Chem. 43 (1981) 2807– 2809. [7] Y. Ni, J.M. Hughes, A.N. Mariano, Am. Mineral. 80 (1995) 21– 26.
[8] PPMS: Physical Property Measurement System, Quantum Design, San Diego, 1999. [9] A.E. Miller, F.J. Jelinek, K.A. Gschneidner, B.C. Gerstein, J. Chem. Phys. 55 (1971) 2647.