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Magnetic properties of NaFeP2O7 studied by neutron diffraction and Mössbauer resonance techniques
Ph,vsica 136B (1986) 447-450 North-Holland, Amsterdam
MAGNETIC PROPERTIES OF NaFeP207 STUDIED BY NEUTRON DIFFRACTION AND MOSSBAUER RESONANCE TECHNIQUES J.L. S O U B E Y R O U X * , R. S A L M O N , L. F O U R N E S and G. L E F L E M Laboratoire de Chimie du Solide du CNRS, 351 Cours de la Libdration, 33405 Talence Cedex, France
The magnetic properties of NaFeP207 have been experimentally studied using neutron diffraction and M6ssbauer spectroscopy. The antiferromagnetic structure has been determined in space group P21/c' with q = [0, 0, 0] and /~ = 4.53(5)~B. The presence of short-range ordering has been discussed on the basis of the relaxation times. A dynamical ordering process has been proposed to account for the observations over a wide range around the critical temperature.
1. Introduction The magnetic properties of oxygenated compounds whose structure consists of a three-dimensional array of (FeO6) octahedra linked by (XO4) tetrahedra (X = S, Mo, P) have been studied by neutron diffraction and M6ssbauer spectroscopy. They are Fe2(SO4) 3 [1], Fez(MoO4) 3 [2-3] and Na3Fez(PO4) 3 [4]; however, no experiments have been performed on compounds where (FeO6) octahedra are linked by pyrophosphate groups: (P207). As the first series of compounds leads to important covalency effects and time dependent phenomenon, one can expect for pyrophosphate compounds accentuated effects as more numerous bridges O - P - O connect two iron +3 ions. We have undertaken a detailed investigation of NaFeP20 7 whose structure is described in ref. 5 with a study of magnetic properties by susceptibility and M6ssbauer spectroscopy measurements. The results of the neutron diffraction experiments are compared with other magnetic results.
2. Experimental NaFeP20 7 has been prepared in its high temperature form of monoclinic symmetry P 21/c [5]. The X-ray powder diffraction pattern of the sample was in excellent agreement with previous * Permanent address: Institut Laue-Langevin, 156X, 38042, Grenoble, France.
data and showed no trace of impurities. Powder neutron-diffraction data were recorded using the multidetector of the S I L O E reactor in Grenoble. The 0.248 nm wavelength allowed a Q-range of 1.5 to 34 nm i to be explored without moving the detector. The powdered sample was contained in a vanadium can and mounted in a cryostat whose temperature was controlled by carbon-platinum thermistances whose accuracy is - 0 . 2 K in the vicinity of the critical temperature. Data reduction and refinements were performed on the DEC-10 computer at ILL using the P. Wolfer's programs [6]. The M6ssbauer effect spectra were recorded at the Laboratoire de Chimie du Solide, Talence. The study was performed between 293 and 4.2 K and the sample was made up of 95 mg of ground crystals glued in a resin.
3. Neutron-diffraction results The 40 K neutron diffraction pattern was indexed in the space group P21/c with a = 0.7310(1) nm, b = 0.7853(1) nm, c = 0.9552(1) nm and/3 = 111.89°(2). These cell parameters are in very good agreement with those determined using X-ray at room temperature. A simple refinement of the scale factor and of an overall D e b y e Waller factor led to an R-factor of 6.3% for the nuclear pattern and confirm the structure previously determined and shown in fig. 1.
0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
448
J.L. Soubeyroux et al. / Magnetic properties of NaFePzO 7
0
z
./ ,'
_••
/
II I A
Fe(1) " \ J
!I \o='-c
z½ ,'; / 4,----~51
Fe(3)
Fig. 1. Nuclear and magnetic structure of NaFeP207. (Only iron y positions are reported.)
The 1.6K pattern exhibits strong magnetic lines that can be indexed in the nuclear cell (q = 0, 0, 0). A difference pattern (1.6-39 K) was used to refine the magnetic structure on the basis of 14 well-defined lines (fig. 2). The F e 3-~ ions occupy the 4-fold general positions and are numbered in fig. 1, in accordance with the International Tables [7]. The refinement of the magnetic structure indicates a colinear structure with Gz configuration:
Gz = S l z -
S2z + S3z-
S4z.
Table I Observed and calculated magnetic lines for NaFeP207 at 2.5 K
H K L a~
lobs(Sigma)b)
lca~~
010 -1 0 1
43.65(0.5) 36.85(0.75) 13.60(0.75)
43.29 35,95 14.14
I4.80(1.2)
15,42
56.50(4.2)
57,25
5.80(0.5)
5.44
101
012 0 I-2. - 1 2 1" 12-1
-1 0 3 2-1 0 210 -212 2 1-2 121
The refined magnetic moment was /z = 4.53(5)/z B. This is the exact value found in the whole series of compounds, and no difference in covalency can be assumed. An analysis of the symmetry elements allowed with the Gz mode led to the magnetic group P21/c'. Furthermore Gx and Fy are also permitted. The last mode will justify the weak ferromagnetism experimentally detected (0.02/z~), but no significant improve-
1-2 1 030 - 1 2 3} 1 2-3 -301 3 2 00 3 - 2 }
99.9(4.8)
98,98
37.35
40.70
20.85(2.4)
23.08
24.25(2.7)
21.55
18.90(3.1)
20.19
19.40(5.9)
27.30
a~All multiplicities are 2. b)The sigma value is the result of a gaussian fit. C)The intensities are calculated with free iron +3 form factor.
J.L. Soubeyroux et al. / Magnetic properties of NaFeP207
ment of the refinement was observed with the complete set of mode (Gz, Gx, Fy) instead of the initial one, Gz. The intensities of magnetic reflections are given in table I. A study as a function of temperature has been performed increasing the
,,a,ii
IT=49K
I (a,u),
T = 3I. 1K
I
l
i '
i i
i
I (a'u)I ~.~, ~.1~,~!~' .0~,~
1
T = 35.7K
: LI
I (a,u)
32.9K "
t
T= 1.6K tll ' o
a'o
30
4'o b
Fig. 2. Neutron difference patterns (160 K pattern subtracted) of NaFeP207. ( × ) indicates a magnetic reflection.
449
temperature from 1.6 to 160 K. Each pattern has been recorded for three hours after one hour of stable temperature such as no temperature gradient exists in our samples under helium atmosphere. The difference patterns have been calculated at a given temperature by subtracting the pattern obtained at 160 K. Some typical examples are given in fig. 2. Magnetic long-range ordering (Bragg peaks, LRO) is still present at 35.7 K that is to say 7 K above the critical temperature found in Mrssbauer spectroscopy experiments. Magnetic short-range ordering (SRO) is present between 36 and 49 K and corresponds to the modulation of the background (fig. 2). The spinspin correlations study in this range of temperature is under development.
4. M6ssbauer spectroscopy analysis
The Mrssbauer spectroscopy study between 293 and 4.2 K has been the subject of a previous paper [5] and only the main results are reported here with a new insight. The low temperature pattern is characterized by a six-line magnetic spectrum whose parameters are those of high-spin +3 iron in an octahedral crystal field. In all patterns a fraction of the +3 iron remains paramagnetic even at 4.2 K. Patterns recorded at 29 and 30 K are characterized by a quadrupole split line with no important broadening of the peaks as would be expected in the presence of short-range static ordering.
5. Discussion
The magnetic structure of NaFeP20 7 can be described by noting that a given iron atom (e.g., Fe(1) in fig. 1) is connected to ten nearest neighbour iron atoms as follows: the two shortest distances with Fe(3) atoms (d = 0.48nm) have ferromagnetic couplings; the four Fe(2) atoms (d = 0.54 nm) have AF couplings; the four Fe(4) atoms (d = 0.62 nm) have AF couplings. The magnetic properties can be summarized as
450
J.L. Soubeyroux et al. / Magnetic properties of NaFee207
follows: the compound orders antiferromagnetically with a superimposed weak ferromagnetism. The paramagnetism obeys a Curie-Weiss law with 0p = -54 K and indicates a +3 iron state. At 50 K SRO appears on neutron diffraction patterns but this is not detected on the Mrssbauer spectroscopy ones. The maximum in the magnetic susceptibility corresponds well to the ordering temperature (T N = 2 8 . 7 K ) seen by Mrssbauer spectroscopy but LRO is already found at higher temperature by neutron diffraction. These results are consistent with the following time analysis of the magnetic ordering process. On decreasing the temperature, one can follow, for example, the evolution of a ~ 10nm iron domain. From room temperature to 50 K the relaxation time for the spins is shorter than ~- = 10 -12 S, that is to say, less than the time for a neutron (A =0.25nm) to cross the domain. So the compound is supposed to be paramagnetic. In the temperature range 50-36 K, r may be of the order of 10 -12 s and in the neutron pattern shortrange ordering is observed. Mrssbauer effect remains insensitive to such short relaxation times and the compound appears paramagnetic. Between 36 and 28.7 K the correlation length increases and enlarged Bragg peaks grow in the neutron diffraction patterns but no LRO is detected by Mrssbauer spectroscopy whose times of r = 10-11-10-1°s are still not accessible. Some tenth of K just around TN short-range ordering is revealed by Mrssbauer spectroscopy. Below Tn the relaxation times are very long and both techniques show the magnetic LRO effects. If these ordering processes are commonly involved [8], in general the range of temperatures is rather limited and the critical phenomenon can only be studied for few degrees around TN. The weak ferromagnetism observed below T N fulfills the Dzyaloshinky-Moriya symmetry rules for canted spins and the proposed mechanism [1-4] of two
uncompensated sublattices cannot apply here since there is only one sublattice. Paramagnetic free +3 iron has yet to be encountered in compounds where competition between ferro- and antiferromagnetic interactions leads to noncorrelated spins [9]. An inelastic neutron diffraction study of NaFeP207 is planned in order to determine the variation of both the correlation range and the relaxation time with temperature.
Acknowledgments It is a pleasure to acknowledge D. Fruchart for very fruitful discussions and comments on this work. We thank the neutron diffraction group of the Centre d'Etudes Nucleaires de Grenoble for the provision of the neutron facilities.
References [1] G.J. Long, G. Longworth, P. Battle, A.K. Cheetham, R.V. Thundathil and D. Beveridge, Inorg. Chem. 18 (1979) 624. [2] Z. Jirak, R. Salmon, L. Fournes, F. Menil and P. Hagenmuller, Inorg. Chem. 21 (1982) 4218. [3] P.D. Battle, A.K. Cheetham, G.J. Long and G. Longworth, Inorg. Chem. 21 (1982) 4223. [4] D. Beltran-Porter, R. Olazcuaga, L. Fournes, F. Menil and G. Le Flem, Rev. Phys. Appl. 15 (1980) 1155. [5] T. Moya-Pizarro, R. Salmon, L. Fournes, G. Le Flem, B. Wanklyn and P. Hagenmuller, J. Sol. State Chem. 53 (1984) 387. [6] P. Wolfers, Int. Rep. Institut Laue-Langevin, Grenoble (1976). [7] International Table for Crystallography, A.T. Hahn ed., A (1983) 174. [8] L.J. De Jongh and A.R. Miedema, Adv. Phys. 23 (1974) 1. [9] D. Fiorani, S. Viticoli, J.L. Dormann, J.L. Tholenee, J. Hammann, A.P. Murani and J.L. Soubeyroux, J. Phys. C16 (1983) 3175.
MAGNETIC PROPERTIES OF NaFeP207 STUDIED BY NEUTRON DIFFRACTION AND MOSSBAUER RESONANCE TECHNIQUES J.L. S O U B E Y R O U X * , R. S A L M O N , L. F O U R N E S and G. L E F L E M Laboratoire de Chimie du Solide du CNRS, 351 Cours de la Libdration, 33405 Talence Cedex, France
The magnetic properties of NaFeP207 have been experimentally studied using neutron diffraction and M6ssbauer spectroscopy. The antiferromagnetic structure has been determined in space group P21/c' with q = [0, 0, 0] and /~ = 4.53(5)~B. The presence of short-range ordering has been discussed on the basis of the relaxation times. A dynamical ordering process has been proposed to account for the observations over a wide range around the critical temperature.
1. Introduction The magnetic properties of oxygenated compounds whose structure consists of a three-dimensional array of (FeO6) octahedra linked by (XO4) tetrahedra (X = S, Mo, P) have been studied by neutron diffraction and M6ssbauer spectroscopy. They are Fe2(SO4) 3 [1], Fez(MoO4) 3 [2-3] and Na3Fez(PO4) 3 [4]; however, no experiments have been performed on compounds where (FeO6) octahedra are linked by pyrophosphate groups: (P207). As the first series of compounds leads to important covalency effects and time dependent phenomenon, one can expect for pyrophosphate compounds accentuated effects as more numerous bridges O - P - O connect two iron +3 ions. We have undertaken a detailed investigation of NaFeP20 7 whose structure is described in ref. 5 with a study of magnetic properties by susceptibility and M6ssbauer spectroscopy measurements. The results of the neutron diffraction experiments are compared with other magnetic results.
2. Experimental NaFeP20 7 has been prepared in its high temperature form of monoclinic symmetry P 21/c [5]. The X-ray powder diffraction pattern of the sample was in excellent agreement with previous * Permanent address: Institut Laue-Langevin, 156X, 38042, Grenoble, France.
data and showed no trace of impurities. Powder neutron-diffraction data were recorded using the multidetector of the S I L O E reactor in Grenoble. The 0.248 nm wavelength allowed a Q-range of 1.5 to 34 nm i to be explored without moving the detector. The powdered sample was contained in a vanadium can and mounted in a cryostat whose temperature was controlled by carbon-platinum thermistances whose accuracy is - 0 . 2 K in the vicinity of the critical temperature. Data reduction and refinements were performed on the DEC-10 computer at ILL using the P. Wolfer's programs [6]. The M6ssbauer effect spectra were recorded at the Laboratoire de Chimie du Solide, Talence. The study was performed between 293 and 4.2 K and the sample was made up of 95 mg of ground crystals glued in a resin.
3. Neutron-diffraction results The 40 K neutron diffraction pattern was indexed in the space group P21/c with a = 0.7310(1) nm, b = 0.7853(1) nm, c = 0.9552(1) nm and/3 = 111.89°(2). These cell parameters are in very good agreement with those determined using X-ray at room temperature. A simple refinement of the scale factor and of an overall D e b y e Waller factor led to an R-factor of 6.3% for the nuclear pattern and confirm the structure previously determined and shown in fig. 1.
0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
448
J.L. Soubeyroux et al. / Magnetic properties of NaFePzO 7
0
z
./ ,'
_••
/
II I A
Fe(1) " \ J
!I \o='-c
z½ ,'; / 4,----~51
Fe(3)
Fig. 1. Nuclear and magnetic structure of NaFeP207. (Only iron y positions are reported.)
The 1.6K pattern exhibits strong magnetic lines that can be indexed in the nuclear cell (q = 0, 0, 0). A difference pattern (1.6-39 K) was used to refine the magnetic structure on the basis of 14 well-defined lines (fig. 2). The F e 3-~ ions occupy the 4-fold general positions and are numbered in fig. 1, in accordance with the International Tables [7]. The refinement of the magnetic structure indicates a colinear structure with Gz configuration:
Gz = S l z -
S2z + S3z-
S4z.
Table I Observed and calculated magnetic lines for NaFeP207 at 2.5 K
H K L a~
lobs(Sigma)b)
lca~~
010 -1 0 1
43.65(0.5) 36.85(0.75) 13.60(0.75)
43.29 35,95 14.14
I4.80(1.2)
15,42
56.50(4.2)
57,25
5.80(0.5)
5.44
101
012 0 I-2. - 1 2 1" 12-1
-1 0 3 2-1 0 210 -212 2 1-2 121
The refined magnetic moment was /z = 4.53(5)/z B. This is the exact value found in the whole series of compounds, and no difference in covalency can be assumed. An analysis of the symmetry elements allowed with the Gz mode led to the magnetic group P21/c'. Furthermore Gx and Fy are also permitted. The last mode will justify the weak ferromagnetism experimentally detected (0.02/z~), but no significant improve-
1-2 1 030 - 1 2 3} 1 2-3 -301 3 2 00 3 - 2 }
99.9(4.8)
98,98
37.35
40.70
20.85(2.4)
23.08
24.25(2.7)
21.55
18.90(3.1)
20.19
19.40(5.9)
27.30
a~All multiplicities are 2. b)The sigma value is the result of a gaussian fit. C)The intensities are calculated with free iron +3 form factor.
J.L. Soubeyroux et al. / Magnetic properties of NaFeP207
ment of the refinement was observed with the complete set of mode (Gz, Gx, Fy) instead of the initial one, Gz. The intensities of magnetic reflections are given in table I. A study as a function of temperature has been performed increasing the
,,a,ii
IT=49K
I (a,u),
T = 3I. 1K
I
l
i '
i i
i
I (a'u)I ~.~, ~.1~,~!~' .0~,~
1
T = 35.7K
: LI
I (a,u)
32.9K "
t
T= 1.6K tll ' o
a'o
30
4'o b
Fig. 2. Neutron difference patterns (160 K pattern subtracted) of NaFeP207. ( × ) indicates a magnetic reflection.
449
temperature from 1.6 to 160 K. Each pattern has been recorded for three hours after one hour of stable temperature such as no temperature gradient exists in our samples under helium atmosphere. The difference patterns have been calculated at a given temperature by subtracting the pattern obtained at 160 K. Some typical examples are given in fig. 2. Magnetic long-range ordering (Bragg peaks, LRO) is still present at 35.7 K that is to say 7 K above the critical temperature found in Mrssbauer spectroscopy experiments. Magnetic short-range ordering (SRO) is present between 36 and 49 K and corresponds to the modulation of the background (fig. 2). The spinspin correlations study in this range of temperature is under development.
4. M6ssbauer spectroscopy analysis
The Mrssbauer spectroscopy study between 293 and 4.2 K has been the subject of a previous paper [5] and only the main results are reported here with a new insight. The low temperature pattern is characterized by a six-line magnetic spectrum whose parameters are those of high-spin +3 iron in an octahedral crystal field. In all patterns a fraction of the +3 iron remains paramagnetic even at 4.2 K. Patterns recorded at 29 and 30 K are characterized by a quadrupole split line with no important broadening of the peaks as would be expected in the presence of short-range static ordering.
5. Discussion
The magnetic structure of NaFeP20 7 can be described by noting that a given iron atom (e.g., Fe(1) in fig. 1) is connected to ten nearest neighbour iron atoms as follows: the two shortest distances with Fe(3) atoms (d = 0.48nm) have ferromagnetic couplings; the four Fe(2) atoms (d = 0.54 nm) have AF couplings; the four Fe(4) atoms (d = 0.62 nm) have AF couplings. The magnetic properties can be summarized as
450
J.L. Soubeyroux et al. / Magnetic properties of NaFee207
follows: the compound orders antiferromagnetically with a superimposed weak ferromagnetism. The paramagnetism obeys a Curie-Weiss law with 0p = -54 K and indicates a +3 iron state. At 50 K SRO appears on neutron diffraction patterns but this is not detected on the Mrssbauer spectroscopy ones. The maximum in the magnetic susceptibility corresponds well to the ordering temperature (T N = 2 8 . 7 K ) seen by Mrssbauer spectroscopy but LRO is already found at higher temperature by neutron diffraction. These results are consistent with the following time analysis of the magnetic ordering process. On decreasing the temperature, one can follow, for example, the evolution of a ~ 10nm iron domain. From room temperature to 50 K the relaxation time for the spins is shorter than ~- = 10 -12 S, that is to say, less than the time for a neutron (A =0.25nm) to cross the domain. So the compound is supposed to be paramagnetic. In the temperature range 50-36 K, r may be of the order of 10 -12 s and in the neutron pattern shortrange ordering is observed. Mrssbauer effect remains insensitive to such short relaxation times and the compound appears paramagnetic. Between 36 and 28.7 K the correlation length increases and enlarged Bragg peaks grow in the neutron diffraction patterns but no LRO is detected by Mrssbauer spectroscopy whose times of r = 10-11-10-1°s are still not accessible. Some tenth of K just around TN short-range ordering is revealed by Mrssbauer spectroscopy. Below Tn the relaxation times are very long and both techniques show the magnetic LRO effects. If these ordering processes are commonly involved [8], in general the range of temperatures is rather limited and the critical phenomenon can only be studied for few degrees around TN. The weak ferromagnetism observed below T N fulfills the Dzyaloshinky-Moriya symmetry rules for canted spins and the proposed mechanism [1-4] of two
uncompensated sublattices cannot apply here since there is only one sublattice. Paramagnetic free +3 iron has yet to be encountered in compounds where competition between ferro- and antiferromagnetic interactions leads to noncorrelated spins [9]. An inelastic neutron diffraction study of NaFeP207 is planned in order to determine the variation of both the correlation range and the relaxation time with temperature.
Acknowledgments It is a pleasure to acknowledge D. Fruchart for very fruitful discussions and comments on this work. We thank the neutron diffraction group of the Centre d'Etudes Nucleaires de Grenoble for the provision of the neutron facilities.
References [1] G.J. Long, G. Longworth, P. Battle, A.K. Cheetham, R.V. Thundathil and D. Beveridge, Inorg. Chem. 18 (1979) 624. [2] Z. Jirak, R. Salmon, L. Fournes, F. Menil and P. Hagenmuller, Inorg. Chem. 21 (1982) 4218. [3] P.D. Battle, A.K. Cheetham, G.J. Long and G. Longworth, Inorg. Chem. 21 (1982) 4223. [4] D. Beltran-Porter, R. Olazcuaga, L. Fournes, F. Menil and G. Le Flem, Rev. Phys. Appl. 15 (1980) 1155. [5] T. Moya-Pizarro, R. Salmon, L. Fournes, G. Le Flem, B. Wanklyn and P. Hagenmuller, J. Sol. State Chem. 53 (1984) 387. [6] P. Wolfers, Int. Rep. Institut Laue-Langevin, Grenoble (1976). [7] International Table for Crystallography, A.T. Hahn ed., A (1983) 174. [8] L.J. De Jongh and A.R. Miedema, Adv. Phys. 23 (1974) 1. [9] D. Fiorani, S. Viticoli, J.L. Dormann, J.L. Tholenee, J. Hammann, A.P. Murani and J.L. Soubeyroux, J. Phys. C16 (1983) 3175.