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Magnetic ordering in rare-earth formates
Physica B 234-236 (1997)679-681
ELSEV1ER
Magnetic ordering in rare-earth formates V. T r o u n o v a'*, E. T s e r k o v n a y a a, S. G a v r i l o v a, S. V a h r u s h e v b aPNPl RASc, Leningrad district, 188 350 Gatchina, St. Petersburg, Russian Federation bPTI RASc, St.-Petersburg, Russian Federation
Abstract In this report the preliminary results of a study of the structural instability in Tb(HCOO)a and Tm(DCOO)a are presented. The appearance of instability was fixed by the help of the unusual compressing of unit-cell parameter in lowtemperature range and the jump of dielectrical susceptibility. Keywords: Instability; Phase transition; Powder diffraction; Rare-earth metals
1. Introduction Rare-earth element formates in the series from La up to Tm are well-known for its nonlinear optical properties. Formate of Y is included in this isostructural list [1-3]. It was assumed that the optical properties are defined by thin structural features of formate group; that stimulated the interest of structural researches [4-6]. Formates contain light and heavy elements in their structure, and consequently, the use of neutron diffraction is effective and justified. Only with the help of neutrons the existence of hydrogen bond in all isostructural range was experimentally established [5, 6]. In all the structural series no essential distinctions were found that could explain large changes of optical properties [6], down to temperature of thermal decomposition. Thus, it was established that thermal destruction starts with gap in R e - O bond [7].
2. Experiments and received results Tb(HCO0)3 and Tm(DCO0)3 were used for the study. The research was conducted in the temper* Corresponding author.
ature range 23-300 K. In Fig. 1, temperature dependencies of lattice constants for formates of Tm are presented (data for Tb are analogous). The significant nonlinear changes of lattice constants with temperature were found for both compounds. For a lattice constant along hexagonal axis the nonlinear character near the temperature ~< 100 K is the most obvious. No essential changes in diffraction spectra were found, but an opportunity of structural transition, for example, magnetic-type was not excluded. The diffraction spectrum was measured by time-of-flight technique on a backscattering detector in a range of small values of dhkl. Such geometry has weak sensitivity to magnetic scattering because of restrictions due to the magnetic-form factor. Moreover, the correlation device (Fourier technique) has very limited opportunities in registration of weak reflections on a background of strong ones. Measurements of magnetic susceptibility 103-105 Hz were carried out a within a temperature range 60-300 K. Within this temperature range nonlinear effects of susceptibility were not found at the level ~< 10-5. Experiments on diffraction of polarized neutrons were carried out at two temperatures: 106 and 4 K. The geometry of scattering was chosen so that the
0921-4526/97/$17.00 © 1997 Elsevier ScienceB.V. All rights reserved PII S092 1-4526(96)0 1085-X
680
K Trounov et al./PhysicaB234-236H99 ~ 679-681
*< 10.34-
5000-
~ 10.33-
O
4500-
~ 10.32-
O
/
• o
4000-
lO.31
10.30
E =
=
1500 '
0
3.9190-
2
i
,
i
•
i
,
i
,
i
,
i
50 100 150 200 250 300 Temperature, K
-
~ 3.9185~ 3.9150-
/
3'0
;o
s'o
two theta, degree
do
10
Fig. 2. 0-20 scanning with polarized neutrons at T = 4 K; 2 = 2.51 A. SF-spin-flip and NSF - non-spin-flip scattering intensity.
/
"~ 3.9175i
3.9170
2500-
8 2000-
,~ 10.29~
g O
o 3500. 04 -.~ 3000-
10.28
SF / NSFJ
0 ' 5 0 ' 160' 1;0'260'2;0'360 Temperature, K
Fig. 1. Temperature dependence of unit-cell parameters of Tm(HCOO)3. The size of experimental point is equal to errors.
2.05 ZOO' 1.95
I~
-~1.90
"
1.85
•
1,80
70 polarization was unparallel to a vector of scattering. In this case, with availability of magnetic scattering and "spin-flipper" on, only the observation of magnetic reflections is possible. In Fig. 2, the areas of scanning close to (3 3 0) reflection are shown for Tb(HCOO)3. At all temperatures diffraction reflections are absent with "spin-flipper" on. The area of scanning on dhk] was chosen on the basis of processing of time-of-flight diffraction spectrum and the small differences observed between the experimental spectrum and calculated profile. The results of the measurement of the magnetic susceptibility and diffraction of polarized neutrons, as it appears, permits the rejection of the assumption that magnetic ordering at (23 ~< T ~< 100) K is the reason for the sharp change in lattice constants. Next, the measurements of dielectric susceptibility on a pellet prepared from Tm(DCOO)3 were conducted. In a cycle of heating, the abnormal change of susceptibility was found out at T ,-, 76 K
80
90
T,K
100
110
Fig. 3. Temperature dependence of dielectrical susceptibility: real part, nonnormalized values, arbitrary units, f.,=,, = 10 Hz.
(Fig. 3). Because of shortage of time, a more detailed study was not conducted. Therefore these experiments will be continued. Nevertheless, using these data it is possible to make the conclusion that the reason of nonlinear change of lattice constants (up to 4 K) is not of magnetic nature and it is more than likely that the change is connected with elastic properties.
Acknowledgements The authors wish to thank the ECNS'96 Organizing Committee for allocation of the grant and Russian Scientific funds for support.
lL Trounov et aL / Physica B 234-236 (1997) 679-681
References [1] A. Adreychuc, L. Dorozhkin and Yu. Krasilov et al., Crystallography 28 (1983) 922 (in Russian). [2] L. Soboleva, V. Ogadzhanov and L. Hapaeva et al., Crystallography 29 (1984) 581 (in Russian). [3] P.S. Bechthold and S. Haussiihl. Appl. Phys. 14 (1977) 403.
681
[4] H. Furmanova, Z. Razmazanova and L. Soboleva et al., Crystallography 29 (1984) 476 (in Russian). [5] V. Trounov, V. Kudryashev and A. Bulkin et al., Solid State Commun. 59 (1986) 95. [6] R. Bolotovsky, A. Bulkin, V. Kudryashev and V. Trounov et al., Solid State Commun. 76 (1990) 1045. [7] V. Trounov and E. Bessrnertnaya., Preprint PNPI, N2013, SS-71 (1994).
ELSEV1ER
Magnetic ordering in rare-earth formates V. T r o u n o v a'*, E. T s e r k o v n a y a a, S. G a v r i l o v a, S. V a h r u s h e v b aPNPl RASc, Leningrad district, 188 350 Gatchina, St. Petersburg, Russian Federation bPTI RASc, St.-Petersburg, Russian Federation
Abstract In this report the preliminary results of a study of the structural instability in Tb(HCOO)a and Tm(DCOO)a are presented. The appearance of instability was fixed by the help of the unusual compressing of unit-cell parameter in lowtemperature range and the jump of dielectrical susceptibility. Keywords: Instability; Phase transition; Powder diffraction; Rare-earth metals
1. Introduction Rare-earth element formates in the series from La up to Tm are well-known for its nonlinear optical properties. Formate of Y is included in this isostructural list [1-3]. It was assumed that the optical properties are defined by thin structural features of formate group; that stimulated the interest of structural researches [4-6]. Formates contain light and heavy elements in their structure, and consequently, the use of neutron diffraction is effective and justified. Only with the help of neutrons the existence of hydrogen bond in all isostructural range was experimentally established [5, 6]. In all the structural series no essential distinctions were found that could explain large changes of optical properties [6], down to temperature of thermal decomposition. Thus, it was established that thermal destruction starts with gap in R e - O bond [7].
2. Experiments and received results Tb(HCO0)3 and Tm(DCO0)3 were used for the study. The research was conducted in the temper* Corresponding author.
ature range 23-300 K. In Fig. 1, temperature dependencies of lattice constants for formates of Tm are presented (data for Tb are analogous). The significant nonlinear changes of lattice constants with temperature were found for both compounds. For a lattice constant along hexagonal axis the nonlinear character near the temperature ~< 100 K is the most obvious. No essential changes in diffraction spectra were found, but an opportunity of structural transition, for example, magnetic-type was not excluded. The diffraction spectrum was measured by time-of-flight technique on a backscattering detector in a range of small values of dhkl. Such geometry has weak sensitivity to magnetic scattering because of restrictions due to the magnetic-form factor. Moreover, the correlation device (Fourier technique) has very limited opportunities in registration of weak reflections on a background of strong ones. Measurements of magnetic susceptibility 103-105 Hz were carried out a within a temperature range 60-300 K. Within this temperature range nonlinear effects of susceptibility were not found at the level ~< 10-5. Experiments on diffraction of polarized neutrons were carried out at two temperatures: 106 and 4 K. The geometry of scattering was chosen so that the
0921-4526/97/$17.00 © 1997 Elsevier ScienceB.V. All rights reserved PII S092 1-4526(96)0 1085-X
680
K Trounov et al./PhysicaB234-236H99 ~ 679-681
*< 10.34-
5000-
~ 10.33-
O
4500-
~ 10.32-
O
/
• o
4000-
lO.31
10.30
E =
=
1500 '
0
3.9190-
2
i
,
i
•
i
,
i
,
i
,
i
50 100 150 200 250 300 Temperature, K
-
~ 3.9185~ 3.9150-
/
3'0
;o
s'o
two theta, degree
do
10
Fig. 2. 0-20 scanning with polarized neutrons at T = 4 K; 2 = 2.51 A. SF-spin-flip and NSF - non-spin-flip scattering intensity.
/
"~ 3.9175i
3.9170
2500-
8 2000-
,~ 10.29~
g O
o 3500. 04 -.~ 3000-
10.28
SF / NSFJ
0 ' 5 0 ' 160' 1;0'260'2;0'360 Temperature, K
Fig. 1. Temperature dependence of unit-cell parameters of Tm(HCOO)3. The size of experimental point is equal to errors.
2.05 ZOO' 1.95
I~
-~1.90
"
1.85
•
1,80
70 polarization was unparallel to a vector of scattering. In this case, with availability of magnetic scattering and "spin-flipper" on, only the observation of magnetic reflections is possible. In Fig. 2, the areas of scanning close to (3 3 0) reflection are shown for Tb(HCOO)3. At all temperatures diffraction reflections are absent with "spin-flipper" on. The area of scanning on dhk] was chosen on the basis of processing of time-of-flight diffraction spectrum and the small differences observed between the experimental spectrum and calculated profile. The results of the measurement of the magnetic susceptibility and diffraction of polarized neutrons, as it appears, permits the rejection of the assumption that magnetic ordering at (23 ~< T ~< 100) K is the reason for the sharp change in lattice constants. Next, the measurements of dielectric susceptibility on a pellet prepared from Tm(DCOO)3 were conducted. In a cycle of heating, the abnormal change of susceptibility was found out at T ,-, 76 K
80
90
T,K
100
110
Fig. 3. Temperature dependence of dielectrical susceptibility: real part, nonnormalized values, arbitrary units, f.,=,, = 10 Hz.
(Fig. 3). Because of shortage of time, a more detailed study was not conducted. Therefore these experiments will be continued. Nevertheless, using these data it is possible to make the conclusion that the reason of nonlinear change of lattice constants (up to 4 K) is not of magnetic nature and it is more than likely that the change is connected with elastic properties.
Acknowledgements The authors wish to thank the ECNS'96 Organizing Committee for allocation of the grant and Russian Scientific funds for support.
lL Trounov et aL / Physica B 234-236 (1997) 679-681
References [1] A. Adreychuc, L. Dorozhkin and Yu. Krasilov et al., Crystallography 28 (1983) 922 (in Russian). [2] L. Soboleva, V. Ogadzhanov and L. Hapaeva et al., Crystallography 29 (1984) 581 (in Russian). [3] P.S. Bechthold and S. Haussiihl. Appl. Phys. 14 (1977) 403.
681
[4] H. Furmanova, Z. Razmazanova and L. Soboleva et al., Crystallography 29 (1984) 476 (in Russian). [5] V. Trounov, V. Kudryashev and A. Bulkin et al., Solid State Commun. 59 (1986) 95. [6] R. Bolotovsky, A. Bulkin, V. Kudryashev and V. Trounov et al., Solid State Commun. 76 (1990) 1045. [7] V. Trounov and E. Bessrnertnaya., Preprint PNPI, N2013, SS-71 (1994).