Low-temperature heat capacity of dysprosium diboride

Article abstract of DOI:10.1007/s10973-006-8054-8

Heat capacity C _p(T) of dysprosium diboride is experimentally investigated at the temperatures of 5-300 K. On the dependence C _p(T) smooth anomaly at the temperatures of 15-20 K and sharp anomalies at T _m1=47.8 K and T

Full text of DOI:10.1007/s10973-006-8054-8

Journal of Thermal Analysis and Calorimetry, Vol. 88 (2007) 2, 597–599
LOW-TEMPERATURE HEAT CAPACITY OF DYSPROSIUM DIBORIDE
V. V. Novikov* and A. V. Matovnikov
Bryansk State University, Bedjitskaya str., 14, 241036 Bryansk, Russia
Heat capacity Cp(T) of dysprosium diboride is experimentally investigated at the temperatures of 5–300 K. On the dependence
Cp(T) smooth anomaly at the temperatures of 15–20 K and sharp anomalies at Tm1=47.8 K and Tm2=178.8 K are revealed. Anomaly
at Tm1 is caused, obviously, by ferromagnetic phase transition in DyB2. The lattice contribution to DyB2 heat capacity is determined
by comparison with heat capacity of non-magnetic YB2. The estimation of Schottky contribution to DyB2 heat capacity is made, the
scheme of splitting of the ground state of Dy3+ ion is offered.
Keywords: diborides, heat capacity, low temperatures, magnetic phase transitions
Introduction
Experimental
Sample
The diborides of rare-earth elements RB2 represent
the group of borides which is practically not investi-
gated until recently. It is caused obviously by the dif-
ficulty of making single-phase samples of these com-
pounds. It is known that diborides of neodymium,
samarium, gadolinium are formed only at high pres-
sures. The diborides of the heavier RE metals may be
received by direct synthesis from elements at high
temperatures [1].
DyB2 sample was synthesized from elements through
intermediate hydride phase. Synthesis was carried out
according to reactions
2R+3H2®2RH3
RH3+4B®RB2+B2H6
RH3+RB4®RB2+B2H6
The rare-earth diborides are crystallized in hexa-
gonal structure D61h – P6 / mmm, there is one atom of
The received hydride was mixed up stoichio-
metrically with amorphous boron, the mix was frayed
in jasper mortar, was pressed at pressure 8t. The re-
ceived tablet was placed in a molybdenum crucible
and burnt in an atmosphere of argon in two stages.
The first stage of burn lasted 6 h at temperature
T=1200°C. The X-ray spectrum of the created sample
contained mainly diffraction maxima of RB4 and also
RB2.
At the 2nd stage the hydride RH3 was added to the
frayed sample according the scheme: RH3+RB4®
RB2+B2H6. The received mixture was frayed in a
jasper mortar and again pressed under pressure 8t,
then was burnt at temperature T=1200°C during 3 h.
The X-ray powder diffraction pattern of the synthe-
sized sample is given on Fig. 2. The positions and
amplitudes of reflexes of DyB2 phase on the
diffractogram are close to ASTM data. There is an
insignificant quantity (less than 3%) of tetraboride
phases and also the oxide of dysprosium in the
sample. The lattice parameters are calculated from the
X-ray data are: a=0.32915 nm, c=0.385182 nm (on
the data [2] a=0.328 nm, c=0.3845 nm).
metal and two atoms of boron per elementary cell
(Fig. 1). At low temperatures the majority of rare-
earth diborides are ferromagnetics. Curie tempera-
tures TC of rare-earth diborides are in the interval
5–150 K (TC =151 K, TC =55 K, TC =15 K,
TbB
DyB
HoB
2
2
2
TC =16 K) [1, 2].
ErB
2 The purpose of the present work is the experi-
mental research of heat capacity of dysprosium
diboride at the temperatures of 5–300 K.
Fig. 1 Crystal structure of DyB2: 1 – Dy atoms; 2 – B atoms
*
Author for correspondence: vvnovikov@mail.ru
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2007 Akadémiai Kiadó, Budapest

NOVIKOV, MATOVNIKOV
Experimental values of heat capacity Cp(T) of
DyB2 and YB2 plot of temperature are given on Fig. 5.
Practically in all investigated temperature interval the
heat capacity of DyB2 is greater than appropriate values
of the non-magnetic YB2 which are also given on Fig. 3.
On a curve Cp(T) of dysprosium diboride a num-
ber of anomalies is clearly shown: a smooth maxi-
mum in the field of 20 K and two sharp maxima at
temperatures Tm1=47.8 K and Tm2=178.8 K.
Results and discussion
Fig. 2 X-ray powder diffraction pattern of DyB2: 1 – Dy,
2 – DyB2, 3 – Dy2B3, 4 – DyB4
Lattice component of heat capacity Clat(T) of
dysprosium diboride was determined by the method of
conformity of lattice heat capacities of isostructural
compounds by comparison with non-magnetic yttrium
diboride [4]. Thus it is considered, that near to room
temperature isobaric and isochoric heat capacities of
diborides are approximately identical, Cp@Cv, and the
full heat capacity of diborides consists only of lattice
component. In this case it is possible to believe, that
heat capacities of isostructural substances are identical
functions of absolute temperature and characteristic
temperature inherent to every compound. Therefore
The heat capacity at constant pressure of DyB2
was measured in an adiabatic calorimeter with puls
heating [3]. At the temperatures of 5–15 K the
temperature was measured with
a germanium
resistance thermometer with accuracy of 0.05 K. At
higher temperatures the platinum thermometer was
used whose accuracy of measurement was 0.01 K. The
relative error of measurement in the temperature
interval of 5–20
K equals 0.6%, at higher
temperatures – 0.3%. Calibration of the calorimeter
was done on a sample of electrolytic copper which was
burnt in vacuum.
QDyB (T ) QDyB (300 K)
2
2
=
QYB (T ) QYB (300 K)
2
2
Using formula (1) and tables of Debye’s function
the first approximation of lattice contribution to DyB2
heat capacity Clat was calculated. After separation of
lattice component of heat capacity and integration of
1
the given excess heat capacity DC1(T)/T=(C–C )/T
lat1
the value of DyB2 excess entropy DS1(T) was received.
It equals 44.6 J mol–1 K–1 at the temperature 300 K.
This value considerably surpasses the theoretical value
of entropy change DSm=Rln(2J+1) during infringement
of magnetic ordering. For Dy3+ ions J=15/2 and DSm
equals 23.04 J mol–1. Obviously other mechanisms
besides processes of magnetic ordering also contribute
to the full heat capacity of dysprosium diboride. Such
additional contribution may be the heat capacity
caused by Schottky’s effect which is characteristic for
compounds of the majority of rare earths.
Fig. 3 Heat capacity of 1 – DyB2 and 2 – YB2
Since energy levels of ions with even number of
electrons are split by crystal field to singlets and
doublets [5] the statistical sum of energy levels of a
dysprosium ion (Fig. 4) corresponding to the simplified
three-level scheme of splitting is given by the formula:
2 / kT
Z = x1 + x2e–d / kT + x2e–d
1
where xi is degeneration of ith sublevel, di is size of
splitting.
Proceeding from the best conformity to the data of
experiment the following characteristics of splitting are
Fig. 4 Schottky contribution to heat capacity of DyB2
598
J. Therm. Anal. Cal., 88, 2007

HEAT CAPACITY OF DYSPROSIUM DIBORIDE
investigation of DyB2 has shown that at the temp-
erature below 180 K reflexes of another phase appear
on the X-ray powder diffraction patterns. This allows
to consider that in dysprosium diboride besides
magnetic transition a structural transition takes place.
The effect of this transition is the maximum of heat
capacity at the temperature Tm2 and nonzero value of
excess heat capacity at the temperatures of 50–180 K.
Conclusions
Fig. 5 Components of low-temperature heat capacity of DyB2:
1 – CDyB , 2 – Clat , 3 – DC2, 4 – CSch
Calorimetric investigation of low-temperature prop-
erties of dysprosium diboride has allowed to reveal
the series of new, earlier unknown characteristics of
its lattice and magnetic subsystems.
2
2
• The characteristic temperatures of YB2 and DyB2
are quite big (QDyB (300 K)=987 K, QYB (300 K)=
2
2
774 K). This is characteristic for refractory sub-
stances. So the values of isochoric and isobaric heat
capacities of diborides in the investigated interval
of temperatures differ insignificantly.
• It follows from the analysis of temperature depen-
dencies of excess entropy and X-ray data that be-
sides ferromagnetic transition at the temperature
Tm1 in DyB2 a structural phase transition also takes
place at higher temperatures.
T DC
• Determined in this work parameters of splitting of
Dy3+ ion ground level by crystal electric field and
corresponding to them Schottky contribution to
DyB2 heat capacity were determined from the best
correspondence to experimental data and have esti-
mated character.
Fig. 6 Excess entropy of DyB , DS = ò
dT
2
T
0
(DC=C–Clat–CSch) vs. temperature
offered: x1=2, x2=2, x3=12, d1=50 K, d2=340 K. Here
values of xi may comprise united closely located
sublevels. Schottky heat capacity CSch(T) corresponding
to this parameters is given on Fig. 4. The calculated
value CSch(300 K) is 2.5 J mol–1 K–1.
References
The account of this value at calculation of DyB2
lattice heat capacity allowed to determine the corrected
temperature dependence of lattice component Clat (T )
1 K. H. J. Buschov, Boron and Refractory Borides,
V. I. Matkovich, Ed., NY 1977, p. 494.
2 G. Will, K. H. J. Buschov and V. Lehman,
Inst. Phys. Conf. Ser., 37 (1978) 255.
2
given on Fig. 5. In the same figure dependences
C
Sch(T) and DC2(T)=C–C –CSch are given.
3 N. N. Sirota, A. M. Antyukhov, V. V. Novikov and
V. A. Fyodorov, Cryst. Res. Technol., 13 (1982) 279.
4 J. W. Stout and E. Catalano, J. Chem. Phys., 23 (1955) 2013.
5 S. A. Altshuler and B. M. Kozyrev, Electron Paramagnetic
Resonance, Moscow 1961, p. 368.
lat 2
Temperature dependence of excess dysprosium
diboride entropy DS(T) determined from dependence
DC2(T)=C –Clat –CSch is given on Fig. 6. As you
can see from the figure it turns out that the value
DS(300 K) is close to DSm, but nevertheless it surpasses
DSm at temperatures higher than 140 K. The X-ray
DyB2
2
DOI: 10.1007/s10973-006-8054-8
J. Therm. Anal. Cal., 88, 2007
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