Thermochimica Acta 515 (2011) 79–83
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Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Thermodynamic properties of pentalead tris(vanadate) chloride
A.V. Knyazev
∗
, E.N. Bulanov, N.N. Smirnova, N.Yu. Kuznetsova, I.A. Letyanina, E.D. Pryamova
Nizhny Novgorod State University, Gagarin Prospekt 23/2, Nizhny Novgorod 603950, Russia
article info
Article history:
Received 9 November 2010
Received in revised form
17 December 2010
Accepted 22 December 2010
Available online 11 January 2011
Keywords:
Apatite
Vanadinite
Phase transition
Heat capacity
Thermodynamic functions
abstract
In the present work the temperature dependences of pentalead tris(vanadate) chloride heat capacity have
been measured for the first time in the range from 7 to 350 K. The experimental data were used to calculate
standard thermodynamic functions, namely the heat capacity C
p
◦
(T), enthalpy H
◦
(T) - H
◦
(0), entropy
S
◦
(T) - S
◦
(0) and Gibbs function G
◦
(T) - H
◦
(0), for the range from T → 0 to 350 K. The differential scanning
calorimetry in temperature interval 298–1473 K was applied to measure temperatures of phase transition
and melting of compound under study. The high-temperature X-ray diffraction in range 298–1073 K was
used for the determining thermal expansion coefficients.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Pentalead tris(vanadate) chloride Pb
5
(VO
4
)
3
Cl belongs to struc-
tural class of apatite. These compounds have the general formula
M
II
5
(A
V
O
4
)
3
L (M
II
= Ca, Sr, Ba, Cd, Pb; A
V
= P, As, V, Mn, Cr; L = OH,
F, Cl, Br, I) and can be used as matrix for radioactive wastes
(Pb
5
(VO
4
)
3
I) [1], bioactive ceramics (Ca
5
(PO
4
)
3
OH, Ca
5
(PO
4
)
3
F) [2],
inorganic pigments (Ba
5
(MnO
4
)
3
Cl) [3]. In particular synthetic ana-
logue of mineral vanadinite Pb
5
(VO
4
)
3
Cl is the general source of
vanadium. In spite of that there are a few articles consecrated
to study of this compound [4,5]. In the present work we have
described thermodynamics properties of pentalead tris(vanadate)
chloride, which can be used in calculating of solution behavior of
vanadate-ion.
2. Experimental
2.1. Sample
Sample of Pb
5
(VO
4
)
3
Cl was prepared by the solid-state reac-
tion between lead vanadate and lead chlorate. The synthesis was
performed in a porcelain crucible, into which the reaction mix-
ture with the atomic ration 5Pb + 3V + 1Cl was loaded. The mixture
was calcined at 573 and 873 K for 10 h with dispersion in an agate
mortar every 2 h. To prove the atomic ration 5Pb + 3V + 1Cl the
∗
Corresponding author. Tel.: +7 831 462 32 34; fax: +7 831 434 50 56.
E-mail address: knyazevav@gmail.com (A.V. Knyazev).
obtained sample was analyzed on a energo-dispersive roentgen
fluorescent spectrometer. The X-ray data and estimated impurity
content (0.5–1 wt.%) in the substances led us to conclude that the
studied sample was an individual crystalline compound.
2.2. Apparatus and measurement procedure
Shimadzu energo-dispersive roentgen fluorescent spectrome-
ter EDX-900HS (from
11
Na to
92
U) with sensitive detector without
liquid nitrogen was used to prove the atomic ration in compound
under study.
To measure the heat capacity C
p
◦
of the tested substance in the
range from 7 to 370 K a BKT-3.0 automatic precision adiabatic vac-
uum calorimeter with discrete heating was used. The calorimeter
design and the operation procedure were described earlier [6,7].
The calorimeter was tested by measuring the heat capacity of high
purity copper and reference samples of synthetic corundum and
K-2 benzoic acid. The analysis of the results showed that mea-
surement error of the heat capacity of the substance at helium
temperature was within ±2% (5 < T < 20 K), then it decreased to
±0.5% as the temperature was rising to 40 K, and was equal to ±0.2%
at T > 40 K.
Thermal experiments were carried out on a LABSYS Setaram
differential scanning calorimeter with a heating rate of 10 K/min.
The phase individuality of synthesized compounds was moni-
tored by X-ray diffraction. X-ray diffraction patterns were recorded
on a XRD-6000 Shimadzu diffractometer (Cu K
radiation, geome-
try –2) in the 2 range from 10
◦
to 120
◦
with scan increment of
0.02
◦
.
0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2010.12.026