Phase diagram of TlH2(P1-xAsx)O 4 mixed crystals

Full text of DOI:10.1143/JPSJ.77.075002

Journal of the Physical Society of Japan
Vol. 77, No. 7, July, 2008, 075002
#2008 The Physical Society of Japan
SHORT NOTES
sition temperature, TcðxÞ, of KH2(P1ꢁxAsx)O4.23) The down-
ward bowing of the transition temperature was also observed
in the solid solution NH4H2(P1ꢁxAsx)O4 between antiferro-
electric NH4H2PO4 and its isomorphous antiferroelectric
NH4H2AsO4. In the light of these phase diagrams, we
consider that it is worth examining the solid solutions of
TlH2PO4 and its isomorphous TlH2AsO4.
The TlH2PO4/TlH2AsO4 compounds were synthesized
by the reaction Tl2CO3 þ 2H3PO4/H3AsO4 ! 2TlH2PO4=
TlH2AsO4 þ H2O þ CO2. Mixed polycrystalline powders of
TlH2PO4 and TlH2AsO4 were prepared with the starting
molar ratio x0 ¼ 0:50, and then they were dissolved in
the solvent water. Mixed crystals of TlH2(P1ꢁxAsx)O4 were
grown by slow evaporation from aqueous solution at
about 37 ꢃC. The real concentration x in the mixed crystal
was estimated as x ¼ 0:40 by elemental analysis of
solid solutions. This value x ¼ 0:40 in the solid solution is
smaller than the concentration x0 ¼ 0:50 in the aqueous
solution. The similar results have been reported in KH2-
(P1ꢁxAsx)O4.23) The small single crystals were ground to
Phase Diagram of TlH2(P1ÀxAsx)O4
Mixed Crystals
ꢀ
1
Kwang-Sei LEE , Se-Hun KIM ,
2
2
Cheol Eui LEE , and In-Hwan OH
Department of Nano Systems Engineering,
Center for Nano Manufacturing, Inje University,
Gimhae, Gyeongnam 621-749, Korea
1Faculty of Science Education, Cheju National University,
Cheju 690-756, Korea
2Department of Physics and Institute for Nano Science,
Korea University, Seoul 136-713, Korea
(Received April 16, 2008; accepted May 9, 2008;
published June 25, 2008)
KEYWORDS: thallium dihydrogen phosphate, thallium dihydrogen
arsenate, TlH2PO4, TlH2AsO4, solid solution, tran-
sition temperature, phase diagram
DOI: 10.1143/JPSJ.77.075002
Thallium dihydrogen phosphate (TlH2PO4) crystal under- form a disk-shaped pellet for attaching a silver electrode.
goes structural phase transitions in order of descending The TlH2(P0:60As0:40)O4 specimen was cooled down to
temperature at atmospheric pressure: at the upper transition 190 K and the dielectric constant was measured over the
point 357 K (Tc1) from the orthorhombic phase I with the temperature range from 193 to 258 K with a heating rate of
space group Pcan-D to the monoclinic phase II with the 0.2 K minꢁ1 using an impedance analyzer (HP 4192A).
14
2h
space group P21=a ꢁ C25h,1–3) and at the lower transition
Figure 1 shows the temperature dependence of the real
point 230 K (Tc2) to the phase III that is still debatable.4–13) and imaginary part of the dielectric constant, "0 and "00,
The I–II transition is a ferroelastic one,1–3) while the II–III at measuring frequency of 1 MHz in TlH2(P0:60As0:40)O4
transition is believed to be an antiferroelectric one5,6) but its polycrystalline pellet. Upon heating, the "0 shows a con-
ferrielectric7) or nonferroic8,9) transition was suggested. The tinuous but small change of slope at about 235 K (Tch2) and a
crystal system of the phase III was proposed as presumably positive slope above the transition, similarly as in the cases
monoclinic (C-centered cell with 2a ꢂ 2b ꢂ c with respect of TlH2PO4 (x ¼ 0:00) and TlH2AsO4 (x ¼ 1:00). The
to the lattice constants a, b, and c in phase II),6,9) but was feature of the "0 is comparable to the cases of TlH2PO4
1
10–12)
ꢀ
assessed as triclinic with the space group P1-Ci .
Light (x ¼ 0:00) and TlH2AsO4 (x ¼ 1:00) that undergo the III !
scattering, x-ray diffraction and neutron scattering suggested II phase transition near Tch2 (230 K for x ¼ 0:00 and 251 K
that TlD2PO4 undergoes a different kind of phase transition for x ¼ 1:00) on heating with a small discontinuous change
sequence as compared to that of TlH2PO4 (see Fig. 1 in of the "0.4,5,8,13,17,19,20) The smearing of the discontinuity of "0
ref. 13).7,10–13) However, the nature of the upper and lower due to the random mixing of PO4 and AsO4 indicates
transitions still remain controversial, together with the
ambiguous phase diagram of Tl(H1ꢁxDx)2PO4 (see Fig. 5
in ref. 13).5–7,10–13) Upon heating TlH2PO4 above 357 K
(Tc1), the high-temperature dielectric anomaly was observed
at about 425 K (Tp) and was interpreted as an onset of
thermal decomposition (condensation polymerization) rather
than a polymorphic transition.14–16) Thallium dihydrogen
arsenate (TlH2AsO4) belongs to TlH2PO4 family, with
transition temperatures of 391 K (Tc1), 251 K (Tc2), and onset
of thermal decomposition 413 K (Tp).17–19) The phase
diagram of Tl(H1ꢁxDx)2AsO4 on deuteration is also debat-
able.20) Moreover, the mixed phase with coexisting anti-
ferroelectricity and paraelectricity was insisted below 327 K
(Ts) in the ferroelastic phase of TlH2PO4.21,22)
3ꢁ
3ꢁ
Mixed crystals of the KH2PO4 family have been inves-
tigated by many researchers. The substitution of cations,
between ferroelectric RbH2PO4 and antiferroelectric
NH4H2PO4 generates the dipole glass state at low temper-
atures. On the other hand, the anionic replacement between
ferroelectric KH2PO4 and its isomorphous ferroelectric
KH2AsO4 showed a large bowing downwards in the tran-
Fig. 1. Temperature dependence of the real and imaginary part of the
dielectric constant on heating a TlH2(P0:60As0:40) polycrystalline pellet.
Measuring frequency: 1 MHz. The III ! II transition temperature on
ꢀE-mail: kslee@physics.inje.ac.kr
heating, Tch2, is indicated by the vertical arrow.
075002-1

J. Phys. Soc. Jpn., Vol. 77, No. 7
SHORT NOTES
K.-S. LEE et al.
that the III ! II transition of TlH2(P0:60As0:40)O4 is of bonds. The coupling constant Jij stands for the interaction
the second order, while the cases of TlH2PO4 (x ¼ 0:00) between the pseudo-spins and ꢁ is the tunneling frequency
and TlH2AsO4 (x ¼ 1:00) are of first order.4,8,13,17,19) This of the O–H···O protons. In TlH2PO4 and TlH2AsO4, the
implies that the II–III phase transition of TlH2(P1ꢁxAsx)O4 hydrogen bond network is different from that in KH2PO4
is close to a tricritical point. On the other hand, isotope and KH2AsO4/NH4H2PO4 and NH4H2AsO4 that have one
effect and the pressure dependence of the dielectric con- type hydrogen bond connected three-dimensionally. There
stant reveal a change of the II–III transition from first to are three types of hydrogen bonds in TlH2PO4 at room
second on deuteration5,13,20) and vice versa with increasing temperature (phase II).6,10–12) Two hydrogen atoms, H1 and
pressure.5) The imaginary part of the dielectric constant of H2, are in special positions at the center of inversion forming
TlH2(P0:60As0:40)O4 pellet, "00, shows an anomaly similar to hydrogen bonding zigzag chains along the c-axis. The other
that of "0.
hydrogen atom, H3, is in a general position and involved
When cooling the TlH2PO4 (x ¼ 0:00) crystal with the in an asymmetric bond along the b-axis. In spite of the
rate of 0.3 K minꢁ1 the transition temperature was 228.7 K different hydrogen bond network, it is noticeable that
(Tcc2), when heating with the rate of 0.3 K minꢁ1 it was anionic replacement gives rise to the universal trend of
229.7 K (Tch2).8) In the case of TlH2AsO4 (x ¼ 1:00) powder downward bowing of the transition temperatures. The P/As
with the same rate of cooling and heating, the lower transi- size difference may generate random strain fields that couple
tion occurred at 249.8 K (Tcc2) on cooling and at 251.2 K to the effective spin operator, giving random-field Ising
(Tch2) on heating.17) This thermal hysteresis is also indicative model behavior.24)
of the first-order nature of the II–III transition. Irokawa et al.
This work was supported by the Korea Science and
reported by dielectric measurement with cooling and heating Engineering Foundation (NRL Program R0A-2008-000-
rate of about 1 K hꢁ1 (ꢄ0:017 K minꢁ1) that single crystal 20066-0 and User Program of Proton Engineering Frontier
of TlH2AsO4 (x ¼ 1:00) undergoes the lower transition at Project). The measurements at the Korean Basic Science
250.8 K (Tcc2) on cooling and at 250.9 K (Tch2) on heating Institute (KBSI) are acknowledged.
with thermal hysteresis of 0.1 K.19) Combining the III ! II
transition temperature for x ¼ 0:40 with the previous results
for x ¼ 0:00 and 1.00,8,17) the Tch2ðxÞ was plotted as a
1) H. Yoshida, M. Endo, T. Kaneko, T. Osaka, and Y. Makita: J. Phys.
Soc. Jpn. 53 (1984) 910.
function of x in Fig. 2. The III ! II transition temperatures
on heating, Tch2ðxÞ, deviate considerably from the linearity
2) M. Arai, T. Yagi, A. Sakai, M. Komukae, T. Osaka, and Y. Makita:
J. Phys. Soc. Jpn. 59 (1990) 1285.
and show bowing downwards.
3) K. Hanazawa, M. Komukae, T. Osaka, Y. Makita, M. Arai, T. Yagi,
and A. Sakai: J. Phys. Soc. Jpn. 60 (1991) 188.
In the mixed crystals of ferroelectric KH2(P1ꢁxAsx)O4 and
antiferroelectric NH4H2(P1ꢁxAsx)O4, a large bowing down-
wards in the transition temperature, TcðxÞ, was also observed
and could be explained by applying the virtual crystal
approximation to the pseudospin Ising Hamiltonian system
with a transverse field:23)
4) T. Matsuo and H. Suga: Solid State Commun. 21 (1977) 923.
5) N. Yasuda, S. Fujimoto, T. Asano, and K. Yoshino: J. Phys. D 13
(1980) 85.
6) R. J. Nelmes: Solid State Commun. 39 (1981) 741.
´
7) B. Pasquier, N. Le Calve, S. Al Homsi-Teiar, and F. Fillaux: Chem.
Phys. 171 (1993) 203.
8) J.-H. Park, K.-S. Lee, J.-B. Kim, M. Komukae, T. Osaka, and Y.
Makita: J. Phys. Soc. Jpn. 63 (1994) 4195.
X
X
1
2
1
2
H ¼ ꢁ
JijSzi Szj ꢁ ꢁ
Sxi ; Si ¼ ꢅ
;
ð1Þ
i;j
i
9) K.-B. Kim, K.-S. Lee, and C. E. Lee: J. Phys. Soc. Jpn. 67 (1998)
1886.
where Si represents the pseudo-spins corresponding to the
two equilibrium positions of the protons within the O–H···O
10) S. Rios, W. Paulus, A. Cousson, M. Quilichini, and G. Heger: Acta
Crystallogr., Sect. B 54 (1998) 790.
´
11) E. Alvarez-Otero, G. Madariaga, I. Peral, C. L. Folcia, and S. Rios:
Acta Crystallogr., Sect. B 58 (2002) 750.
12) I. H. Oh, M. Merz, S. Mattauch, and G. Heger: Acta Crystallogr., Sect.
B 62 (2006) 719.
13) K.-S. Lee, J.-H. Ko, and C.-S. Kim: Ferroelectrics 272 (2002) 9.
14) K.-S. Lee: J. Phys. Chem. Solids 57 (1996) 333.
15) K.-S. Lee, J.-H. Park, K.-B. Kim, J.-B. Kim, and J.-N. Kim: J. Phys.
Soc. Jpn. 66 (1997) 1268.
16) K.-S. Lee: Ferroelectrics 268 (2002) 369.
17) K.-S. Lee and K.-L. Kim: J. Phys. Soc. Jpn. 60 (1991) 3207.
18) K.-S. Lee and D.-H. Ha: Phys. Rev. B 48 (1993) 73.
19) K. Irokawa, M. Komukae, T. Osaka, and Y. Makita: J. Phys. Soc. Jpn.
63 (1994) 1162.
20) S.-K. Lee and K.-S. Lee: Ferroelectr. Lett. 18 (1994) 69.
21) S.-H. Kim, K. W. Lee, C. E. Lee, K.-S. Lee, D. H. Kim, and D. Y.
Jang: Appl. Phys. Lett. 89 (2006) 102901.
22) S.-H. Kim, K. W. Lee, B. H. Oh, and C. E. Lee: Phys. Rev. B 76
(2007) 172104.
Fig. 2. (Color online) Phase diagram of TlH2(P1ꢁxAsx)O4 solid solutions.
The dashed line is a straight line, while the solid line is a guide to the eye
of the III ! II transition temperatures on heating.
23) K.-S. Lee, S.-M. Ju, and J.-B. Kim: Phys. Rev. B 49 (1994) 9958.
24) D. R. Taylor, J. T. Love, G. T. Topping, and J. G. A. Dane: Phys. Rev.
B 72 (2005) 052109.
075002-2