Journal of the Physical Society of Japan
Vol. 79, No. 2, February, 2010, 025002
#2010 The Physical Society of Japan
SHORT NOTES
1
05
Pressure Effect on Phase Transition
CDP
0.05 GPa
.13 GPa
0 GPa
0.09 GPa
in Partially Deuterated Cs(H1 D ) PO4
Àx x 2
1.2
0
ꢀ
Eisuke MAGOME , Satoshi TOMIOKA,
0
.22 GPa
Yusuke TAO, and Masaru KOMUKAE
0
0
0
.8
.4
.0
0
.24 GPa
Department of Applied Physics, Faculty of Science,
ε
Tokyo University of Science, Shinjuku, Tokyo 162-8601, Japan
(
Received November 16, 2009; accepted December 28, 2009;
published February 10, 2010)
KEYWORDS: CsH2PO4, phase transition, pressure effect, dielectric
110
130
150
170
Temperature [K]
Fig. 1. Temperature dependence of the dielectric constant along the b-axis
of CDP measured at 15.8 kHz. The vertical arrows indicate ferroelectric
transitions and the values indicate the hydrostatic pressures at TC.
Hydrogen-bonded compounds, in which the hydrogen
bonds are relatively weak intermolecular interactions,
exhibit many unusual and important properties. Modification
of the molecular potential by the application of a hydrostatic
pressure changes the physical properties of a compound. were grown by a slow evaporation method after per-
Consequently, there has been great interest in hydrogen- forming recrystallization several times in order to obtain
bonded ferroelectric and antiferroelectric crystals, particu- single crystals with the desired deuterium concentration.
larly on studying the hydrostatic pressure effect in them. In Measurement of the dielectric constants of CsH2PO4,
AH BO -type crystals [A ¼ K, Rb, Cs, Tl, NH , (CH ) NH ; Cs(H0:78
D
) PO , and Cs(H0:15D ) PO (abbreviated
0:22 2 4 0:85 2 4
2
4
4
3 2
2
B ¼ P, As], which constitute a large and important group of as CDP, 22%d-CDP, and 85%d-CDP, respectively) under
hydrogen-bonded ferroelectrics, the transition temperature a hydrostatic pressure was carried out along the b-axis with
TC increases considerably on deuteration, indicating that the an LCR meter (HP4284A) at a frequency of 15.8 kHz. A
hydrogen bonds in these crystals play an important role in Cu–Be high-pressure vessel containing Fluorinate as the
the phase transition. The crystals of KH2PO4, RbH2PO4, pressure transmitting fluid was used to apply hydrostatic
and NH4H2PO4 belong to the tetragonal system, whereas pressures to the sample. The value of pressure was obtained
the crystals of CsH2PO4, TlH2PO4, and (CH3)2NH2H2PO4 using a calibrated Manganin resistance gauge during the
belong to the monoclinic system. The tetragonal and measurement. The temperature of sample was measured
monoclinic AH BO -type crystals have extremely different with a copper–constantan thermocouple placed near the
4
2
physical properties associated with the phase transition. sample, which was surrounded by the Manganin resistance
2
Experimental results on the effects of hydrostatic pressure gauge. The samples were about 2 ꢂ 2 mm in area and
on the crystals of KH2PO4, RbH2PO4, and NH4H2PO4 have about 1 mm thick, and silver paste was used as electrodes.
shown large isotope effects on both the transition temper- The sample temperature was changed at a rate of 0.25
ature and the pressure derivatives of the transition temper- K/min.
ature dTC=dp.1 As an example, the values of dTC=dp are
–3)
Figure 1 shows the temperature dependence of the dielec-
ꢁ
46 and ꢁ25 K/GPa for KH PO and KD PO , respective- tric constant measured for CDP during heating at various
2
4
2
4
4)
ly. Thus, the pressure derivative of the transition temper- pressures below 0.24 GPa. The dielectric constant shows
ature of KH PO decreases by about 50% on deuteration. a maximum at the transition temperature. The transition
2
4
However, Yasuda et al. have reported that the values of temperature decreases monotonically with increasing pres-
dTC=dp for both undeuterated and deuterated CsH2PO4 sure. The pressure within the vessel diminishes at a rate of
5
)
ꢁ3
crystals are the same, being ꢁ85 K/GPa. On the other about 1:08 ꢂ 10 GPa/K with decreasing temperature. The
hand, Gesi et al. have reported that the values of dTC=dp for vertical arrows in Fig. 1 indicate the transition temperatures
undeuterated and deuterated CsH2PO4 crystals are ꢁ56 and under various hydrostatic pressures and the values indicate
6)
ꢁ
85 K/GPa, respectively. These two results appear to be the hydrostatic pressures at the transition temperatures. The
contradictory. It is interesting to consider whether the lack effect of pressure on the ferroelectric transition temperatures
of an isotope effect on dT =dp is an intrinsic characteristic of 22%d-CDP and 85%d-CDP was also measured at various
C
of CsH2PO4, in contrast with KH2PO4, RbH2PO4, and pressures, in order to compare with the results for CDP
NH4H2PO4. It is, therefore, necessary to investigate the shown in Fig. 1. The transition temperatures of both 22%d-
effect of hydrostatic pressure on the transition temperature CDP and 85%d-CDP decrease monotonically with increas-
of Cs(H1ꢁxDx)2PO4 with deuterium concentration x.
Crystals of CsH2PO4 were grown by a slow evaporation dependences of the transition temperature of CDP, 22%d-
method from an aqueous solution containing Cs CO CDP, and 85%d-CDP are shown in Fig. 3. The plots in
and H PO in a molar ratio of 1 : 2. Partially deuterated Fig. 3 indicate the results obtained from Figs. 1 and 2. The
ing pressure, as shown in Fig. 2. The hydrostatic pressure
2
3
3
4
Cs(H1 D ) PO crystals were grown from a H O and D O transition temperatures of CDP, 22%d-CDP, and 85%d-CDP
ꢁx x 2
solution of CsH2PO4. Single crystals of a solid solution are 156, 180, and 253 K at atmospheric pressure, respec-
4
2
2
tively. The obtained transition temperatures agree well with
7)
ꢀ
E-mail: magome@rs.kagu.tus.ac.jp
those reported by Deguchi et al. This result indicates a
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