Phase Transition and Crystal Structure of CsHSeO 4 and CsDSeO 4 Crystals

DSC, TG-DTA and X-ray diffraction measurements have been performed on cesium hydrogen selenate CsHSeO4 and deuterated CsDSeO4 crystals. The superionic phase transitions for the proton and deuterated compounds were found to occur at 402.6 and 398.1 K, respectively. The thermal decomposition accompanied by hydrolysis in both compounds started at around the transition temperature, and the maximum rate of weight loss from the reaction was observed at around 490 K. The space group symmetry (monoclinic P21/c) and structural parameters were determined at 298 and 355 K. The expansion of the O-H-O hydrogen bond at room temperature by the substitution of deuterium for hydrogen was observed to be 0.015(7) Å. The geometric isotope effect on the hydrogen bond structure by deuteration was realized in the CsHSeO4 crystal. The experimental results denied the existence of a phase transition from phase II to III in the proton and deuterated compounds.


Introduction
Alkali (or ammonium) ions (M + = K + , Rb + , Cs + , or NH 4 + ) and sulfate (or selenate) ions (XO 4 2-, where X = S or Se) exist generally in five types of compounds with the following chemical formulas: M 2 XO 4 , MHXO 4 , M 3 H(XO 4 ) 2 , M 5 H 3 (XO 4 ) 4 , and M 3 H 5 (XO 4 ) 4 .Many crystals of these types are superionic conductors at high temperature.Some of them are characterized by their isomorphism, ferroelasticity, ferroelectricity and sequential structural phase transitions.The physical properties and phase transition mechanisms for these types have been widely studied by using many experimental methods.
The transition temperature T I-II in all published papers for the CsHSeO 4 crystal is very close to 401 K, almost without exception.However, the transition temperature T II-III is in the range of 323-370 K depending on the type of investigation (Baranov et al., 1982;Checa et al., 2009;Colomban et al., 1986;Luspin et al., 1995Luspin et al., , 2000;;Ortiz et al., 2008;Pham-Thi et al., 1985).The reason for the wide spread of T II-III is most likely to be due to sample quality.For example, three endothermic peaks in DSC curves have been described in the previous paper by Ortiz et al. (2008), despite that the presence of either one or two transitions has already been reported in many papers.This anomaly is inferred to be caused by using low-quality samples in their experiments, and this type of mistake can be eliminated by inspecting the crystal structure of sample.By removing obviously deviant data the phase transition temperature T II-III from phase II to III can be modified to be in the range of 323-350 K (Checa et al., 2009;Colomban et al., 1986;Luspin et al., 1995Luspin et al., , 2000;;Pham-Thi et al., 1985).
In contrast to CsHSeO 4 crystal, studies on deuterated cesium hydrogen selenate (CsDSeO 4 ) crystal are extremely rare.The velocity of sound and the elastic constants for the deuterated crystal have been obtained at 293 K, and the space groups in room-and high-temperature phases have been determined to be monoclinic P2 1 /c and tetragonal I4 1 /amd, respectively (Balagurov et al., 1986;Lushnikov et al., 1987).Moreover, structural information on partially deuterated crystal with deuteration level x = 70% has been obtained at room temperature by analyzing neutron powder diffraction data using the Rietveld method (Balagurov et al., 1987).However, the accurate crystal structure of CsDSeO 4 and the isotope effect on properties of CsHSeO 4 by substitution of deuterium for hydrogen have not yet been reported.
The purpose of this paper is to report the phase transition from phase II to III in CsHSeO 4 and CsDSeO 4 crystals, and to determine the crystal structure of the room-temperature phase of CsDSeO 4 .Isotope effects on the structure and properties of the CsHSeO 4 crystal by deuteration have been studied.

Thermal Measurements
Differential scanning calorimetry (DSC) and thermogravimetric-differential thermal analysis (TG-DTA) measurements were carried out in the temperature range of 100-600 K using DSC7020 and TG/DTA7300 systems from Seiko Instruments Inc, respectively.The sample amounts for the DSC and TG-DTA measurements varied between 3.28 and 10.97 mg, and the heating and cooling rates were 5 or 10 K/min with flowing dry N 2 gas.

X-Ray Crystal Structure Determination
The X-ray diffraction measurements were carried out at 298 (phase III) and 355 (phase II) K on a Rigaku Saturn CCD X-ray diffractometer with graphite monochromated Mo K α radiation (λ = 0.71073 Å).Diffraction data were collected by using an ω scan mode with a detector distance of 40 mm to the sample crystal, and the data were processed using the CrystalClear software package.Intensity data were corrected for Lorentz polarization and absorption effects.The structures were solved with direct methods of SIR2008 and refined on F 2 by full-matrix least-squares methods using the SHELXL-97 program in the WinGX program package (Burla et al., 2007;Farrugia, 1999;Scheldrick, 1997).A summary of crystal data, intensity data collections, and structure refinements is given in Table 1.

Thermal Analysis
Figure 1 shows the DSC curves of (a) CsHSeO 4 and (b) CsDSeO 4 crystals for heating and cooling in the temperature range from room temperature to 455 K.The endothermic and exothermic peaks in the DSC curves respectively are clearly seen at 403.6 and 390.7 K for the proton compound, and at 399.3 and 384.8K for the deuterated compound.The temperature hysteresis between the endothermic and exothermic peak temperatures in both compounds is about 14 K.Moreover, there are slight decreases in the DSC peak temperature between the two compounds.The decreases in the peak temperature of the heating and cooling curves by deuteration are about 4 and 6 K, respectively.The decrease of the DSC peak temperature can also be seen in the previously reported papers of Na 3 H(SO 4 ) 2 and (NH 4 ) 3 H(SeO 4 ) 2 crystals (Fukami & Chen, 1999, 2003).The onset temperatures of the endothermic and exothermic peaks are respectively determined to be 402.6 and 392.0 K for the proton compound, and 398.1 and 385.6 K for the deuterated compound.The onset temperature in the heating curve for the proton compound is very close to the I-II transition temperature (401 K) (Kamazawa et al., 2010;Komukae et al., 1990;Luspin et al., 1995Luspin et al., , 2000;;Yokota et al., 1982).Generally, it is believed that a clear peak in the DSC chart is attributed to the change of exchange energy at phase transition in almost all cases.A first-order phase transition is characterized by a sharp endothermic peak at transition and is accompanied by a thermal hysteresis with transition temperature.Therefore, we concluded that the proton and deuterated crystals undergo a first-order structural phase transition at 402.6 and 398.1 K, respectively.However, the DSC peak corresponding to the II-III phase transition previously reported by some investigators can not be seen in the curves for both compounds, as shown in Figure 1.Moreover, no significant endothermic or exothermic peaks in DSC curves were observed in the temperature range of 100 K to room temperature.Therefore, these results indicate that there is no phase transition in the temperature range of 100-400 K for both compounds.
The bond length of the O-H-O hydrogen bond at 298 and 355 K has the same value of 2.608(5) Å.However, the length of the O-D-O hydrogen bond decreases from 2.623(6) to 2.613(5) Å with increasing temperature.This implies that the O atoms of the O-D-O hydrogen bond are slightly displaced into a more stable position with increasing temperature.In all the observed structures, the SeO 4 tetrahedra are slightly disorder from a regular tetrahedron because the magnitudes of the O(1)-O(4) length and O(1)-Se-O(4) angle differ from those of the other lengths and angles in the SeO 4 tetrahedra, respectively.Moreover, the two bond distances between the Se atom and the O atoms (O(1) and O(4)) ended to the H(D) atom (the distance of the Se-O(-H(D)) bond) are longer than that of the other Se-O bonds.These characteristics of bond length and angle can be seen in all the four structures, and the magnitudes in these differences are almost the same value.Thus, it is considered that there exists a bonding strength between the O atoms involved in the O(1)-H(D)-O(4) hydrogen bond, and the bonding strength is not affected by deuteration.The bonding strength has also been reported in previous papers on Na 3 H(SO 4 ) 2 and (NH 4 ) 3 H(SeO 4 ) 2 crystals (Fukami & Chen, 1999, 2003)., 1978, 2000).about 2.43 to 2 5 Å.   to the f the Figure 1 and CsD

Table 1 .
Crystal data, intensity collections and structure refinements for CsHSeO 4 and CsDSeO 4 at 298 and 355K Single crystals of CsHSeO 4 and CsDSeO 4 were grown at room temperature by slow evaporation from aqueous solutions containing a molar ratio 1:2 of Cs 2 CO 3 and H 2 SeO 4 in desiccators over P 2 O 5 .The deuterated crystals thus obtained were recrystallized five times from a mixed D 2 O solution by the evaporation method.The grown crystals had platelike shapes.