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Superprotonic behavior of Cs2(HSO4)(H2PO4) – a new solid acid in the CsHSO4–CsH2PO4 system
Solid State Ionics 136–137 (2000) 229–241 www.elsevier.com / locate / ssi
Superprotonic behavior of Cs 2 (HSO 4 )(H 2 PO 4 ) – a new solid acid in the CsHSO 4 –CsH 2 PO 4 system Calum R.I. Chisholm, Sossina M. Haile* Materials Science 138 -78, California Institute of Technology, Pasadena, CA 91125, USA
Abstract Investigations into the CsHSO 4 –CsH 2 PO 4 system have yielded a new solid acid, Cs 2 (HSO 4 )(H 2 PO 4 ), with a superprotonic phase transition that occurs over the temperature range 61–1058C. In the room temperature structure, the SO 4 and PO 4 groups are randomly arranged on a single tetrahedral anion site. Hydrogen bonds are distributed through the structure so as to generate a two-dimensional network quite different from that of other cesium sulfate phosphate solid acids. The transition in Cs 2 (HSO 4 )(H 2 PO 4 ) takes place by a unique two-step process, occurs at an unusually low temperature, is accompanied by a large heat of transformation, DH 5 4462 J / g, and exhibits significant hysteresis. High temperature X-ray powder diffraction (XRD) and infrared (IR) spectroscopy revealed that the high temperature phase is cubic, with ˚ and likely takes on a CsCl structure, with Cs atoms at the corners of a simple cubic unit cell, and XO 4 a o 5 4.926(5) A, groups (X 5 P or S) at the center. The conductivity in the high temperature phase at 1108C is 3 3 10 23 V21 cm 21 , and the activation energy for proton transport is 0.37(1) eV. These values suggest that proton transport is facilitated by rapid XO 4 group reorientations in the cubic phase of Cs 2 (HSO 4 )(H 2 PO 4 ), as is known to occur in the high temperature, tetragonal phase of CsHSO 4 . 2000 Elsevier Science B.V. All rights reserved. Keywords: Ionic conductivity; Hydrogen bonding; IR spectroscopy; Proton conductivity; Anisotropy; Phase transition; Cesium hydrogen sulfate
1. Introduction Many solid acid (or acid salt) compounds have been examined as superprotonic conductors ever since Baranov and coworkers [1] demonstrated that the conductivity of CsHSO 4 increases by several orders of magnitude upon transformation of the material from a monoclinic phase to a high-temperature tetragonal phase. Proton transport in the superionic phase is facilitated by the rapid reorientation of SO 4 groups [2]. While a relatively large *Corresponding author. Fax: 11-626-578-0058. E-mail address: [email protected] (S.M. Haile).
number of superprotonic conducts have been identified in recent years, virtually all fall within two categories of compositional analogs. The first is typified by the afore-mentioned CsHSO 4 compound, and includes CsHSeO 4 [1] and NH 4 SeO 4 [3]. The second is typified by Rb 3 H(SeO 4 ) 2 [4] and includes Cs 3 H(SeO 4 ) 2 and (NH 4 ) 3 H(SeO 4 ) 2 [4]. In this latter case, the structure transforms from a monoclinic to a rhombohedral phase on passing through the superprotonic transition. Because of the limited number of structure-types known to exhibit superprotonic behavior, it has been difficult to assess the role of the hydrogen bond network and / or local hydrogen bond geometry on proton transport and transformation
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00315-5
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mechanisms. To address this limitation, and determine the extent to which other structure types may also undergo similar transitions, we have carried out, over the last several years, extensive investigations of the CsHSO 4 –CsH 2 PO 4 system. Four mixed sulfate–phosphate compounds, each with a unique structure and unique hydrogen bond network have been discovered, and their structures reported [5–8]. Two of these compounds were found to exhibit superprotonic transformations, and those results have also been previously reported [9,10]. In the present work, we describe the high temperature behavior of Cs 2 (HSO 4 )(H 2 PO 4 ), the fourth compound to be discovered in this system. Our preliminary thermal investigations, which accompanied the structure report [7], indicated a gradual, or perhaps two-stage, transition over the temperature range 60–1058C. We here demonstrate, by conductivity measurements, infrared (IR) spectroscopy, and X-ray powder diffraction (XRD), all performed at elevated temperatures, that the transition in Cs 2 (HSO 4 )(H 2 PO 4 ) is superprotonic in nature.
2. Experimental Single crystals of Cs 2 (HSO 4 )(H 2 PO 4 ) were grown from aqueous solutions of cesium carbonate, sulfuric acid and phosphoric acid in which the Cs:SO 4 :PO 4 mole ratio was fixed at 10:5:5, as described previously [7]. Because such solutions often yielded multiphase crystalline products, each crystal used for subsequent analysis was first confirmed to be Cs 2 (HSO 4 )(H 2 PO 4 ) by single crystal X-ray methods. For this, a Syntex 4-circle single-crystal diffractometer (MoKa radiation) was employed. Further X-ray structural characterization was carried out on finely ground samples utilizing a Scintag XDS 2000 powder diffractometer (CuKa radiation, liquid Nitrogen cooled germanium solid state detector). High temperature diffraction data were collected on this instrument under ambient atmosphere using a resistively heated ruthinium–platinum hot-stage. A thermocouple was placed at the Ru–Pt foil to monitor the temperature. Because the sample was mounted onto a glass slide which rested on the heating foil, the sample temperature was lower than the measured value, by about 58C at 558C and 508C at 1958C. The sample was heated rapidly to the
desired temperature and then allowed to equilibrate for approximately 20 min before the diffraction pattern was recorded. Samples were examined at room temperature, at nominal temperatures of 53, 75, 95, 125, 150, and 1958C, and again at room temperature. Infrared (IR) spectroscopy was carried out with a Nicolet Magna 860 FTIR over the wave number range 400–4000 cm 21 . Approximately 1 mg of sample and 300 mg of KBr were ground, then pressed in a 12.7 mm die at 1800 psi under vacuum for 2 min to yield an optically transparent pellet. A rudimentary heating stage was employed to enable the measurement of high temperature IR spectra. The approximate temperature of the sample was recorded using a thermocouple in contact with the sample holder, next to the sample. Because of poor thermal contact between the pellet and the sample holder, the temperature was known only to within an accuracy of 608C. The conductivity of Cs 2 (HSO 4 )(H 2 PO 4 ) was measured by a.c. methods using single crystals with a known orientation relative to the applied field. Data were collected along three independent crystallo¢ a¢ 2 c¢ and a¢ 1 c. ¢ Silver paint graphic directions: b, (Ted Pella, cat. no. 16032) served as the electrode material and was applied to opposite sides of the sample. Because the aqueous growth yielded smooth, plate-like crystals parallel to (1 1¯ 0), measurements along the a¢ 2 c¢ direction could be easily performed. Measurements along the other two directions were more prone to a variety of errors because the small geometric factor A /L (where A5area and L5length) resulted in high absolute resistances, the irregularities in crystal morphologies led to large errors in the measured A /L (perhaps as much as 20%), and the faces to which the silver paint electrodes were applied were rather rough. Despite these complications, the temperature dependence of the conductivity was determined to a high degree of accuracy. Impedance measurements were performed with an HP 4284A LCR (inductance-capacitanceresistance) meter. The applied voltage was 1 V, and the a.c. frequency range 20 Hz–1 MHz. Data were collected over several heating and cooling cycles (between room temperature and 1708C), under stagnant air atmospheres. Heating rates were typically 308C / h, whereas cooling rates ranged from 1008C / h to 18C / h.
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3. Results The conductivity of Cs 2 (HSO 4 )(H 2 PO 4 ), as mea¢ a¢ 1 c¢ and a¢ 2 c¢ directions, is sured along the b, shown in Arrhenius form in Figs. 1–3, respectively. Data from the first and final heating / cooling cycles are presented (where three cycles were measured for ¢ ¢ directions and six for the a¢ 1 c¢ the b¢ and a–c direction). Conductivities measured in intermediate cycles fell between the extremes shown, and did not change significantly after the third cycle. The slight increase in conductivity with cycling in the case of the a¢ 1 c¢ measurement is likely an experimental artifact, reflecting evolution of the electrodes rather than a true change in the sample properties. In Fig. 4 the conductivities along the three directions examined are directly compared. It is readily apparent from the data in Figs. 1–4 that, upon heating, the compound undergoes a gradual or perhaps two-step transition to a high conductivity phase. The activation energy, Ea , and preexponential term, A, for proton transport, determined by fitting the data to s 5(A /T ) exp(2Ea /k b T ),
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where k b and T have their usual meanings, are listed in Table 1. The X-ray powder diffraction patterns and the IR spectra obtained from Cs 2 (HSO 4 )(H 2 PO 4 ), both collected at selected temperatures, are presented in Figs. 5 and 6, respectively. Diffraction data collected at temperatures between ambient and a nominal temperature of 1258C were similar to the lower curve in Fig. 5, and those collected at high temperatures were similar to the upper curve. The nominal temperatures of the X-ray diffraction experiments (listed in Fig. 5) were 538C and 1958C, respectively, however, the actual temperature for the latter is closer to 1458C. Similarly, the IR experiments nominally performed at 1518C and 1998C were more likely to represent actual sample temperatures of approximately 908C and 1408C, respectively. The peak positions and intensities obtained from the high-temperature diffraction pattern, and from the IR spectra are listed in Tables 2 and 3, respectively. The increase in background intensity with temperature in the high frequency region of the IR spectra is an experimental artifact due to changes in the KBr
Fig. 1. The conductivity of Cs 2 (HSO 4 )(H 2 PO 4 ) as measured along the b¢ axis during first heating / cooling cycle and third heating cycle. (Data from the cooling portion of the third cycle were not collected).
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Fig. 2. The conductivity of Cs 2 (HSO 4 )(H 2 PO 4 ) as measured along the a¢ 1 c¢ direction during the first and sixth heating / cooling cycles.
pellet, which gradually became opaque upon heating. In addition, the apparent peak in the spectra at 400 cm 21 is due to noise in the system and is not due to the sample. The results shown in Figs. 5–6 demonstrate that Cs 2 (HSO 4 )(H 2 PO 4 ) undergoes a structural transition upon heating, and reveal that the high temperature phase is of much higher symmetry than the room temperature structure. This observation is further confirmed by differential scanning calorimetry, the results of which have been reported previously [7]. It is noteworthy that decomposition / dehydration of Cs 2 (HSO 4 )(H 2 PO 4 ) occurs at much higher temperatures, beginning at 1908C [7], than the temperature of the superprotonic transition, and H 2 O loss cannot be responsible for any of the structural or physical changes reported here.
4. Discussion In order to provide a meaningful interpretation of
the conductivity, X-ray diffraction and IR results obtained here, it is necessary to first briefly describe the room temperature structure of Cs 2 (HSO 4 )(H 2 PO 4 ) [7]. The compound crystallizes in space group P2 1 / n, with lattice parameters a5 ˚ and b 5 7.856(8), b57.732(7), c57.826(7) A, 99.92(4)8. Sulfate and phosphate tetrahedra are randomly distributed over the single crystallographic XO 4 anion site. Much as in CsHSO 4 -II, and the room temperature structures of b-Cs 3 (HSO 4 ) 2 (H 22x (S x P12x )O 4 ) and a-Cs 3 (HSO 4 ) 2 (H 2 PO 4 ), XO 4 groups are linked via asymmetric hydrogen bonds to form zig-zag chains. In Cs 2 (HSO 4 )(H 2 PO 4 ) the chains extend along the crystallographically unique axis, Fig. 7, rather than perpendicular to it, as is the case for the other compounds. The HXO 4 chains are further linked to one another via symmetric hydrogen bonds to form hydrogen-bonded layers that are parallel to (1 1¯ 0), Fig. 8. Thus, two of the conductivity measurements, those along the b¢ and a¢ 1 c¢ directions, were made along directions lying within the hydrogen bonded layers, and the third,
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¢ ¢ direction during the first and third heating / cooling cycles. Fig. 3. The conductivity of Cs 2 (HSO 4 )(H 2 PO 4 ) as measured along the a–c
that along the a¢ 2 c¢ direction, made along a direction lying normal to these layers. Each XO 4 group within the H 3 [(S,P)O 4 ] 2 layers participates in forming three hydrogen bonds. Two of these are along the zig-zag chain, and the bond configuration along these chains is given by: –[O(3)–(P,S)–O(4)–H(2) . . . O(39)] n –, whereas that between the chains is given by: O(2)–(P,S)–O(1) . . . H(1) . . . O(19)–(P,S9)–O(29). The O(1) . . . O(19) bond quite probably has a double minimum in the potential well for the proton, and thus the H(1) proton is proposed to be evenly distributed over two sites slightly displaced from the center of symmetry relating the two oxygen atoms. Moreover, the O(4) . . . O(3) bond also appears, despite its asymmetry, to have a double-minimum well, suggesting that a proton interstitial site exists at O(4) . . . H(2a)–O(3). Details of the hydrogen bond geometries are given in Table 4.
4.1. High temperature structure The high temperature X-ray powder diffraction pattern of Cs 2 (HSO 4 )(H 2 PO 4 ) can be indexed to a simple cubic cell with refined lattice parameter a o 5 4.926(5). Peak indices are given in both Fig. 4 and Table 2 assuming this cell. The diffraction pattern is rather similar to that reported for the high temperature phase of a-Cs 3 (HSO 4 ) 2 (H 2 PO 4 ) [8] and for a transient, high-temperature phase of CsH 2 PO 4 [11]. The effective volume of a single Cs(XO 4 ) unit is the monoclinic phase of Cs 2 (HSO 4 )(H 2 PO 4 ) is 117.1(2) ˚ suggesting that there should be one formula unit A, of CsH 1.5 (S 0.5 P0.5 O 4 ) per unit cell in the high temperature cubic phase (the volume of the cubic ˚ 3 ). This can be most easily unit cell is 119.5 A accommodated in a CsCl-type structure, in which the Cs atoms reside at the corners of the unit cell, and the XO 4 group resides at the body center, in analogy to the high temperature structure proposed for CsH 2 PO 4 [11]. Knowledge of the oxygen positions within the
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Fig. 4. Comparison of conductivities along the three directions examined: (a) first heat, (b) final heat.
C.R.I. Chisholm, S.M. Haile / Solid State Ionics 136 – 137 (2000) 229 – 241 Table 1 The activation energy and pre-exponential term for proton conduction in the high temperature, superprotonic phase of Cs 2 (HSO 4 )(H 2 PO 4 ) determined from a fit of the data to s 5(A / T ) exp[Ea /k b T ] a Crystallographic direction
Ea (eV)
log[A] (V21 cm 21 K)
s (1108C) (V21 cm 21 )
b¢ a¢ 1 c¢ ¢ ¢ a–c Average
0.38 0.37 0.36 0.37(1)
5.0 4.8 4.8 4.8(1)
2.8310 23 2.6310 23 2.9310 23 2.8(1)310 23
a The conductivity at 1108C is also given. The ‘crystallographic direction’ of the measurement refers to the direction of the applied field relative to the crystallographic axes of the room temperature, monoclinic phase. Data from the final cooling cycles were employed to obtain the values in the table except in the case of the b¢ direction measurement, for which only data from the first cooling cycle were available.
high temperature structure is critical to understanding the proton transport mechanism. If the presumption of a high temperature cubic structure is correct, then the oxygen atoms must be distributed over many sites, and accordingly the XO 4 group must exhibit multiple orientations, as demonstrated by the following argument. The only manner in which the struc-
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ture could maintain cubic symmetry and an XO 4 group of fixed orientation, would be if oxygen atoms resided at positions x, x, x, along the body diagonals of the unit cell, in space group P23 or P4¯ 3m. However, because the distance between X atoms and ˚ in this direction, there is Cs atoms is only 4.26 A simply not enough room to accomodate an oxygen atom directly between them. All other reasonable oxygen positions in any space group compatible with a simple cubic crystal system have site multiplicities greater than four (more than four equivalent sites per unit cell are generated by the symmetry operations of the space group), and thus imply site occupancies of less than one. This, in turn, implies XO 4 groups with multiple orientations. Preisinger and coworkers sug¯ and oxygen gested a structure in space group Pm3m atoms in the 24 l position at 1 / 2 0.250 0.366 for the transient, high temperature phase of CsH 2 PO 4 [11]. The site multiplicity of 24 implies that the XO 4 groups exist over six possible orientations. The coordinates proposed by these authors are, however, not ideal in that they yield an unrealistically short ˚ (a o for CsH 2 PO 4 is 4.961 X–O distance of 1.41 A ˚A). Adjustment of the position to 1 / 2 0.250, 0.323
Fig. 5. The X-ray powder diffraction patterns for Cs 2 (HSO 4 )(H 2 PO 4 ) in both its (a) room temperature and (b) high temperature, superprotonic phases.
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Fig. 6. Infrared spectra obtained for Cs 2 (HSO 4 )(H 2 PO 4 ) at nominal temperatures of 25, 151, and 1998C. The actual temperatures were more probably 25, 90, and 1408C due to poor thermal contact between the sample pellet and the sample holder.
Table 2 X-ray powder diffraction peak positions and integrated intensities for the high temperature, superprotonic phase of Cs 2 (HSO 4 )(H 2 PO 4 ) h
k
l
Icalc (%)
Iobs (%)
Icalc-obs (%)
2ucalc (8)
2uobs a (8)
2ucalc-obs (8)
d calc ˚ (A)
d obs a ˚ (A)
d calc-obs ˚ (A)
1 1 1 2 2 2 2 2 3
0 1 1 0 1 1 2 2 1
0 0 1 0 0 1 0 1 0
18.2 100.0 11.8 10.1 14.6 14.7 2.8 2.8 2.8
22.5 100.0 16.1 16.1 14.7 15.8 3.0 2.3 3.4
24.3 0.0 24.3 26.0 20.1 21.1 20.2 10.5 20.6
17.97 25.51 31.37 36.37 40.83 44.92 52.34 55.77 59.06
17.97 25.48 31.39 36.39 40.83 44.93 52.33 55.78 59.05
0.00 0.03 20.02 20.02 0.00 20.01 0.01 20.01 0.01
4.932 3.488 2.849 2.468 2.208 2.016 1.747 1.647 1.563
4.932 3.493 2.847 2.466 2.208 2.016 1.747 1.647 1.563
0 20.004 0.002 0.002 0 0 0 0 0
a
Corrected for an obvious displacement error due to the sample stage configuration.
˚ and Cs–O distances gives X–O distances of 1.52 A, ˚ of 3.20 A, both typical values. In Table 2 the measured peak intensities are compared with calculated values determined assuming these optimized oxygen coordinates. The agreement between the observed and calculated values is rather good, indicating that this preliminary structural model is a reasonable representation of the true structure. Both the conductivity and optical spectroscopy results are consistent with a structural transition to a high-temperature phase that is cubic. For a cubic material, one expects that the conductivity measured along all crystallographic directions to be equal, i.e. that the material be isotropic. Such an equivalence is
evident in Fig. 4. While the conductivities along different directions differ significantly at low temperatures, they coalesce to the same value at temperatures above 1058C. Moreover, the very low activation energy for proton transport, 0.37(1) eV, in the high temperature phase, is consistent with a transport mechanism in which XO 4 group reorientation facilitates proton motion, as has been directly observed for CsHSO 4 and CsHSeO 4 . In the respective superprotonic phases of these compounds the activation energies for proton transport are 0.33(1) and 0.35(2) eV [1]. With respect to the optical spectra, the five-atom XO 4 group is expected to conform to ideal T d
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Table 3 Infrared peak positions and heights measured for Cs 2 (HSO 4 )(H 2 PO 4 ) in its high temperature phase and at room temperature High temperature Wavenumbers (cm 21 )
Room temperature Peak intensity % absorbance
Wavenumbers (cm 21 )
Qualitative assignment Peak intensity % absorbance
495 513
0.52 0.47
474 496 517
0.46 0.64 0.56
599 620
0.37 0.76
968
0.80
601 623 641 670 971 1012
0.38 0.33 0.48 0.20 1.05 0.70
1109 1124 1147
1.42 1.39 1.24
1065 1091 1173 1286
0.80 0.81 1.10 0.47
1700 2340 2760
n / aa n / aa n / aa
1715 2350 2945 a
n / aa n / aa n / aa
Parent mode in T d symmetry
jn2 (E) OXOasymmetric bend (IR inactive) bending modes
X–OH stretch X–O stretch
jn4 (F 2 ) OXO-symmetric bend jn1 (A 1 ) XOsymmetric stretch (IR inactive) jn3 (F 2 ) XOasymmetric stretch
jX–O–H in-plane bending mode jn OH(C) jn OH(B) jn OH(A)
Unavailable due to large experimental background in this area of the spectrum.
Fig. 7. The structure of Cs 2 (HSO 4 )(H 2 PO 4 ). For clarity, Cs atoms are shown with isotropic displacement parameters and protons are omitted. Hydrogen bonds are indicated with heavy dotted lines. Zig-zag chains of hydrogen bonded XO 4 groups extend along b¢ and are further linked to one another via the O(1) . . . O(1) hydrogen bonds.
symmetry in the cubic phase. The group exhibits nine vibrational modes, only four of which are independent. These are the n1 (A 1 ) or XO 4 symmetric stretch, the n2 (E32) or O–X–O asymmetric bend, the n3 (F 2 33) or XO 4 asymmetric stretch, and the n4 (F 2 33) or O–X–O symmetric bend. In the case of the SO 422 group, these peaks appear at 980, 451, 1104 and 613 cm 21 , respectively, whereas in the case of the PO 32 group, they appear at 970, 358, 4 1080 and 500 cm 21 , respectively [12]. In the IR spectra of Cs 2 (HSO 4 )(H 2 PO 4 ), absorbance bands are clearly visible in these regions, (although for ideal T d symmetry the n1 and n2 modes are infrared inactive). In the high temperature spectrum, there are four primary peaks associated with these four vibrational modes, centered at 968, 500, 1125 and 620 cm 21 . The n2 and n4 peaks appear as doublets, and the n3 peak as a triplet (Table 3). This is interpreted to result from local chemical differences between SO 4 and PO 4 groups, and differences in the proton occupations at various XO 4 groups, rather than a global deviation from cubic symmetry. The degree of splitting at these peaks is small, and suggests that the local crystal-chemical differences between one XO 4 group and the next are correspondingly small.
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Fig. 8. The structure of Cs 2 (HSO 4 )(H 2 PO 4 ) shown in projection in a direction perpendicular to (1¯ 0 1). For clarity, Cs atoms are omitted. The hydrogen bonding between XO 4 groups creates a 2-dimensional network, with planes parallel to (1¯ 0 1). The H(1) proton is distributed over two crystallographically equivalent sites within the O(1) . . . O(1) hydrogen bond. The H(2) proton resides at a site close to O(4), the donor oxygen atom in the O(4) . . . O(3) hydrogen bond; the position H(2a) represents a probable interstitial site.
Table 4 ˚ and angles [8] between atoms involved in hydrogen bonds in Cs 2 (HSO 4 )(H 2 PO 4 ) [7] Interatomic distances [A]
H(1) H(2)
O Neighbors
d(O . . . O) ˚ (A)
d(O D –H) ˚ (A)
d(OA –H) ˚ (A)
/(O D HOA ) (8)
O(1)A / D –O(1)A / D O(3)A –O(4) D
2.541(6) 2.552(8)
1.000 0.999
1.567 1.658
163.3 146.5
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The high frequency peaks in the IR spectra, at 1700, 2340, and 2760 cm 21 can be readily assigned to OH stretching modes within the O–H . . . O bonds; the peak at |3400 cm 21 is due to water that was in the atmosphere and / or on the sample surface, and quickly disappeared upon gentle heating. The appearance of three broad, high frequency absorption bands is common in strongly hydrogen-bonded systems, and these are often referred to as the ‘ABC bands’ [13]. In CsHSO 4 (at 258C) the bands appear 21 at 1720, 2430 and 2900 cm , respectively [14], and in CsH 2 PO 4 (at 258C) they appear at |1700, |2300 and |2600 cm 21 [15], values which are in good agreement with those observed here for Cs 2 (HSO 4 )(H 2 PO 4 ). The similarity of the peak positions in the high temperature and room temperature phases of Cs 2 (HSO 4 )(H 2 PO 4 ), Table 3, suggest that the O–H . . . O bond strengths are not significantly different in the two structures, despite the certain rotational disorder of the tetrahedral groups in the former.
4.2. Room temperature structures While the room temperature structure of Cs 2 (HSO 4 )(H 2 PO 4 ) has been well-established from X-ray diffraction studies [7], the optical spectra and measurements of the conductivity tensor provide new insights into local structural and chemical features. Although there is only one crystallographically distinct XO 4 anion site in the P2 1 / n structure, there are a tremendous number of absorbance peaks in the 21 1400–400 cm region attributable to tetrahedral group internal modes. This is, of course, due to the low symmetry (the P/ S atoms resides at a general position in the unit cell and all O are crystallographically distinct), the fact that only three of the four oxygen atoms are involved in forming hydrogen bonds, the extensive disorder in proton positions, and, as pointed out above for the high temperature structure, local chemical differences between PO 4 and SO 4 anions and their respective vibration modes. On the basis of the coalescence of various absorption peaks during heating of the sample, qualitative assignment of these peaks to internal vibration modes was possible, Table 3. The five peaks present in the frequency range 1220–1050 cm 21 are taken to derive from n3 asymmetric stretching modes. On
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heating, coalescence of these peaks resulted in two doublets at an intermediary temperature, Fig. 6, curve b, and then a single triplet at high temperature, curve c. By analogy to peak assignments for CsHSO 4 [14] it is reasonable to assign these five peaks to X–OH stretching modes in the XO 4 H 1.5 group. From similar observations of peak coalescence, two peaks in the room temperature spectrum present in the range 1020–950 cm 21 are taken to derive from n1 symmetric stretching modes, and, again, from comparison to CsHSO 4 , these are presumed to correspond to X–O stretching modes. Four peaks present in the range 680–580 likely derive from n4 symmetric bending modes, and three peaks present in the range 530–460 to derive from n2 asymmetric bending modes. A peak at 1286 cm 21 is present in the room temperature spectrum which, rather than coalescing with other peaks at high temperature, simply diminished in intensity. This peak is likely due to X–O–H bending motion [16], and has no counterpart in an ideal XO 4 molecule. Moreover, such a peak similarly disappears upon heating from CsHSO 4 [14]. The difference in frequencies between the primary n (X–O) and n (X–OH) peaks in Cs 2 (HSO 4 )(H 2 PO 4 ) is approximately 120 cm 21 . In the case of CsHSO 4 the difference is significantly greater, with the main n (X–O) peak at 850 cm 21 and the main n (X–OH) peak at 1000 cm 21 . This would suggest that the difference in chemical environments about hydrogenbonded and non-hydrogen-boned oxygen atoms in Cs 2 (HSO 4 )(H 2 PO 4 ) is smaller than in CsHSO 4 , and is in agreement with the structure determinations for the two compounds. In the case of CsHSO 4 , there is one hydrogen bond, O(3) . . . H–O(4) and the oxygen donor, O(4), and acceptor, O(3), atoms in this bond are clearly distinguishable. The S–O(4) dis˚ much larger than the mean of the tance is 1.570 A, ˚ [17]. In remaining three S–O distances of 1.445 A Cs 2 (HSO 4 )(H 2 PO 4 ) three oxygen atoms serve as mixed donor acceptor atoms. The mean X–OA / D ˚ and this is rather similar to the distance is 1.517 A, ˚ remaining X–O distance of 1.462 A. Turning to the electrical properties, it is apparent from Figs. 1 to 4 that Cs 2 (HSO 4 )(H 2 PO 4 ) does not transform back to its as-synthesized P2 1 / n monoclinic phase upon cooling from high temperature. X-ray powder diffraction data collected from a
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sample after cooling from 1458C (at which high temperature diffraction data were collected) also indicated that the reverse transformation to the assynthesized structure does not occur. This diffraction pattern (not shown) contained many more peaks than evident in Fig. 5 curve a, and suggested that a two phase material may have been obtained. The conductivity data, Figs. 1–4, further indicate that there is some evolution in the room temperature structure with cycling; specifically, with increased cycling the room temperature conductivity increased. It is probable that the P2 1 / n phase is metastable, that the single crystal samples retain some ‘memory’ of the P2 1 / n → cubic transition, and that on cooling some P2 1 / n phase is formed, but on repeated heating and cooling cycles this ‘memory’ is lost and eventually only the more stable room temperature phase is obtained. The room temperature IR spectrum of a sample exposed to a single heating cycle (not shown) was indistinguishable from the pattern present in Fig. 6 curve (a), despite the clear structural and physical changes that took place on temperature cycling. This suggests that the stable room temperature phase is also monoclinic, and also demonstrates that one should be cautious when drawing conclusions about crystalline structure solely from optical spectroscopy. While these hypotheses regarding transitions on heating and cooling are consistent with the data, further experimentation will be necessary to unequivocally determine the nature of the stable room temperature structure.
4.3. Anisotropy and transformation mechanisms There is clear anisotropy in both the room temperature conductivity of Cs 2 (HSO 4 )(H 2 PO 4 ) and the temperature dependence of the conductivity, Fig. 4. At low temperatures (lower than approximately 608C at which a transition is first detected by thermal methods), the resistances of the samples were too high to permit reliable measurement of the activation energy for proton transport and these data are not included in the figure. Nevertheless, it was observed without ambiguity that at room temperature and on ¢ ¢ ). the first heating cycle s (a¢ 1 c¢ ). s (b¢ )4 s (a–c This observation leads us to conclude that proton transport within the hydrogen bonded planes is easier than transport from one layer to the next, as might be expected (recall that the b¢ and a¢ 1 c¢ directions lie
¢ ¢ within the hydrogen-bonded planes, whereas the a–c direction, lies normal to them). Within the planes, the higher value of s (a¢ 1 c¢ ) over s (b¢ ) may indicate that proton jumps between neighboring H(2) bond sites which involve pairs of distinct XO 4 groups are easier than jumps between H(1) and H(2) bond sites, which involve, in effect, proton motion along the edge of an XO 4 group. The latter type of jump may require the proton to come into close proximity of the positively charged X cation and thus be unfavorable. Note that, because all hydrogen bonds appear to have double-minima potential wells, as described above, all should be capable of accepting proton interstitials. ¢ ¢ Upon heating, the conductivity along the a–c direction increased sharply over the temperature range 60–728C, whereas that along the b¢ and a¢ 1 c¢ directions exhibited only slight increases, Fig. 4a. This temperature range is coincident with the sharp peak observed in the DSC data [7]. These observations suggest that a structural transformation takes place over this low temperature range in which the hydrogen-bonded layers change position relative to one another, perhaps shortening proton jump distances in the direction normal to the planes, without undergoing significant internal changes. Over the temperature range 80–908C the conductivity along all three directions examined increased dramatically, ¢ by approximately five orders of magnitude along b, three to four orders of magnitude along a¢ 1 c¢ and ¢ ¢ At 908C and still two orders of magnitude along a–c. on the first heating cycle the conductivity was isotropic, however, it is unlikely that the structure had fully reached its high temperature phase. The apparent activation energy for proton transport from 908C to 1058C is relatively high, and there is a second, broad DSC peak in the calorimetry data over precisely this temperature range [7]. Without further characterization, it is difficult to hypothesise what kind of structural transitions may or may not occur from 908C to 1058C. It must also be noted that no obvious transient phase was detected by high temperature X-ray powder diffraction. Beyond 1058C, Cs 2 (HSO 4 )(H 2 PO 4 ) clearly exists in its superprotonic cubic phase. On all cooling cycles, the high temperature phase persisted to temperatures well below 1058C. In general, a transformation to a lower conductivity
C.R.I. Chisholm, S.M. Haile / Solid State Ionics 136 – 137 (2000) 229 – 241
phase was observed at about 308C, but this was as much time dependent as it was temperature dependent. Often, the cubic phase was retained at room temperature for up to 2 h, and typically the total transformation time (time below 1058C) was |12 h. On the final heating cycles, the conductivities along ¢ ¢ directions were similar, the nominally a¢ 1 c¢ and a–c whereas that along the b¢ axis was almost an order of magnitude lower than the other two directions, Fig. 4b. One can only conclude that the sequence of phases encountered upon heating from the metastable P2 1 / n phase are different than those encountered upon heating from the stable room temperature phase. The anisotropies evident in the final heating cycle suggest that an intermediate phase with tetragonal or hexagonal symmetry is encountered over the temperature range 70–1058C, and the unique axis of this intermediary phase is parallel to the unique axis of the metastable monoclinic phase.
241
Footnotes ‡ and § given in the Table refer to these hydrogen atoms, not O(3) and O(4). The isotropic ˚ 2. hydrogen thermal parameters were fixed at 0.08 A
Acknowledgements The authors are pleased to acknowledge the financial support the National Science Foundation via a National Young Investigator Award to SMH. Additional funding was provided by the Irvine Foundation. The authors thank Professor George Rossman and Liz Miura at the California Institute of Technology for their kind guidance with infrared analyses and Paul Wagner for his assistance with high temperature powder diffractometry.
References 5. Conclusions The main conclusions from this work can be summarized as follows. Cs 2 (HSO 4 )(H 2 PO 4 ) undergoes a gradual, and perhaps two-step superprotonic transition over the temperature range 61–1058C. The high temperature, superprotonic phase is cubic, with a CsCl-type structure. The four oxygen atoms are distributed over 24 l sites at 1 / 2 0.250 0.323, and XO 4 groups undergo reorientational disorder in this cubic phase. The activation energy for proton conduction in the high temperature phase is 0.37(1) eV, and proton transport is facilitated by rapid XO 4 group reorientation. The high conductivity of Cs 2 (HSO 4 )(H 2 PO 4 ), 33 10 23 V21 cm 21 at 1108C, suggests the material may have technological as well as scientific relevance. Note added in proof Due to an editorial error, the proton coordinates in Table 2 of Ref. [7] were omitted. These co-ordinates are: H(1)‡ : H(2)
§
0.05387(4), 0.0329(6), 0.512(6) 0.8558(2), 2 0.0469(3), 2 0.2449(2).
[1] A.I. Baranov, L.A. Shuvalov, N.M. Shchagina, JETP Lett. 36 (1982) 459. ¨ [2] W. Munch, K.-D. Kreuer, U. Traub, J. Maier, Solid State Ionics 77 (1995) 10. [3] Yu.N. Moskvich, A.A. Sukhovsky, O.V. Rozanov, Sov. Phys. Solid State 26 (1984) 21. [4] A. Pawlowski, Cz. Pawlaczyk, B. Hilczer, Solid State Ionics 44 (1990) 17. [5] S.M. Haile, P.M. Calkins, D. Boysen, J. Solid State Chem. 139 (1998) 373. [6] S.M. Haile, P.M. Calkins, J. Solid State Chem. 140 (1998) 251. [7] C.R.I. Chisholm, S.M. Haile, Acta Crystallogr. B55 (1999) 937. [8] S.M. Haile, K.-D. Kreuer, J. Maier, Acta Cryst. B51 (1995) 680. [9] S.M. Haile, P.M. Calkins, D. Boysen, Solid State Ionics 97 (1997) 145. [10] S.M. Haile, G. Lentz, K.-D. Kreuer, J. Maier, Solid State Ionics 77 (1995) 128. [11] A. Preisinger, K. Meriter, W. Bronowska, Mater. Sci. Forum 166–169 (1994) 511. [12] D.E. Irish, T. Ozeki, in: J.G. Grasselli, B.J. Bulkin (Eds.), Chemical Analysis, Vol. 114, Chapter 4, Wiley, New York, 1991. [13] P. Sandorfy, G. Schuster, C. Zundel, The Hydrogen Bond, North Holland Publishing Co, New York, 1976. [14] M. Pham-Thi, Ph. Colomban, A. Novak, R. Blinc, J. Raman Spectrosc. 18 (1987) 185. [15] B. Marchon, A. Novak, J. Chem. Phys. 78 (1983) 2105. ¨ [16] U. Dobler, H. Happ, Phys. Stat. Sol. (b) 144 (1987) 853. [17] K. Itoh, T. Ukeda, T. Ozaka, E. Nakamura, Acta Crystallogr. C46 (1990) 358.
Superprotonic behavior of Cs 2 (HSO 4 )(H 2 PO 4 ) – a new solid acid in the CsHSO 4 –CsH 2 PO 4 system Calum R.I. Chisholm, Sossina M. Haile* Materials Science 138 -78, California Institute of Technology, Pasadena, CA 91125, USA
Abstract Investigations into the CsHSO 4 –CsH 2 PO 4 system have yielded a new solid acid, Cs 2 (HSO 4 )(H 2 PO 4 ), with a superprotonic phase transition that occurs over the temperature range 61–1058C. In the room temperature structure, the SO 4 and PO 4 groups are randomly arranged on a single tetrahedral anion site. Hydrogen bonds are distributed through the structure so as to generate a two-dimensional network quite different from that of other cesium sulfate phosphate solid acids. The transition in Cs 2 (HSO 4 )(H 2 PO 4 ) takes place by a unique two-step process, occurs at an unusually low temperature, is accompanied by a large heat of transformation, DH 5 4462 J / g, and exhibits significant hysteresis. High temperature X-ray powder diffraction (XRD) and infrared (IR) spectroscopy revealed that the high temperature phase is cubic, with ˚ and likely takes on a CsCl structure, with Cs atoms at the corners of a simple cubic unit cell, and XO 4 a o 5 4.926(5) A, groups (X 5 P or S) at the center. The conductivity in the high temperature phase at 1108C is 3 3 10 23 V21 cm 21 , and the activation energy for proton transport is 0.37(1) eV. These values suggest that proton transport is facilitated by rapid XO 4 group reorientations in the cubic phase of Cs 2 (HSO 4 )(H 2 PO 4 ), as is known to occur in the high temperature, tetragonal phase of CsHSO 4 . 2000 Elsevier Science B.V. All rights reserved. Keywords: Ionic conductivity; Hydrogen bonding; IR spectroscopy; Proton conductivity; Anisotropy; Phase transition; Cesium hydrogen sulfate
1. Introduction Many solid acid (or acid salt) compounds have been examined as superprotonic conductors ever since Baranov and coworkers [1] demonstrated that the conductivity of CsHSO 4 increases by several orders of magnitude upon transformation of the material from a monoclinic phase to a high-temperature tetragonal phase. Proton transport in the superionic phase is facilitated by the rapid reorientation of SO 4 groups [2]. While a relatively large *Corresponding author. Fax: 11-626-578-0058. E-mail address: [email protected] (S.M. Haile).
number of superprotonic conducts have been identified in recent years, virtually all fall within two categories of compositional analogs. The first is typified by the afore-mentioned CsHSO 4 compound, and includes CsHSeO 4 [1] and NH 4 SeO 4 [3]. The second is typified by Rb 3 H(SeO 4 ) 2 [4] and includes Cs 3 H(SeO 4 ) 2 and (NH 4 ) 3 H(SeO 4 ) 2 [4]. In this latter case, the structure transforms from a monoclinic to a rhombohedral phase on passing through the superprotonic transition. Because of the limited number of structure-types known to exhibit superprotonic behavior, it has been difficult to assess the role of the hydrogen bond network and / or local hydrogen bond geometry on proton transport and transformation
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00315-5
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mechanisms. To address this limitation, and determine the extent to which other structure types may also undergo similar transitions, we have carried out, over the last several years, extensive investigations of the CsHSO 4 –CsH 2 PO 4 system. Four mixed sulfate–phosphate compounds, each with a unique structure and unique hydrogen bond network have been discovered, and their structures reported [5–8]. Two of these compounds were found to exhibit superprotonic transformations, and those results have also been previously reported [9,10]. In the present work, we describe the high temperature behavior of Cs 2 (HSO 4 )(H 2 PO 4 ), the fourth compound to be discovered in this system. Our preliminary thermal investigations, which accompanied the structure report [7], indicated a gradual, or perhaps two-stage, transition over the temperature range 60–1058C. We here demonstrate, by conductivity measurements, infrared (IR) spectroscopy, and X-ray powder diffraction (XRD), all performed at elevated temperatures, that the transition in Cs 2 (HSO 4 )(H 2 PO 4 ) is superprotonic in nature.
2. Experimental Single crystals of Cs 2 (HSO 4 )(H 2 PO 4 ) were grown from aqueous solutions of cesium carbonate, sulfuric acid and phosphoric acid in which the Cs:SO 4 :PO 4 mole ratio was fixed at 10:5:5, as described previously [7]. Because such solutions often yielded multiphase crystalline products, each crystal used for subsequent analysis was first confirmed to be Cs 2 (HSO 4 )(H 2 PO 4 ) by single crystal X-ray methods. For this, a Syntex 4-circle single-crystal diffractometer (MoKa radiation) was employed. Further X-ray structural characterization was carried out on finely ground samples utilizing a Scintag XDS 2000 powder diffractometer (CuKa radiation, liquid Nitrogen cooled germanium solid state detector). High temperature diffraction data were collected on this instrument under ambient atmosphere using a resistively heated ruthinium–platinum hot-stage. A thermocouple was placed at the Ru–Pt foil to monitor the temperature. Because the sample was mounted onto a glass slide which rested on the heating foil, the sample temperature was lower than the measured value, by about 58C at 558C and 508C at 1958C. The sample was heated rapidly to the
desired temperature and then allowed to equilibrate for approximately 20 min before the diffraction pattern was recorded. Samples were examined at room temperature, at nominal temperatures of 53, 75, 95, 125, 150, and 1958C, and again at room temperature. Infrared (IR) spectroscopy was carried out with a Nicolet Magna 860 FTIR over the wave number range 400–4000 cm 21 . Approximately 1 mg of sample and 300 mg of KBr were ground, then pressed in a 12.7 mm die at 1800 psi under vacuum for 2 min to yield an optically transparent pellet. A rudimentary heating stage was employed to enable the measurement of high temperature IR spectra. The approximate temperature of the sample was recorded using a thermocouple in contact with the sample holder, next to the sample. Because of poor thermal contact between the pellet and the sample holder, the temperature was known only to within an accuracy of 608C. The conductivity of Cs 2 (HSO 4 )(H 2 PO 4 ) was measured by a.c. methods using single crystals with a known orientation relative to the applied field. Data were collected along three independent crystallo¢ a¢ 2 c¢ and a¢ 1 c. ¢ Silver paint graphic directions: b, (Ted Pella, cat. no. 16032) served as the electrode material and was applied to opposite sides of the sample. Because the aqueous growth yielded smooth, plate-like crystals parallel to (1 1¯ 0), measurements along the a¢ 2 c¢ direction could be easily performed. Measurements along the other two directions were more prone to a variety of errors because the small geometric factor A /L (where A5area and L5length) resulted in high absolute resistances, the irregularities in crystal morphologies led to large errors in the measured A /L (perhaps as much as 20%), and the faces to which the silver paint electrodes were applied were rather rough. Despite these complications, the temperature dependence of the conductivity was determined to a high degree of accuracy. Impedance measurements were performed with an HP 4284A LCR (inductance-capacitanceresistance) meter. The applied voltage was 1 V, and the a.c. frequency range 20 Hz–1 MHz. Data were collected over several heating and cooling cycles (between room temperature and 1708C), under stagnant air atmospheres. Heating rates were typically 308C / h, whereas cooling rates ranged from 1008C / h to 18C / h.
C.R.I. Chisholm, S.M. Haile / Solid State Ionics 136 – 137 (2000) 229 – 241
3. Results The conductivity of Cs 2 (HSO 4 )(H 2 PO 4 ), as mea¢ a¢ 1 c¢ and a¢ 2 c¢ directions, is sured along the b, shown in Arrhenius form in Figs. 1–3, respectively. Data from the first and final heating / cooling cycles are presented (where three cycles were measured for ¢ ¢ directions and six for the a¢ 1 c¢ the b¢ and a–c direction). Conductivities measured in intermediate cycles fell between the extremes shown, and did not change significantly after the third cycle. The slight increase in conductivity with cycling in the case of the a¢ 1 c¢ measurement is likely an experimental artifact, reflecting evolution of the electrodes rather than a true change in the sample properties. In Fig. 4 the conductivities along the three directions examined are directly compared. It is readily apparent from the data in Figs. 1–4 that, upon heating, the compound undergoes a gradual or perhaps two-step transition to a high conductivity phase. The activation energy, Ea , and preexponential term, A, for proton transport, determined by fitting the data to s 5(A /T ) exp(2Ea /k b T ),
231
where k b and T have their usual meanings, are listed in Table 1. The X-ray powder diffraction patterns and the IR spectra obtained from Cs 2 (HSO 4 )(H 2 PO 4 ), both collected at selected temperatures, are presented in Figs. 5 and 6, respectively. Diffraction data collected at temperatures between ambient and a nominal temperature of 1258C were similar to the lower curve in Fig. 5, and those collected at high temperatures were similar to the upper curve. The nominal temperatures of the X-ray diffraction experiments (listed in Fig. 5) were 538C and 1958C, respectively, however, the actual temperature for the latter is closer to 1458C. Similarly, the IR experiments nominally performed at 1518C and 1998C were more likely to represent actual sample temperatures of approximately 908C and 1408C, respectively. The peak positions and intensities obtained from the high-temperature diffraction pattern, and from the IR spectra are listed in Tables 2 and 3, respectively. The increase in background intensity with temperature in the high frequency region of the IR spectra is an experimental artifact due to changes in the KBr
Fig. 1. The conductivity of Cs 2 (HSO 4 )(H 2 PO 4 ) as measured along the b¢ axis during first heating / cooling cycle and third heating cycle. (Data from the cooling portion of the third cycle were not collected).
232
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Fig. 2. The conductivity of Cs 2 (HSO 4 )(H 2 PO 4 ) as measured along the a¢ 1 c¢ direction during the first and sixth heating / cooling cycles.
pellet, which gradually became opaque upon heating. In addition, the apparent peak in the spectra at 400 cm 21 is due to noise in the system and is not due to the sample. The results shown in Figs. 5–6 demonstrate that Cs 2 (HSO 4 )(H 2 PO 4 ) undergoes a structural transition upon heating, and reveal that the high temperature phase is of much higher symmetry than the room temperature structure. This observation is further confirmed by differential scanning calorimetry, the results of which have been reported previously [7]. It is noteworthy that decomposition / dehydration of Cs 2 (HSO 4 )(H 2 PO 4 ) occurs at much higher temperatures, beginning at 1908C [7], than the temperature of the superprotonic transition, and H 2 O loss cannot be responsible for any of the structural or physical changes reported here.
4. Discussion In order to provide a meaningful interpretation of
the conductivity, X-ray diffraction and IR results obtained here, it is necessary to first briefly describe the room temperature structure of Cs 2 (HSO 4 )(H 2 PO 4 ) [7]. The compound crystallizes in space group P2 1 / n, with lattice parameters a5 ˚ and b 5 7.856(8), b57.732(7), c57.826(7) A, 99.92(4)8. Sulfate and phosphate tetrahedra are randomly distributed over the single crystallographic XO 4 anion site. Much as in CsHSO 4 -II, and the room temperature structures of b-Cs 3 (HSO 4 ) 2 (H 22x (S x P12x )O 4 ) and a-Cs 3 (HSO 4 ) 2 (H 2 PO 4 ), XO 4 groups are linked via asymmetric hydrogen bonds to form zig-zag chains. In Cs 2 (HSO 4 )(H 2 PO 4 ) the chains extend along the crystallographically unique axis, Fig. 7, rather than perpendicular to it, as is the case for the other compounds. The HXO 4 chains are further linked to one another via symmetric hydrogen bonds to form hydrogen-bonded layers that are parallel to (1 1¯ 0), Fig. 8. Thus, two of the conductivity measurements, those along the b¢ and a¢ 1 c¢ directions, were made along directions lying within the hydrogen bonded layers, and the third,
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233
¢ ¢ direction during the first and third heating / cooling cycles. Fig. 3. The conductivity of Cs 2 (HSO 4 )(H 2 PO 4 ) as measured along the a–c
that along the a¢ 2 c¢ direction, made along a direction lying normal to these layers. Each XO 4 group within the H 3 [(S,P)O 4 ] 2 layers participates in forming three hydrogen bonds. Two of these are along the zig-zag chain, and the bond configuration along these chains is given by: –[O(3)–(P,S)–O(4)–H(2) . . . O(39)] n –, whereas that between the chains is given by: O(2)–(P,S)–O(1) . . . H(1) . . . O(19)–(P,S9)–O(29). The O(1) . . . O(19) bond quite probably has a double minimum in the potential well for the proton, and thus the H(1) proton is proposed to be evenly distributed over two sites slightly displaced from the center of symmetry relating the two oxygen atoms. Moreover, the O(4) . . . O(3) bond also appears, despite its asymmetry, to have a double-minimum well, suggesting that a proton interstitial site exists at O(4) . . . H(2a)–O(3). Details of the hydrogen bond geometries are given in Table 4.
4.1. High temperature structure The high temperature X-ray powder diffraction pattern of Cs 2 (HSO 4 )(H 2 PO 4 ) can be indexed to a simple cubic cell with refined lattice parameter a o 5 4.926(5). Peak indices are given in both Fig. 4 and Table 2 assuming this cell. The diffraction pattern is rather similar to that reported for the high temperature phase of a-Cs 3 (HSO 4 ) 2 (H 2 PO 4 ) [8] and for a transient, high-temperature phase of CsH 2 PO 4 [11]. The effective volume of a single Cs(XO 4 ) unit is the monoclinic phase of Cs 2 (HSO 4 )(H 2 PO 4 ) is 117.1(2) ˚ suggesting that there should be one formula unit A, of CsH 1.5 (S 0.5 P0.5 O 4 ) per unit cell in the high temperature cubic phase (the volume of the cubic ˚ 3 ). This can be most easily unit cell is 119.5 A accommodated in a CsCl-type structure, in which the Cs atoms reside at the corners of the unit cell, and the XO 4 group resides at the body center, in analogy to the high temperature structure proposed for CsH 2 PO 4 [11]. Knowledge of the oxygen positions within the
234
C.R.I. Chisholm, S.M. Haile / Solid State Ionics 136 – 137 (2000) 229 – 241
Fig. 4. Comparison of conductivities along the three directions examined: (a) first heat, (b) final heat.
C.R.I. Chisholm, S.M. Haile / Solid State Ionics 136 – 137 (2000) 229 – 241 Table 1 The activation energy and pre-exponential term for proton conduction in the high temperature, superprotonic phase of Cs 2 (HSO 4 )(H 2 PO 4 ) determined from a fit of the data to s 5(A / T ) exp[Ea /k b T ] a Crystallographic direction
Ea (eV)
log[A] (V21 cm 21 K)
s (1108C) (V21 cm 21 )
b¢ a¢ 1 c¢ ¢ ¢ a–c Average
0.38 0.37 0.36 0.37(1)
5.0 4.8 4.8 4.8(1)
2.8310 23 2.6310 23 2.9310 23 2.8(1)310 23
a The conductivity at 1108C is also given. The ‘crystallographic direction’ of the measurement refers to the direction of the applied field relative to the crystallographic axes of the room temperature, monoclinic phase. Data from the final cooling cycles were employed to obtain the values in the table except in the case of the b¢ direction measurement, for which only data from the first cooling cycle were available.
high temperature structure is critical to understanding the proton transport mechanism. If the presumption of a high temperature cubic structure is correct, then the oxygen atoms must be distributed over many sites, and accordingly the XO 4 group must exhibit multiple orientations, as demonstrated by the following argument. The only manner in which the struc-
235
ture could maintain cubic symmetry and an XO 4 group of fixed orientation, would be if oxygen atoms resided at positions x, x, x, along the body diagonals of the unit cell, in space group P23 or P4¯ 3m. However, because the distance between X atoms and ˚ in this direction, there is Cs atoms is only 4.26 A simply not enough room to accomodate an oxygen atom directly between them. All other reasonable oxygen positions in any space group compatible with a simple cubic crystal system have site multiplicities greater than four (more than four equivalent sites per unit cell are generated by the symmetry operations of the space group), and thus imply site occupancies of less than one. This, in turn, implies XO 4 groups with multiple orientations. Preisinger and coworkers sug¯ and oxygen gested a structure in space group Pm3m atoms in the 24 l position at 1 / 2 0.250 0.366 for the transient, high temperature phase of CsH 2 PO 4 [11]. The site multiplicity of 24 implies that the XO 4 groups exist over six possible orientations. The coordinates proposed by these authors are, however, not ideal in that they yield an unrealistically short ˚ (a o for CsH 2 PO 4 is 4.961 X–O distance of 1.41 A ˚A). Adjustment of the position to 1 / 2 0.250, 0.323
Fig. 5. The X-ray powder diffraction patterns for Cs 2 (HSO 4 )(H 2 PO 4 ) in both its (a) room temperature and (b) high temperature, superprotonic phases.
C.R.I. Chisholm, S.M. Haile / Solid State Ionics 136 – 137 (2000) 229 – 241
236
Fig. 6. Infrared spectra obtained for Cs 2 (HSO 4 )(H 2 PO 4 ) at nominal temperatures of 25, 151, and 1998C. The actual temperatures were more probably 25, 90, and 1408C due to poor thermal contact between the sample pellet and the sample holder.
Table 2 X-ray powder diffraction peak positions and integrated intensities for the high temperature, superprotonic phase of Cs 2 (HSO 4 )(H 2 PO 4 ) h
k
l
Icalc (%)
Iobs (%)
Icalc-obs (%)
2ucalc (8)
2uobs a (8)
2ucalc-obs (8)
d calc ˚ (A)
d obs a ˚ (A)
d calc-obs ˚ (A)
1 1 1 2 2 2 2 2 3
0 1 1 0 1 1 2 2 1
0 0 1 0 0 1 0 1 0
18.2 100.0 11.8 10.1 14.6 14.7 2.8 2.8 2.8
22.5 100.0 16.1 16.1 14.7 15.8 3.0 2.3 3.4
24.3 0.0 24.3 26.0 20.1 21.1 20.2 10.5 20.6
17.97 25.51 31.37 36.37 40.83 44.92 52.34 55.77 59.06
17.97 25.48 31.39 36.39 40.83 44.93 52.33 55.78 59.05
0.00 0.03 20.02 20.02 0.00 20.01 0.01 20.01 0.01
4.932 3.488 2.849 2.468 2.208 2.016 1.747 1.647 1.563
4.932 3.493 2.847 2.466 2.208 2.016 1.747 1.647 1.563
0 20.004 0.002 0.002 0 0 0 0 0
a
Corrected for an obvious displacement error due to the sample stage configuration.
˚ and Cs–O distances gives X–O distances of 1.52 A, ˚ of 3.20 A, both typical values. In Table 2 the measured peak intensities are compared with calculated values determined assuming these optimized oxygen coordinates. The agreement between the observed and calculated values is rather good, indicating that this preliminary structural model is a reasonable representation of the true structure. Both the conductivity and optical spectroscopy results are consistent with a structural transition to a high-temperature phase that is cubic. For a cubic material, one expects that the conductivity measured along all crystallographic directions to be equal, i.e. that the material be isotropic. Such an equivalence is
evident in Fig. 4. While the conductivities along different directions differ significantly at low temperatures, they coalesce to the same value at temperatures above 1058C. Moreover, the very low activation energy for proton transport, 0.37(1) eV, in the high temperature phase, is consistent with a transport mechanism in which XO 4 group reorientation facilitates proton motion, as has been directly observed for CsHSO 4 and CsHSeO 4 . In the respective superprotonic phases of these compounds the activation energies for proton transport are 0.33(1) and 0.35(2) eV [1]. With respect to the optical spectra, the five-atom XO 4 group is expected to conform to ideal T d
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237
Table 3 Infrared peak positions and heights measured for Cs 2 (HSO 4 )(H 2 PO 4 ) in its high temperature phase and at room temperature High temperature Wavenumbers (cm 21 )
Room temperature Peak intensity % absorbance
Wavenumbers (cm 21 )
Qualitative assignment Peak intensity % absorbance
495 513
0.52 0.47
474 496 517
0.46 0.64 0.56
599 620
0.37 0.76
968
0.80
601 623 641 670 971 1012
0.38 0.33 0.48 0.20 1.05 0.70
1109 1124 1147
1.42 1.39 1.24
1065 1091 1173 1286
0.80 0.81 1.10 0.47
1700 2340 2760
n / aa n / aa n / aa
1715 2350 2945 a
n / aa n / aa n / aa
Parent mode in T d symmetry
jn2 (E) OXOasymmetric bend (IR inactive) bending modes
X–OH stretch X–O stretch
jn4 (F 2 ) OXO-symmetric bend jn1 (A 1 ) XOsymmetric stretch (IR inactive) jn3 (F 2 ) XOasymmetric stretch
jX–O–H in-plane bending mode jn OH(C) jn OH(B) jn OH(A)
Unavailable due to large experimental background in this area of the spectrum.
Fig. 7. The structure of Cs 2 (HSO 4 )(H 2 PO 4 ). For clarity, Cs atoms are shown with isotropic displacement parameters and protons are omitted. Hydrogen bonds are indicated with heavy dotted lines. Zig-zag chains of hydrogen bonded XO 4 groups extend along b¢ and are further linked to one another via the O(1) . . . O(1) hydrogen bonds.
symmetry in the cubic phase. The group exhibits nine vibrational modes, only four of which are independent. These are the n1 (A 1 ) or XO 4 symmetric stretch, the n2 (E32) or O–X–O asymmetric bend, the n3 (F 2 33) or XO 4 asymmetric stretch, and the n4 (F 2 33) or O–X–O symmetric bend. In the case of the SO 422 group, these peaks appear at 980, 451, 1104 and 613 cm 21 , respectively, whereas in the case of the PO 32 group, they appear at 970, 358, 4 1080 and 500 cm 21 , respectively [12]. In the IR spectra of Cs 2 (HSO 4 )(H 2 PO 4 ), absorbance bands are clearly visible in these regions, (although for ideal T d symmetry the n1 and n2 modes are infrared inactive). In the high temperature spectrum, there are four primary peaks associated with these four vibrational modes, centered at 968, 500, 1125 and 620 cm 21 . The n2 and n4 peaks appear as doublets, and the n3 peak as a triplet (Table 3). This is interpreted to result from local chemical differences between SO 4 and PO 4 groups, and differences in the proton occupations at various XO 4 groups, rather than a global deviation from cubic symmetry. The degree of splitting at these peaks is small, and suggests that the local crystal-chemical differences between one XO 4 group and the next are correspondingly small.
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Fig. 8. The structure of Cs 2 (HSO 4 )(H 2 PO 4 ) shown in projection in a direction perpendicular to (1¯ 0 1). For clarity, Cs atoms are omitted. The hydrogen bonding between XO 4 groups creates a 2-dimensional network, with planes parallel to (1¯ 0 1). The H(1) proton is distributed over two crystallographically equivalent sites within the O(1) . . . O(1) hydrogen bond. The H(2) proton resides at a site close to O(4), the donor oxygen atom in the O(4) . . . O(3) hydrogen bond; the position H(2a) represents a probable interstitial site.
Table 4 ˚ and angles [8] between atoms involved in hydrogen bonds in Cs 2 (HSO 4 )(H 2 PO 4 ) [7] Interatomic distances [A]
H(1) H(2)
O Neighbors
d(O . . . O) ˚ (A)
d(O D –H) ˚ (A)
d(OA –H) ˚ (A)
/(O D HOA ) (8)
O(1)A / D –O(1)A / D O(3)A –O(4) D
2.541(6) 2.552(8)
1.000 0.999
1.567 1.658
163.3 146.5
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The high frequency peaks in the IR spectra, at 1700, 2340, and 2760 cm 21 can be readily assigned to OH stretching modes within the O–H . . . O bonds; the peak at |3400 cm 21 is due to water that was in the atmosphere and / or on the sample surface, and quickly disappeared upon gentle heating. The appearance of three broad, high frequency absorption bands is common in strongly hydrogen-bonded systems, and these are often referred to as the ‘ABC bands’ [13]. In CsHSO 4 (at 258C) the bands appear 21 at 1720, 2430 and 2900 cm , respectively [14], and in CsH 2 PO 4 (at 258C) they appear at |1700, |2300 and |2600 cm 21 [15], values which are in good agreement with those observed here for Cs 2 (HSO 4 )(H 2 PO 4 ). The similarity of the peak positions in the high temperature and room temperature phases of Cs 2 (HSO 4 )(H 2 PO 4 ), Table 3, suggest that the O–H . . . O bond strengths are not significantly different in the two structures, despite the certain rotational disorder of the tetrahedral groups in the former.
4.2. Room temperature structures While the room temperature structure of Cs 2 (HSO 4 )(H 2 PO 4 ) has been well-established from X-ray diffraction studies [7], the optical spectra and measurements of the conductivity tensor provide new insights into local structural and chemical features. Although there is only one crystallographically distinct XO 4 anion site in the P2 1 / n structure, there are a tremendous number of absorbance peaks in the 21 1400–400 cm region attributable to tetrahedral group internal modes. This is, of course, due to the low symmetry (the P/ S atoms resides at a general position in the unit cell and all O are crystallographically distinct), the fact that only three of the four oxygen atoms are involved in forming hydrogen bonds, the extensive disorder in proton positions, and, as pointed out above for the high temperature structure, local chemical differences between PO 4 and SO 4 anions and their respective vibration modes. On the basis of the coalescence of various absorption peaks during heating of the sample, qualitative assignment of these peaks to internal vibration modes was possible, Table 3. The five peaks present in the frequency range 1220–1050 cm 21 are taken to derive from n3 asymmetric stretching modes. On
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heating, coalescence of these peaks resulted in two doublets at an intermediary temperature, Fig. 6, curve b, and then a single triplet at high temperature, curve c. By analogy to peak assignments for CsHSO 4 [14] it is reasonable to assign these five peaks to X–OH stretching modes in the XO 4 H 1.5 group. From similar observations of peak coalescence, two peaks in the room temperature spectrum present in the range 1020–950 cm 21 are taken to derive from n1 symmetric stretching modes, and, again, from comparison to CsHSO 4 , these are presumed to correspond to X–O stretching modes. Four peaks present in the range 680–580 likely derive from n4 symmetric bending modes, and three peaks present in the range 530–460 to derive from n2 asymmetric bending modes. A peak at 1286 cm 21 is present in the room temperature spectrum which, rather than coalescing with other peaks at high temperature, simply diminished in intensity. This peak is likely due to X–O–H bending motion [16], and has no counterpart in an ideal XO 4 molecule. Moreover, such a peak similarly disappears upon heating from CsHSO 4 [14]. The difference in frequencies between the primary n (X–O) and n (X–OH) peaks in Cs 2 (HSO 4 )(H 2 PO 4 ) is approximately 120 cm 21 . In the case of CsHSO 4 the difference is significantly greater, with the main n (X–O) peak at 850 cm 21 and the main n (X–OH) peak at 1000 cm 21 . This would suggest that the difference in chemical environments about hydrogenbonded and non-hydrogen-boned oxygen atoms in Cs 2 (HSO 4 )(H 2 PO 4 ) is smaller than in CsHSO 4 , and is in agreement with the structure determinations for the two compounds. In the case of CsHSO 4 , there is one hydrogen bond, O(3) . . . H–O(4) and the oxygen donor, O(4), and acceptor, O(3), atoms in this bond are clearly distinguishable. The S–O(4) dis˚ much larger than the mean of the tance is 1.570 A, ˚ [17]. In remaining three S–O distances of 1.445 A Cs 2 (HSO 4 )(H 2 PO 4 ) three oxygen atoms serve as mixed donor acceptor atoms. The mean X–OA / D ˚ and this is rather similar to the distance is 1.517 A, ˚ remaining X–O distance of 1.462 A. Turning to the electrical properties, it is apparent from Figs. 1 to 4 that Cs 2 (HSO 4 )(H 2 PO 4 ) does not transform back to its as-synthesized P2 1 / n monoclinic phase upon cooling from high temperature. X-ray powder diffraction data collected from a
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sample after cooling from 1458C (at which high temperature diffraction data were collected) also indicated that the reverse transformation to the assynthesized structure does not occur. This diffraction pattern (not shown) contained many more peaks than evident in Fig. 5 curve a, and suggested that a two phase material may have been obtained. The conductivity data, Figs. 1–4, further indicate that there is some evolution in the room temperature structure with cycling; specifically, with increased cycling the room temperature conductivity increased. It is probable that the P2 1 / n phase is metastable, that the single crystal samples retain some ‘memory’ of the P2 1 / n → cubic transition, and that on cooling some P2 1 / n phase is formed, but on repeated heating and cooling cycles this ‘memory’ is lost and eventually only the more stable room temperature phase is obtained. The room temperature IR spectrum of a sample exposed to a single heating cycle (not shown) was indistinguishable from the pattern present in Fig. 6 curve (a), despite the clear structural and physical changes that took place on temperature cycling. This suggests that the stable room temperature phase is also monoclinic, and also demonstrates that one should be cautious when drawing conclusions about crystalline structure solely from optical spectroscopy. While these hypotheses regarding transitions on heating and cooling are consistent with the data, further experimentation will be necessary to unequivocally determine the nature of the stable room temperature structure.
4.3. Anisotropy and transformation mechanisms There is clear anisotropy in both the room temperature conductivity of Cs 2 (HSO 4 )(H 2 PO 4 ) and the temperature dependence of the conductivity, Fig. 4. At low temperatures (lower than approximately 608C at which a transition is first detected by thermal methods), the resistances of the samples were too high to permit reliable measurement of the activation energy for proton transport and these data are not included in the figure. Nevertheless, it was observed without ambiguity that at room temperature and on ¢ ¢ ). the first heating cycle s (a¢ 1 c¢ ). s (b¢ )4 s (a–c This observation leads us to conclude that proton transport within the hydrogen bonded planes is easier than transport from one layer to the next, as might be expected (recall that the b¢ and a¢ 1 c¢ directions lie
¢ ¢ within the hydrogen-bonded planes, whereas the a–c direction, lies normal to them). Within the planes, the higher value of s (a¢ 1 c¢ ) over s (b¢ ) may indicate that proton jumps between neighboring H(2) bond sites which involve pairs of distinct XO 4 groups are easier than jumps between H(1) and H(2) bond sites, which involve, in effect, proton motion along the edge of an XO 4 group. The latter type of jump may require the proton to come into close proximity of the positively charged X cation and thus be unfavorable. Note that, because all hydrogen bonds appear to have double-minima potential wells, as described above, all should be capable of accepting proton interstitials. ¢ ¢ Upon heating, the conductivity along the a–c direction increased sharply over the temperature range 60–728C, whereas that along the b¢ and a¢ 1 c¢ directions exhibited only slight increases, Fig. 4a. This temperature range is coincident with the sharp peak observed in the DSC data [7]. These observations suggest that a structural transformation takes place over this low temperature range in which the hydrogen-bonded layers change position relative to one another, perhaps shortening proton jump distances in the direction normal to the planes, without undergoing significant internal changes. Over the temperature range 80–908C the conductivity along all three directions examined increased dramatically, ¢ by approximately five orders of magnitude along b, three to four orders of magnitude along a¢ 1 c¢ and ¢ ¢ At 908C and still two orders of magnitude along a–c. on the first heating cycle the conductivity was isotropic, however, it is unlikely that the structure had fully reached its high temperature phase. The apparent activation energy for proton transport from 908C to 1058C is relatively high, and there is a second, broad DSC peak in the calorimetry data over precisely this temperature range [7]. Without further characterization, it is difficult to hypothesise what kind of structural transitions may or may not occur from 908C to 1058C. It must also be noted that no obvious transient phase was detected by high temperature X-ray powder diffraction. Beyond 1058C, Cs 2 (HSO 4 )(H 2 PO 4 ) clearly exists in its superprotonic cubic phase. On all cooling cycles, the high temperature phase persisted to temperatures well below 1058C. In general, a transformation to a lower conductivity
C.R.I. Chisholm, S.M. Haile / Solid State Ionics 136 – 137 (2000) 229 – 241
phase was observed at about 308C, but this was as much time dependent as it was temperature dependent. Often, the cubic phase was retained at room temperature for up to 2 h, and typically the total transformation time (time below 1058C) was |12 h. On the final heating cycles, the conductivities along ¢ ¢ directions were similar, the nominally a¢ 1 c¢ and a–c whereas that along the b¢ axis was almost an order of magnitude lower than the other two directions, Fig. 4b. One can only conclude that the sequence of phases encountered upon heating from the metastable P2 1 / n phase are different than those encountered upon heating from the stable room temperature phase. The anisotropies evident in the final heating cycle suggest that an intermediate phase with tetragonal or hexagonal symmetry is encountered over the temperature range 70–1058C, and the unique axis of this intermediary phase is parallel to the unique axis of the metastable monoclinic phase.
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Footnotes ‡ and § given in the Table refer to these hydrogen atoms, not O(3) and O(4). The isotropic ˚ 2. hydrogen thermal parameters were fixed at 0.08 A
Acknowledgements The authors are pleased to acknowledge the financial support the National Science Foundation via a National Young Investigator Award to SMH. Additional funding was provided by the Irvine Foundation. The authors thank Professor George Rossman and Liz Miura at the California Institute of Technology for their kind guidance with infrared analyses and Paul Wagner for his assistance with high temperature powder diffractometry.
References 5. Conclusions The main conclusions from this work can be summarized as follows. Cs 2 (HSO 4 )(H 2 PO 4 ) undergoes a gradual, and perhaps two-step superprotonic transition over the temperature range 61–1058C. The high temperature, superprotonic phase is cubic, with a CsCl-type structure. The four oxygen atoms are distributed over 24 l sites at 1 / 2 0.250 0.323, and XO 4 groups undergo reorientational disorder in this cubic phase. The activation energy for proton conduction in the high temperature phase is 0.37(1) eV, and proton transport is facilitated by rapid XO 4 group reorientation. The high conductivity of Cs 2 (HSO 4 )(H 2 PO 4 ), 33 10 23 V21 cm 21 at 1108C, suggests the material may have technological as well as scientific relevance. Note added in proof Due to an editorial error, the proton coordinates in Table 2 of Ref. [7] were omitted. These co-ordinates are: H(1)‡ : H(2)
§
0.05387(4), 0.0329(6), 0.512(6) 0.8558(2), 2 0.0469(3), 2 0.2449(2).
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