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Intermediate-temperature polymorphic phase transition in KH2PO4: A synchrotron X-ray diffraction study
Journal of Physics and Chemistry of Solids 71 (2010) 1576–1580
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Intermediate-temperature polymorphic phase transition in KH2PO4: A synchrotron X-ray diffraction study Cristian E. Botez a,n, David Carbajal a, Venkata A.K. Adiraju a, Ronald J. Tackett a, Russell R. Chianelli b a b
Department of Physics, University of Texas at El Paso, 500W University Avenue, El Paso, TX 79968, USA Department of Chemistry, University of Texas at El Paso, 500W University Avenue, El Paso, TX 79968, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 26 March 2010 Received in revised form 8 August 2010 Accepted 10 August 2010
We have used synchrotron X-ray diffraction to investigate the structural and chemical changes undergone by polycrystalline KH2PO4 (KDP) upon heating within the 30–250 1C temperature interval. Our data show evidence of a polymorphic transition at T 190 1C from the room-temperature tetragonal KDP phase to a new intermediate-temperature monoclinic KDP modification (spacegroup ˚ and b ¼ 107.361). The monoclinic RDP P21/m and lattice parameters a ¼ 7.590, b ¼ 6.209, c ¼4.530 A, polymorph remains stable upon further heating to 235 1C, and is isomorphic to its RbH2PO4 and CsH2PO4 counterparts. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds B. Chemical synthesis C. X-ray diffraction D. Phase transitions
1. Introduction Upon cooling to Tc ¼–150 1C, the fully hydrogen bonded solid acid KH2PO4 (KDP) undergoes a paraelectric-to-ferroelectric phase transition that corresponds to the transformation of the room temperature tetragonal (I-42d) KDP phase into a low temperature orthorhombic (Fdd2) modification. This behavior has been extensively investigated, and widespread agreement exists on both its micro-structural and macroscopic-property aspects [1,2]. On the other hand, much less is understood about the structural, chemical, and physical-property changes that occur in KDP upon heating from room temperature toward its melting point. Thermal events observed around Tp 185 1C, for example, have been attributed by some authors to a polymorphic phase transition to an intermediate-temperature KDP modification [3], while others have claimed that the behavior at Tp is in fact due to chemical changes such as dehydration and onset of partial polymerization of the room-temperature tetragonal KDP phase [4,5]. The interest in understanding the structural changes undergone upon heating by fully-hydrogen-bonded solid acids such as CsH2PO4 (CDP), RbH2PO4 (RDP), and KH2PO4 (KDP) has recently been renewed by the successful use of CDP as a fuel cell electrolyte at temperatures above 200 1C [6]. This remarkable application is based on CDP’s so-called ‘‘superprotonic’’ behavior—an abrupt several-order-ofmagnitude jump in its proton conductivity upon heating above a temperature threshold [7]. It has been demonstrated that the
n
Corresponding author. Fax: + 915 747 5447. E-mail address: [email protected] (C.E. Botez).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.08.004
above-mentioned proton conductivity enhancement of CDP upon heating is associated with a polymorphic phase transition (occurring at 235 1C) from its room temperature monoclinic (P21/m) phase to a high-temperature dynamically disordered cubic (Pm3m) CDP modification [8,9]. Interestingly, the Rb-based compound (RDP) also exhibits a superprotonic transition at 293 1C, although room-temperature RDP is not monoclinic (as CDP), but tetragonal (I-42d). Very recently, temperatureresolved synchrotron X-ray studies have demonstrated that heating RDP towards its superprotonic transition leads to an intermediate-temperature (T 120 1C) change of the RDP tetragonal phase into a monoclinic modification that, remarkably, is isomorphic (crystallographically identical) to the monoclinic CDP phase [10]. This is important because it strongly suggests that the same monoclinic (P21/m)-cubic (Pm3m) polymorphic transition triggers the superprotonic behavior in both CDP and RDP, and, consequently, a general (cation-independent) type of highlyefficient proton mechanism might be at work in the hightemperature phases of these phosphates. In this context, the results obtained so far on the K-based phosphate solid acid are particularly interesting. KDP was observed not to exhibit a superprotonic behavior upon heating [11], although the room temperature KDP phase is isomorphic to its tetragonal RDP counterpart [12]. There is another phosphate, NH4H2PO4, which crystallizes at room temperature in tetragonal spacegrooup I-42d, and, upon heating, does not show an abrupt enhancement of its proton conductivity [13]. For KDP, a possible explanation for this behavior is that either this compound does not undergo an intermediate-temperature tetragonal-monoclinic transition (similar to the one observed in RDP), or, if such a transition does
C.E. Botez et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1576–1580
2. Experimental procedure KDP crystals were grown by slow evaporation from an aqueous solution prepared mixing stoichiometric amounts of H3PO4 and K2CO3. The crystals were subsequently ground to a fine powder and initially characterized at room temperature by laboratory X-ray diffraction using a Siemens D5000 diffractometer. This confirmed that an impurity-free tetragonal I-42d KDP phase ˚ was synthesized. (lattice parameters a ¼7.460 and c ¼6.982 A) Temperature-resolved XRD data were then collected on the X7B beamline at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory using X-rays of wavelength 0.922 A˚ selected by a double flat-crystal monochromator. A Mar345 flat-plate detector was employed in the transmission geometry to detect the diffracted beam. Images were collected upon heating KDP polycrystals in 15 1C steps from room temperature to 250 1C. An exposure time of 45 s was used at each temperature. Eventually, the images were processed into intensity vs. diffraction angle (2y) patterns by integrating over the projections of the Debye–Scherrer cones onto the flat detector using the FIT2D software [22].
10000
8000
Intensity (arb. units)
occur, the monoclinic KDP and RDP polymorphs have different crystal structures. Previous investigations of the structural changes of the title material upon heating to 200 1C [14,15] did reveal a transition (at Tp 185 1C) to a KDP modification that, according to these studies, is crystallographically different from the monoclinic RDP polymorph. Yet, there is substantial disagreement in terms of the actual structure of the intermediatetemperature KDP phase. Itoh et al. [14], for example, reported a monoclinic structure (P21 or P21/m) having a rather long c unit ˚ and g ¼92.21. These authors also noted that cell edge (c ¼14.49 A) the indexing of their Weissenberg photograph only works if a twinning model is assumed. On the other hand, Subramony et al. [15] reported that KDP becomes triclinic above Tp (spacegroup P-1 ˚ a ¼88.4891, and lattice parameters a¼7.438, b¼7.392, c ¼7.199 A, b ¼86.9201, and g ¼87.7831), modification that persists up to 233 1C. In view of these results, it appears that further investigations aimed at clarifying the structural changes undergone by KDP upon heating above Tp are worth carrying out. Here we present a temperature-resolved synchrotron X-ray diffraction (XRD) study of polycrystalline KDP, where the crystal structure evolution of this material is followed as the temperature is raised from 30 to 250 1C. X-ray scattering methods enhanced by the use of synchrotron radiation have been successfully used to reveal structural information about a wide variety of physical systems from metallic surfaces [16,17] to magnetic systems [18] and organic compounds [19,20]. In particular, powder XRD has significantly benefited from the use of synchrotron X-rays [21]. This method is also very well suited for our present investigation as heating-induced crystal twinning or cracking (previously reported in phosphate solid acids [14]) does affect powder XRD. Our data evidence a polymorphic transition from the roomtemperature tetragonal KDP phase to an intermediate-temperature monoclinic KDP modification at T 190 1C, and, contrary to previous reports [14,15], we found that the new RDP polymorph is isomorphic with both monoclinic RDP and monoclinic CDP. Moreover, we observe that monoclinic KDP is stable up to 235 1C, which indicates that a monoclinic (P21/m)-cubic (Pm3m) transition upon further heating – similar to the one responsible for the superprotonic behavior of CDP and RDP – is not precluded by the intermediate-temperature structural behavior of the title compound. Therefore, the reported lack of superprotonic behavior in KDP [11] is most likely due to ionsize effects and not to crystal-structure considerations.
1577
KH2PO4
70°C < T < 235°C
6000
4000
2000
8
12
16
20
tth (deg.) Fig. 1. XRD patterns collected upon heating KDP from 70 to 235 in 15 1C steps. Datasets are shifted vertically and horizontally for clarity. The vertical bars indicate the 2y positions of the Bragg reflections from the room-temperature tetragonal (I-42d) KDP phase. The data show that a structural transition occurs at about 190 1C.
3. Results and discussion Fig. 1 shows XRD patterns collected within the 7.51–20.51 2y-range upon heating a polycrystalline KDP sample from 70 to 235 in 15 1C steps. The eleven data sets are shifted both laterally and vertically for clarity. The temperature-resolved data demonstrate that the tetragonal (I-42d) KDP phase (whose Bragg reflections are marked by vertical bars) is the only phase present in the sample up to T 190 1C, temperature above which new robust peaks appear in the XRD pattern indicating that a transition to a lower-symmetry structure occurs. At 205 1C, the transition is complete as demonstrated by the fact that all peaks corresponding to the room temperature phase completely vanish. Further heating to 235 1C does not result in major modifications of the XRD patterns, which points toward the stability of the new structure up to this temperature. Heating to 250 1C, however, results in chemical decomposition and melting as indicated by the disappearance of the diffraction peaks. We indexed the peak positions corresponding to the structure observed at 205 1C using a single phase monoclinic unit cell with spacegroup P21/m and lattice parameters a ¼7.579, b¼6.219, ˚ and b ¼106.671. Then, in order to demonstrate that c¼4.548 A, this phase is indeed a new KDP polymorph and confirm its crystal structure, we carried out Rietveld refinements against the powder XRD pattern collected at 205 1C. This was done using the General Structure Analysis System (GSAS) [23]. In a Rietveld fit one adjusts not only the unit cell parameters and the peak profiles, but also the atom positions and the Debye–Waller thermal parameters to obtain the best agreement with the data. Fig. 2 shows the results of this analysis, where the empty symbols represent the XRD data collected at 205 1C, the solid line is the best Rietveld fit, and the vertical bars are the Bragg reflection markers. The lower trace represents the difference curve between the calculated and the observed XRD patterns: Icalc–Iobs. The starting parameters for the Rietveld refinement included the above-mentioned lattice parameters and spacegroup obtained from indexing, as well as the initial positions for the non-hydrogen atoms obtained by assuming that the structure being refined is a monoclinic KDP polymorph crystallographically similar with monoclinic RDP and CDP. Due to the limited 2y range over which data was collected, soft constraints were imposed on the P–O bond lengths and O–P–O angles in the PO4 tetrahedra. The refinement
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converges upon the simultaneous variation of 15 independent parameters to a whole-pattern residual Rwp ¼4.22%, yielding the non-hydrogen atom fractional coordinates shown in Table 1. The corresponding crystal structure of the monoclinic KDP polymorph is shown in the inset of Fig. 2. The most important result of the above-described structural analysis is that the newly found monoclinic KDP polymorph is isomorphic both to monoclinic CDP (i.e. the room-temperature 8000
T = 205°C
Intensity (arb. units)
6000
4000
2000
KH2PO4
0
8
12
16
20
tth (deg.) Fig. 2. Structural refinement of the monoclinic KDP phase. The empty symbols represent the XRD data collected at 205 1C, the solid line in the best Rietveld fit, and the vertical bars are the Bragg reflection markers. The lower trace represents the difference curve between the calculated and the observed XRD patterns: Icalc–Iobs. The inset shows the non-hydrogen atoms in the crystal structure of monoclinic KDP, including the PO4 tetrahedra and the K ions (light spheres).
CDP phase) and to monoclinic RDP (i.e. the intermediatetemperature RDP phase). This isomorphism between monoclinic KDP and RDP is clearly illustrated in Fig. 3, where the atom arrangement in the unit cells of these phases is shown. In addition, Table 2 presents a comparison between the unit cell parameters and PO4 tetrahedral bond distances and angles in monoclinic KDP and RDP. This is significant because there is strong evidence that the superprotonic behavior of CDP and RDP is associated with a heating-induced structural phase transition from their monoclinic phases to a disordered cubic modification that permits a highly efficient proton transport. Thus, the observation that KDP does not exhibit a superprotonic behavior upon heating [11] has been so far explained on the basis of previous reports [14,15] that KDP does not transform to an intermediate-temperature monoclinic phase similar to its CDP and RDP counterparts. However, our results clearly demonstrate that a monoclinic-to-cubic transition in KDP, like the one responsible for the proton conductivity enhancement in the Cs- and Rb-based phosphates, is not precluded by the structural behavior of the title material upon heating. Indeed, we find not only that KDP transforms (at T 190 1C) from its room temperature tetragonal (I-42d) phase into a monoclinic (P21/m) polymorph that is crystallographically identical to monoclinic CDP and RDP, but also that the new monoclinic KDP phase remains stable up to 235 1C. Further heating to 250 1C under ambient pressure and humidity conditions results in the thermal decomposition and eventual melting (marked by the disappearance of the diffraction peaks) of the monoclinic KDP phase. This behavior is similar to that observed in CDP and RDP. For these later phosphates, however, heating the monoclinic phase under special conditions (either a saturated water vapor atmosphere or high pressure of about 1 GPa) results in a three-order-of magnitude jump in their proton conductivity (superprotonic behavior), which, as mentioned above, is associated to the structural modification of the monoclinic phase into a disordered cubic
Table 1 Fractional atomic coordinates in the monoclinic KDP phase. Numbers in parentheses are statistically estimated standard deviations (ESDs) from the Rietveld fit. Atomic Fractional Coordinates Atom X
Y
Z
Multiplicity
Wyckoff letter
Occupancy
K P O(1) O(2) O(3)
0.25 0.75 0.75 0.75 0.5540(1)
0.06928(3) 0.5293(1) 0.5022(1) 0.8447(1) 0.4178(1)
2 2 2 2 4
e e e e e
1 1 1 1 1
0.2338(5) 0.2370(1) 0.3965(2) 0.3220(1) 0.1266(1)
Fig. 3. Comparison between the atom positions in the unit cells of RDP and KDP demonstrating the isomorphism of the two crystal structures.
C.E. Botez et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1576–1580
Table 2 Comparison between unit cell parameters and PO4 tetrahedral bond distances and angles in the monoclinic phases of RDP and KDP. Spacegroup
RDP P21/m
˚ a (A) ˚ b (A) ˚ c (A)
KDP P21/m
7.868(6)
7.590(2)
6.299(5)
6.209(5)
4.871(4)
4.530(5)
109.15(3)
b (deg.)
107.36(5)
Phosphate group tetrahedral bond distances and angles 1.622(19) ˚ P–O(1) (A) ˚ P–O(2) (A) ˚ P–O(3) (A) O(1)-P–O(2) O(1)-P–O(3) O(2)-P–O(3) O(3)-P–O(3)
(deg.) (deg.) (deg.) (deg.)
1.253(14)
1.590(18)
1.381(19)
1.558(7)
1.479(2)
101.1(9) 109.6(6) 113.4(6) 109.4(7)
96.2(4) 115.6(1) 113.3(1) 110.8(1)
We used synchrotron X-ray diffraction to investigate the structural and chemical modifications undergone by KDP upon heating from room temperature to 250 1C. We found that the room-temperature KDP phase (tetragonal I-42d with lattice ˚ persists up to T 190 1C, parameters a ¼7.460 and c¼6.982 A) where our data indicate that a polymorphic phase transition to an intermediate-temperature monoclinic KDP phase occurs. The new monoclinic KDP polymorph crystallizes in space group P21/m ˚ (lattice parameters a¼ 7.590, b¼6.209, c¼4.530 A, and b ¼ 107.361), and, contrary to previous reports, is isomorphic to its CDP and RDP counterparts. Consequently, the absence of a superprotonic behavior upon further heating of KDP is not due to the lack of a P21/m phase, similar to the one that makes the monoclinic-cubic superprotonic transition possible in CDP in RDP, but most likely to cation-size effects, where small cations might not be able to support the particular anion–cation arrangement in the superprotonic phase.
β (deg.)
c (Å)
110 108 106 5 4.9 4.8 4.7 4.6 6.4 6.35
b (Å)
superprotonic behavior, one may speculate that the K cation might simply be too small to be part of the structural framework (disordered cubic Pm3m phase) required to trigger the superprotonic behavior. It is important to mention, however, that in order to positively rule out the existence of a monoclinic-to-cubic structural transition in KDP upon heating above 235 1C, the heating needs to be carried out under the high humidity or high pressure conditions mentioned above. This is due to the fact that dehydration and chemical decomposition are expected to occur upon heating under ambient atmosphere, and are also likely to mask a potential transition to a cubic phase, as it actually happens in both RDP and CDP. Experiments aimed at investigating the structural behavior of monoclinic KDP upon heating above 235 1C under 1 GPa of pressure are currently underway.
4. Summary
112
6.3 6.25
Acknowledgements
6.2 8 7.9 a (Å)
1579
7.8 7.7
K
Rb
Cs
7.6 7.5 1.4
1.5
1.6
1.7
Ionic radius (Å) Fig. 4. Lattice parameters of the monoclinic phases of KDP, RDP, and CDP as a function of the radius of the cation.
polymorph. In view of our current results, the lack of a superprotonic behavior in KDP can no longer be attributed to an intermediate-temperature structural behavior different from the one in the Rb- and Cs-based phosphates. It is also worth noting that the enthalpies of formation within the MH2PO4 (M¼Cs, Rb, K) series differ from one member to another by less that 0.3%, while the differences between lattice energies are about one order of magnitude greater. In addition, ion-size effects might play an important role. The ionic radius of K is 10% and 17.5% smaller than that of Rb and Cs, respectively. Consequently, as shown in Fig. 4, the lattice parameters of the monoclinic KDP phase are reduced with respect to their RDP and CDP isomorphs. Since phosphates for which the cation radii are even smaller than that of K, such as NaH2PO4 or LiH2PO4, have been reported not to exhibit a
CEB, DC, VAKA, and RJT would like to acknowledge support from the Texas Advanced Research Program under Award no. 003661-0010-2007, the Research Corporation under Award no. 7749, and the Donors of the American Chemical Society Petroleum Research Fund under Research Grant no. 45854-GB10. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract no. DE-AC02-98CH10886. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
B.C. Frazer, R. Pepinsky, Acta Crystallogr. 6 (1953) 273. E. Rapoport, J. Chem. Phys. 53 (1970) 311. ¨ J. Grunberg, S. Levin, I. Pelah, D. Gerlich, Phys. Status Solidi B 49 (1972) 857. J.-K. Lee, J. Phys. Chem. Solids 57 (1996) 333. E. Ortiz, A. Vargas, B.-E. Mellander, J. Phys. Chem. Solids 59 (1998) 333. D.A. Boysen, T. Uda, C.R.I. Chisholm, S.M. Haile, Science 303 (2004) 68. A.I. Baranov, V.P. Khiznichenko, L.A. Shuvalov, Ferroelectrics 100 (1989) 135. D.A. Boysen, S.M. Haile, H. Liu, R.A. Secco, Chem. Mater. 15 (2003) 727. C.E. Botez, J.D. Hermosillo, J. Zhang, J. Qian, Y. Zhao, J. Majzlan, R.R. Chianelli, C. Pantea, J. Chem. Phys. 127 (2007) 194701. C.E. Botez, H. Martinez, R.J. Tackett, R.R. Chianelli, J. Zhang, Y. Zhao, J. Phys.: Condens. Matter 21 (2009) 325401. D.A. Boysen, S.M. Haile, H. Liu, R.A. Secco, Chem. Mater. 16 (2004) 693. J. West, Z. Kristallogr. 10 (1930) 306. E. Torijano, R.A. Vargas, J.E. Diosa, B.E. Mellander, Phys. Status Solidi B 220 (2000) 659. K. Itoh, T. Matubayashi, E. Nakamura, H. Motegi, J. Phys. Soc. Jpn. 39 (1975) 843. J.A. Subramoni, S. Lovell, B. Kahr, Chem. Mater. 10 (1998) 2055. R. Feidenhans’l, Surf. Sci. Rep. 10 (1989) 105.
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[17] C.E. Botez, K. Li, E.D. Lu, W.C. Elliott, E.H. Conrad, P.W. Stephens, Appl. Phys. Lett. 81 (2002) 4718. [18] J.T.E. Galindo, A.H. Adair, C.E. Botez, V.C. Flores, D.B. Baques, L.F. Cobas, J.A. Matutes-Aquino, Appl. Phys. A 87 (2007) 743. [19] H.N. de Armas, O.M. Peeters, N. Blaton, G. Van der Mooter, D.J.A. De Rider, H. Schenk, J. Pharm. Sci. 10 (2006) 2123. [20] C.E. Botez, P.W. Stephens, C. Nunes, R. Suryanarayanan, Powder Diffr. 18 (2003) 214.
[21] Structure determination form powder diffraction data, IUCr Monographs in Crystallography, (Eds.), W.I.F. David, K. Shankland, L.B.McCusker, Ch. Baerlocher, Oxford Science Publications, 2002. [22] A.P. Hammersley, S.O. Svenson, M. Hanfland, A.N. Fitch, D. Hauserman, High Pressure Res. 14 (1996) 235. [23] A.C. Larson and R.B. Von Dreele, Los Alamos National Laboratory Report No. LA-UR-86-784, Los Alamos, 1987, Program and documentation available from /http://public.lanl.gov/gsasS.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Intermediate-temperature polymorphic phase transition in KH2PO4: A synchrotron X-ray diffraction study Cristian E. Botez a,n, David Carbajal a, Venkata A.K. Adiraju a, Ronald J. Tackett a, Russell R. Chianelli b a b
Department of Physics, University of Texas at El Paso, 500W University Avenue, El Paso, TX 79968, USA Department of Chemistry, University of Texas at El Paso, 500W University Avenue, El Paso, TX 79968, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 26 March 2010 Received in revised form 8 August 2010 Accepted 10 August 2010
We have used synchrotron X-ray diffraction to investigate the structural and chemical changes undergone by polycrystalline KH2PO4 (KDP) upon heating within the 30–250 1C temperature interval. Our data show evidence of a polymorphic transition at T 190 1C from the room-temperature tetragonal KDP phase to a new intermediate-temperature monoclinic KDP modification (spacegroup ˚ and b ¼ 107.361). The monoclinic RDP P21/m and lattice parameters a ¼ 7.590, b ¼ 6.209, c ¼4.530 A, polymorph remains stable upon further heating to 235 1C, and is isomorphic to its RbH2PO4 and CsH2PO4 counterparts. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds B. Chemical synthesis C. X-ray diffraction D. Phase transitions
1. Introduction Upon cooling to Tc ¼–150 1C, the fully hydrogen bonded solid acid KH2PO4 (KDP) undergoes a paraelectric-to-ferroelectric phase transition that corresponds to the transformation of the room temperature tetragonal (I-42d) KDP phase into a low temperature orthorhombic (Fdd2) modification. This behavior has been extensively investigated, and widespread agreement exists on both its micro-structural and macroscopic-property aspects [1,2]. On the other hand, much less is understood about the structural, chemical, and physical-property changes that occur in KDP upon heating from room temperature toward its melting point. Thermal events observed around Tp 185 1C, for example, have been attributed by some authors to a polymorphic phase transition to an intermediate-temperature KDP modification [3], while others have claimed that the behavior at Tp is in fact due to chemical changes such as dehydration and onset of partial polymerization of the room-temperature tetragonal KDP phase [4,5]. The interest in understanding the structural changes undergone upon heating by fully-hydrogen-bonded solid acids such as CsH2PO4 (CDP), RbH2PO4 (RDP), and KH2PO4 (KDP) has recently been renewed by the successful use of CDP as a fuel cell electrolyte at temperatures above 200 1C [6]. This remarkable application is based on CDP’s so-called ‘‘superprotonic’’ behavior—an abrupt several-order-ofmagnitude jump in its proton conductivity upon heating above a temperature threshold [7]. It has been demonstrated that the
n
Corresponding author. Fax: + 915 747 5447. E-mail address: [email protected] (C.E. Botez).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.08.004
above-mentioned proton conductivity enhancement of CDP upon heating is associated with a polymorphic phase transition (occurring at 235 1C) from its room temperature monoclinic (P21/m) phase to a high-temperature dynamically disordered cubic (Pm3m) CDP modification [8,9]. Interestingly, the Rb-based compound (RDP) also exhibits a superprotonic transition at 293 1C, although room-temperature RDP is not monoclinic (as CDP), but tetragonal (I-42d). Very recently, temperatureresolved synchrotron X-ray studies have demonstrated that heating RDP towards its superprotonic transition leads to an intermediate-temperature (T 120 1C) change of the RDP tetragonal phase into a monoclinic modification that, remarkably, is isomorphic (crystallographically identical) to the monoclinic CDP phase [10]. This is important because it strongly suggests that the same monoclinic (P21/m)-cubic (Pm3m) polymorphic transition triggers the superprotonic behavior in both CDP and RDP, and, consequently, a general (cation-independent) type of highlyefficient proton mechanism might be at work in the hightemperature phases of these phosphates. In this context, the results obtained so far on the K-based phosphate solid acid are particularly interesting. KDP was observed not to exhibit a superprotonic behavior upon heating [11], although the room temperature KDP phase is isomorphic to its tetragonal RDP counterpart [12]. There is another phosphate, NH4H2PO4, which crystallizes at room temperature in tetragonal spacegrooup I-42d, and, upon heating, does not show an abrupt enhancement of its proton conductivity [13]. For KDP, a possible explanation for this behavior is that either this compound does not undergo an intermediate-temperature tetragonal-monoclinic transition (similar to the one observed in RDP), or, if such a transition does
C.E. Botez et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1576–1580
2. Experimental procedure KDP crystals were grown by slow evaporation from an aqueous solution prepared mixing stoichiometric amounts of H3PO4 and K2CO3. The crystals were subsequently ground to a fine powder and initially characterized at room temperature by laboratory X-ray diffraction using a Siemens D5000 diffractometer. This confirmed that an impurity-free tetragonal I-42d KDP phase ˚ was synthesized. (lattice parameters a ¼7.460 and c ¼6.982 A) Temperature-resolved XRD data were then collected on the X7B beamline at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory using X-rays of wavelength 0.922 A˚ selected by a double flat-crystal monochromator. A Mar345 flat-plate detector was employed in the transmission geometry to detect the diffracted beam. Images were collected upon heating KDP polycrystals in 15 1C steps from room temperature to 250 1C. An exposure time of 45 s was used at each temperature. Eventually, the images were processed into intensity vs. diffraction angle (2y) patterns by integrating over the projections of the Debye–Scherrer cones onto the flat detector using the FIT2D software [22].
10000
8000
Intensity (arb. units)
occur, the monoclinic KDP and RDP polymorphs have different crystal structures. Previous investigations of the structural changes of the title material upon heating to 200 1C [14,15] did reveal a transition (at Tp 185 1C) to a KDP modification that, according to these studies, is crystallographically different from the monoclinic RDP polymorph. Yet, there is substantial disagreement in terms of the actual structure of the intermediatetemperature KDP phase. Itoh et al. [14], for example, reported a monoclinic structure (P21 or P21/m) having a rather long c unit ˚ and g ¼92.21. These authors also noted that cell edge (c ¼14.49 A) the indexing of their Weissenberg photograph only works if a twinning model is assumed. On the other hand, Subramony et al. [15] reported that KDP becomes triclinic above Tp (spacegroup P-1 ˚ a ¼88.4891, and lattice parameters a¼7.438, b¼7.392, c ¼7.199 A, b ¼86.9201, and g ¼87.7831), modification that persists up to 233 1C. In view of these results, it appears that further investigations aimed at clarifying the structural changes undergone by KDP upon heating above Tp are worth carrying out. Here we present a temperature-resolved synchrotron X-ray diffraction (XRD) study of polycrystalline KDP, where the crystal structure evolution of this material is followed as the temperature is raised from 30 to 250 1C. X-ray scattering methods enhanced by the use of synchrotron radiation have been successfully used to reveal structural information about a wide variety of physical systems from metallic surfaces [16,17] to magnetic systems [18] and organic compounds [19,20]. In particular, powder XRD has significantly benefited from the use of synchrotron X-rays [21]. This method is also very well suited for our present investigation as heating-induced crystal twinning or cracking (previously reported in phosphate solid acids [14]) does affect powder XRD. Our data evidence a polymorphic transition from the roomtemperature tetragonal KDP phase to an intermediate-temperature monoclinic KDP modification at T 190 1C, and, contrary to previous reports [14,15], we found that the new RDP polymorph is isomorphic with both monoclinic RDP and monoclinic CDP. Moreover, we observe that monoclinic KDP is stable up to 235 1C, which indicates that a monoclinic (P21/m)-cubic (Pm3m) transition upon further heating – similar to the one responsible for the superprotonic behavior of CDP and RDP – is not precluded by the intermediate-temperature structural behavior of the title compound. Therefore, the reported lack of superprotonic behavior in KDP [11] is most likely due to ionsize effects and not to crystal-structure considerations.
1577
KH2PO4
70°C < T < 235°C
6000
4000
2000
8
12
16
20
tth (deg.) Fig. 1. XRD patterns collected upon heating KDP from 70 to 235 in 15 1C steps. Datasets are shifted vertically and horizontally for clarity. The vertical bars indicate the 2y positions of the Bragg reflections from the room-temperature tetragonal (I-42d) KDP phase. The data show that a structural transition occurs at about 190 1C.
3. Results and discussion Fig. 1 shows XRD patterns collected within the 7.51–20.51 2y-range upon heating a polycrystalline KDP sample from 70 to 235 in 15 1C steps. The eleven data sets are shifted both laterally and vertically for clarity. The temperature-resolved data demonstrate that the tetragonal (I-42d) KDP phase (whose Bragg reflections are marked by vertical bars) is the only phase present in the sample up to T 190 1C, temperature above which new robust peaks appear in the XRD pattern indicating that a transition to a lower-symmetry structure occurs. At 205 1C, the transition is complete as demonstrated by the fact that all peaks corresponding to the room temperature phase completely vanish. Further heating to 235 1C does not result in major modifications of the XRD patterns, which points toward the stability of the new structure up to this temperature. Heating to 250 1C, however, results in chemical decomposition and melting as indicated by the disappearance of the diffraction peaks. We indexed the peak positions corresponding to the structure observed at 205 1C using a single phase monoclinic unit cell with spacegroup P21/m and lattice parameters a ¼7.579, b¼6.219, ˚ and b ¼106.671. Then, in order to demonstrate that c¼4.548 A, this phase is indeed a new KDP polymorph and confirm its crystal structure, we carried out Rietveld refinements against the powder XRD pattern collected at 205 1C. This was done using the General Structure Analysis System (GSAS) [23]. In a Rietveld fit one adjusts not only the unit cell parameters and the peak profiles, but also the atom positions and the Debye–Waller thermal parameters to obtain the best agreement with the data. Fig. 2 shows the results of this analysis, where the empty symbols represent the XRD data collected at 205 1C, the solid line is the best Rietveld fit, and the vertical bars are the Bragg reflection markers. The lower trace represents the difference curve between the calculated and the observed XRD patterns: Icalc–Iobs. The starting parameters for the Rietveld refinement included the above-mentioned lattice parameters and spacegroup obtained from indexing, as well as the initial positions for the non-hydrogen atoms obtained by assuming that the structure being refined is a monoclinic KDP polymorph crystallographically similar with monoclinic RDP and CDP. Due to the limited 2y range over which data was collected, soft constraints were imposed on the P–O bond lengths and O–P–O angles in the PO4 tetrahedra. The refinement
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converges upon the simultaneous variation of 15 independent parameters to a whole-pattern residual Rwp ¼4.22%, yielding the non-hydrogen atom fractional coordinates shown in Table 1. The corresponding crystal structure of the monoclinic KDP polymorph is shown in the inset of Fig. 2. The most important result of the above-described structural analysis is that the newly found monoclinic KDP polymorph is isomorphic both to monoclinic CDP (i.e. the room-temperature 8000
T = 205°C
Intensity (arb. units)
6000
4000
2000
KH2PO4
0
8
12
16
20
tth (deg.) Fig. 2. Structural refinement of the monoclinic KDP phase. The empty symbols represent the XRD data collected at 205 1C, the solid line in the best Rietveld fit, and the vertical bars are the Bragg reflection markers. The lower trace represents the difference curve between the calculated and the observed XRD patterns: Icalc–Iobs. The inset shows the non-hydrogen atoms in the crystal structure of monoclinic KDP, including the PO4 tetrahedra and the K ions (light spheres).
CDP phase) and to monoclinic RDP (i.e. the intermediatetemperature RDP phase). This isomorphism between monoclinic KDP and RDP is clearly illustrated in Fig. 3, where the atom arrangement in the unit cells of these phases is shown. In addition, Table 2 presents a comparison between the unit cell parameters and PO4 tetrahedral bond distances and angles in monoclinic KDP and RDP. This is significant because there is strong evidence that the superprotonic behavior of CDP and RDP is associated with a heating-induced structural phase transition from their monoclinic phases to a disordered cubic modification that permits a highly efficient proton transport. Thus, the observation that KDP does not exhibit a superprotonic behavior upon heating [11] has been so far explained on the basis of previous reports [14,15] that KDP does not transform to an intermediate-temperature monoclinic phase similar to its CDP and RDP counterparts. However, our results clearly demonstrate that a monoclinic-to-cubic transition in KDP, like the one responsible for the proton conductivity enhancement in the Cs- and Rb-based phosphates, is not precluded by the structural behavior of the title material upon heating. Indeed, we find not only that KDP transforms (at T 190 1C) from its room temperature tetragonal (I-42d) phase into a monoclinic (P21/m) polymorph that is crystallographically identical to monoclinic CDP and RDP, but also that the new monoclinic KDP phase remains stable up to 235 1C. Further heating to 250 1C under ambient pressure and humidity conditions results in the thermal decomposition and eventual melting (marked by the disappearance of the diffraction peaks) of the monoclinic KDP phase. This behavior is similar to that observed in CDP and RDP. For these later phosphates, however, heating the monoclinic phase under special conditions (either a saturated water vapor atmosphere or high pressure of about 1 GPa) results in a three-order-of magnitude jump in their proton conductivity (superprotonic behavior), which, as mentioned above, is associated to the structural modification of the monoclinic phase into a disordered cubic
Table 1 Fractional atomic coordinates in the monoclinic KDP phase. Numbers in parentheses are statistically estimated standard deviations (ESDs) from the Rietveld fit. Atomic Fractional Coordinates Atom X
Y
Z
Multiplicity
Wyckoff letter
Occupancy
K P O(1) O(2) O(3)
0.25 0.75 0.75 0.75 0.5540(1)
0.06928(3) 0.5293(1) 0.5022(1) 0.8447(1) 0.4178(1)
2 2 2 2 4
e e e e e
1 1 1 1 1
0.2338(5) 0.2370(1) 0.3965(2) 0.3220(1) 0.1266(1)
Fig. 3. Comparison between the atom positions in the unit cells of RDP and KDP demonstrating the isomorphism of the two crystal structures.
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Table 2 Comparison between unit cell parameters and PO4 tetrahedral bond distances and angles in the monoclinic phases of RDP and KDP. Spacegroup
RDP P21/m
˚ a (A) ˚ b (A) ˚ c (A)
KDP P21/m
7.868(6)
7.590(2)
6.299(5)
6.209(5)
4.871(4)
4.530(5)
109.15(3)
b (deg.)
107.36(5)
Phosphate group tetrahedral bond distances and angles 1.622(19) ˚ P–O(1) (A) ˚ P–O(2) (A) ˚ P–O(3) (A) O(1)-P–O(2) O(1)-P–O(3) O(2)-P–O(3) O(3)-P–O(3)
(deg.) (deg.) (deg.) (deg.)
1.253(14)
1.590(18)
1.381(19)
1.558(7)
1.479(2)
101.1(9) 109.6(6) 113.4(6) 109.4(7)
96.2(4) 115.6(1) 113.3(1) 110.8(1)
We used synchrotron X-ray diffraction to investigate the structural and chemical modifications undergone by KDP upon heating from room temperature to 250 1C. We found that the room-temperature KDP phase (tetragonal I-42d with lattice ˚ persists up to T 190 1C, parameters a ¼7.460 and c¼6.982 A) where our data indicate that a polymorphic phase transition to an intermediate-temperature monoclinic KDP phase occurs. The new monoclinic KDP polymorph crystallizes in space group P21/m ˚ (lattice parameters a¼ 7.590, b¼6.209, c¼4.530 A, and b ¼ 107.361), and, contrary to previous reports, is isomorphic to its CDP and RDP counterparts. Consequently, the absence of a superprotonic behavior upon further heating of KDP is not due to the lack of a P21/m phase, similar to the one that makes the monoclinic-cubic superprotonic transition possible in CDP in RDP, but most likely to cation-size effects, where small cations might not be able to support the particular anion–cation arrangement in the superprotonic phase.
β (deg.)
c (Å)
110 108 106 5 4.9 4.8 4.7 4.6 6.4 6.35
b (Å)
superprotonic behavior, one may speculate that the K cation might simply be too small to be part of the structural framework (disordered cubic Pm3m phase) required to trigger the superprotonic behavior. It is important to mention, however, that in order to positively rule out the existence of a monoclinic-to-cubic structural transition in KDP upon heating above 235 1C, the heating needs to be carried out under the high humidity or high pressure conditions mentioned above. This is due to the fact that dehydration and chemical decomposition are expected to occur upon heating under ambient atmosphere, and are also likely to mask a potential transition to a cubic phase, as it actually happens in both RDP and CDP. Experiments aimed at investigating the structural behavior of monoclinic KDP upon heating above 235 1C under 1 GPa of pressure are currently underway.
4. Summary
112
6.3 6.25
Acknowledgements
6.2 8 7.9 a (Å)
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7.8 7.7
K
Rb
Cs
7.6 7.5 1.4
1.5
1.6
1.7
Ionic radius (Å) Fig. 4. Lattice parameters of the monoclinic phases of KDP, RDP, and CDP as a function of the radius of the cation.
polymorph. In view of our current results, the lack of a superprotonic behavior in KDP can no longer be attributed to an intermediate-temperature structural behavior different from the one in the Rb- and Cs-based phosphates. It is also worth noting that the enthalpies of formation within the MH2PO4 (M¼Cs, Rb, K) series differ from one member to another by less that 0.3%, while the differences between lattice energies are about one order of magnitude greater. In addition, ion-size effects might play an important role. The ionic radius of K is 10% and 17.5% smaller than that of Rb and Cs, respectively. Consequently, as shown in Fig. 4, the lattice parameters of the monoclinic KDP phase are reduced with respect to their RDP and CDP isomorphs. Since phosphates for which the cation radii are even smaller than that of K, such as NaH2PO4 or LiH2PO4, have been reported not to exhibit a
CEB, DC, VAKA, and RJT would like to acknowledge support from the Texas Advanced Research Program under Award no. 003661-0010-2007, the Research Corporation under Award no. 7749, and the Donors of the American Chemical Society Petroleum Research Fund under Research Grant no. 45854-GB10. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract no. DE-AC02-98CH10886. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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