CRYSTAL GROWTH & DESIGN
Structural Ferroelectric Phase Transition and Polymorphism in 2-Aminopyridine Dihydrogen Phosphate
2008 VOL. 8, NO. 5 1635–1639
Ivana Radosavljevic Evans,* Judith A. K. Howard, and John S. O. Evans Department of Chemistry, Durham UniVersity, Science Site, South Road, Durham DH1 3LE, England, UK ReceiVed October 31, 2007
ABSTRACT: The structural ferroelectric phase transition in 2-aminopyridine dihydrogen phosphate (2APP) has been studied by single crystal and powder X-ray diffraction between room temperature and 16 K. It has been shown that R-aminopyridine dihydrogen phosphate (R-2APP) undergoes a transition from the centrosymmetric space group C2/c in the paraelectric phase to the polar space group Cc in the ferroelectric phase. This is a second-order phase transition associated with ordering of protons in short O-H · · · O hydrogen bonds. This system is found to exhibit rich polymorphism: depending on crystallization conditions, three anhydrate forms and one monohydrate can be isolated. 2APP hydrate and γ-2APP have been identified for the first time, and their structures have been solved from single crystal diffraction data. Introduction Inorganic, organic, and hybrid ferroelectrics are technologically important materials, with current and future potential applications in electronics and optics. Owing to properties such as large dielectric constants and reversible polarization in electric fields, they can be used, for example, in capacitors, actuators, nonlinear optical devices, piezoelectric and nonvolatile memory elements. One important class of ferroelectric materials is the KH2PO4 (KDP) family of hydrogen-bonded ferroelectrics, which has been extensively studied for decades.1 In addition to practical applications, another interesting aspect of KDP-type materials is the study of fundamental physical phenomena that accompany the ferroelectric phase transition.2 A reversible ferroelectric phase transition in 2-aminopyridine dihydrogen phosphate (2APP) was detected by electrical permittivity measurements and reported by Czapla et al.3 together with the crystal structure of the material in the paraelectric phase at room temperature. It should be noted, however, that in our hands the material prepared according to the procedure they reported, by crystallization of 2-aminopyridine and phosphorous acid,3 in fact yields 2-aminopyridine dihydrogen phosphite, a compound whose composition, properties, and crystal structure differ from the title phase.4 Since then, a second polymorph of 2APP has been prepared, which crystallizes in the noncentrosymmetric polar space group P21.5 In this paper, we report new details of polymorphism in this system, including the preparation and crystal structures of two new forms. We also present a variable temperature single crystal and powder diffraction study of the original 2-aminopyridine dihydrogen phosphate material3 (referred to as the R form), a detailed comparison of the paraelectric and ferroelectric phases and provide insight into the structural aspect of this phase transition.
Scheme 1
the solution to evaporate slowly at room temperature. Clear blockshaped crystals formed after a few days. After a further week, the liquid was completely evaporated and the dry polycrystalline material was recrystallized from water, a 1:1 mixture of water and DMF and from methanol. Single crystals of different habits, suitable for single crystal X-ray diffraction work, were obtained in all batches. X-ray Diffraction. All powder X-ray diffraction measurements, including room temperature scans used for phase identification and variable temperature work, were carried out on a Bruker D8 Advance diffractometer with Cu KR1 radiation, an incident beam Ge(111) monochromator and a Braun linear PSD detector, equipped with a Phenix closed cycle He cryostat. In the variable temperature experiment, data were collected between room temperature and 16 K during continuous cooling and heating at a rate of 14 K/h, in the range from 5 to 70 °2θ. Data collection time was 56 min per range and a total of 72 patterns were collected on cooling and heating. Average temperatures for each data collection were extracted using in-house data processing procedures.6 All data analysis was carried out using Topas Academic software.7 All standard, single temperature single crystal diffraction data on different polymorphs of 2APP were collected on Bruker Smart 6000 diffractometer with a Mo source and a CCD detector. The variable temperature measurements on R-2APP at 30 and 295 K were carried out on a Bruker Smart 1000 CCD instrument with a Mo source, equipped with a Helix He cooling device. In a typical experiment, a full sphere of data was collected using ω scans, with a frame width of 0.3°, and a frame exposure time between 10 and 20 s, depending on the size of the crystal. Data reduction was carried out using the SAINT software suite.8 The crystal structures were solved by direct methods using SIR929 and refined in the Crystals software package.10
Results Experimental Procedures Sample Preparation. 2-Aminopyridine dihydrogenphosphate (hereafter referred to as 2APP, shown in Scheme 1) was prepared from equimolar quantities of reagents, by adding 2-aminopyridine (0.0106 mol, 99%+, Sigma Aldrich) to 2 ml diluted phosphoric acid and leaving * To whom correspondence should be addressed. E-mail: ivana.radosavljevic@ durham.ac.uk.
Polymorphism in 2APP. Our initial crystal growth from an aqueous solution of 2-aminopyridine and phosphoric acid yielded colorless block-shaped crystals that were analyzed by single crystal diffraction and identified as the previously reported β-2APP.5 Interestingly, previous workers reported that β-2APP crystals could only be obtained in a hydrothermal reaction of 2-aminopyridine and phosphoric acid in the presence of ZnO, while the direct reaction of 2-aminopyridine and phosphoric acid
10.1021/cg701076s CCC: $40.75 2008 American Chemical Society Published on Web 03/27/2008
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Figure 1. Pawley refinement of R-2APP obtained by evaporation of the solution from which 2APP monohydrate crystals were grown: observed, calculated, and difference11 profiles are given in blue, red and gray, respectively. Tick marks show allowed peak positions. Table 1. Crystallographic Details for Different Polymorphic Forms of 2APP at 120 K phase
R-2APP
β-2APP
γ-2APP
2APP · H2O
formula mol weight (amu) crystal system space group a (Å) b (Å) c (Å) β (°) V (Å3) Z T (K) calc density (g/cm-3) µ (mm-1) no of obs reflections no. of parameters R (%) wR (%)
(C5H7N3)(H2PO4) 192.11 monoclinic C2/c 13.384(5) 10.168(3) 12.544(4) 111.227(8) 1591.2(9) 8 120 1.604 0.32 2438 143 3.03 6.92
(C5H7N3)(H2PO4) 192.11 monoclinic P21 9.0168(15) 4.5093(8) 9.9301(17) 98.678(3) 399.13(12) 2 120 1.598 0.32 1884 145 3.42 6.75
(C5H7N3)(H2PO4) 192.11 monoclinic P21/c 9.9910(6) 10.4605(7) 8.0467(5) 91.419(2) 840.71(9) 4 120 1.518 0.31 1857 145 3.89 9.47
(C5H7N3)(H2PO4)(H2O) 210.13 monoclinic P21/c 7.3727(4) 15.9120(9) 7.5660(4) 90.274(2) 887.59(8) 4 120 1.572 0.31 1546 162 3.62 8.51
produced the known R form. We carried out a single crystal structure determination of β-2APP at 120 K, and the results do not differ significantly from the published room temperature structure; hence, no further details of this form will be discussed (apart from the basic crystallographic data given for comparison of all polymorphs in Table 1). When the solution from which β-2APP crystals were grown completely evaporated, the dry product left behind was identified by powder X-ray powder diffraction as R-2APP. Small portions of this material were then dissolved in demineralized water, in a mixture of water and DMF, and in methanol. Recrystallization from water yielded colorless crystals in the form of elongated
prisms. Unit cell parameters determined from single crystal XRD were a ) 7.3727(4) Å, b ) 15.9120(9) Å, c ) 7.5660(4) Å, and β 90.274(2)°, indicating this to be a new form of 2APP,
Figure 2. Formation of the different polymorphs of 2APP.
Figure 4. Hydrogen bonding motifs in γ-2APP.
Figure 3. Asymmetric unit and the atom numbering scheme for γ-2APP. Atomic displacement parameters drawn at the 50% probability level.
Ferroelectric Phase Transition in 2-APP
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Figure 5. Crystal packing in γ-2APP: (a) down the c-axis; (b) down the b-axis. Table 2. Crystallographic Details for r-2APP at 290 and 30 K
Figure 6. Asymmetric unit and the atom numbering scheme for 2APP monohydrate. Atomic displacement parameters drawn at the 50% probability level.
and the subsequent structure solution and refinement suggested this was 2APP monohydrate. When this solution was completely evaporated, the remaining dry polycrystalline product was pure R-2APP, as confirmed by powder XRD and the Pawley refinement11 shown in Figure 1. Recrystallization from a mixture of water and DMF gave platy crystals identified as R-2APP. Crystal growth from methanol initially resulted in the formation of elongated rods which gave unit cell parameters of a ) 9.9910(6) Å, b ) 10.4605(7) Å, c ) 8.0467(5) Å, and β ) 91.419(2)°, a new polymorph hereafter referred to as γ-2APP. After a few days, block-shaped crystals of R-2APP started appearing, and those of the γ form disappeared. The formation of different phases in this system is summarized in Figure 2, and crystallographic data are reported in Table 1.
phase
paraelectric R-2APP
ferroelectric R-2APP
T (K) crystal system space group a (Å) b (Å) c (Å) β (°) V (Å3) Z R (%) wR (%)
290 monoclinic C2/c 13.4693(8) 10.2186(6) 12.5884(7) 111.002(1) 1617.5(2) 8 3.20 5.82
30 monoclinic Cc 13.3953(9) 10.1674(7) 12.5422(8) 111.287(1) 1591.6(2) 8 3.06 5.95
The sequence of formation and transformation, as well as the density data, suggest that the R form is the stable polymorph of 2APP. It has the highest density and appears as the final product in all crystallizations and desolvations performed. The β- and γ-forms are kinetically favored when crystallization is carried out from water and methanol, respectively. Crystal structure of γ-2APP. γ-2APP crystallizes in monoclinic space group P21/c, with one aminopyridinium cation and one dihydrogen phosphate anion in the asymmetric unit (Figure 3). Bond lengths in the tetrahedral dihydrogen phosphate group reflect the distribution of protons. There are two shorter P-O bonds of 1.506(1) and 1.509(1) Å, to terminal oxygen atoms O3 and O5, respectively. Two longer bonds of 1.568(1) and 1.567(1) Å are to proton carrying oxygen atoms O2 and O4, respectively.
Figure 7. Crystal packing in 2APP monohydrate: (a) down the c-axis; (b) down the b-axis.
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Evans et al.
Figure 8. The asymmetric unit and atom numbering scheme for R-2APP: (a) at 290 K; (b) at 30 K.
γ-2APP displays hydrogen bonding in three dimensions, and this is different from that found in the other two anhydrous forms. In R-2APP, the main feature is dihydrogen phosphate units connected into chains running along the [110] directions by short O-H · · · O hydrogen bonds (O · · · O distance of 2.47 Å), with a shared proton occupying an inversion center halfway between the P atoms. In the β-form, triplets of H2PO4tetrahedra are connected into R33(12) loops12 forming double chains that run along the [010] direction. In γ-2APP, each H2PO4- group has two OH groups that act as donors and two oxygen atoms that act as acceptors of hydrogen bonds to two adjacent anions, with O · · · O distances of 2.513(2) and 2.615(2) Å (Figure 4). These asymmetric hydrogen bonds form R22(8) motifs that link up into chains running parallel to the c-axis (Figure 5b). These chains are connected to the aminopyridinium cations via O-H · · · N hydrogen bonds principally directed along the b-axis (Figures 4 and 5a). Crystal Structure of 2APP Monohydrate. The asymmetric unit of 2APP monohydrate with the atom numbering scheme is shown in Figure 6. It consists of one aminopyridinium cation, one dihydrogen phosphate group, and one water molecule. The H2PO4- tetrahedra display two longer bonds to protonated oxygen atoms (1.577(2) and 1.576(2) Å) and two shorter bonds to terminal oxygen atoms (1.503(2) and 1.504(2) Å). In regard to the hydrogen bonding, there are similarities between the monohydrate and the γ-2APP polymorph, and this is reflected in the packing arrangements depicted in Figure 7. Acting as donors and acceptors in two hydrogen bonds each, the H2PO4- groups again form essentially symmetric R22(8) motifs, with O · · · O distances of 2.545(2) and 2.547(2) Å. These are linked together into chains running along the c-axis (Figure 7b), and they are connected to the aminopyridinium cations by O-H · · · N bonds of 2.803(2) and 2.849(2) Å. Water molecules are located in channels down the c-axis (Figure 7a) and stabilize the structure by acting as hydrogen bond donors to two H2PO4groups (O · · · O distances of 2.809(2) and 2.822(2) Å) and hydrogen bond donors to adjacent aminopyridinium cations, with the O · · · N distance of 2.730(2) Å. Structural Ferroelectric Phase Transition in r-2APP. A ferroelectric phase transition in R-2APP at 103 K was detected and characterized by electric permittivity and spontaneous polarization measurements.3,13 Temperature dependencies of these quantities have indicated this to be a continuous second-
Figure 9. Arrangement of protons in short O-H · · · O hydrogen bonds principally directed along the a-axis for R-2APP: (a) centrosymmetric arrangement at 290 K; (b) polar arrangement at 30 K. Large red spheres represent oxygen atoms; smaller black spheres represent hydrogen atoms.
order phase transition. We have performed variable temperature diffraction studies of this material by both single crystal and powder methods, in order to gain insight into the structural aspects of this ferroelectric transition. The same single crystal was used for data collections at 290 and 30 K. Structure solution from the low temperature data indicated that the system had undergone a structural phase transition into space group Cc, which is polar and a subgroup of C2/c. In the analysis of both data sets, all hydrogen atoms were located in difference Fourier maps and subsequently treated using a riding model. Basic crystallographic data and agreement factors obtained are given in Table 2. In the ferroelectric phase, the asymmetric unit consists of two aminopyridinium cations and two H2PO4- anions (Figure 8). Except for the expected effects of thermal contraction, the cations remain unchanged.
Ferroelectric Phase Transition in 2-APP
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Figure 10. Temperature variation of unit cell volume and cell parameters a, b, and c in R-2APP. Both cooling (blue) and heating (red) data points are plotted.
The main structural change occurs in the arrangement of protons in dihydrogen phosphate groups. In the high temperature paraelectric form, each H2PO4- tetrahedron contains one terminal O atom with a short P-O bond (1.51 Å), one long P-OH bond of 1.57 Å, and two intermediate distances to oxygen atoms bonded to protons located at inversion centers, shared by adjacent tetrahedra (Figure 8a). This arrangement is consistent with the centrosymmetric space group symmetry of the crystal. In the low temperature ferroelectric form, the protons order so that each unique H2PO4- group contains two short P-O bonds (1.49/1.52 Å and 1.51/1.54 Å) and two long P-OH bonds (1.57/1.58 Å for both groups) (Figure 8b). As a result, the symmetry of the crystal structure is lowered and the asymmetric O-H · · · O hydrogen bonds impart polarity that is primarily directed along the a crystallographic axis (Figure 9). To obtain detailed information about the unit cell parameters variation with temperature, we have collected powder XRD patterns between 293 and 16 K and analyzed the data using the Rietveld method.14 The changes in the O-H · · · O hydrogen bonds at around 100 K are clearly manifested in the temperature dependence of the unit cell parameters (Figure 10). While the cell edges b and c show smooth trends, a significant change of slope is evident in the temperature dependence of the unit cell edge a, the most strongly affected by the changes in hydrogen bonding at the phase transition. The electric permittivity anomaly
measured along the a-axis is much larger than in the other two crystallographic directions.3 Our structural model for the ferroelectric phase transition in R-2APP is thus consistent with the physical measurements on this material.
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