Pyroelectric Potassium Hydrogen Malate with Polarized Hydrogen Bond Chains and Aligned Dipole Moments

Pyroelectric potassium hydrogen malate with polarized hydrogen bond chains and aligned dipole moments E. de Matos Gomes,1,a兲M. S. Belsley,1A. Criado,2V. H. Rodrigues,3and M. M. R. Costa3 1Departamento de Física, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal 2Departamento de Física de la Materia Condensada, Instituto de Ciencia de Materiales, CSIC, Universidad de Sevilla, Apartado 1065, Sevilla 41012, Spain 3Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, 3004-516 Coimbra, Portugal 共Received 7 October 2008; accepted 5 November 2008; published online 2 December 2008兲 A spontaneous polarization of 16 ␮ C/cm2at room temperature has been found in the semiorganic crystal potassium hydrogen malate 共KHC4H4O5·H2O兲. This high polarization results from a structural arrangement of oriented dipoles formed by the potassium cations and the COO−group from the malate anion, and long hydrogen bonded polarized chains created by the hydrogen L-malate anion via O–H¯O interactions, directed along the polar sixfold axis. The material shows a second harmonic generation with a rotationally averaged effective second order susceptibility equal to 0.70 of potassium dihydrogen phosphate at an incident wavelength of 1064 nm. © 2008 American Institute of Physics.关DOI: 10.1063/1.3036010兴 Hydrogen bonds are often used in crystal structure engineering to create molecular assemblies with specific properties, such as polar materials, which display technologically important properties such as pyroelectricity and ferroelectricity. Pyroelectric crystals are currently used as thermal detectors and low-power x-ray sources.1–5One crystal engineering approach takes advantage of the strong directional orientation capability of hydrogen bond interactions between molecules to control molecular self-assembly during crystallization and to induce polarity in the crystal structure.6–9A particular example is that of hydrogen malate anions, which are able to create rigid one and two dimensional aggregates that direct the orientation of the cations in the solid state, capable of creating hydrogen bonded materials with exceptional linear and nonlinear dielectric properties.10 The role played by electrostatic dipole-dipole interactions, particularly important for hydrogen bonded ferroelectric materials, has been estimated and a ferroelectricity dipole model was found to be quite accurate in the case of sodium potassium tartrate11,12 Cesium hydrogen malate hydrate 共CsLM兲has been reported as an excellent pyroelectric, electro-optic, and piezoelectric material.13,14 In this crystal, the polar properties are related to the structural macroscopic dipole moments formed by cesium cations and COO−groups from the malate anions, but there was no contribution from hydrogen bonded chains as they are oriented almost perpendicular to the polar twofold axis. In contrast, the semiorganic 1,4- diazabicyclo关2.2.2.兴octane perrhenate has been identified as a ferroelectric compound where the spontaneous polarization is derived from straight parallel hydrogen bonds and identical orientation of the polar cations.15 The crystal was revealed as a hydrogen bonded ferroelectric where no compensation of the hydrogen bond polarization exists, so polarized hydrogen bonded chains contributed directly to the high spontaneous polarization displayed by the material. Here we report the crystal structure and outstanding pyroelectric properties of potassium hydrogen malate monohydrate KHC4H4O6·H2O共KLM兲, which combines the directionality of hydrogen bonds with structural dipole moments to give rise to a very high spontaneous polarization. KLM crystals were grown from an equimolar aqueous solution of potassium hydroxide and L-malic acid. Controlled evaporation at 290 K yielded colorless plates perpendicular to the ccrystallographic axis. Single crystal x-ray data were collected on a CAD4 diffractometer using Mo K ␣ radiation 共␭ =0.710 73 Å兲at 293 K. The pyroelectric current along the sixfold crystallographic axis was measured with a programmable electrometer Keithley 617. A c-cut plate 共with thickness of 1 and surface area of 5 mm2兲with silver paste electrodes was placed in an appropriate cell, and its temperature swept at rates of 2 K/min over the temperature range of 300–380 K. Electric currents of opposite sign of the order of several picoamperes were detected over the heating and cooling runs. The spontaneous polarization was obtained by integrating the pyroelectric current over the temperature range measured. Optical second harmonic generation was measured on polycrystalline KLM samples using the well known Kurtz and Perry16 test. Samples with an overall 3 mm diameter, 0.5 mm thickness, and a nominal grain size ranging from 50 to 160 ␮ m were compared with polycrystalline potassium dihydrogen phosphate 共KDP兲with the same grain size and similar sample preparation. The second harmonic signal generated by KLM was observed to increase with grain size up to a nominal grain size of approximately 90 ␮ m, above a兲Electronic mail: [email protected]. FIG. 1. 共Color online兲Hydrogen bonded chains along the chiral cpolar axis 共K atoms and water molecules not shown for clarity兲. APPLIED PHYSICS LETTERS 93, 222903 共2008兲 0003-6951/2008/93共22兲/222903/2/$23.00 © 2008 American Institute of Physics93, 222903-1 10 June 2025 13:33:57 which it remains roughly constant, indicative that the crystal is phase matchable. Overall, the optical second harmonic signal was measured to be approximately 0.7 of KDP. At room temperature KLM is hexagonal, space group of P61, with a=b=7.5980共10兲Å, c=21.301共4兲Å, V =1064.9共3兲Å3, and Z=6. A head-to-tail arrangement of translational equivalent units, in which the carboxylic group 共tail兲COOH of each malate anion is strongly hydrogen bonded to one oxygen atom of the carboxylate group COO− group 共head兲of the next anion, creates infinite oriented chains along 关001兴共Fig. 1兲. Such hydrogen bonded chains, polarized parallel to the polar crystallograpic axis, are interconnected via hydrogen bonds to the oxygen atoms of the water molecules, giving rise to an infinite three dimensional hydrogen-bond packing network. Reinforcing these oriented hydrogen bonded chains, there is a dipole moment of 82 D 共2.73⫻10−28 Cm兲per unit cell, formed by K+cations and the COO−groups from the malate anions, with a net component along the hexagonal polar axis, a situation similar to that found in CsLM 共Fig. 2兲. The calculated spontaneous polarization, as a function of temperature, is shown in Fig. 3. The maximum value of Ps, 16 ␮ C/cm2at room temperature, is more than five times higher than in triglycine sulfate crystals 共2.8 ␮ C/cm2兲, nearly an order of magnitude higher than in sodium potassium tartrate17 共0.25 ␮ C/cm2兲and tris-sarcosine calcium chloride18 共0.27 ␮ C/cm2兲. In summary, we found that potassium hydrogen malate has a high spontaneous polarization, which results from the contribution of both parallel hydrogen bonds and structural dipole moments along the polar hexagonal axis. The excellent polar properties of KLM make this compound a strong candidate for use in pyroelectric applications. This is a crystalline compound where a structural arrangement of polarized hydrogen bonds chains and structural dipoles along the crystalline polar axis combine to produce the highest spontaneous polarization so far reported for a semiorganic crystal. This work was supported by Spanish DGICYT under Project No. BFM2002-03327, FEDER funds, and Portuguese Fundação para a Ciência e Tecnologia 共FCT兲. The authors thank G. Arunmozhi for the pyroelectric measurements. 1S. Lang, Phys. Today 58 共8兲,31共2005兲. 2O. Auciello, J. F. Scott, and R. Ramesh, Phys. Today 51共7兲,22共1998兲. 3R. Kumai, S. Horiuchi, H. Sagayama, T. Arima, M. Watanabe, Y. Noda, and Y. Tokura, J. Am. Chem. Soc. 129, 12920 共2007兲. 4F. Ishii, N. Nagaosa, and K. Terakura, Phys. Rev. B 73, 212105 共2006兲. 5J. A. Geuther and Y. Danon, J. Appl. Phys. 97, 104916 共2005兲. 6M. Muthuraman, R. Masse, J. F. Nicoud, and G. R. Desiraju, Chem. Mater. 13, 1473 共2001兲. 7R. Kumai, S. Horiuchi, H. Sagayama, T. Arima, M. Watanabe, Y. Noda, and Y. J. Tokura, J. Am. Chem. Soc. 129, 12920 共2007兲. 8M. C. Etter and K. S. Huang, Chem. Mater. 4, 824 共1992兲. 9Supramolecular Assembly via Hydrogen Bonds II: Structure and Bonds edited by D. M. P. Mingos 共St. Edmund Hall, Oxford/Springer, Berlin, 2004兲. 10C. B. Aakeroy and P. B. Hitchcock, J. Mater. Chem. 3, 1129 共1993兲. 11M. Tokunaga, J. Phys. Soc. Jpn. 57, 4275 共1988兲. 12J. F. Scott, J. Phys. Soc. Jpn. 58, 4487 共1989兲. 13E. de Matos Gomes, C. F. Gonçales, M. S. Belsley, F. Ferreira, M. M. R. Costa, V. H. Rodrigues, and A. Criado, Proc. SPIE 5912, 5912OD1 共2005兲. 14D. V. Isakov, F. P. Ferreira, J. Barbosa, J. L. Ribeiro, E. de Matos Gomes, and M. S. Belsley, Appl. Phys. Lett. 90, 073505 共2007兲. 15M. Szafrański, A. Katrusiak, and G. J. McIntyre, Phys. Rev. Lett. 89, 215507 共2002兲. 16S. K. Kurtz and T. T. J. Perry, Appl. Phys. 共Berlin兲19, 3768 共1968兲. 17F. Jona and G. Shirane, Ferroelectric Crystals 共Dover, New York, 1993兲. 18G. V. Kozlov, A. A. Volkov, J. F. Scott, G. E. FeldKamp, and J. Petzet, Phys. Rev. B 28, 255 共1983兲. FIG. 2. 共Color online兲Structural dipole moments 共dashed arrows兲formed by K+cations 共balls兲and COO−groups. 310 320 330 340 350 360 370 0.00 0.05 0.10 0.15 0 . 20 Ps C/m2 T ( K ) FIG. 3. Spontaneous polarization as a function of temperature calculated from measurements of the pyroelectric current. 222903-2 de Matos Gomes et al. Appl. Phys. Lett. 93, 222903 共2008兲 10 June 2025 13:33:57