Letter pubs.acs.org/JPCL
Negative Linear Compressibility in Organic Mineral Ammonium Oxalate Monohydrate with Hydrogen Bonding Wine-Rack Motifs Yuancun Qiao,† Kai Wang,† Hongsheng Yuan,† Ke Yang,‡ and Bo Zou*,† †
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201203, China
‡
S Supporting Information *
ABSTRACT: Negative linear compressibility (NLC) is a relatively uncommon phenomenon and rarely studied in organic systems. Here we provide the direct evidence of the persistent NLC in organic mineral ammonium oxalate monohydrate under high pressure using synchrotron X-ray powder diffraction, Raman spectroscopy and density functional theory (DFT) calculation. Synchrotron X-ray powder diffraction measurement reveals that ammonium oxalate monohydrate shows both positive and negative linear compressibility along b-axis before 11.5 GPa. The red shift of the external Raman modes and abnormal changes of several selected internal modes in high-pressure Raman spectra further confirmed the NLC. DFT calculations demonstrate that the NH···O hydrogen bonding “wine-rack” motifs result in the NLC along b-axis in ammonium oxalate monohydrate. We anticipate the highpressure study of ammonium oxalate monohydrate may represent a promising strategy for accelerating the pace of exploitation and improvement of NLC materials especially in organic systems.
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mechanisms that produce the unexpected NLC mechanical property. Ferroelastic phase transition, polyhedral tilts, “winerack”, and “honeycomb” framework motifs have been identified to rationalize this highly unusual NLC mechanical behavior in multifarious materials.1,9 Furthermore, systematic computational procedures gradually applied to predict or design new NLC materials.24−29 Although these models and mechanisms provide the accesses to seeking new NLC materials, the studies of NLC are confined in simple inorganic compounds,15−22 metallic cyanide frameworks,2,6,10,23 and MOFs.4,8,30−32 Up till now, merely one pure organic material showing NLC effects reported by Fortes et al. in methanol monohydrate. However, methanol monohydrate is only stable at the temperature below 160 K.3 In the past few years, primary efforts were concentrated on improving the magnitude of NLC materials,2 but few efforts were on extending the range.23 Comparing with inorganic materials, organic materials are soft compounds for the characteristic weak intermolecular interactions in their structure. In addition, the weak intermolecular interactions in organic materials have a moderate response to external stimuli. We anticipate that the NLC in organic materials may represent a persistent pressure range. Organic NLC materials as bionics function materials may have a practical application in biomechanical engineering. Given the lack of research knowledge about the NLC effect in organic systems and the potential
he counterintuitive phenomenon of uni- or biaxial expansion along with a positive volume compression on increasing hydrostatic pressure is known as negative linear compressibility (NLC)1−9 or negative area compressibility (NAC).10,11 NLC is a rare but highly attractive mechanical property and has a number of important applications in fields such as ultrasensitive pressure-sensing devices, artificial muscles and next-generation body armor.1,9,12−14What’s more, lots of biological species in nature are thought to make use of NLC, like the contraction of elephant trunk, movement squid, and octopus.9,12 Materials with NLC have been intensively expanded in the past few decades. The earlier studies about NLC concentrated on some simple inorganic compounds, like Se,15−17 TeO2,18 NaVO4,19,20 LaNbO4,21 BAsO4,22 and so on. The values of NLC in these inorganic compounds appear relatively small. With a further investigation of the geometric mechanism, materials with NLC effects have been gradually found in metallic cyanide framework in recent years. For example, the largest value (−76 TPa−1) of NLC was reported by Goodwin et al. in metallic cyanide framework Ag3[Co(CN)6].2 Soon after, a structural analogue of Ag3[Co(CN)6], KMn[Ag(CN)2]3 was detected to exhibit strong persistent NLC.23 More recently, Monica Kosa et al. identified a NLC behavior with a mechanical anisotropy along its c axis in metal organic frameworks (MOFs) [NH4][Zn(HCOO)3].4 In molecular complexes, Shepherd et al. reported and illustrated the NLC in [Fe(dpp)2(NCS)2]·py which also shows the large negative thermal expansion (NTE) properties.5 Meanwhile, extensive efforts have been made toward the rationalization of specific structures and geometric © XXXX American Chemical Society
Received: May 29, 2015 Accepted: June 28, 2015
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DOI: 10.1021/acs.jpclett.5b01129 J. Phys. Chem. Lett. 2015, 6, 2755−2760
Letter
The Journal of Physical Chemistry Letters
bonded network. Because ammonium cations, oxalate anions, and water molecules are all involved in NH···O hydrogen bonds, the three parts change cooperatively in response to the external stimuli. High-pressure X-ray powder diffraction data was collected on crystals of ammonium oxalate monohydrate from ambient to 11.5 GPa at room temperature (Supporting Information Figure S1). Analysis of the unit cell parameters reveals that the compression is highly anisotropic with the c-axis direction being the most compressible (Supporting Information Figure S2). As seen in Figure 2a, the b axis shows an intriguing behavior that it undergoes two inverse regions in the pressure range from 0 to 11.5 GPa. In the region I, it decreases slightly with increasing pressure up to 4.0 GPa, and then it shows a counterintuitive persistent NLC with the value of KII = −2.3(7) TPa−1 over pressure range from 5.1 to 11.5 GPa in region II. The compressibility values KI and KII in this paper were obtained by PASCal based on the relationship (Kl = (∂(Inl)/∂p)T).45 To understand the NLC properties in ammonium oxalate monohydrate, we make a comparison with some representative NLC compounds. As can be seen in Supporting Information Table S1, most of compounds have a relatively small pressure range (
