Phase Transition In Single Crystals Research Articles

Selective Gas Adsorption in the Flexible Metal–Organic Frameworks Cu(BDTri)L (L=DMF, DEF)

Use of the ditopic ligand 1,4-benzenedi(1H-1,2,3-triazole) (H(2)BDTri) enabled isolation of two new three-dimensional metal-organic frameworks of formulae Cu(BDTri)L in which L=DMF (1) and diethylformamide (DEF; 2). These compounds have the same primary structure, featuring one-dimensional channels with the bridging DMF or DEF molecules pointing into the cavity. Upon exposure to solvent vapors, both display a reversible flexibility, as characterized by single-crystal to single-crystal phase transitions in 1. The O(2) adsorption isotherms for the compounds show a two-step adsorption behavior associated with a permanent microporosity and a pore-opening process. In the case of N(2) adsorption, only 1 exhibits a two-step adsorption isotherm, whereas 2 does not present any pore opening, demonstrating that design of a flexible framework cavity can control the pore opening and thereby possibly enhance O(2)/N(2) separation.

Comparison of the effects of pressure on three layered hydrates: a partially successful attempt to predict a high-pressure phase transition

We report the effect of pressure on the crystal structures of betaine monohydrate (BTM), L-cysteic acid monohydrate (CAM) and S-4-sulfo-L-phenylalanine monohydrate (SPM). All three structures are composed of layers of zwitterionic molecules separated by layers of water molecules. In BTM the water molecules make donor interactions with the same layer of betaine molecules, and the structure remains in a compressed form of its ambient-pressure phase up to 7.8 GPa. CAM contains bi-layers of L-cysteic acid molecules separated by water molecules which form donor interactions to the bi-layers above and below. This phase is stable up to 6.8 GPa. SPM also contains layers of zwitterionic molecules with the waters acting as hydrogen-bond donors to the layers above and below. SPM undergoes a single-crystal to single-crystal phase transition above 1 GPa in which half the water molecules reorient so as to form one donor interaction with another water molecule within the same layer. In addition, half of the S-4-sulfo-L-phenylalanine molecules change their conformation. The high-pressure phase is stable up to 6.9 GPa, although modest rearrangements in hydrogen bonding and molecular conformation occur at 6.4 GPa. The three hydrates had been selected on the basis of their topological similarity (CAM and SPM) or dissimilarity (BTM) with serine hydrate, which undergoes a phase transition at 5 GPa in which the water molecules change orientation. The phase transition in SPM shows some common features with that in serine hydrate. The principal directions of compression in all three structures were found to correlate with directions of hydrogen bonds and distributions of interstitial voids.

Optical and dielectric studies of phase transitions in single crystals of NaNbO3-Gd1/3NbO3 solid solutions

The temperature dependences of the permittivity ɛ and the false-color image patterns obtained by the rotating polarizer method for single crystals of (1 − x)NaNbO3−xGd1/3NbO3 (x = 0.003, 0.090) solid solutions with different degrees of diffuseness of the phase transition are investigated. A multifractal analysis of the false-color images has revealed anomalies in the temperature dependences of the parameter ƒ∞ of the multifractal spectrum. For a sample with a sharp phase transition (x ≈ 0.003), the temperature of this anomaly is in good agreement with the temperature of the jumps in the permittivity ɛ(T) and birefringence. For an NNG crystal with x ≈ 0.09, which exhibits a diffuse maximum of ɛ(T), the temperatures of the anomalies of ƒ∞(T) differ in the central and peripheral regions, which correlates with the distribution of Gd over the crystal.

Efflorescence of mirabilite, epsomite and gypsum traced by automated monitoring on-site

Abstract Short crystallisation and deliquescence cycles of mirabilite, epsomite and gypsum are identified by using an automated monitoring system on-site. It consists of digital cameras and RH/T sensors which are computer-controlled and connected to the Internet. Digital images are processed to form a time-lapse movie, so that surface alterations are easily recognised. High resolution imaging (6 mega pixels) in combination with various lenses (12–24 mm zoom, 28–105 mm zoom, 20 mm inversed) results in a wide range of precise observation from 7 to 500 mm width of image. That is essential for discriminating phase transitions of single crystals at the same time as monitoring the evolution of the efflorescence as a whole. Crystallisation-deliquescence cycles are induced by small temperature and humidity variations. Mirabilite and epsomite undergo phase changes from crystallisation to deliquescence and vice versa at about 83% RH (15–20 °C). These equilibrium relative humidities are considerably reduced compared to the pure salt systems as a consequence of mixed salt solutions. Observations confirm predictions by ECOS simulation based on chemical analyses of water extracts from the site. Dehydration of mirabilite is observed when the relative humidity drops below 65% RH. Mirabilite exhibits the most pronounced and rapid phase changes in response to indoor climate variations. As an example, whisker crystals of 1–3 mm length grow within one up to some days. Epsomite responds fainter and slower. Crystallisation of gypsum is distinguished by the growth of new crystals on the stone surface. They have a size of about 10 μ – which is close to the image resolution – when they appear on the image. Crystallisation is associated with an increased granular disaggregation of the stone surface. Rhythmical crystallisation of gypsum appears to correlate with small relative humidity variations of ±2–5% within a band from 65 to 75% RH in the room. However, this needs to be clarified in detail.

Temperature- and Pressure-Induced Proton Transfer in the 1:1 Adduct Formed between Squaric Acid and 4,4′-Bipyridine

We have applied a combination of spectroscopic and diffraction methods to study the adduct formed between squaric acid and bypridine, which has been postulated to exhibit proton transfer associated with a single-crystal to single-crystal phase transition at ca. 450 K. A combination of X-ray single-crystal and very-high flux powder neutron diffraction data confirmed that a proton does transfer from the acid to the base in the high-temperature form. Powder X-ray diffraction measurements demonstrated that the transition was reversible but that a significant kinetic energy barrier must be overcome to revert to the original structure. Computational modeling is consistent with these results. Modeling also revealed that, while the proton transfer event would be strongly discouraged in the gas phase, it occurs in the solid state due to the increase in charge state of the molecular ions and their arrangement inside the lattice. The color change is attributed to a narrowing of the squaric acid to bipyridine charge-transfer energy gap. Finally, evidence for the possible existence of two further phases at high pressure is also presented.

Mean field statistical mechanics of model Hamiltonian for hydrogen bonded phase transitions

AbstractRecently, a new prototype model has been proposed to explain structural phase transitions in single crystals of organic hydrogen bonded phenol–amine adducts. A variational calculation of the thermodynamic properties was able to explain the main features of the phase transition. We report an alternative statistical mechanical calculation which confirms the results of the earlier calculation even though self‐energy terms appear in the present calculation. The interpretation of the results within the pseudo spin representation is clarified further. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of pressure and substituents on the size of pseudo-macrocyclic cavities in salicylaldoxime ligands

The effect of pressure on the crystal structures of 3-chloro-, 3-methoxy-, 3-methyl- and 3-tert-butylsalicylaldoximes has been investigated. The compounds all form the dimeric structure found in salicylaldoxime form I at ambient pressure, which is based on intermolecular hydrogen bonds between the oximic hydrogen and the phenolic oxygen atoms across an inversion centre. These intermolecular interactions, along with intramolecular phenolic hydrogen to oximic nitrogen atom hydrogen bonds form a pseudo-macrocycle with a R44(10) ring motif. These hydrogen bonding motifs pre-organize an arrangement of four potential donor atoms for a metal cation which, when the phenol groups are deprotonated, provides an N2O22− pocket well suited to the binding of planar transition metal ions. The radius of the cavity defined by the donor atoms in the dimers is dependent on the nature of the 3-substituent, varying from 1.949 A in the 3-methoxy- to 2.037 A in the 3-tBu-derivative. Anisotropic compression of the crystals on the application of pressure results in significant changes in the radii of the cavities in the dimers which decrease by ca. 10% at 6 GPa. During compression of the 3-tBu-derivative a single crystal to single crystal phase transition was observed between 0.2 and 1.0 GPa to a new polymorph, 3-tert-butylsalicylaldoxime-II. The phase transition produces an increase in symmetry as the space group changes from P-1 to I2/a, but the intermolecular interactions remain essentially unchanged. No phase transitions were observed in the compression of 3-Cl-, 3-Me- or 3-MeO-salicylaldoxime up to 6.2 GPa.

Single-crystal to single-crystal phase transition of cucurbit[5]uril hydrochloride hydrates: large water-filled channels transforming to layers of unusual stability

Cucurbit[5]uril hydrochloride hydrate crystals with large water-filled channels transform to a highly stable layer structure via a single-crystal to single-crystal mechanism; (129)Xe NMR showed that porosity in CB[5] samples depends critically on the method of preparation.

Magnetoelectric interactions in rare-earth ferroborates

Magnetic-field-induced phase transitions in single crystals of the rare-earth ferroborates GdFe3(BO3)4 and NdFe3(BO3)4 have been studied by measuring magnetoelectric dependences and torque curves. These phase transitions can serve as one of the possible mechanisms for magnetic control of electric polarization. Magnetic phase transitions in GdFe3(BO3)4 are analyzed theoretically.

Heteropolynuclear Metamagnet Showing Spin Canting and Single-Crystal to Single-Crystal Phase Transformation

The two-dimensional heteropolynuclear metal-organic framework [Co3Zn2(m-BDC)4(μ3-OH)2(μ2-OH2)2(H2O)4]·4.5H2O (1) (m-H2BDC = 1,3-benzenedicarboxylic acid) has been hydrothermally synthesized and characterized by single-crystal X-ray diffraction, infrared spectrum, X-ray powder diffraction, and elemental analysis. Our experimental results signify that the formation of compound 1 is highly medium-selective and temperature-sensitive in the hydrothermal system. Interestingly, 1 undergoes a single-crystal to single-crystal structural phase transition from orthorhombic Pmna to monoclinic P21/c (1‘) at very low temperature of about 156.5 K. In this process, we found two topologically different water tapes. More importantly, the magnetic property of the target compound was also intensively investigated in the temperature range of 2−300 K and shows the coexistence of spin canting and field-induced metamagnetism.

Single-crystal to single-crystal phase transition with a large deformation in Zn(OH) 2 under high-pressure

High-pressure behavior of Zn(OH) 2 with a techto-silicates related structure was investigated under hydrostatic condition by X-ray diffraction method for both powdered and single-crystal specimens. A quick single-crystal to single-crystal transition was discovered at 1.1 GPa. The transition occurred with 10% volume change and a large deformation in the sharp. The high-pressure phase has an α-cristobalite related structure, in which ZnO 4 tetrahedrons build up a three-dimensional framework. To explain all these facts a simple displacive mechanism by tilting ZnO 4 tetrahedrons is proposed for the transition.

Single-crystal to single-crystal phase transitions of bis(N-phenylisonicotinamide)silver(i) nitrate reveal cooperativity properties in porous molecular materials

Two single-crystal to single-crystal phase transitions in trans-[Ag(NPI)(2)](NO(3)).2CH(3)OH (), (NPI = N-phenylisonicotinamide) were characterized with X-ray crystallography; the first transition is reversible and arises from a desolvation transition induced by vacuum and generated ; the second transition was induced by heat at 140 degrees C and generated .

Effect of pressure on the crystal structure of salicylaldoxime-I, and the structure of salicylaldoxime-II at 5.93 GPa

The effect of pressure on the crystal structure of salicylaldoxime has been investigated. The ambient-pressure phase (salicylaldoxime-I) consists of pairs of molecules interacting through oximic OH...O hydrogen bonds; taken with phenolic OH...N intramolecular hydrogen bonds, these dimers form a pseudo-macrocycle bounded by an R4 4(10) motif. The dimers interact principally via pi...pi stacking contacts. Salicylaldoxime derivatives are used industrially as selective solvent extractants for copper; the selectivity reflects the compatibility of the metal ion with the pseudo-macrocycle cavity size. On increasing the pressure to 5.28 GPa the size of the cavity was found to decrease by an amount comparable to the difference in hole sizes in the structures of the Cu2+ salicylaldoximato complex and its Ni2+ equivalent. On increasing the pressure to 5.93 GPa a new polymorph, salicylaldoxime-II, was obtained in a single-crystal to single-crystal phase transition. PIXEL calculations show that the phase transition is driven in part by relief of intermolecular repulsions in the dimer-forming OH...O-bonded ring motif, and the ten-centre hydrogen-bonding ring motif of the phase I structure is replaced in phase II by a six-centre ring formed by oximic OH...N hydrogen bonds. The transition also relieves repulsions in the pi...pi stacking contacts. The intramolecular OH...N hydrogen bond of phase I is replaced in phase II by a intermolecular phenolic OH...O hydrogen bond, but the total interaction energy of the pairs of molecules connected by this new contact is very slightly repulsive because the electrostatic hydrogen-bond energy is cancelled by the repulsion term. The intra- to intermolecular hydrogen-bond conversion simply promotes efficient packing rather than contributing to the overall lattice energy.

Multilayer optic based components for synchrotron sources

The response of crystal structures of L-serine and DL-serine to an increase in hydrostatic pressure was shown to be radically different: reversible phase transitions were observed in L-serine at 5.3 GPa (reverse transition at 4.0 GPa) and 7.8 GPa, whereas the crystal of DL-serine was stable at high pressures up to 8 GPa. Structural distortion and phase transitions were followed by Raman spectroscopy, X-ray single-crystal X-ray diffraction (a laboratory radiation source), high-resolution X-ray diffraction (synchrotron radiation). The structures of the two high-pressure phases were solved and refined from single-crystal and highresolution powder diffraction data; the results were in a good agreement. The anisotropy of strain within the range of stability of the same phases was compared for L - and DL -serine. Pressure-induced changes were radically different from those on cooling. Phase transitions in single crystals of L-serine were shown to proceed in a single-crystal-to-single-crystal mode, via a rapid propagation of an interface after a pronounced induction period. In powder samples, the two phases - a low-pressure and a high pressure phase - co-exist from about 5 up to 10 GPa, and this shows, that the phase transitions in the powder samples are to a large extent kinetically controlled. Acknowledgments: Financial support has been obtained from Alexander von Humboldt Foundation, RFBR (05-03-32468), BRHE Program, and a grant from SB RAS.

Infrared reflectivity study of the phase transitions in sodium ammonium sulfatedihydrate

This work reports a detailed infrared reflectivity investigation of the phase transitions insingle crystals of sodium ammonium sulfate dihydrate (SASD). The polarizedreflectivity spectra allow us to follow the temperature dependence of the polarvibrational modes and detect the critical behaviour of the vibrational parametersthrough the two low temperature structural phase transitions observed in thecompound. The results obtained show that the mechanism of the transitions inSASD is complex, involving a strong coupling between pseudo-spins and phonons.In the paraelectric phase, the driving mechanism of the first phase transition(Tc1 = 95 K) appears to be related to a relaxation with a characteristic frequency that is much lower thanthe phonon frequencies. In the temperature range corresponding to the first ferroelectric phase(Tc1>T>Tc2 = 79 K),the dynamics of the lattice change considerably and the parameters characterizing several vibrationalmodes display anomalous temperature dependences. The second phase transition occurring atTc2 is marked by an important and discontinuous change of the spectral shape, indicating thata considerable lattice distortion is involved.

Comparative study of the effect of low-dose gamma irradiation on phase transitions in single crystals of triglycine sulfate and its deuterated analog

The effect of low γ-radiation doses (< 0.5 MR) on the specific heat Cp of crystals of triglycine sulfate and its deuterated analog was studied using precision vacuum adiabatic calorimetry in the temperature range 80–350 K. Appreciable changes in the behavior of Cp of nominally pure and γ-irradiated crystals were detected near the phase transition temperature (Tc), which decreased as the dose was lowered.

High-pressure polymorphism in L-cysteine: the crystal structures of L-cysteine-III and L-cysteine-IV

The crystal structure of the orthorhombic phase of L-cysteine (hereafter L-cysteine-I) consists of chains of molecules linked via NH...O hydrogen bonds. The chains are linked into a layer by other NH...O hydrogen bonds, forming R4(4)(16) ring motifs. The layers are linked by further NH...O and disordered SH...S/SH...O interactions. The main effects of compression to 1.8 GPa are to contract voids in the middle of the R4(4)(16) rings and to reduce S...S distances from 3.8457 (10) to 3.450 (4) angstroms. The latter is at the lower limit for S...S distances and we suggest that strain about the S atom is responsible for the formation of a new phase of L-cysteine, L-cysteine-III, above 1.8 GPa. The phase transition is accompanied by a change in the NCCS torsion angle from ca 60 to ca -60 degrees and small positional displacements, but with no major changes in the orientations of the molecules. The structure of L-cysteine-III contains similar R-type ring motifs to L-cysteine-I, but there are no S...S contacts within 3.6 angstroms. L-Cysteine-III was found to be stable to at least 4.2 GPa. On decompression to 1.7 GPa, another single-crystal to single-crystal phase transition formed another previously uncharacterized phase, L-cysteine-IV. This phase is not observed on increasing pressure. The structure consists of two crystallographically independent cysteine molecules in the same conformations as those found in L-cysteine-I and L-cysteine-III. The structure separates into zones with are alternately phase I-like and phase III-like. L-Cysteine-IV can therefore be thought of as an unusual example of an intermediate phase. Further decompression to ambient pressure generates L-cysteine-I.

Crystal engineering with ethynylbenzenes : Part 2. Structures of 4-trimethylsilylethynyl-N,N-dimethylaniline, and 4-ethynyl-N,N-dimethylaniline with Z′ = 12 and a single-crystal to single-crystal phase transition at 122.5 ± 2 K

The crystal and molecular structures of 4-ethynyl-N,N-dimethylaniline (1), and 4-(trimethylsilylethynyl)-N,N-dimethylaniline (2), have been obtained from X-ray diffraction data. Crystals of 1 exhibit a phase transition at 122.5 ± 2 K. Both polymorphs are triclinic with Z′ = 12, and the molecules are linked into dodecamers by weak hydrogen bonds involving C–H groups. The orthorhombic crystals of 2 show no phase changes and have Z′ = 1.5 (3 half molecules) without short intermolecular contacts.

Isostructural phase transition in m-carboxyphenylammonium monohydrogenphosphite

Crystals of m-carboxyphenylammonium monohydrogenphosphite, C7H8NO2+.H2PO3- (m-CPAMP), space group P2(1)/c, grown from aqueous solution undergo a reversible first-order single-crystal phase transition at Tc = 246 (2) K with a hysteresis of 3.6 K. The thermal behaviour of the sample was characterized by differential scanning calorimetry (DSC) experiments. Variations of the unit-cell parameters versus temperature between 100 and 320 K are reported. The transition from the higher-temperature phase (HTP) to the lower-temperature phase (LTP) is characterized by a unit-cell volume contraction of 1.77%. The average structure and unit-cell packing of m-CPAMP at lower temperature (100 K) are reported from accurate X-ray data sets and compared with those of the higher-temperature phase (293 K) in order to investigate the mechanism of the phase transition. The reciprocal lattice reconstruction showed a few very weak satellite reflections which will be discussed in a forthcoming paper.

The Mössbauer study of magnetic phase transition in single crystal (Nd 1− xSm x) 1/3Sr 2/3FeO 3

(Nd 1− x Sm x ) 1/3Sr 2/3FeO 3 were synthesized and their charge ordering (CO) transition related to lattice distortion was systematically investigated. The canted antiferromagnetic spin ordering exists below Néel temperature ( T N). This phase transition is accompanied by charge disproportionation into nominally Fe 3+ and Fe 5+. The CO, a sequence of Fe +3 Fe +3 Fe +5 Fe +3 Fe +3Fe +5, which exists align the [1 1 1] direction of pseudo cubic perovskite. The three kinds of iron, Fe 3+, Fe 5+ and Fe 4+, are found below T N. The amount of Fe 4+ increases from 13% to 66% as temperature increases. This can be interpreted to mean that the charge ordering and disordering phase coexists. The charge ordering state is realized by strong hybridization between Fe and O atoms. The Néel temperature decreases with the increase of the Sm concentration.