Characterization of the high-pressure structures and phase transformations in SnO2. A density functional theory study

Theoretical investigations concerning the high-pressure polymorphs, the equations of state, and the phase transitions of SnO2 have been performed using density functional theory at the B3LYP level. Total energy calculations and geometry optimizations have been carried out for all phases involved, and the following sequence of structural transitions from the rutile-type (P42/mnm) driven by pressure has been obtained (the transition pressure is in parentheses): --> CaCl2-type, Pnnm (12 GPa) --> alpha-PbO2-type, Pbcn (17 GPa) --> pyrite-type, Pa (17 GPa) --> ZrO2-type orthorhombic phase I, Pbca (18 GPa) --> fluorite-type, Fmm (24 GPa) --> cotunnite-type orthorhombic phase II, Pnam (33 GPa). The highest bulk modulus values, calculated by fitting pressure-volume data to the second-order Birch-Murnaghan equation of state, correspond to the cubic pyrite and the fluorite-type phases with values of 293 and 322 GPa, respectively.     

High-pressure-induced phase transitions in pentaerythritol: X-ray and Raman studies

The high-pressure response of pentaerythritol crystals has been examined to 10 GPa in diamond-anvil cells using angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy. The results reveal two first-order phase transitions: one at 4.8 GPa from phase I, tetragonal I(), to phase II, orthorhombic Pnn2C2v10, with a small approximately 0.5% volume change, and the other at 7.2 GPa to phase III with an unknown crystal structure. We found that phase I exhibits a large crystallographic anisotropy which rapidly decreases with increasing pressure: the ratio of linear compressibilities between two primary crystal axes decreases from betao= 8.1 at 1 atm to betaP = 2.6 at 4 GPa. We suggest that this apparent decrease in crystal anisotropy is due to the disruption of hydrogen bonding in the (001) plane of phase I and eventually leads to an orthorhombic distortion from a quadrilateral network structure in phase I to a quasi one-dimensional structure in phase II. The crystal structure of phase III exhibits a disordered character, and it is likely a conformational variant of phase II.     

Raman scattering study on pressure-induced phase transformation of marokite (CaMn2O4)

An in-situ Raman Spectroscopic study was conducted to explore the pressure-induced phase transformation of CaMn₂O₄ to pressures of 73.7 GPa. Group theory yields 24 Raman active modes for CaMn₂O_4, of which 20 are observed at ambient conditions. With the slight compression below 5 GPa, the pressure-induced contraction compensates the structural distortion induced by a Jahn–Teller (JT) effect, resulting in the occurrence of the zero pressure shifts of the JT-related Raman modes. Upon elevation of pressure to nearby 35 GPa, these Raman modes start to display a significant variation in pressure shift, implying the appearance of a pressure-induced phase transformation. Group factor analyses on all possible structure polymorphs indicate that the high-pressure phase is preferentially assigned to an orthorhombic structure, having the CaTi₂O₄ structure. The cooperative JT distortion is continuously reduced in the CaMn₂O₄ polymorph up to 35 GPa. Beyond 35 GPa, it is found that the JT effect was completely suppressed by pressure in the newly formed high-pressure phase. Upon release of pressure, this high-pressure phase transforms to the original CaMn₂O₄ phase, and continuously remains stable to ambient conditions.     

Evaluation of the physical stability and local crystallization of amorphous terfenadine using XRD-DSC and micro-TA

It is very difficult to follow rapid changes in polymorphic transformation and crystallization and to estimate the species recrystallized from the amorphous form. The aim of this study was to clarify the structural changes of amorphous terfenadine and to evaluate the polymorphs crystallized from amorphous samples using XRD-DSC and an atomic force microscope with a thermal probe (micro-TA). Amorphous samples were prepared by grinding or rapid cooling of the melt. The rapid structural transitions of samples were followed by the XRD-DSC system. On the DSC trace of the quenched terfenadine, two exotherms were observed, while only one exothermic peak was observed in the DSC scan of a ground sample. From the in situ data obtained by the XRD-DSC system, the stable form of terfenadine was recrystallized during heating of the ground amorphous sample, whereas the metastable form was recrystallized from the quenched amorphous sample and the crystallized polymorph changed to the stable form. Obtained data suggested that recrystallized species could be related to the homogeneity of samples. When the stored sample surface was scanned by atomic force microscopy (AFM), heterogeneous crystallization was observed. By using micro-TA, melting temperatures at various points were measured, and polymorph forms I and II were crystallized in each region. The percentages of the crystallized form I stored at 120 and 135 °C were 47 and 79%, respectively. This result suggested that increasing the storage temperature increased the crystallization of form I, the stable form, confirming the temperature dependency of the crystallized form. The crystallization behavior of amorphous drug was affected by the annealing temperature. Micro-TA would be useful for detecting the inhomogeneities in polymorphs crystallized from amorphous drug.     

Two polymorphs of safinamide,a selective and reversible inhibitor of monoamine oxidase B

Two polymorphs of safinamide {systematic name: (2S)‐2‐[4‐(3‐fluorobenzyloxy)benzylamino]propionamide}, C₁₇H₁₉FN₂O₂, a potent selective and reversible monoamine oxidase B (MAO‐B) inhibitor, are described. Both forms are orthorhombic and regarded as conformational polymorphs due to the differences in the orientation of the 3‐fluorobenzyloxy and propanamide groups. Both structures pack with layers in the ac plane. In polymorph (I), the layers have discrete wide and narrow regions which are complementary when located next to adjacent layers. In polymorph (II), the layer has long flanges protruding from each side, which interdigitate when packed with the adjacent layers. N—H...O hydrogen bonds are present in both structures, whereas N—H...F hydrogen bonding is seen in polymorph (I), while N—H...N hydrogen bonding is seen in polymorph (II).     

Giant Volume Change and Topological Gaps in Temperature‐ and Pressure‐Induced Phase Transitions: Experimental and Computational Study of ThMo2O8

By applying high temperature (1270 K) and high pressure (3.5 GPa), significant changes occur in the structural volume and crystal topology of ThMo₂O₈, allowing the formation of an unexpected new ThMo₂O₈ polymorph (high‐temperature/high‐pressure (HT/HP) orthorhombic ThMo₂O₈). Compared with the other three ThMo₂O₈ polymorphs prepared at the ambient pressure (monoclinic, orthorhombic, and hexagonal phases), the molar volume for the quenched HT/HP–orthorhombic ThMo₂O₈ is decreased by almost 20 %. As a result of such a dramatic structural transformation, a permanent high‐pressure quenchable state is able to be sustained when the pressure is released. The crystal structures of the three ambient ThMo₂O₈ phases are based on three‐dimensional (3D) frameworks constructed from corner‐sharing ThO_x (x=6, 8, or 9) polyhedra and MoO₄ tetrahedra. The HT/HP–orthorhombic ThMo₂O₈, however, crystallizes in a novel structural topology, exhibiting very dense arrangements of ThO₁₁ and MoO₄₊₁ polyhedra connecting along the crystallographic c axis. The phase transitions among all four of these ThMo₂O₈ polymorphs are unveiled and fully characterized with regard to the structural transformation, thermal stability, and vibrational properties. The complementary first principles calculations of Gibbs free energies reveal the underlying energetics of the phase transition, which support the experimental findings.     

High-pressure synthesis of the polymorph of layer structured compounds MNX (M = Zr, Hf; X = Cl, Br, I)

Transition metal nitride halides MNX (M = Zr, Hf; X = Cl, Br, I) have two types of layer structured polymorphs, the alpha-form with the FeOCl type and the beta-form with the SmSI type. Both polymorphs consist of corrugated double M-N layers sandwiched between halogen layers, but with different atomic arrangements within the layers. The beta-form had been considered to be a high-temperature polymorph, because some beta-forms were obtained by thermal treatment of the corresponding alpha-forms. Here, the alpha-form was successfully transformed into the beta-form under high-pressure and high-temperature conditions; the new members of the beta-form were prepared for the first time from alpha-HfNBr, alpha-ZrNI, and alpha-HfNI using a high pressure of 3-5 GPa at 900 degrees C. The beta-form should be characterized as the high-pressure form rather than the corresponding high-temperature polymorph. This is the first high-pressure study on the polymorphs of metal nitride systems.     

Pressure‐Dependent Polymorphism and Band‐Gap Tuning of Methylammonium Lead Iodide Perovskite

We report the pressure‐induced crystallographic transitions and optical behavior of MAPbI₃ (MA=methylammonium) using in situ synchrotron X‐ray diffraction and laser‐excited photoluminescence spectroscopy, supported by density functional theory (DFT) calculations using the hybrid functional B3PW91 with spin‐orbit coupling. The tetragonal polymorph determined at ambient pressure transforms to a ReO₃‐type cubic phase at 0.3 GPa. Upon continuous compression to 2.7 GPa this cubic polymorph converts into a putative orthorhombic structure. Beyond 4.7 GPa it separates into crystalline and amorphous fractions. During decompression, this phase‐mixed material undergoes distinct restoration pathways depending on the peak pressure. In situ pressure photoluminescence investigation suggests a reduction in band gap with increasing pressure up to ≈0.3 GPa and then an increase in band gap up to a pressure of 2.7 GPa, in excellent agreement with our DFT calculation prediction.     

DSC and adiabatic calorimetry study of the polymorphs of paracetamol

Monoclinic (I) and orthorhombic (II) polymorphs of paracetamol were studied by DSC and adiabatic calorimetry in the temperature range 5 - 450 K. At all the stages of the study, the samples (single crystals and powders) were characterized using X-ray diffraction. A single crystal → polycrystal II→ I transformation was observed on heating polymorph II, after which polymorph I melted at 442 K. The previously reported fact that the two polymorphs melt at different temperatures could not be confirmed. The temperature of the II→I transformation varied from crystal to crystal. On cooling the crystals of paracetamol II from ambient temperature to 5 K, a II→ I transformation was also observed, if the 'cooling-heating' cycles were repeated several times. Inclusions of solvent (water) into the starting crystals were shown to be important for this transformation. The values of the low-temperature heat-capacity of the I and II polymorphs of paracetamol were compared, and the thermodynamic functions calculated for the two polymorphs. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dynamics of the intermolecular hydrogen bonds in the polymorphs of paracetamol in relation to crystal packing and conformational transitions: a variable-temperature polarized Raman spectroscopy study

The properties of the intermolecular hydrogen bonds in the monoclinic (Form I) and the orthorhombic (Form II) polymorphs of paracetamol, C(8)H(9)NO(2), have been studied by single crystal polarized Raman spectroscopy (40 to 3700 cm(-1)) in a wide temperature range (5 K < T < 300 K) in relation to the dynamics of methyl-groups of the two forms. A detailed analysis of the temperature dependence of the wavenumbers, bandwidths and integral intensities of the spectral bands has revealed an essential difference between the two polymorphs in the strength and ordering of OH···O and NH···O hydrogen bonds. The compression of intermolecular hydrogen bonds is interrelated with crystal packing and the dynamics of methyl-groups. On structural compression of the orthorhombic polymorph on cooling, a compromise is to be sought between the shortening of OH···O and NH···O bonds, attractive CH···O and repulsive CH···H contacts in the crystal structure. As a result of a steric conflict at temperatures below 100 K, N-H···O hydrogen bonds become significantly disordered, and an extended intramolecular transition from the conformation "staggered" with respect to the C=O bond to the one "staggered" with respect to the NH bond is observed. In most of the studied crystals this transition was only about 60% complete even at 5 K, but in some of the crystals the orientation of all the methyl-groups became staggered with respect to the NH bond at low temperatures. This complete transition was coupled to a sharp shortening of the OH···O and NH···O hydrogen bonds at <100 K, the appearance of new additional positions of the protons in these H-bonds, and a slight strengthening of the C-HO bonds formed by methyl-groups. The same conformational transition has been observed also in the monoclinic polymorph at T < 80 K. The crystal packing in Form I prevents the O-H···O hydrogen bonds from adopting the optimum geometry, and they are significantly disordered at all the temperatures, especially at ≤200 K. The packing of molecules in Form I is also not favourable to form C-H···O hydrogen bonds involving methyl-groups. One can conclude from the comparison of diffraction and spectroscopic data that the higher stability of Form I results not from a larger strength of individual OH···O and NH···O hydrogen bonds, but is a cumulative effect: all the hydrogen bonds together stabilize the structure of the monoclinic polymorph more than that of the orthorhombic polymorph.     

Zoledronic acid: monoclinic and triclinic polymorphs from powder diffraction data

The crystal structures of the monoclinic and triclinic polymorphs of zoledronic acid, C₅H₁₀N₂O₇P₂, have been established from laboratory powder X‐ray diffraction data. The molecules in both polymorphs are described as zwitterions, namely 1‐(2‐hydroxy‐2‐phosphonato‐2‐phosphonoethyl)‐1H‐imidazol‐3‐ium. Strong intermolecular hydrogen bonds (with donor–acceptor distances of 2.60 Å or less) link the molecules into layers, parallel to the (100) plane in the monoclinic polymorph and to the (10) plane in the triclinic polymorph. The phosphonic acid groups form the inner side of each layer, while the imidazolium groups lie to the outside of the layer, protruding in opposite directions. In both polymorphs, layers related by translation along [100] interact through weak hydrogen bonds (with donor–acceptor distances greater than 2.70 Å), forming three‐dimensional layered structures. In the monoclinic polymorph, there are hydrogen‐bonded centrosymmetric dimers linked by four strong O—H...O hydrogen bonds, which are not present in the triclinic polymorph.     

Synthesis, characterization and X-ray crystal structures of two polymorphs of a cobalt(III) complex with a N3O2 pentadentate Schiff base ligand, and benzylamine: spectral and electrochemical studies

The title complex [Co^III{(naph)₂dpt}(bzlan)]BPh₄, where (naph)₂dpt stands for bis-(2-hydroxy-1-naphthaldimine)-N-dipropylenetriamine dianion and bzlan for benzylamine, has been synthesized and characterized by elemental analyses, IR, UV–Vis and ¹H NMR spectroscopy. The structures of two possibly concomitant polymorphs, obtained from methanolic solution, [polymorph I (monoclinic, centrosymmetric, red-brown, thickish prismatic platelets) and polymorph II (orthorhombic, noncentrosymmetric, ochre olive-green, thin laths)] have been determined by X-ray crystallography. The geometry around the central cobalt ion is distorted octahedral in both polymorphs, but the benzylamine shows different kinds of orientation and disorder.     

Intermolecular interactions in polymorphs of trinuclear gold(I) complexes: insight into the solvoluminescence of Au(I)3(MeN=COMe)3

The trinuclear complex, Au(I)3(MeN=COMe)3, which displays a number of remarkable properties including solvoluminescence, has been found to crystallize as three polymorphs. The new triclinic and monoclinic polymorphs crystallized as colorless blocks, whereas the original hexagonal polymorph formed colorless needles. These polymorphs differ in the manner in which the nearly planar molecules pack and in the nature of the aurophilic interactions between them. Each of the three polymorphs of Au(I)3(MeN=COMe)3 shows a distinctive emission spectrum, but only the original hexagonal polymorph shows the low-energy emission that is responsible for its solvoluminescence. Colorless Au(I)3(n-PentN=COMe)3 crystallized from diethyl ether as needles of an orthorhombic polymorph and blocks of a triclinic polymorph. These polymorphs differ in the orientation of the n-Pent substituents, in the orientation of the trimers with respect to one another, and in the nature of the aurophilic interactions between the molecules. Only the triclinic polymorph of Au(I)3(n-PentN=COMe)3 shows luminescence at room temperature, but it is not solvoluminescent. Colorless Au(I)3(i-PrN=COMe)3 has also been prepared and crystallographically characterized. The isopropyl groups protrude out of the plane of the nine-membered ring and prevent self-association. The closest Au...Au contact between molecules is 6.417 A. Crystalline Au(I)3(i-PrN=COMe)3 is not luminescent at room temperature.     

Raman and X-ray scattering studies of high-pressure phases of urea

Single-crystal and polycrystalline urea samples were compressed to 12 GPa in a diamond-anvil cell. Raman-scattering measurements indicate a sequence of four structural phases occurring over this pressure range at room temperature. The transitions to the high-pressure phases take place at pressures near 0.5 GPa (phase I --> II), 5.0 GPa (II --> III), and 8.0 GPa (III --> IV). Lattice parameters in phase I (tetragonal, with 2 molecules per unit cell, space group P42(1)m (D3(2d))) and phase II (orthorhombic, 4 molecules per unit cell, space group P2(1)2(1)2(1) (D2(4))) were determined using angle-dispersive X-ray diffraction experiments. For phases III and IV, the combined Raman and diffraction data indicate that the unit cells are likely orthorhombic with four molecules per unit cell. Spatially resolved Raman measurements on single-crystal samples in phases III and IV reveal the coexistence of two domains with distinct spectral features. Physical origins of the spatial domains in phases III and IV are examined and discussed.     

Crystal polymorphs of 6‐[(phenylcarbamoyl)oxy]hexa‐2,4‐diyn‐1‐yl isonicotinate

The title compound, C₁₉H₁₄N₂O₄, was found to have two crystal polymorphs, in which the molecular structures of the diacetylenic compound are broadly similar. The main structural difference between the polymorphs concerns the intermolecular hydrogen‐bonding motifs adopted, namely a one‐dimensional zigzag polymer linked by N—H…N(py) (py is pyridine) interactions in polymorph I and a centrosymmetric dimeric motif formed by N—H…O=C interactions in polymorph II. The diacetylene cores of the molecules stack along the a and b axes in polymorphs I and II, respectively. It was found that only the molecular arrangement in polymorph II satisfies Baughman's criterion to afford polydiacetylenes (PDAs) by thermal annealing or irradiation with light. This predicted polymerization activity was confirmed by experiment.     

Zinc selenium oxochloride, β‐Zn2(SeO3)Cl2, a synthetic polymorph of the mineral sophiite

Dizinc selenium dichloride trioxide, β‐Zn₂(SeO₃)Cl₂, a monoclinic polymorph of the orthorhombic mineral sophiite, has a structure built of distorted ZnO₄Cl₂ octahedra, ZnO₂Cl₂ tetrahedra and SeO₃E tetrahedra (E being the 4s² lone pair of the Se^IV ion), joined through shared edges and corners to form charge‐neutral layers. The Cl atoms and the Se lone pairs protrude from each layer towards adjacent layers. The main structural difference between the mineral and synthetic polymorphs lies in the packing of the layers.     

A comparative study of two polymorphs of bis(1‐hydroxy‐2‐methylpropan‐2‐aminium) carbonate

Alkanolamines have been known for their high CO₂ absorption for over 60 years and are used widely in the natural gas industry for reversible CO₂ capture. In an attempt to crystallize a salt of (RS)‐2‐(3‐benzoylphenyl)propionic acid with 2‐amino‐2‐methylpropan‐1‐ol, we obtained instead a polymorph (denoted polymorph II) of bis(1‐hydroxy‐2‐methylpropan‐2‐aminium) carbonate, 2C₄H₁₂NO⁺·CO₃²⁻, (I), suggesting that the amine group of the former compound captured CO₂ from the atmosphere forming the aminium carbonate salt. This new polymorph was characterized by single‐crystal X‐ray diffraction analysis at low temperature (100 K). The salt crystallizes in the monoclinic system (space group C2/c, Z = 4), while a previously reported form of the same salt (denoted polymorph I) crystallizes in the triclinic system (space group P, Z = 2) [Barzagli et al. (2012). ChemSusChem, 5 , 1724–1731]. The asymmetric unit of polymorph II contains one 1‐hydroxy‐2‐methylpropan‐2‐aminium cation and half a carbonate anion, located on a twofold axis, while the asymmetric unit of polymorph I contains two cations and one anion. These polymorphs exhibit similar structural features in their three‐dimensional packing. Indeed, similar layers of an alternating cation–anion–cation neutral structure are observed in their molecular arrangements. Within each layer, carbonate anions and 1‐hydroxy‐2‐methylpropan‐2‐aminium cations form planes bound to each other through N—H…O and O—H…O hydrogen bonds. In both polymorphs, the layers are linked to each other via van der Waals interactions and C—H…O contacts. In polymorph II, a highly directional C—H…O contact (C—H…O = 156°) shows as a hydrogen‐bonding interaction. Periodic theoretical density functional theory (DFT) calculations indicate that both polymorphs present very similar stabilities.     

Twinning in 5‐fluorosalicylic acid: description of a new polymorph

The crystal structure of 5‐fluorosalicylic acid is known from the literature [Choudhury & Guru Row (2004). Acta Cryst. E 60 , o1595–o1597] as crystallizing in the monoclinic crystal system with space‐group setting P2₁/n and with one molecule in the asymmetric unit (polymorph I). We describe here a new polymorph which is again monoclinic but with different unit‐cell parameters (polymorph II). Polymorph II has two molecules in the asymmetric unit. Its structure was modelled as a twin, with a pseudo‐orthorhombic C‐centred twin cell.     

Crystal structures of sulfathiazole polymorphs in the temperature range 100–295 K: A comparative analysis

The response of four polymorph modifications of sulfathiazole C₉H₉N₃O₂S₂ to variation of temperature was examined in the range 295–100 K by single crystal X-ray diffraction. No phase transitions occur in this temperature range; all the structures exhibit anisotropic contraction. The metastable sulfathiazole modification I is drastically different from the other modifications (II, III, and IV) in the anisotropy of structure distortions and changes in the intra-and intermolecular geometry, although bulk thermal expansion is virtually similar for all polymorphs within the temperature range studied.     

Conformational polymorphs of 3‐cyclopropyl‐5‐(3‐methyl‐[1,2,4]triazolo[4,3‐a]pyridin‐7‐yl)‐1,2,4‐oxadiazole

The dipharmacophore compound 3‐cyclopropyl‐5‐(3‐methyl‐[1,2,4]triazolo[4,3‐a]pyridin‐7‐yl)‐1,2,4‐oxadiazole, C₁₂H₁₁N₅O, was studied on the assumption of its potential biological activity. Two polymorphic forms differ in both their molecular and crystal structures. The monoclinic polymorphic form was crystallized from more volatile solvents and contains a conformer with a higher relative energy. The basic molecule forms an abundance of interactions with relatively close energies. The orthorhombic polymorph was crystallized very slowly from isoamyl alcohol and contains a conformer with a much lower energy. The basic molecule forms two strong interactions and a large number of weak interactions. Stacking interactions of the `head‐to‐head' type in the monoclinic structure and of the `head‐to‐tail' type in the orthorhombic structure proved to be the strongest and form stacked columns in the two polymorphs. The main structural motif of the monoclinic structure is a double column where two stacked columns interact through weak C—H…N hydrogen bonds and dispersive interactions. In the orthorhombic structure, a single stacked column is the main structural motif. Periodic calculations confirmed that the orthorhombic structure obtained by slow evaporation has a lower lattice energy (0.97 kcal mol^?1) compared to the monoclinic structure.