Phase behaviour of the thermotropic cubic mesogen 1,2-bis-(4- n -octyloxybenzoyl)hydrazine under pressure

The phase transition behaviour of an optically isotropic, thermotropic cubic mesogen 1,2-bis-(4-n-octyloxybenzoyl)hydrazine, BABH(8), was investigated under pressures up to 200 MPa using a high pressure differential thermal analyser, wide-angle X-ray diffraction and a polarizing optical microscope equipped with a high pressure optical cell. The phase transition sequence, low temperature crystal (Cr₂)-high temperature crystal (Cr ₁)- cubic (Cub)-smectic C (SmC)-isotropic liquid (I) observed at atmospheric pressure, is seen in the low pressure region below about 30 MPa. The cubic phase disappears at high pressures above 30–40 MPa, in conjunction with the disappearance of the Cr₁ phase. The transition sequence changes to Cr₂-SmC-I in the high pressure region. Since only the Cub-SmC transition line among all the phase boundaries has a negative slope (dT/dP) in the temperature-pressure phase diagram, the temperature range for the cubic phase decreases rapidly with increasing pressure. As a result, a triple point was estimated approximately as 31.6 ±2.0 MPa, 147.0±1.0°C for the SmC, Cub and Cr₁ phases, indicating the upper limit of pressure for the observation of the cubic phase. Reversible changes in structure and optical texture between the Cub and SmC phases were observed from a spot-like X-ray pattern and dark field for the cubic phase to the Debye-Sherrer pattern and sand-like texture for the SmC phase both in isobaric and isothermal experiments.     

In situ observation of the pressure-induced mesophase for 4′-n-hexadecyloxy-3′-nitrobiphenyl-4-carboxylic acid

In situ observation of the optical texture, and X-ray patterns of the pressure-induced mesophase seen for 4′-n-hexadecyloxy-3′-nitrobiphenyl-4-carboxylic acid (ANBC-16) was performed under hydrostatic pressures up to 100MPa using a polarizing optical microscope equipped with a high pressure hot stage and a wide angle X-ray diffractometer equipped with a high pressure vessel respectively. It was found that the pressure-induced mesophase (hereafter refered to as ‘X’) appeared at pressures above 60 MPa, and exhibits a birefringent broken-fan or a sand-like texture that remain unaltered in the SmC phase. The POM-transmitted light intensity curve measured on heating clearly showed the Cr₄ → Cr₁ → SmC → ‘X’ → SmA → I transition sequence at 80 MPa. The optical texture and the POM-transmitted light intensity measured during a pressure cycle at 185°C showed a reversible change between the cubic and ‘X’ phases. The WAXD pattern of the ‘X’ phase showed a spot-like pattern, suggesting no layered structure for this phase, and also revealed a substantial decrease in the d-spacing of the low angle reflection at 80 and 100 MPa, compared with the d-spacings of the (0 0 1) reflection of the SmC phase and also the (2 1 1) reflection of the cubic phase. It is concluded from these data that the ‘X’ phase is a birefringent hexagonal columnar phase.     

The role of disclinations in two phase changes induced by temperature and pressure in crystalline solids: melting and the brittle–ductile transition

It has long been known that the melting temperature T _m of close-packed metals correlates well with the mono-vacancy formation energy. However, with the possible exception of the face-centered-cubic metals, there is a prior phase transition from a mechanically brittle solid phase to a ductile phase. Here the likely role of disclinations in the brittle-ductile phase change is stressed. The present picture may help to understand the brittle–ductile transition not only in crystalline materials but also in amorphous phases. The structure of such phases can probably be characterized in terms of a disordered disclination network. As examples of elemental crystalline solids, Si and graphite are finally discussed, with the melting under pressure of graphite being quantified.     

Phase behavior of a thermotropic cubic mesogen 4'-n-hexadecyloxy-3'-nitro- biphenyl-4-carboxylic acid under pressure

The phase behavior of an optically isotropic cubic mesogen 4'-n-hexadecyloxy-3'-nitrobiphenyl-4-carboxylic acid (ANBC-16) was investigated under hydrostatic pressures up to 200 MPa using a high-pressure DTA, a polarizing optical microscope equipped with a high-pressure hot-stage and a wide-angle X-ray diffractometer equipped with a high-pressure vessel. In the T vs. P phase diagram constructed in the heating mode, a triple point exists at 54±1 MPa and 205±1°C for the SmC, cubic, and SmA phases. A new mesophase, denoted here as X, appears in place of the cubic phase under pressures above about 60 MPa, while the X phase appears on cooling in the whole pressure region studied. Thus the X phase is a monotropic (metastable) phase between the SmA and Cub phases in the low pressure region, while being an enantiotropic phase between the SmA and SmC phases in the high pressure range. The X phase exhibits broken-fan or sand-like textures under pressure and a spot-like diffraction pattern, indicating the birefringent feature and no layered structure. It is suggested that the X phase is tetragonal or hexagonal columnar phase. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pressure-induced transition in Tl2MoO4

Tl₂MoO₄ has been studied under high-pressure by X-ray diffraction, Raman spectroscopy, and optical absorption measurements. A first-order phase transition is observed at 3.5±0.5 GPa. The nature (ordered vs. disordered) of the high-pressure phase strongly depends on the local hydrostatic conditions. Optical absorption measurements tend to show that this transition is concomitant with an electronic structure transformation. Prior to the transition, single crystal X-ray diffraction shows that pressure induces interactions between MoO₄ fragments and the Mo coordination number tends to increase. In addition, the stereoactivity of the lone-pair electrons on the three symmetrically independent Tl-sites is not uniform; while for two sites the stereoactivity decreases with increasing pressures for the third site the stereoactivity increases.     

Molecular dynamics studies of the kinetics of phase changes in clusters III: structures, properties, and crystal nucleation of iron nanoparticle Fe331

Molecular dynamics (MD) computer simulations have been carried out to study the structures, properties, and crystal nucleation of iron nanoparticles with 331 Fe atoms or with diameter around 2 nm. Structure information for the nanoparticles was analyzed from the MD simulations. Three crystalline phases and one amorphous phase were obtained by cooling the nanoparticles from their molten droplets at different cooling rates or with different lengths of cooling time periods. Molten droplets froze into three different solid phases and a solid-solid transition from a disordered body-centered cubic (BCC) phase to an ordered BCC phase were observed during the slow cooling and the quenching processes. Properties of nanoparticle Fe₃₃₁, such as melting point, freezing temperature, heat capacity, heat of fusion, heat of crystallization, molar volume, thermal expansion coefficient, and diffusion coefficient, have been estimated. Nucleation rates of crystallization to two solid phases for Fe₃₃₁ at temperatures of 750, 800, and 850 K are presented. Both classical nucleation theory and diffuse interface theory are used to interpret our observed nucleation results. The interfacial free energy and the diffuse interface thickness between the liquid phase and two different solid phases are estimated from these nucleation theories.     

Thermal behaviour of the thermotropic cubic mesogen 4'-n-hexadecyloxy-3'-nitrobiphenyl-4-carboxylic acid (ANBC-16) under hydrostatic pressure

The phase behaviour of 4'-n-hexadecyloxy-3'-nitrobiphenyl-4-carboxylic acid (ANBC-16) was investigated under hydrostatic pressures up to 200 MPa using high pressure differential thermal analysis. The phase transition sequence crystal 4 (Cr4)-crystal 3 (Cr3)-crystal 2 (Cr₂)-crystal 1 (Cr₁)-smectic C (SmC)-Cubic (Cub)-smectic A (SmA)-'structured liquid' (I₁)-isotropic liquid (I₂) was observed for a virgin sample on heating at atmospheric pressure. The stable temperature region of the optically isotropic cubic phase becomes narrower on increasing pressure and disappears at pressures above 65 MPa. The T vs. P phase diagram exhibits the existence of a triple point (65 MPa, 207.6°C) for the cubic phase, a new mesophase (X), and the SmA phase, indicating the upper limit for the cubic phase. The new mesophase, denoted here as X, appears in place of the cubic phase at pressures above 65 MPa. The phase diagram also indicates that the Cr₄-Cr₃, Cr₃-Cr₂, and Cr₂-Cr₁ transition lines merge at about 40-50 MPa and then only the Cr₄-Cr₁ transition is observed in the solid state at higher pressures. Thus the phase transition process on heating changes from the sequence Cr₄-Cr₃-Cr₂-Cr₁-SmC-Cub-SmA-I₁-I₂ at atmospheric pressure to Cr₄-Cr₁-SmC-X-SmA-I₁-I₂ in the high pressure region above 65 MPa, via Cr₄-Cr₃-Cr₂-Cr₁-SmC-(X)-Cub-SmA-I₁-I₂ in the low pressure region.     

Amorphous polymerization of nitrogen in compressed cupric azide

Metal azides have attracted increasing attention as precursors for synthesizing polymeric nitrogen. In this article, we report the amorphous polymerization of nitrogen by compressing cupric azide. The ab initio molecular dynamics simulations show that crystalline cupric azide transforms into a disordered network composed of singly bonded nitrogen at a hydrostatic pressure of 40 GPa and room temperature. The transformation manifests the formation of a π delocalization along the disordered Cu-N network, thus resulting in a semiconductor–metal transition. The estimated heat of formation of the amorphous polymeric nitrogen system is comparable to conventional high-energy-density materials. The amorphization provides an alternative route to the polymerization of nitrogen under moderate conditions.     

High temperature crystal structures and superionic properties of SrCl2, SrBr2, BaCl2 and BaBr2

The structural properties of the binary alkaline-earth halides SrCl₂, SrBr₂, BaCl₂ and BaBr₂ have been investigated from ambient temperature up to close to their melting points, using the neutron powder diffraction technique. Fluorite-structured SrCl₂ undergoes a gradual transition to a superionic phase at 900–1100 K, characterised by an increasing concentration of anion Frenkel defects. At a temperature of 920(3) K, the tetragonal phase of SrBr₂ undergoes a first-order transition to a cubic fluorite phase. This high temperature phase shows the presence of extensive disorder within the anion sublattice, which differs from that found in superionic SrCl₂. BaCl₂ and BaBr₂ both adopt the cotunnite crystal structure under ambient conditions. BaCl₂ undergoes a first-order structural transition at 917(5) K to a disordered fluorite-structured phase. The relationship between the (disordered) crystal structures and the ionic conductivity behaviour is discussed and the influence of the size of the mobile anion on the superionic behaviour is explored.     

Inversion of the phase sequence between the cubic and smectic C phases under pressure

The phase behaviour of a thermotropic cubic mesogen of 1,2-bis(4′-n-tetradecyloxybenzoyl)hydrazine BABH-14 was studied under hydrostatic pressure using a polarising optical microscope equipped with a high-pressure optical cell, and the P–T phase diagram was constructed. BABH-14 shows the Cr–Cub–I transition sequence under atmospheric and lower pressures, but the Cub phase is replaced completely by the high-pressure SmC, SmC(hp), phase under higher pressures. There is a narrow intermediate-pressure region between the low- and high-pressure regions, in which the Cr–SmC(hp)–Cub–I phase sequence is recognised. The SmC(hp)–Cub transition line has a positive slope with pressure and there are two triple points: one is for the Cr, Cub and SmC(hp) phases and the other is for the I, Cub and SmC(hp) phases. Comparing the phase sequence of BABH-14 with those for BABH-8 and -10, the pressure-induced inversion of the phase sequence between the cubic and SmC phases occurs in the BABH-n homologous compounds. Another new phenomenon is the formation of the monotropic cubic phase on cooling in the intermediate- and high-pressure regions, and an intriguing phenomenon of the cubic phase appearing twice, i.e. I–Cub–SmC(hp)– Cub–Cr phase transition, occurs in the intermediate-pressure region.     

Order and disorder in Ca2ND0.90H0.10-A structural and thermal study

The structure of calcium nitride hydride and its deuterided form has been re-examined at room temperature and studied at high temperature using neutron powder diffraction and thermal analysis. When synthesised at 600 °C, a mixture of both ordered and disordered Ca₂ND_0.90H_0.10 phases results. The disordered phase is the minor component and has a primitive rocksalt structure (spacegroup Fm3m) with no ordering of D/N on the anion sites and the ordered phase is best described using the rhombohedral spacegroup R-3m with D and N arranged in alternate layers in (111) planes. This mixture of ordered and disordered phases exists up to 580 °C, at which the loss of deuterium yields Ca₂ND_0.85 with the disappearance of the disordered phase. In the new ordered phase there exists a similar content of vacancies on both anion sites; to achieve this balance, a little N transfers onto the D site, whereas there is no indication of D transferring onto the N-sites. These observations are thought to indicate that the D/N ordering is difficult to achieve with fully occupied anion sites. It has previously been reported that Ca₂ND has an ordered cubic cell with alternating D and N sites in the [100] directions [1]; however, for the samples studied herein, there were clearly two coexisting phases with apparent broadening/splitting of the primitive peaks but not for the ordered peaks. The rhombohedral phase was in fact metrically cubic; however, all the observed peaks were consistent with the rhombohedral unit cell with no peaks requiring the larger ordered cubic unit cell to be utilised. Furthermore this rhombohedral cell displays the same form of N-D ordering as the Sr and Ba analogues, which are metrically rhombohedral.     

Phase behaviour of thermotropic banana-shaped compounds under pressure

The phase behaviour of two achiral bent core banana-shaped compounds, the hexyloxy (compound I) and decyloxy (compound II) members of the 1,3-phenylene bis[N-(2-hydroxy-4-n-alkoxybenzylidene)-4′-aminobenzoate] series was investigated under hydrostatic pressures up to 300?MPa using high pressure differential thermal analysis and light transmission methods. The reversible transition sequence crystal (Cr₁)–B₁ phase–isotropic liquid (I), observed at room pressure for compound I, remains in the pressure region up to c 70?MPa. At higher pressures a pressure-induced crystalline phase (Cr_i) appears between the Cr₁ and B₁ phases, its temperature region becoming wider with increasing pressure. The temperature vs. pressure phase diagram shows a triple point of 72.9?MPa and 160.3°C for the Cr₁, Cr_i and B₁ phases, indicating the lower limit of pressure for the Cr_i phase. In compound II the reversible transition sequence crystal (Cr₁)–B₂ phase–I is seen over the whole pressure region, and the temperature range of the B₂ phase remains unaltered. It is concluded that both the B₁ and B₂ banana phases are stable over the whole pressure region studied.     

Calculated structures of asymmetric modifications of cyanospinels

Structural mechanisms of formation of low-symmetry phases of cyanospinels are described in terms of the phenomenological theory of phase transitions. These are examined for transitions from a spinel structure to two cubic and four rhombohedral phases, induced by automorphic irreducible representations 9-2 and 9-3. Atomic coordinates and the symmetry types of occupied positions in asymmetric phases are determined. Criteria for discrimination between the two as well as between the four phases are formulated. The structures of complex cyanides Tl₂Zn(CN)₄ , K₂Zn(CN)₄, K₂Hg(CN)₄, and Rb₂Hg(CN)₄ are calculated. The displacements and irreducible representations inducing transitions from spinel structures to these phases are determined. Novocherkassk State Technological University. Translated fromZhumal Struktumoi Khimii, Vol. 35, No. 5, pp. 41–49, September–October, 1994. Translated by L. Smolina     

Structural Transformations and Ionic Mobility in CsSbF<Subscript>3</Subscript>(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)

¹H, ¹⁹F, ³¹P NMR, DSC, and XRD methods are used to study ionic mobility and structural transformations in the CsSbF₃(H₂PO₄) compound (I). Radical changes in ¹H, ¹⁹F, ³¹P NMR spectra above 390 K are associated with a crystalline disordered phase which forms in I at 400–420 K. This phase demonstrates high ionic mobility and further transforms (above 425 K) into the amorphous (glassy) phase. We have determined the types of ionic mobility in this compound and in its amorphous product. According to the NMR data, the diffusion in the proton sublattice of the disordered and amorphous phases proceeds even at room temperature.     

Porous framework of T2[Fe(CN)6]·xH2O with T=Co, Ni, Cu, Zn, and H2 storage

The materials under study were prepared from aqueous solutions of ferrocyanic acid and salts of the involved transition metals and their crystal structure solved and refined from X-ray powder diffraction data. Complementary information from thermogravimetric, infrared and Mössbauer data was also used for the structural study. Three different crystal structures were found: hexagonal (P-3) for Zn with the zinc atom coordinated to three N ends of CN groups plus a water molecule, cubic (Pm-3m) for Ni and Cu, and monoclinic (P2₁/m) for Co. For Ni and Cu the obtained solids have an open channel framework related to 50% of vacancies for the building unit, [Fe(CN)₆]. In the as-synthesized material the framework free volume is occupied by coordinated and hydrogen-bonded water molecules. These of hexacyanoferrates (II) have received certain attention as prototype of materials for the hydrogen storage. In the anhydrous phase of Ni and Cu, 50% of the metal (T) coordination sites, located at the cavities surface, will be available to interact with the hydrogen molecule. However, when the crystal waters are removed the porous frameworks collapse as it is suggested by H₂ and CO₂ adsorption data. For Co, a structure of stacked layers was found where the cobalt atoms have both tetrahedral and octahedral coordination. The layers remain together through a network of hydrogen-bonding interactions between coordinated and weakly bonded water molecules. No H₂ adsorption was observed in the anhydrous phase of Co. For Zn, the porous framework remains stable on the water removal but with a system of narrow channels and a small available volume, also inaccessible to H₂.     

Order-Disorder Structural Phase Transitions in Alkali Perchlorates

Order-disorder structural phase transitions in alkali perchlorates MClO₄ (M=Na, K, Rb, Cs) are investigated using molecular dynamics simulation. The potentials in the simulations are based on the Gordon-Kim modified electron gas formalism extended to molecular ions. The simulations yield first-order phase transitions in perchlorates from low temperature orthorhombic structures to high temperature cubic NaCl structures. The perchlorate ions are found to be orientational disordered in the high temperature phases.     

Nature of phase transitions in crystalline and amorphous GeTe-Sb2Te3 phase change materials

The thermodynamic nature of phase stabilities and transformations are investigated in crystalline and amorphous Ge(1)Sb(2)Te(4) (GST124) phase change materials as a function of pressure and temperature using high-resolution synchrotron x-ray diffraction in a diamond anvil cell. The phase transformation sequences upon compression, for cubic and hexagonal GST124 phases are found to be: cubic → amorphous → orthorhombic → bcc and hexagonal → orthorhombic → bcc. The Clapeyron slopes for melting of the hexagonal and bcc phases are negative and positive, respectively, resulting in a pressure dependent minimum in the liquidus. When taken together, the phase equilibria relations are consistent with the presence of polyamorphism in this system with the as-deposited amorphous GST phase being the low entropy low-density amorphous phase and the laser melt-quenched and high-pressure amorphized GST being the high entropy high-density amorphous phase. The metastable phase boundary between these two polyamorphic phases is expected to have a negative Clapeyron slope.     

甘油单油酸脂/水立方液晶体系流变性质的研究

Monoglyceride (MO) can form various liquid crystalline phases spontaneously in the presence of various amount of water at room temperature. The appropriate compositions from binary phase diagram of MO/H2O were selected to form cubic phases. The selected systems were studied at different salt concentrations and pH value using rheological methods. There was a weak effect of salt on viscoelastic properties of cubic phases formed from MO/H2O system. Hexagonal phase was formed when pH value was decreased or increased. The viscoelasticity of cubic phases was different from that of hexagonal liquid crystals. Rheological properties of MO/H2O cubic phases were stable at pH and salt concentration similar to physiological condition.     

Phase progression via phonon modes in lanthanide dioxides under pressure

The present paper reports the phase progression in nano-crystalline oxides PrO₂ and CeO₂ up to pressures of 49 GPa and 35 GPa, respectively, investigated via in situ Raman spectroscopy at room temperature. The samples were characterized at ambient conditions using X-ray diffraction (XRD), AFM, and Raman spectroscopy and were found to be cubic with fluorite structure. With an increase in applied pressure the cubic bands were seen to steadily shift to higher wavenumbers for both the samples. However, we observed the appearance of a number of new peaks around a pressure of about 34.7 GPa in CeO₂ and 33 GPa in PrO₂ which were characteristic of an orthorhombic α-PbCl₂ type structure. The mode Gruneisen parameters for both the phases were obtained from the pressure dependence of frequency shifts. On decompression, the high pressure phase existed down to a total release of pressure.     

Phase behaviour of the thermotropic cubic mesogens 1,2-bis-(4-n-undecyl- and 4-n-dodecyl-oxybenzoyl)hydrazine under pressure

The phase transition behaviour of two optically isotropic, thermotropic cubic mesogens 1,2-bis-(4-n-undecyloxy- and 4-n-dodecyloxy-benzoyl)hydrazine, BABH(11) and BABH(12), was investigated under hydrostatic pressures up to 300 MPa using a high pressure differential thermal analyser, a wide angle X-ray diffractometer and a polarizing optical microscope equipped with a high pressure optical cell. It is found that for BABH(11) and BABH(12), a smectic C (SmC) phase is induced between the isotropic liquid (I) and the cubic (Cub) phases by applying pressures above 10-12 and 16-17 MPa, respectively. A sea-island texture consisting of bright sand-like sea regions (SmC phase) and areas of dark islands (Cub phase) appears in the mesophase under pressures up to 140 MPa, while the sand-like texture of the SmC phase is formed predominantly on cooling under pressure. These observations indicate the destabilization of the cubic phase with increasing pressure. The phase transition sequence of BABH(11) and BABH(12), Cr-Cub-I at atmospheric pressure, changes to Cr-Cub-SmC-I under intermediate pressures and would change to Cr-SmC-I under elevated pressure.