Transformation of three-connected silicon nets in CaSi2

Up to 40 kbar and 1100°C, CaSi₂ is dimorphic. Trigonal/rhombohedral CaSi₂I (CaSi₂-type structure) with corrugated layers of three-connected Si atoms can be transformed by a high pressure-high temperature treatment into tetragonal CaSi₂II (α-ThSi₂-type structure) with a three-dimensional net of three-connected Si atoms. The silicon net of CaSi₂II is slightly distorted from the topologically simplest tetragonal three-dimensional three-connected net derived on a geometrical basis. In order to correlate crystal chemical with thermochemical data the transformation between both polymorphs of CaSi₂ has been studied at equilibrium and nonequilibrium conditions. The pressure-temperature phase diagram of CaSi₂ has been investigated by X-ray technique in quenched samples. From the slope of the equilibrium line and the change in molar volume the approximate values of the entropy and energy of transformation CaSi₂(I-II) have been determined ΔS = 3.2 e.u., ΔU = 4.9 kcal/mole. Under nonequilibrium conditions the transformation CaSi₂(II-I) yielded ΔH = ?4.2 kcal/mole at 500°C and ambient pressure in a DTA apparatus. Complete transformation of metastable CaSi₂II can be achieved within 5 min at a heating rate of 20°C/min. Due to the relatively high speed of transformation simple structural relations between both polymorphs of CaSi₂ are discussed.     

Preparation and ionic conductivity of (100−x)(0.8Li2S·0.2P2S5)·xLiI glass–ceramic electrolytes

The glass–ceramic electrolytes of (100?x)(0.8Li₂S·0.2P₂S₅)·xLiI (in mole percent; x?=?0, 2, 5, 10, 15, 20, and 30) were prepared by mechanical milling and subsequent heat treatment. Crystalline phases analogous to the thio-LISICON region II or III in the Li₂S–GeS₂–P₂S₅ system were precipitated. The thio-LISICON III analog phase was mainly precipitated at the composition x?=?0, and the thio-LISICON II analog phase was precipitated in the composition range from x?=?2 to 15. The X-ray diffraction peaks of the thio-LISICON II analog phase shifted to the lower diffraction angle side with increasing the LiI content. High conductivities above 2?×?10^?3?S?cm^?1 at room temperature were observed in the glass–ceramics at the wide composition range from x?=?2 to 15. The glass–ceramic electrolyte at x?=?5 with the highest conductivity of 2.7?×?10^?3?S?cm^?1 showed a wide electrochemical window of about 10 V. The addition of LiI to the 80Li₂S·20P₂S₅ (in mole percent) glass was effective in crystallizing the thio-LISICON II analog phase with high conductivity from the glass.     

Solvent and external pressure effects on the ratio of the cyanoisopropyl radical recombination and disproportionation rates

A GC-MS analysis of the azobisisobutyronitrile thermal decomposition products of in solutions at 80°C showed that the ratio of recombination and disproportionation rates of the cyanoisopropyl radical does not depend on the medium viscosity, but increases when the internal pressure of the solvent increases according to the log(k _dispr/k _rec) = ?1.25 + 0.096 P _int ^0.5 law. This means that the activation volume corresponding to recombination is larger than that corresponding to disproportionation. It follows from the relationship log(k _dispr/k _rec) = (ΔV _rec ^≠ ? Δv _dispr ^≠ )ΔP/RT that, for the decomposition of the substrate in benzene under a pressure of 0.5–4.0 kbar, the difference between the activation volumes is ΔV _rec ^≠ ? ΔV _dispr ^≠ = 8 cm³/mol.     

The system BaOGeO2 at high pressures and temperatures,with special reference to high-pressure transformations in BaGeO3, BaGe2O5, and Ba2Ge5O12

Phase relations in the system BaOGeO₂ were investigated in the pressure range 20–70 kbar in the temperature range 750–1200°C. Several new phases were identified in this system: an atmospheric phase of BaGe₂O₅ (monoclinic BaGe₂O₅ I), two high-pressure phases of BaGe₂O₅ (monoclinic BaGe₂O₅ II and tetragonal BaGe₂O₅ III), and a high-pressure phase of Ba₂Ge₅O₁₂. The phase boundary curve between BaGe₂O₅ II and BaGe₂O₅ III was preliminarily determined as P(kbar) = 7.7 + 0.047T (°C). The high-pressure phases of BaGeO₃, which were previously reported by Y. Shimizu, Y. Syono, and S. Akimoto (High Temp.-High Pressures2, 113 (1970)) in the pressure range 15–95 kbar, were interpreted to be not single-phase materials but complicated mixtures of more than two phases in the system BaOGeO₂. X-Ray powder diffraction data for the new compounds synthesized in this study are given.     

Equilibrium anionic polymerization of the isopropenyl monomers containing aromatic heterocycles

The anionic polymerization of three monomers, 2-isopropenyl-4,5-dimethyloxazole(I), 2-isopropenylthiazole(II), and 2-isopropenylpyridine(III), was studied in THF. These monomers produced red-colored living polymers on addition of sodium naphthalene or living α-methylstyrene tetramer as an initiator. It was observed that a considerable amount of monomer remained in the respective living polymer–monomer system, indicating that an equilibrium between the polymer and the monomer existed as in the case of α-methylstyrene. At lower temperatures, the conversion of the monomer to the polymer increased. The equilibrium monomer concentrations [M_e] were determined at different temperatures, and the heats (ΔH) and the entropies (ΔS°) of polymerization were obtained by plotting In(1/[M_e]) against 1/T as ΔH = ?9.4, ?6.8, and ?6.2 kcal/mole, ΔS°S = ?22.9, ?16.5, and ?16.6, eu for I, II, and III, respectively.     

Solid state electrochemical study of the phase diagram and thermodynamics of the ternary system CuGeO

Coulometric titrations using solid zirconia ionic conductors have been employed to determine the phase diagram of the ternary system CuGeO in the temperature range from 750 to 950°C. CuGeO₃ was found to be the only existing ternary compound in the system. It is in equilibrium with Cu₂O, CuO, GeO₂, and oxygen of atmospheric pressure. Cu and Cu₂O may coexist with GeO₂. The standard Gibbs energy of formation of CuGeO₃ was found to be ΔG°_f (CuGeO₃) = ?424.5 kJ/mole at 900°C. The standard enthalpy and entropy of formation are ΔH⁰_f = ?756.8 kJ/mole and ΔS°_f = ?283 J/mole·K, respectively.     

Phase behavior of Li2WO4 at high pressures and temperatures

The phase diagram of Li₂WO₄, previously studied by Yamaoka et al. (J. Solid State Chem.6, 280 (1973)) has been revised. Li₂WO₄ II is stable at atmospheric pressure below ～310°C. This phase appears to be a modified spinel, and is tetragonal, a, c = 11.941, 8.409Å, Z = 16, space group $\text{I4}{}_1\text{amd}$. The melting curve of phenacite-type Li₂WO₄ I rises with pressure with a slope of 0.9°C/kbar to the III/I/liquid triple point at 3.1 kbar, 743°C, beyond which the melting curve of orthorhombic Li₂WO₄ III rises steeply with pressure (initial slope 31°C/kbar). The Li₂WO₄$\text{I}\text{III}$ transition line at 3 kbar is almost independent of temperature, i.e., the $\text{I}\text{III}$ transition entropy is zero. Li₂WO₄ II is 21.3% denser than Li₂WO₄ I at ambient conditions.     

Conformational analysis of intramolecular bonded amino alcohols : The conformational free energies of some intramolecular OH ⋯ N hydrogen bonds

Equilibrium positions between intramolecular OH ? N hydrogen bonded and free OH forms of some 3-piperidinols, decahydroisoquinolinols, a decahydroquinolinol, lupinine and N-methyl-3-piperidinemethanol have been determined from dilute solution IR spectral data at 33°. Conformational free energies of the H-bonds (ΔG°_OH?N, attractive) have been calculated. The results suggest a linear relationship between the apparent value of ΔG°_OH?N, as defined by the method of calculation, and the strength of the OH ? N bond expressed as Δν, within the limits of 0·5 ± 0·2kcal/mole per 100 cm^?1, from Δν 90 to 350 cm^?1. For cis-decahydroisoquinoline (N-Me or N-H) systems, a 0·4 kcal/mole difference has been calculated between the two possible ring-fused conformations, in favor of the so-called steroid form. For the corresponding cis-decahydroqumoline equilibrium, a 0·8 kcal/mole difference has been calculated, in favor of the nonsteroid form.     

Melting curve and high-pressure polymorphism of NH4HF2

The melting curve of NH₄HF₂ I rises from 125.2°C at atmospheric pressure to a triple point II/I/liquid at 9.3 kbar, 220°C. The I/II phase boundary is terminated at a triple point III/I/II at ∼45 kbar, 295°C. The melting curve of the new phase NH₄HF₂ II passes through a broad maximum at ∼39 kbar, 306°C, and is terminated at a triple point III/II/liquid at 46.3 kbar, 301°C. The melting curve of NH₄HF₂ III rises with pressure. The NH₄HF₂ III may be a dense hydrogen-bonded phase. Liquid NH₄HF₂ appears to be anomalous in several respects, and has a high compressibility relative to the solid phases.     

γ-radiation-initiated polymerization of bulk styrene at high pressure

The polymerization of styrene in bulk at pressures up to 273 MPa and temperatures between 3 and 49°C with the use of γ-radiation as the initiator has been studied. The polymerization rate and the molecular weight of the polymer increased with increasing pressure; the molecular weight increased at a slightly faster rate. The difference in the rate is a theoretical expectation which has not previously been observed because chain-transfer reactions obscure the effect in chemically initiated systems. A small but significant retardation of the initiation reaction occurs as the pressure is increased. The results of previous workers are critically reviewed. Chain transfer at 25°C for pressures below 220 MPa is negligible when γ-radiation is the initiator. The activation energy for bulk polymerization decreased with increasing pressure from 28.1 kJ/mole at 0.1013 MPa to 22.3 kJ/mole at 203 MPa. Volumes of activation at 25°C for 0.1013 < p < 273 MPa were calculated to be Initiation, +4.0 < ΔV < +4.4 cm³/mole; polymerization; ?Δ = ?20.9 cm³/mole; degree of polymerization; ΔV = ?25.3 cm³/mole; propagation/termination; ?ΔV = ?22.7 cm³/mole.     

Pressure effects on cation migration in 2,5‐di‐tert‐butyl‐1,4‐benzoquinone radical anion

Pressure effects on the two‐site jumping of sodium and potassium cations in a 2,5‐di‐tert‐butyl‐1,4‐benzoquinone ion pair have been studied using a high‐pressure EPR technique. The rate constants of the intramolecular and intermolecular migrations for Na⁺ and K⁺ were determined from an EPR spectral simulation. The migration rates were found to be accelerated by increasing the external pressure. Using the pressure dependence of the migration rates, we estimated the activation volumes of the intramolecular (ΔV₁^?) and intermolecular (ΔV₂^?) processes for the Na⁺ and K⁺ migrations: ΔV₁^? = ?5.3 cm³ mol^?1 and ΔV₂^? = ?29 cm³ mol^?1 for Na⁺, and ΔV₁^? = ?8.3 cm³ mol^?1 and ΔV₂^? = ?0.85 cm³ mol^?1 for K⁺. Based on the results, the mechanisms for the two‐site jumping of Na⁺ and K⁺ are discussed in terms of volume. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 397–401, 2001     

Spectroscopic studies of 1,4-dibromobutyne-2

Infrared spectra of 1,4-dibromobutyne-2 have been recorded over the 4000-200 cm^?1 region in the vapour, liquid, amorphous and crystalline states Raman spectra were extended to ca. 20 cm^?1 in the same states of aggregation, except for the non-recorded vapour phase spectrum. The temperature was varied between ?190 and 160 °C, and the pressure up to 10 kbar.A high proportion of the molecules exhibited free, internal rotation in the vapour and liquid phases, but to a smaller extent in the amorphous state at ?190 °C. For those molecules not being excited beyond the potential barrier, an unsymmetric conformation was preferred, whereas in the crystalline state the molecules possessed the anti conformation (C_2h) both at low temperature and at high pressure at ambient temperature.A vibrational analysis based upon force field calculations was carried out and the mean amplitudes of vibration computed. The data have been related to preliminary results from dipole moment and electron diffraction investigations.     

The constitution of the grignard reagent : VIII. The structure of complexes between ethylmagnesium bromide and 1-ethoxy-2-methylbutane

From measurements of the degree of association and of the optical rotation of solutions of ethylmagnesium bromide in (+)(S)-1-ethoxy-2-methylbutane [(+)-(S)-L] and benzene it is concluded that these solutions contain the species EtMgBr·2L (I), (EtMgBr)₂·3L (III) and (EtMgBr·L)₂ (II) which have molecular rotations [M]²⁵_D of 3.79°, 5.92° and 4.26° respectively. Equilibrium constants of 103 mole/l and 1.41 mole/l at 25° were calculated for the equilibria between these species.     

Radiation-Induced Polymerization of Methyl Methacrylate at High Pressure

Radiation-induced polymerization of methyl methacrylate (MMA) was studied up to 7500 kg/cm² at 20°C. The rate of polymerization increased to 3000 kg/cm² with overall activation volume ΔV_pol? of -23.6 cm³/mole, and then the pressure dependence of the rate was very small in the pressure range between 3000 and 3700 kg/cm². The rate of polymerization increased again above 3700 kg/cm² up to the crystallization pressure of MMA (5500 kg/cm²) with ΔV_pol? of -13.7 cm³/ mole with increasing pressure. The volume contraction by polymerization decreased with increasing pressure up to 3000 kg/cm² but hardly decreased with increasing pressure above 3000 kg/cm². The stereoregulzarity (triad probability) of PMMA changed slightly at 3000 kg/cm; above 3000 kg/cm², syndiotactic addition decreased and heterotactic addition increased. Marked change in P-V isotherms of MMA, however, was not observed about 3000 kg/cm². We concluded from these facts that an alignment of monomer molecules, which does not cause large volume change, was realized about 3000 kg/cm². Polymerization proceeded above the crystallization pressure by long time irradiation, and isotactic addition increased clearly in the solid-state polymerization.     

ESR study of pressure dependence of free-radical decay in irradiated polystyrene

Radicals giving the usual triplet ESR spectrum have been generated in polystyrene by γ-irradiation at room temperature. The decay of the radicals has been investigated in the temperature interval between 90° and 200° and pressures ranging from 1 to 8000 atm. The effect of pressure on the mechanism of the free-radical decay is discussed. There are two regions of free-radical decay showing different activation volumes: V_I = 11.5 cm³/mole and V_II = 66 cm³/mole. The correlation between molecular motion in the α-relaxation region and radical decay is pointed out.     

Heat capacity of pure and doped V2O3 single crystals

Heat capacities have been measured for single crystals of V₂O₃, either pure or doped with 1 and 1.4 mole% Cr₂O₃ and Al₂O₃ over the temperature range 100–700°K. V₂O₃ undergoes a fairly sharp transition at low temperatures (～170°K) but fails to exhibit any thermal anomaly above 300°K. The thermal behavior of (M_xV_1?x)₂O₃, M = Cr, Al, is manifested by two transitions: one at low temperatures, 170–180°K for x = 0.01 and 180–190°K for x = 0.014, and the other at high temperatures. For x = 0.01, the high-temperature (HT) anomaly extended over the range 325–345°K (Cr-doped V₂O₃) and 345–365°K (Al-doped V₂O₃), respectively. The corresponding ranges for x = 0.014 were found to be 260–280°K and 270–290°K, respectively. Further, the HT anomaly was characterized by a large hysteresis (～50°K). The values of lattice heat capacity of pure and doped V₂O₃ were, however, found to be almost the same and could be empirically represented by the Debye (D)?Einstein (E) function $\text{D}\text{(}\text{580}\text{T}\text{) + 4}\text{E}\text{(}\text{θ}\text{T}\text{)}$ with θ values 430°K (T = 100–230°K) and 465°K (T > 230°K), respectively. Further, the enthalpy change ΔH associated with the HT anomaly in doped V₂O₃ (80 ≤ ΔH ≤ 510 J/mole) was 5–10 times smaller than the ΔH corresponding to the lower-temperature transition. The results cited here appear incompatible with the Mott transition model that has been invoked to explain the HT anomaly.     

Transformation of three-dimensional three-connected silicon nets in SrSi2

Dimorphic SrSi₂ is the first compound for which the two simplest three-dimensional three-connected nets are found in its polymorphs. The cubic net of three-connected silicon atoms (SrSi₂ type of structure) can be transformed into the tetragonal one (α-ThSi₂ type of structure) by a high-pressure-high-temperature treatment. The tetragonal phase is quenchable. Heating of this phase to 600–700°C at ambient pressure results in transformation into the cubic one. At a heating rate of 20°C/min complete transformation can be achieved within 5 min in a DTA apparatus. The energy of transformation has been obtained from the peak areas of the DTA curves to ?1.6 ± 0.3 kcal/mole. Although the transformation between the three-dimensional three-connected sets in SrSi₂ must be formally classified as a reconstructive one, a relatively small entropy change (ΔS = 1 ·₁ cal/deg · mole) has been calculated from the change in molar volume and p-T equilibrium conditions. Therefore, structural relations between the cubic and the tetragonal nets are discussed.     

Thermodynamic study of nonstoichiometric tungsten trioxide WO3−x by EMF measurements at high temperature

The values of ΔG(O₂), ΔH(O₂), and ΔS(O₂) have been determined from electrochemical cell measurements, within the whole homogeneity range of WO_3?x, between 700 and 900°C. The samples have been previously prepared by equilibration of WO₃ pellets with COCO₂ mixtures and their composition has been determined by thermogravimetry. A single phase has been found between WO₃ and WO_2.9760. The results may be understood by considering a structure involving point defects, singly ionized oxygen vacancies V^·_O between WO₃ and WO_2.9880. For larger departure from stoichiometry, the variations of ΔH(O₂) and ΔS(O₂) suggest the formation of more complex defects. The enthalpy of formation of V^·_O has been calculated: 78 kcal · mole^?1.     

Synthesis and crystal structure of complexes of manganese(II), cobalt(II), and nickel(II) isothiocyanates with ?-caprolactam

The possibility of ?-caprolactam (CPL) to coordinate to manganese(II), cobalt(II), and nickel(II) rhodanides has been investigated. New complexes trans-[M(CPL)₄(NCS)₂], where M = Mn (I), Co (II), and Ni (III), have been synthesized. The complexes have been studied by chemical analysis and IR spectroscopy. According to X-ray diffraction, complexes are isostructural to each other and crystallize in monoclinic space group P2₁/c, Z = 2. For I: a = 6.9457(2) ?, b = 17.7751(6) 0A, c = 12.8999(4) 0A, ?? = 104.2670(10)°, V = 1543.51(8) ?³, ??_calc = 1.342 g/cm³, R ₁ = 0.0426. For II: a = 6.8925(2) ?, b = 17.8189(8) ?, c = 12.7278(6) ?, ?? = 104.421(2)°, V = 1513.93(11) ?³, ??_calc = 1.377 g/cm³, R ₁ = 0.0280. For III: a = 6.7804(2) ?, b = 18.4631(4) ?, c = 12.4841(3) ?, ?? = 105.2950(10)°, V = 1507.49(7) ?³, ??_calc = 1.382 g/cm³, R ₁ = 0.0273. Structures I?CIII are molecular; the metal atom in each of them coordinates four CPL molecules and two NCS groups via oxygen and nitrogen atoms, respectively.     

Correlations of the thermodynamic quantities with the frequency shifts of a lattice mode close to the I–II transition in NH4Br

We correlate here the specific heat C_p with the frequency shifts (1/V) (?V/?T)_p and the thermal expansivity α_p with the (1/ν) (?V/?P)_T close to the I–II transition in NH₄Br. This correlation is performed for the Raman mode of ν_s (140 cm^?1) using the molar volume data for NH₄Br. It is shown here that spectroscopic modifications of the Pippard relations are applied satisfactorily to the I–II phase transition by using a lattice mode of NH₄Br.