A second order phase transition between the space groups D182h and D174hleads to the high temperature tetragonal phase of (CH3NH3)2MnCl4. Similar transitions to tetragonal phases exist also in the ethyl- and propyl-compounds. Transition temperature increase with an increasing carbon chain length. Very low ΔH- and ΔS-values are compatible with a transition model obtained from nuclear resonance experiments. Further thermoanalytical results bear evidence on the complex role of alkyl-ammonium groups. 相似文献
Calcium tartrate tetrahydrate (L-CTT) is reported to have point group symmetry mmm at room temperature. Second harmonic generation experiments indicate that calcium tartrate tetrahydrate prepared from pure L-tartaric acid has the noncentrosymmetric point group mm2. All the nonlinear optical d-coefficients are determined. Linear optical properties of this material such as: refractive indices and absorption constant are measured in the range 200 nm < λ < 2000 nm and the phase matching conditions for optical frequency doubling are derived. Non critical type I phase matching can be achieved in L-CTT at room temperature for a fundamental wavelength of λ = 816 nm. 相似文献
The infrared and Raman spectra of polycrystalline PbHPO4, PbHAsO4, CaHPO4, and BaHPO4 were measured at room temperature in the range between 4000 and 30 cm−1, and the temperature dependence of the dielectric constant of these materials was studied. The O H O hydrogen bonds in PbHPO4 and PbHAsO4 are found to be very similar to those in KH2PO4 and KH2AsO4 and the XO4 groups (X = P, As) are – in contrast to CaHPO4 and BaHPO4 – as well close to being tetrahedral. The symmetry of the soft mode in PbHPO4 is deduced and it is found that the ferroelectric transition could be of second order as far as group theory is concerned. A microscopic model of the ferroelectric orderdisorder phase transition based on the interaction between the chains of O H O bonds linking the PO4 groups is proposed. 相似文献
(CH3NH3)2CdCl4 is found to be isotructural at room temperature with (CH3NH3)2MnCl4 having the orthorhombic space group Cmca. A high temperature phase transition to a tetragonal phase could not be observed because of a decomposition slightly above 383 K. Two low temperature phase transitions are detected by means of DTA at about 283 and 173 K. The first one leads to the space group P42/ncm. A microscopic model for this phase transition is proposed. 相似文献
An enzymatic tandem reaction is described in which the enzymes phosphorylase and Deinococcus geothermalis glycogen branching enzyme (Dg GBE) catalyze the synthesis of branched polyglucans from glucose‐1‐phosphate (G‐1‐P). Phosphorylase consumes G‐1‐P and polymerizes linear amylose while Dg GBE introduces branching points on the α‐(1 → 6) positions by reshuffling short oligosaccharides. The resulting branched polyglucans have an unusually high degree of branching of 11%.
