No abstract

Crystals of phase II K2Cr2O7, potassium dichromate, space group P1 , grown from aqueous solution undergo a first-order transition to phase I, space group reportedly P21/n, at a phase-transition temperature, TPT, of 544 (2) K on first heating; the corresponding transition on cooling is at 502 (2) K. The endotherm on subsequent heatings occurs reproducibly at TPT = 531 (2) K. Mass loss between ca 531 and 544 K, identified as included water, is rapid and continues more slowly to higher temperatures for a total loss of ca 0.20%. The higher TPT on first heating is associated with the increasing pressure of superheated water occupying inclusion defects. The latent diagonal glide plane in phase II allows the structure of phase I to be inferred. The triclinic structure at 296 K has been independently redetermined. Normal probability analysis shows high consistency between the resulting and previous atomic coordinates, but with uncertainties reduced by a factor of ca 2. The earlier uncertainties are systematically underestimated by a comparable factor. The structure of phase IIb, space group A2/a on transposing axes, was determined at ca 300 K by Krivovichev et al. [Acta Cryst. (2000), C56, 629-630]. The first-order transition between phases I and II arises from the ca 60 degrees relative rotation of terminal O atoms in each tetrahedron as the n glide plane is gained or lost. A transition between phases IIb and I, also of first order, is likely but not between phases II and IIb. An intermediate phase may exist between phases IIb and I.

Ion migration can cause severe damage to high power semiconductor devices. One of the main sources of mobile ions is the encapsulation material such as molding compounds. Within this work a method will be described to evaluate the risk of field induced ion migration of different molding compounds depending on the temperature and field strength. This method allows the qualitative and quantitative determination of ion mobility in molding compounds, without the interference of electron conductivity.

Synthesis of the title compounds by present literature methods produces compositional mixtures that are difficult to separate. Controlled temperatures, times, and reactant concentrations produce gram quantities of pure, colorless, transparent crystals. Thermogravimetry and differential scanning calorimetry show that heating to 400 °C in a nitrogen atmosphere causes stepwise decomposition involving first the loss of water (for the hydrated compounds), followed by liberation of HF, then decomposition to ZrF4.