Variable Temperature X-ray Powder Diffractometry Research Articles

Applications of pressure differential scanning calorimetry in the study of pharmaceutical hydrates: I. Carbamazepine dihydrate

The dehydration of carbamazepine dihydrate (C 15H 12N 2O · 2H 2O) was studied by both conventional differential scanning calorimetry (DSC) and by pressure differential scanning calorimetry. Variable temperature powder X-ray diffractometry (VTXRD) and thermogravimetric analysis were used as complementary techniques. By performing DSC at elevated pressures, the dehydration and vaporization endotherms were separated and it was possible to determine the enthalpy of dehydration. Over the range of 100–600 psi, the enthalpy of dehydration was unaffected by pressure. However, the solid-state of the anhydrous phase formed was influenced by the DSC conditions. At ambient pressure, dehydration resulted in the formation of the γ-form of anhydrous carbamazepine while at elevated pressures, the anhydrous β-form appeared which converted to the γ-form at higher temperatures. At high pressures, the water liberated on dehydration is not immediately removed and its presence appears to be responsible for the formation of β-carbamazepine. VTXRD permitted in situ study of dehydration and this confirmed the DSC results. A specially fabricated sample holder permitted VTXRD studies at elevated pressures.

Structural aspects of metal-oxide-pillared sheet silicates. An investigation by magic-angle-spinning nuclear magnetic resonance, fourier-transform infrared spectroscopy and powder X-ray diffractometry

Variable-temperature powder X-ray diffractometry, Fourier-transform infrared spectroscopy and 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectroscopy have been used to elucidate both the nature of alumina pillars introduced into sheet aluminosilicate and to monitor changes in structure of the host matrix following the introduction of the pillars. The pillars are neutral oxide columns which are structurally similar to γ-alumina; they are linked through oxygen to aluminium and magnesium atoms within the octahedral layer of the clay sheet. The layer charge of the clay is balanced by protons, released during calcination, residing in the aluminosilicate structure.

The appearance of a rhombohedral distortion in zeolite A

Variable temperature X-ray powder diffractometry shows that the recently identified rhombohedral distortion is sodium-exchanged zeolite A (Na-A) occurs when the material is dehydrated and maintained at temperatures lower than ∼50°C. Interconversion between the cubic and rhombohedral forms is sluggish and displays hysteresis. The degree of distortion is also dependent upon the sol Si Al ratio and the nature of the exchangeable cation.