High Temperature

Phase transitions occur upon variation of the temperature of a sample. According to Buerger they are classified into

• Reconstructive phase transitions where the pattern of chemical bonds change.
• Displacive phase transitions where atoms shift only slightly in their positions.
• Order-disorder phase transitions where the occupancy of a position becomes disordered.

Observing what lies beyond a phase transition can be important for fundamental science i.e., understanding the transition, its driving force and its impact on symmetry. Moreover, it can be important for industrial applications, for example for probing the thermal stability of materials. Another observable phase transition upon changing the temperature is the change of a physical property, classically known from the block of iron losing its magnetism when heated. Usually, the high or low temperature phases can be characterized after quenching them to room temperature. Some phases however, are evasive and cannot be characterized by such methods, but can be accessed by diffraction studies directly at the appropriate temperature.

STOE provides two options for conducting powder diffraction at elevated temperatures. The HT1 and HT2 are capillary ovens specifically designed to achieve temperatures exceeding 1000 K, which are compatible with all STOE powder diffraction instruments. Notably, the HT2 model offers the added capability of introducing reactive gases to monitor in-situ reactions. This comprehensive solution empowers researchers to explore and analyze materials under controlled temperature conditions with exceptional precision and accuracy.

To measure single crystals at high temperatures, they are best encapsulated between two capillaries to prevent oxidation. These capillaries can then as usually be mounted on a pin, which is subsequently mounted on the goniometer head. For heating, STOE offers a dedicated setup, the HEATSTREAM, which can be easily mounted in STOE’s well known “plug and play” manner. It sits on the bottom end of the Eulerian cradle and can be kept on during all measurements or removed when not in use. Temperatures are measured with a thermocouple sitting inside the heating head, which can be calibrated against a second thermocouple mounted directly in the beam center. To prevent the accumulation of fumes, the HEATSTREAM comes with an exhaust tube including a fan, which can be mounted through the diffractometer ceiling. Lastly, a detector shield is delivered with the setup, to protect the detector window from the hot nitrogen gas.
The HEATSTREAM offers a temperature stability of ±1 K and can be used in combination with the STADIVARI and the IPDS II /2T diffractometers, giving users outstanding temperature and movement precision in one instrument.

Low Temperature

Transitions as described above can also occur upon cooling samples for example when compounds become (super)conducting or magnetic.

The most common use of low temperature measurements is however to freeze in effects, which reduce the quality of room temperature measurements. A classical example are rotating groups like tert butyl or trifluoro methyl rests, but there is a plethora of disorder effects that can be ameliorated by cooling.

STOE commonly implements the nitrogen and helium cryo systems from Oxford Cryosystems, e.g. Cryostream 1000, Cobra, N helix, Chimera and PheniX, where the latter two are exclusively for powder diffraction in reflection geometry while the Cryostream 1000 and the Cobra can be used for powder and single crystal measurements. All of the above mentioned cooling systems are seamlessly integrated in STOE’s powder and single crystal instruments, providing excellent temperature control in combination with precision perfected diffractometers.