SYSTEM FOR ELECTRON-PARAMAGNETIC-RESONANCE MEASUREMENTS AT HIGH-PRESSURE AND LOW-TEMPERATURE

Major improvements have been made on the sensitive high pressure elect ron paramagnetic resonance (EPR) system developed previously in 1978 a t this laboratory. These changes allow low temperature measurements an d provide a more stable signal. The high pressure EPR cell is placed i nside a vacuum chamber to provide thermal isolation, so that the sq st em may be cooled by a closed cycle refrigerator to temperatures betwee n 45 and 60 K, depending on the energy input to the modulation coil. T he combination of high pressure and low temperature greatly expands th e thermodynamic range over which EPR measurements can be made. An impr oved and effective method of forming a conductive surface to define th e microwave cavity is presented. This method successfully avoids the d eterioration of the sapphire's polished surface which causes premature breaking of the sapphire high pressure anvil, and therefore significa ntly improves the reliability of the system at high pressure. Other mo difications to the system, such as the microwave coupling method, the modulation coil, and selecting of a hydrostatic pressure fluid, are di scussed. EPR measurements on BaTiO3 have been made at temperatures ran ging from 233 to 353 K and pressures from 0 to 4.4 GPa. High quality s ignals can be realized in the entire pressure and temperature range. ( C) 1997 American Institute of Physics.