HYDROGEN DESORPTION FROM CRYSTALLINE QUARTZ AND SOME RELATED DIFFERENTIAL-SCANNING CALORIMETRY AND CONDUCTIVITY PHENOMENA

Temperature-programmed desorption (TPD) and differential-scanning calo rimetry (DSC) analyses were performed on synthetic crystalline quartz, both ''as received'' and ''hydrogen swept'', from 200 to 600 degrees C. The TPD measurements allowed us to establish that interstitial hydr ogen ions, present as charge compensators near trivalent aluminium ion s substitutional for silicon, are removed from the crystal in the cons idered temperature range, mostly in the form of H-2 molecules. In 2 th e hydrogen-swept samples, release of small amounts of other molecular groups, namely SiH4 and NaOH, was also observed. The DSC studies revea led the existence of a weak exothermic peak (enthalpy change similar o r equal to 0.90 J g(-1)) in the temperature range from 350 to 400 degr ees C. The exothermic process, only observable in hydrogen-swept sampl es, may tentatively be attributed to the total-energy variation involv ed in processes leading to H-2 formation and to SiO4 tetrahedra relaxa tion after H-2 release. The above results have been compared with the intensity variations, following annealing treatments, of near-infrared OH stretching vibrations and with time-transient effects observed in ionic conductivity measurements. The analysis contributes to a deeper understanding of the hydrogen-impurity dynamics in crystalline quartz. Copyright (C) 1996 Elsevier Science Ltd