Low-temperature ultrasonic attenuation in rapidly cooled niobium containing oxygen and hydrogen

Measurements of ultrasonic attenuation and velocity in dilute Nb-O-H alloys were made as a function of temperature, frequency, polarization, annealing temperature, isotope, and defect concentration. In addition to the stable 2.5 K relaxation peak found earlier an additional peak at 6.3 K at 10 MHz was found when the specimen was rapidly cooled to He temperature. It is concluded that peak 2 at 6.3 K arises from an OH complex. The simplest interpretation is that it represents an OH2 complex. In a slow cooling process, one oxygen traps only one hydrogen and this OH pair is rather stable. In a rapid cooling process, some of the oxygen traps more than one hydrogen, forming OH2. However, the second hydrogen is less strongly bound than the first. During annealing, detrapping of one of the two hydrogens occurs and an OH2 becomes an OH with freed hydrogen going into precipitates.