Structural, elastic, and optical properties of holmium–ytterbium-doped borotellurite glass for possible applications in optical fiber sensing

A series of mixed glass formers (MGF) with varying compositions denoted as (79- x )B 2 O 3 - x TeO 2 -20Li 2 O-0.5Ho 2 O 3 -0.5Yb 2 O 3 ( x = 0–50 mol%) were prepared using the melt-quenching technique. This aim of this study was to examine the effect of combining glass formers TeO 2 and B 2 O 3 on the structural, DC conductivity, elastic, and optical properties of the glass system. XRD measurements confirmed the amorphous nature of the glass samples. Structural analysis revealed a rivalry between the TeO 2 and B 2 O 3 formers. It was observed that the bridging oxygen (BO) indicated by the BO 4 functional group decreases gradually at x ≥ 40 mol%, implying that non-bridging oxygen (NBO) units, denoted by BO 3 , become dominant beyond 40 mol%. The variation in DC conductivity showed a non-linear behaviour upon addition of TeO 2 , with the conductivity increasing to a maximum value at x = 30 mol% before decreasing at x > 30 mol%. Interestingly, the conductivity showed a slight decline at x = 40 mol%, possibly resisting a decrease due to the mixed glass former effect (MGFE). Elastic moduli such as C L , K e and Y exhibited a non-linear decrease between 10 ≤ x ≤ 30 mol% and reached a lowest value for x = 40 mol%, which coincided with the maximum of DC conductivity attributed to MGFE. Quantitative analysis of ultrasonic data, utilizing bulk compression and ring deformation models, revealed that the K bc / K e value is maximum at x = 40 mol%, implying a reduction in ring deformation within the MGFE region. UV–Vis spectroscopy demonstrated that with increasing TeO 2 content, E opt and E opt’ decreased except for x = 40 mol%. The refractive index, n also increased except at x = 40 mol%. The alternating dominance of BO and NBO in the MGFE region led to this conclusion.