There have been reports on a correlation between structural rigidity and connectivity, that allows easy assessment of phosphor host potential based on a combination of large Debye temperature and a wide band gap, or nearly ideal bond valence sum (BVS). Based on these criteria, we can estimate the efficiency of Ba5Y13[SiO4]8O8.5 as a potential phosphor host. Ba5Y13[SiO4]8O8.5 is characterized by a band gap of 5.8(4) eV (see Figure ). Since the heat capacity was modelled using two Debye terms, the weighted average of both (θD 696 K) will be used for the assessment. According to the correlation plot given by Brgoch et al., these values correspond to the estimated quantum yield of 80-90 % (compared to experimentally measured materials with Ce 3+ as the dopant). At the same time, one should be cautious as these are only the estimates and real phosphor efficiency can be proved only after successful experiments. For example, recent studies on Ba2-xSrxSiO4:Eu 2+ showed no clear correlation between the structural rigidity (accessed via θD values) and the thermal stability of the luminescence. The bond-valence calculations for the atoms in the structure of Ba5Y13[SiO4]8O8.5 yielded values close to the ideal numbers (see Table ). Depending on the size, the rare-earth ions can be doped on both barium and yttrium sites. For these atoms, the BVS deviate slightly from the ideal values (Figure ). In the case of yttrium, the Y3 site yields the largest deviation of ~+3.24 (compared to ideal valence of +3). Similarly, Ba2 is underbonded with BVS of ~+1.839 (should be +2). At the same time, Y1 and Ba1 atoms show almost ideal bond valence sums of ~+3.05 and ~+2.00 satisfying a condition for an effective phosphor host. It should be noted that the BVS values can be further tuned by substituting barium and yttrium sites by related atoms (e.g., Sr for Ba or Ho for Y).