A pellet of Ba5Y13[SiO4]8O8.5 (Phase A) was processed to >88% relative density (with respect to the theoretical density) via spark plasma sintering at 1843 K and 35 MPa uniaxial pressure (see Figure ) which enabled measurement of thermal transport properties. The thermal conductivity of Ba5Y13[SiO4]8O8.5 was measured from 298 to 1073 K and has a low value of 1.04(5) W m -1 K -1 at room temperature (Figure ). Temperature independence over this wide temperature range is observed, where κlatt at 1073 K is 1.03(5) W m -1 K -1 . This temperature independence is characteristic of glass-like phonon transport behaviour, in which a solid behaves like a disordered material, despite Ba5Y13[SiO4]8O8.5 being a crystalline material. Interestingly, these measured values are comparable directly to SiO2-Al2O3-Y2O3 glasses (≈0.8-1.0 W m -1 K -1 at room temperature). The thermal conductivity of Ba5Y13[SiO4]8O8.5 is lower than that of other silicates such as Y2SiO5 (≈1.6 W m -1 K -1 ), β-Y2Si2O7 (≈5.2 and 1.84 W m -1 K -1 at 300 K respectively 1473 K) and γ-Y2Si2O7 (≈5.1 and 1.9 W m -1 K -1 at 300 K respectively 1273 K) ceramics. The phonon-glass behaviour of Ba5Y13[SiO4]8O8.5 is further supported by the observation of excess heat capacity at low temperatures (Figure ), which often arises from localised vibrations. The measured heat capacity of Ba5Y13[SiO4]8O8.5 is modelled with a combination of two Debye temperatures, three Einstein temperatures, and a linear contribution (Table ). The two Debye temperatures (θD) of 910 (10) and 300 (10) K are necessary to fit the data in the high temperature region, while the three Einstein temperatures (θE) of 90(5), 21(5), and 10.5(2) K are necessary to model the excess heat capacity observed at low temperatures. Models with fewer terms are not able to accurately reproduce the data (Figure ). The low κlatt, the glass-like temperature dependence, and the extracted Debye and Einstein temperatures can be related to the crystal structure of Ba5Y13[SiO4]8O8.5 through consideration of the structural motifs and bonding types present. The lower Debye temperature θD2 of 300(10) K is comparable to that reported for Ba2SiO4 (332 K), and is associated with the A and B motifs within the structure of Ba5Y13[SiO4]8O8.5 (Figure , a) which consist of high atomic mass Ba with ionic, low-forceconstant Ba-O bonds, and isolated SiO4 tetrahedra respectively. The higher frequency contributions of θD2 (910(10) K) likely result from the condensed blocks of edge-sharing Y-O environments that resemble high-pressure high-temperature forms of Y2O3. This combination of multiple structural motifs with distinct bonding is likely responsible for the complex heat capacity observed at low temperature, for which the three Einstein temperatures are necessary. These represent nonpropagating phonon modes, associated with localised oscillators of singular vibrational frequencies. The two low frequency terms of 21(5) and 10.5(2) K can be associated with the higher mass Ba species which occupy large coordination 8-and 9-fold coordination environments, and particularly the disordered distribution of Ba1 sites. The high frequency Einstein temperature of θE1 = 90(5) K could arise from the lightweight isolated SiO4 tetrahedra which may act as localised oscillators. In particular, it is noted that the three oxygen sites coordinated to Si1 sites in the A motifs (O2, O5, and O6) exhibit the highest isotropic displacement parameters of all oxygen sites (Table ). The combination of multiple structural motifs within Ba5Y13[SiO4]8O8.5 with diverse atomic environments that distinct types of bonding culminate in increased phonon scattering to yield the low glass-like observed thermal conductivity. ). (c) Deviation of the BVS for the atoms in the structure of Ba5Y13[SiO4]8O8.5 from the ideal values. For the list of all values, see Table . (d) Arrhenius plot of ionic conductivity (σ) of Ba5Y13[SiO4]8O8.5 measured in air (black squares), argon (blue triangles), and oxygen (red circles) yielding and activation energy (Ea) of ~1.74-1.91 eV (see Table ).