We plot the extracted resonance wavelengths with T in Figure . The separation between the consecutive orders of resonance, known as the free spectral range (FSR), is approximately 5 nm which corresponds to a group index of the optical mode 𝑛 𝑔 = 2.56. We do not observe any significant change in 𝑛 𝑔 and FSR with temperature (T). The blue-shift of the resonances is sharp (large magnitude of dλ0 / dT) at T ≤ 320 K and T ≥ 360 K with a region of negligible resonance shift at T ≈ 330 K. This temperature coincides with the phase transition temperature of T = 332 K observed in the DSC and magnetic measurements as reported earlier (Figure ). Since the refractive index is the square root of the dielectric constant, we believe the zero-shift region in Figure is due to the same phase transition as shown in Figure . The corresponding variation of dλ0/dT with T is shown in Figure . The values of dλ0/dT at T ≤ 320 K and T ≥ 360 K are observed to be ≈ -200 pm/K, which is an order of magnitude larger than the thermo-optic shifts observed in conventional integrated photonic systems in silicon and silicon nitride platforms (with positive thermo-optic effect) as well as in polymer platforms (with negative thermo-optic effect). Based on eq. ( ), we understand that d𝑛 𝑒𝑓𝑓 /dT has large negative value at T ≤ 320 K and T ≥ 360 K and tends to zero around T ≈ 330 K, which has shown in Figure . The extracted values of d𝑛 𝑒𝑓𝑓 /dT ≈ -0.4 RIU/K at room temperature are an order of magnitude larger than the reported d𝑛 𝑒𝑓𝑓 /dT ≈ 0.001-0.01 RIU/K in conventional integrated photonic platforms. Since the 𝑛 𝑒𝑓𝑓 is dominated by the material 1 in the cladding (evident from the large blue shift opposed to the weaker red shift in bare silicon systems ), we can conclude that d𝑛/dT ≈ d𝑛 𝑒𝑓𝑓 /dT ≈ -0.4 RIU/K at room temperature and d𝑛/dT ≈ d𝑛 𝑒𝑓𝑓 /dT ≈ 0 RIU/K at T ≈ 330 K.