“…The results prove that higher S A can be achieved in the two‐phase coexistence system contrasting to the single host due to the different thermal response behaviors of the two hosts. Compared with other Er 3+ ‐based optical thermometers presented in Table 4, the S A is comparable to those in Ba 3 Y 4 O 9 , 15 NaYF 4 , 61 Ba 2 In 2 O 5 , 62 NaGaF 4 , 63 and NaYTiO 4 64 hosts, but lower than those in Gd 2 ZnTiO 6 , 18 Bi 5 IO 7 , 54 and Na 0.5 Gd 0.5 MoO 4 60 hosts. On the other hand, the maximum S A is obtained at lower temperature with the increasing of La 3+ concentration (520 K for x = 0.2 and 0.4, 450 K for x = 0.6, and 420 K for x = 0.8).…”