ABSTRACT
In response to the ongoing quest for new, highly sensitive upconverting luminescent thermometers, this article introduces, for the first time, upconverting luminescent thermometers based on thermally induced structured phase transitions. As demonstrated, the transition from the low-temperature monoclinic to the high-temperature tetragonal structures of LiYO2:Yb3+,Er3+ induces multifaceted modification in the spectroscopic properties of the examined material, influencing the spectral positions of luminescence bands, energy gap values between thermally coupled energy levels, and the red-to-green emission intensities ratio. Moreover, as illustrated, both the color of the emitted light and the phase transition temperature (from 265 K, for LiYO2:Er3+, 1%Yb3+, to 180 K, for 10%Yb3+), and consequently, the thermometric parameters of the luminescent thermometer can be modulated by the concentration of Yb3+ sensitizer ions. Establishing a correlation between the phase transition temperature and the mismatch of ion radii between the host material and dopant ions allows for smooth adjustment of the thermometric performance of such a thermometer following specific application requirements. Three different thermometric approaches were investigated using thermally coupled levels (SR = 1.8%/K at 180 K for 1%Yb3+), green to red emission intensities ratio (SR = 1.5%/K at 305 K for 2%Yb3+), and single band ratiometric approach (SR = 2.5%/K at 240 K for 10%Yb3+). The thermally induced structural phase transition in LiYO2:Er3+,Yb3+ has enabled the development of multiple upconverting luminescent thermometers. This innovative approach opens avenues for advancing the field of luminescence thermometry, offering enhanced relative thermal sensitivity and adaptability for various applications.
ABSTRACT
Water-presence dependent switchable ferroelectricity was discovered in the hybrid organic-inorganic zinc oxalate 1D coordination polymer (DABCOH2)[Zn(C2O4)2]·3H2O (DZnOH, where DABCOH2: diprotonated 1.4-diazoniabicyclo[2.2.2]octane). The compound undergoes a reversible para-ferroelectric phase transition at 207 K from room temperature centrosymmetric phase I (space group P21/n) to low-temperature non-centrosymmetric phase II (space group P21). The microscopic mechanism of the phase transition is directly associated with the reconstruction of the hydrogen-bond network. On heating, the crystals exhibit a reversible single-crystal to single-crystal transformation concerned with the removal of all water molecules giving anhydrous DABCO zinc oxalate (DABCOH2)[Zn(C2O4)2] (DZnO). The dehydrated compound does not show ferroelectric properties.
ABSTRACT
The crystal structure has been determined for (CH3NH3)2[KCo(CN)6] at temperatures 100 and 443 K. It crystallizes in the monoclinic space group C2/c (LT) and in the cubic one Fm3[combining macron]m (HT). The dielectric response has been investigated for single crystals of pure K3[A'(CN)6] and guest-hosts of (CH3NH3)2[KA'(CN)6], where A' is a trivalent metal, Co or Fe. Their dielectric properties were measured in the frequency range between 135 Hz and 2 MHz in a wide temperature range between 200 and 460 K. The anisotropy of the electric permittivity for single crystals of (CH3NH3)2[KCo(CN)6] was analysed in terms of the crystal structure. The relaxation processes were observed in the case of the pure host K3Fe(CN)6 and the guest/host crystal (CH3NH3)2[KFe(CN)6]. The activation energies of the dielectric relaxation have been estimated to be equal to 44 and 40 kJ mol-1 for (CH3NH3)2[KFe(CN)6] and K3Fe(CN)6, respectively. The mechanism of the phase transitions found at 425 K and 421 K for (CH3NH3)2[KFe(CN)6] and (CH3NH3)2[KCo(CN)6], respectively, has been postulated. From the dielectric losses the value of the electric ac conductivity has been estimated and analyzed in terms of the activation process of the current carriers.
