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Metal-organic frameworks are of great interest to scientists from various fields. This group also includes organic–inorganic hybrids with a perovskite structure. Recently their structural, phonon, and luminescent properties have been paid much attention. However, a new way of characterization of these materials has become luminescence thermometry. Herein, we report the structure, luminescence, and temperature detection ability of formate organic–inorganic perovskite [C(NH2)3]M(HCOO)3 (Mg2+, Mn2+, Zn2+) doped with Cr3+ ions. Crystal field strength (Dq/B) and Racah parameters were determined based on diffuse reflectance spectra. It was shown that Cr3+ ions are positioned in the intermediate crystal field or close to it with a Dq/B range of 2.29–2.41. The co-existence of the spin-forbidden and spin-allowed transitions of Cr3+ ions enable the proposal of an approach for remote readout of the temperature. The relative sensitivity (Sr) can be easily modified by sample composition and Cr3+ ions concentration. The luminescent thermometer based on the 2E/4T2g transitions has the relative sensitivity Sr of 2.08%K−1 at 90 K for [C(NH2)3]Mg(HCOO)3: 1% Cr3+ and decrease to 1.20%K−1 at 100 K and 1.08%K−1 at 90 K for Mn2+ and Zn2+ analogs, respectively.

An unrelenting focus on improving stability and optoelectronic performance of lead halide three-dimensional hybrid organic-inorganic perovskites (3D HOIPs, ABX3 formula) leads to the usage of many chemical engineering techniques, including X-site alloying. Formerly we have shown the usefulness of this approach in 3D HOIPs comprising methylhydrazinium (MHy+); this time we prove it again in the layered (A2BX4) perovskites. Herein we report the synthesis, crystal structure and physicochemical characterization of MHy2PbBr4-xIx in a wide concentration range (x = 0.25, 0.3, 0.4, 1.1, 2.35, 2.5, 3.1). Reported compounds crystallise in the Ruddlesden-Popper phase with corner-sharing [PbX6]4- (X = Br, I) octahedra, forming (010) layers, separated by MHy+. An abundant temperature-driven polymorphism is observed, starting from the Br-rich compounds with a sequence of Pnmm, Pnma and P21/c phases on cooling, through the region with the P[1 with combining macron] low-temperature phase, and ending with the I-rich compounds with an incommensurately modulated Pnma(00γ)0s0 superspace group. We also report, for the first time in the MHy-based perovskites, the 2D → 3D transition to Pm3 with combining macron]m cubic symmetry and a 3D perovskite phase with a dominant presence of iodine. With the use of Raman spectroscopy, the dynamics of MHy+ cations in particular phases is thoroughly analysed, revealing the positional disorder in room-temperature and high-temperature zones and its freezing at low temperatures. Linear optical studies demonstrate the decrease of the energy band gap (2.91 eV to 2.31 eV) and a strong change in photoluminescence colour (bluish-green to yellow-green) along with increasing iodine contribution. [ABSTRACT FROM AUTHOR]

Recently, hybrid double-perovskite structures have attracted attention due to their versatile multifunctional properties originating from the variety of different constituent units in these materials. Here, we report the synthesis and comprehensive multitechnique characterization of a novel hybrid double-perovskite formate-based material [CH3CH2NH3]2NaGa(HCOO)6. The heat capacity measurements indicate that this compound has a structural phase transition at 379 K. In the low-temperature phase, the compound crystallizes in the non-centrosymmetric polar Pn structure, which exhibits a long-range order of ethylammonium (EA+) cations. Surprisingly, the Pn space group is not affected by the transition to the high-temperature disordered phase, which indicates that the transitions are isosymmetric making this compound a unique member of the formate-based double-perovskite family. The presence of the second-harmonic generation response in both phases confirms their non-centrosymmetric nature, while the dielectric spectroscopy experiments reveal that the transitions have a continuous order–disorder character. The observation of the pyroelectric current, but the absence of the electric polarization switching indicate that this compound is not properly ferroelectric. The electron paramagnetic resonance experiments of a compound slightly doped with paramagnetic Fe3+ impurities confirm the continuous character of the transition and allow us to probe the EA+ cation and framework dynamics in this system. The vibrational spectroscopy data confirm that the phase transition is primarily driven by the progressive ordering of EA+ cations and the resulting changes in the hydrogen bond strength. The temperature-dependent luminescence studies show that the [EA]2NaCr0.931Ga0.069(HCOO)6 perovskite is a promising material for noncontact temperature monitoring. [ABSTRACT FROM AUTHOR]

Metal-organic frameworks are of great interest to scientists from various fields. This group also includes organic–inorganic hybrids with a perovskite structure. Recently their structural, phonon, and luminescent properties have been paid much attention. However, a new way of characterization of these materials has become luminescence thermometry. Herein, we report the structure, luminescence, and temperature detection ability of formate organic–inorganic perovskite [C(NH2)3]M(HCOO)3 (Mg2+, Mn2+, Zn2+) doped with Cr3+ ions. Crystal field strength (Dq/B) and Racah parameters were determined based on diffuse reflectance spectra. It was shown that Cr3+ ions are positioned in the intermediate crystal field or close to it with a Dq/B range of 2.29–2.41. The co-existence of the spin-forbidden and spin-allowed transitions of Cr3+ ions enable the proposal of an approach for remote readout of the temperature. The relative sensitivity (Sr) can be easily modified by sample composition and Cr3+ ions concentration. The luminescent thermometer based on the 2E/4T2g transitions has the relative sensitivity Sr of 2.08%K−1 at 90 K for [C(NH2)3]Mg(HCOO)3: 1% Cr3+ and decrease to 1.20%K−1 at 100 K and 1.08%K−1 at 90 K for Mn2+ and Zn2+ analogs, respectively. [ABSTRACT FROM AUTHOR]

Metal-organic frameworks with perovskite structures have recently attracted increasing attention due to their structural, optical, and phonon properties. Herein, we report the structural and luminescence studies of a series of six heterometallic perovskite-type metal-organic frameworks with the general formula [EA] 2 NaCr x Al 1- x (HCOO) 6 , where x = 1, 0.78, 0.57, 0.30, 0.21, and 0. The diffuse reflectance spectral analysis provided valuable information, particularly on crystal field strength ( D q/ B ) and energy band gap ( E g ). We showed that the D q/ B ions. The results showed a strong correlation between the surrounding temperature, composition, and spectroscopic properties, allowing one to design a temperature sensing model. The temperature-dependent luminescence of the Cr 3+ ions. The results showed a strong correlation between the surrounding temperature, composition, and spectroscopic properties, allowing one to design a temperature sensing model. The temperature-dependent luminescence of the Cr 3+ ions makes the investigated materials promising candidates for noncontact thermometers.