The amino acids (AA) have been considered as building blocks of proteins rather so far than the hybrid organic-inorganic materials (HOIM), like polymers or perovskites (HOIP). Yet, here we present the structure-property relation results for four HOIMs based on a simple AA-sarcosine (N-methylglycine) and metal halides MX2, where M=Sr, Ba and X=Cl, Br. The studies reveal a presence of reversible phase transitions above room-temperature in each crystal associated with crystal transformation into different symmetries, caused by a rearrangement of the metal coordination spheres. These materials constitute a cheap, bio-based alternative for organic-inorganic perovskites.

Recently, a wide variety of organic eutectic mixtures, with their intriguing physicochemical properties, found wide application in medicine, pharmaceutical and pharmacological fields for the synthesis of advanced medicine formulations and development of improved drug delivery systems. However, the mechanisms of phase transitions and underlying molecular interactions in eutectic mixtures are poorly studied so far due to their extremely complicated nature. To fill the gap, here we report for the first time the investigation of molecular dynamics in solid fatty-acid binary mixtures with near-eutectic composition. Three binary mixtures of octadecanoic and docosanoic acids, of different mixing proportions, were investigated over temperature range 20-100 degrees C using temperature-variable FTIR spectroscopy correlated with differential scanning calorimetry (DSC). Previous DSC values were re-examined and it was revealed that mixtures with small amount of longer acid show a eutectic behaviour with a single solid-to-isotropic transition at temperature similar to 62 degrees C that is much lower than the melting point of parent acids. The main transition preceded with a weak endotherm 5-10 degrees C below the melting point corresponding to a "pre-melting" process. The analysis of temperature-variable FTIR spectra showed that small changes in composition of the mixtures are responsible for their different phase behaviour, such as the temperatures of the beginning of hydrocarbon chains trans-gauche disorder, as well as the sequential activation of shorter and longer methylene chains rotational motions. Due to the importance of fatty acids, the obtained results would be useful for advanced knowledge in engineering of novel organic eutectic mixtures with application potential in different fields of industries.

Layered hybrid organic-inorganic lead halides have gained a lot of attention for optoelectronic applications. A notable subset within this category is perovskites comprising halogenated amines since they may exhibit reduced band gap or polar order. We synthesized three compounds comprising 2-chloroethylammonium (CEA+) cations, with the chemical formula CEA2PbX4 (X = Cl, Br, I). X-ray diffraction studies show that at room temperature (RT), CEA2PbBr4 and CEA2PbI4 crystallize in Pbnm symmetry, with ordered CEA+ cations. CEA2PbBr4 and CEA2PbI4 undergo one structural phase transition (PT) into a disordered Pmnm phase near 315 and 360 K, respectively. CEA2PbCl4 shows a different packing of CEA+ with the organic chains oriented perpendicularly to the perovskite layers. It undergoes two PTs at 332 and 203 K from the high-temperature (HT) disordered I4/mmm phase to the partially ordered intermediate Pbnm phase and completely ordered low-temperature (LT) phase of the unknown space group. All compounds emit photoluminescence (PL): orange, yellow-green, and yellow for CEA2PbI4, CEA2PbCl4, and CEA2PbBr4, respectively, and bromide exhibits a very high quantum efficiency of 48%. Overall, our findings show that halide engineering strongly modulates hydrogen and halogen bonding strength, affecting the structural arrangement of building units, molecular dynamics, and thus optoelectronic properties.

For the first time, the room temperature polarized IR reflection and Raman spectra of sarcosine phosphate (sar)& sdot; H3PO4, in the form of an oriented single crystal have been measured. The polarized spectra are discussed with respect to the crystal structure (X-ray diffraction) data and the literature data on the normal coordinate analysis of the zwitterionic and cationic forms of the sarcosine molecule. Based on the analysis of the spectra and transition dipole moment calculations for each internal mode with respect to the experimental X(a), Y(b) and Z(c) axes, we have been able to determine the directionality and strength of the complicated hydrogen bonds network in the crystal. The A-B-C band structure of the OH stretching vibrations in the strongest hydrogen bonds (HBs) has been assigned. Independent interactions of each HB with light are confirmed based on their polarization properties. The differences within the band of carboxyl group components are ascribed to a rather strong Davydov-type interaction between the carboxyl groups in the title crystal, they may also be caused by participation of the carboxyl-oxygen atom in the medium-strong HB with phosphate-oxygen. These studies are interesting from the point of view of designing strategy for the molecular engineering of new compounds with strong hydrogen bonds, new phase-change materials, as well as crystals with improved optical, magnetic and nonlinear optical properties.

The remarkable sensitivity of the luminescent properties of Eu3+ ions to structural changes in host materials has been well-explored for years. However, the application of this feature of Eu3+ in materials exhibiting thermally induced structural phase transitions for the development of luminescent thermometers has only recently been proposed. The narrow operating range of such thermometers necessitates the exploration of new host materials. In response to this demand, this study carefully analyzes the spectroscopic properties of X as a function of temperature and dopant ion concentration. As demonstrated, X undergoes a phase transition from a low-temperature monoclinic phase to a high-temperature trigonal structure, resulting in significant changes in both the emission spectrum shape of Eu ions and the depopulation kinetics of the 5D0 level. Consequently, X can be utilized as both a ratiometric and a lifetime-based luminescence thermometer, achieving maximal relative sensitivities of 3.4 and 1.0 or the respective approaches. Additionally, this work highlights how increasing the concentration of Eu3+ ions enables the tuning of the thermal operating range to achieve optimal thermometric performance. Moreover, an implementation of ratiometric approach of temperature sensing and imaging with X using digital camera without filters was demonstrated. This is the first report that demonstrates thermal imaging using Eu3+-solely doped phosphor. This finding underscores the potential of X as a versatile host material for advanced luminescent thermometry applications.

The luminescent properties of Eu3+ ions are highly sensitive to changes in their local crystal environment. While this feature has been widely studied, its application in thermometers based on thermally induced structural phase transitions is a recent development. These thermometers often suffer from a narrow thermal operating range, prompting the search for new host materials. In this context, Na3Sc2(PO4)3:Eu3+ as a function of temperature and dopant ion concentration. As demonstrated, Na3Sc2(PO4)3:Eu3+ was investigated as a potential candidate. This material undergoes a reversible phase transition from a monoclinic to a trigonal structure, leading to significant changes in both the emission spectra and the luminescence decay of Eu3+ ions. These effects enable the development of both ratiometric and lifetime-based luminescent thermometers, achieving maximum relative sensitivities of 3.4% K-1 and 1.0% K-1, respectively. Furthermore, the thermal operating range can be tuned by adjusting the Eu3+ concentration. Importantly, this study demonstrates, for the first time, temperature imaging using only Eu3+-doped phosphor via a digital camera without the use of optical filters. These results position Na3Sc2(PO4)3:Eu3+ as a promising multifunctional material for advanced applications in contactless temperature sensing and thermal imaging.

The ratiometric approach is the most commonly employed readout technique in luminescence thermometry. To address the trade-off between the risk of measurement disturbance in thermometers with high spectral separation of emission bands (due to dispersion in the surrounding medium) and the low sensitivity observed in ratiometric thermometers based on Stark level thermalization, we propose a thermometer based on the structural phase transition in . The use of Yb3+ ions as dopants and the changes in Stark level energies associated with the thermally induced monoclinic-to-trigonal phase transition enable the development of a thermometer with high relative sensitivity, achieving at 340K for N. Additionally, as demonstrated, the structural transition alters the probability of radiative depopulation of the 2F5/2 state of Yb3+, allowing the development of a lifetime-based luminescence thermometer. Furthermore, the phase transition temperature and consequently the thermometric performance of can be modulated by varying the Yb3+ ion concentration, offering additional tunability for specific applications.

Among the various techniques used in luminescence thermometry, luminescence kinetics is considered the least sensitive to perturbations related to the optical properties of the medium containing the phosphor. For this reason, temperature sensing and imaging using lifetime-based luminescence thermometers are of high interest for a wide range of specific applications. However, for most such thermometers, an increase in temperature leads to a shortening in lifetime, which can hinder the specificity and accuracy of the readout. In this work, an approach is presented that utilizes a thermally induced increase in the symmetry of the host material associated with a structural phase transition in LiYO2:Yb3+. Consequently, the lifetime of the excited level 2F5/2 of the Yb3+ ion is thermally prolonged, achieving a relative sensitivity of 0.5%/K. The phase transition temperature can be controlled by adjusting the dopant concentration. Additionally, thermal changes in the emission spectrum enable the use of LiYO2:Yb3+ for ratiometric temperature readout with a relative sensitivity of 5.3%/K at 280K for LiYO2:5%Yb3+.

Expanding the operational range of luminescent thermometers that utilize thermally induced structural phase transitions in lanthanide-doped materials necessitates the exploration of novel host matrices with diverse thermal behaviors. In line with this objective, this study offers a comprehensive analysis of the temperature-dependent spectroscopic properties of Li3Sc2(PO4)3:Eu3+. The findings reveal that the studied material undergoes two reversible phase transitions: the gamma LT -> alpha/beta phase transition at approximately 160 K, followed by a beta -> gamma HT transition around 550 K. These transitions are evidenced by notable alterations in the emission spectra and luminescence decay kinetics of Eu3+ ions. By employing an appropriate luminescence intensity ratio, the sensitivity was determined to be 7.8% K-1 at 160 K for 0.1% Eu3+ and 0.65% K-1 at 550 K for 0.5% Eu3+. Furthermore, the study demonstrates that the phase transition temperature in Li3Sc2(PO4)3:Eu3+ can be modulated through variations in dopant ion concentration and annealing conditions, which in turn influence the material's morphology. These strategies enable the fine-tuning of thermometric performance in phase transition-based luminescent thermometers. To the best of our knowledge, this represents the first report in the literature of a luminescent thermometer exhibiting dual thermal operating ranges.

We have investigated polymorphism in p-cresol using the FT-IR spectroscopy and differential scanning calorimetry. The present results show that in addition to the well-known two crystalline phases of p-cresol, which melts at 307.6 and 309.2 K, we discovered the existence of a new crystalline phase, which melts at 302.9 K. For the first time we have received the FT-IR spectra of three polymorphs and their temperature dependencies in the region 300-12 K. Comparison between the FT-IR spectra of three polymorphs shows that they are completely different.

Phase transition-based thermometers, which are widely known for their remarkable sensitivity to temperature changes, exhibit a narrow temperature range owing to the stoichiometry of the host material. This limits the applicability of optical sensors utilizing structural phase transitions. In this study, we introduce a co-doping method for modulating the properties of phase transition-based luminescent thermometers to influence the phase transition temperature. We demonstrate that by adjusting the ionic radius of the dopant and its concentration, the transition temperature can be finely tuned across a broad temperature spectrum. The proposed technique enables the customization of luminescent thermometers with enhanced sensitivities and practical temperature ranges tailored to specific user requirements. This study represents a crucial advancement towards the development of personalized luminescence thermometers.

Thermal stability and the kinetics of thermal decomposition of the thermally reduced graphene oxide (TRGO) treated by a pulsed high-frequency discharge in a hydrogen atmosphere have been studied. The modified Hummers method was used for obtaining the initial graphite oxide from graphite powder. Thermal exfoliation of the graphene oxide powder has been done in vacuum conditions with a heating rate of 5–7 degrees per minute to a temperature of 300 °С. TRGO has been treated by pulsed high-frequency discharge in a hydrogen atmosphere for partial graphene hydrogenation (chemical addition of atomic hydrogen) that leads to structural changes in the carbon planes and formation of C–H sp3 bonds. The thermogravimetry analysis measurements of the mass loss have been carried from room temperature to 1000 °C in a nitrogen atmosphere with a nitrogen flow rate of 20 mL/min and different heating rates: 50, 75 100, 125, 150, and 200 K/min, respectively. Kissinger’s multiple heating rate method has been used to determine the activation energy for decomposing substances. Activation energies Ea1, Ea2, and Ea3 equal 28, 50, and 148 kJ/mol, respectively, have been compared with the energies of the activation of thermal defunctionalization of multiwalled carbon nanotubes (MWCNTs). The activation energy Ea3 = 148 kJ/mol is close to that of the thermal decomposition of anhydride functional groups in MWCNT. The value of Ea2 = 50 kJ/mol indicates the presence of the keto and hydroxy acid’s function groups on TRGO. Activation energy Ea1 = 28 kJ/mol related with all other groups including the lighter C–H bonds that destructed due to dehydrogenation of the TRGO. Obtained experimental results are useful for further proposing the kinetic model of the mechanism of the most probable reaction of TRGO decomposition.

We present the first experimental spectroscopy and calorimetry proof of the existence of polymorphism in m -cresol. The crystallization of the stable phase occurs on cooling of the liquid phase in the region of 240 K and metastable phase in the supercooled region at 240 K upon heating of the glassy phase. Their melting tempera-tures are 281 and 265 K, respectively. The temperature at which the nucleation starts in the supercooled liquid state is in the range 1.29-1.18 Tg. The first time we have received the FT-IR spectra of both polymorphs, which are quite different especially at low temperatures.

Two mixed surfactants comprising cationic octadecyltrimethylammonium bromide CH 3 (CH (2) ) N-17 + (CH (3) ) (3) Br - (C (18) TAB) modified with dodecanoic C H-12 (24) O (2) (DDA) or hexadecanoic C (16) H (32) O (2) (HDA) acids have been studied in the solid state in a wide temperature range (25-150?). The molecular dynamics and thermotropic properties of acid-modified asymmetric DDA-C (18) TAB and symmetric HDA-C (18) TAB surfactants were investigated using DSC measurements and temperature-variable FTIR spectroscopy complemented by room-temperature powder X-ray diffraction. XRD showed a formation of a new layered compound, the acid-modified C (18) TAB mixed surfactant comprising a hydrophobic moiety formed by two alkyl chains of different lengths. DSC measurements revealed the difference in thermal behavior between symmetric and asymmetric mixed surfactants showing successive phase transitions and several polymorphs in the solid state. With FT-IR spectroscopy, the phase transitions seen in DSC were identified as related to subtle lattice rearrangements in the low-temperature phases C (1) -C- 2 , the trans-gauche conformational disorder and rotational motions of methylene chains followed with lattice instabilities in the intermediate phases C (3) and C (4) , and finally rotational disorder of the alkyl chains and the ionic layer in the high temperature phase H . For the first time, the stabilization effect of DDA acid on the bilayer structure of C (18) TAB was found.

The 4-pentyl-4′-cyanobiphenyl (5CB) molecule conformation and structure alignment have been investigated by means of temperature variable Fourier transform infrared (FTIR) and photoluminescence (PL) spectroscopy in a wide temperature range (from −90 to 50 °C) covering the 5CB glass-like, metastable and stable crystalline, nematic and isotropic phases defined by differential scanning calorimetry measurements. The corresponding FTIR absorption and PL emission bands reflect a strong dipolar association and a formation of at least two types of 5CB dimeric structures with antiparallel or parallel orientation in the planar configuration. No formation of hydrogen bonds between the cyano groups and hydrogen atoms of the adjacent benzene ring has been found. Besides, a small amount of the 5CB monomer structure has been shown to be present in all phases. The observed temperature-induced changes in the IR and PL spectra can be used as an independent source of experimental information on conformational equilibria and structure alignment of 5CB in different states under the phase transitions.

Possible changes in the temperature dependence of the coefficient of thermal conductivity of the hydroxyapatite (HA) composite with multi-walled carbon nanotubes (MWCNTs) with small amounts of MWCNTs additives (0.5 wt%) were analyzed in comparison with the dependence of HA matrix without additives. It was found that MWCNTs can contribute to the increase of the thermal conductivity coefficient. The magnitude of the effect of increasing thermal conductivity varies from relatively small values (about 1%) to a significant increase up to ∼ 20 times that depending on the interaction at the HA–MWCNTs interface and the degree of homogeneity of the MWCNTs distribution in the HA matrix. It was found that long-term high-temperature annealing can lead to changes in the anisotropy, structure, and properties of MWCNTs due to oxidation effects, which can influence the resulting composite properties.

Currently available temperature measurements or imaging at nano-micro scale are limited to fluorescent molecules and luminescent nanocrystals, whose spectral properties respond to temperature variation. The principle of operation of these conventional temperature probes is typically related to temperature induced multiphonon quenching or temperature dependent energy transfers, therefore, above 12%/K sensitivity and high thermal resolution remain a serious challenge. Here we demonstrate a novel class of highly sensitive thermographic phosphors operating in room temperature range with milikelvin thermal resolution, whose temperature readings are reproducible, luminescence is photostable and brightness is not compromised by thermal quenching. Corroborated with phase transition structural characterization and high spatio-temporal temperature imaging, we demonstrated that optically active europium ions are highly and smoothly susceptible to monoclinic to tetragonal phase transition in LiYO2 host, which is evidenced by changed number and the splitting of Stark components as well as by smooth variation of contribution between magnetic and electric dipole transitions. Further, reducing the size of phosphor from bulk to nanocrystalline matrix, shifted the phase transition temperature from 100oC down to room temperature. These findings provide insights into the mechanism underlaying phase transition based luminescence nanothermometry and motivate future research toward new, highly sensitive, high temporal and spatial resolution nano-thermometers aiming at precise studying heat generation or diffusion in numerous biological and technology applications.

Almost all existing luminescent thermometers rely on the temperature-dependent processes such as multi-phonon relaxation and phonon-assisted energy transfers, thermal population, or coupling between energy levels of ground and excited states of luminescent species (lanthanides, transition metals, quantum dots, fluorescent molecules, etc.). Although such phenomena are in principle suitable for straightforward calibration, aiming to offer high temperature sensitivity, high temperature resolution and the widest possible temperature sensitivity range, their performance is often dependent on the excitation intensity or sample dispersive properties and often suffers from insufficient brightness, which further becomes dimmer at increased temperatures. Exploiting temperature-dependent continuous phase transitions that modify the same near-infrared (NIR) emission band under the same NIR excitation wavelength may provide an alternative reading method for temperature sensing. Here, such a new principle of luminescent nano-thermometry (LNT) using a Nd3+ doped nanocrystalline LiYO2 matrix is studied, significant sensitivities of up to 6%/K are achieved, and other issues found in conventional LNT are circumvented. Due to the hysteresis found in this class of LNT, they may find applications in studies of temperature gradients and can be integrated with modern nanophotonic devices.

The thermal analysis and infrared (IR) spectroscopy were used to study cyclohexanol irreversible transformations from an orientationally disordered plastic state to metastable III and then to a stable II crystalline phase. It was found that the orientational glass transition in cyclohexanol occurs at Tg similar to 163.5 K. Solid-solid transition from orientationally disordered phase I to metastable phase III occurs at 195 K. Crystal phase III nucleates at Ta = 203 K and converts into phase I at T = 233.7 K. The stable crystal phase II is formed by nucleation at 213 K. It converts into phase I at T = 250 K. It is shown by IR spectroscopy that the phase transitions in solid cyclohexanol are correlated with conformational changes in the cyclohexanol molecule.