Mn-based Perovskites Research Articles

Research progress of the preparation of perovskite oxide catalysts and their catalytic performances for soot combustion

Soot particles derived from the exhaust of diesel engines is one of the main sources of urban haze, which seriously pollute the environment and endanger people’s health. It is one of the most effective ways to eliminate soot particles by catalytic purification technology. In this way, the design and development of the catalysts with excellent performances is the most critical factor to determine whether this technology can be widely applied. Recently, perovskite oxide catalysts have been become a research hotspot in the catalytic combustion of soot particles due to the advantages of high activity, low costs and high stability. This review focuses on summarizing the preparation methods of perovskite oxide catalysts commonly used in recent years, such as sol-gel method, template method, coprecipitation method, hydrothermal synthesis method, and solid-state synthesis method. Meanwhile, perovskite catalysts with high activities for the soot catalytic combustion, such as Co-based perovskite, Mn-based perovskite, Fe-based perovskite, Ti-based perovskite, are also elaborated in detail. Moreover, the future research and development directions of perovskite oxide catalysts for the diesel soot catalytic combustion are prospected.

Electrochemical Reduction of Oxygen and Nitric Oxide on Mn-Based Perovskites with Different A-Site Cations

Four LnMnO3+δ (Ln = La, Pr, Sm, and Gd) perovskites were synthesized and characterized by powder XRD. It was shown that the perovskite lattice became more and more distorted when lowering the size of the A-site cation. The manganite-based perovskites were evaluated for the ability to electrochemically reduce oxygen and nitric oxide in the temperature range of 200 to 400°C. At the lowest temperature, the electrodes were better at reducing nitric oxide than oxygen. At higher temperatures, the activity for the reduction of oxygen and nitric oxide became similar. The activation energies for the reduction of oxygen and nitric oxide were markedly different for LaMnO3+δ and PrMnO3+δ whereas it was similar for SmMnO3+δ and GdMnO3+δ.

A-site deficient/excessive effects of LaMnO3 perovskite as bifunctional oxygen catalyst for zinc-air batteries

Recently, many works have demonstrated that tuning the A-site deficient and excessive stoichiometry can yield the positive effects on the catalytic activity of the ABO3 perovskite toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Whereas, the universality of the deficient or excessive effects and their resulting improving mechanisms on ABO3 perovskite are still ambiguous and need to be clarified. In this work, the simplest Mn-based perovskite (LaMnO3) is selected to elucidate the deficient/excessive effects and improving mechanisms on both ORR and OER. We find that A-site deficient stoichiometry is favor to the catalytic activity and stability of LaMnO3 toward both ORR and OER, whereas A-site excessive stoichiometry is deleterious to the oxygen catalytic activity and stability of LaMnO3. The high oxygen catalytic activity of La0·9MnO3 (La90) with A-site deficiency toward ORR and OER can be related to its proper Mn cation valence, large amount of oxygen vacancies, upper shift of d-band center and strong adsorption capacity to oxygenated species. The results of this work highlight the A-site deficient Mn-based perovskite as the high efficient and commercially viable bifunctional catalyst for aqueous and solid-state flexible zinc-air battery (ZAB) applications.

A new family of Mn-based perovskite (La1-xYxMnO3) with improved oxygen electrocatalytic activity for metal-air batteries

The sluggish reaction kinetics occurring at the cathodes limits the performances of the metal-air batteries. Therefore, developing the oxygen electrocatalysts which can accelerate oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is a critical issue. Mn-based perovskite has drawn extensive interests though their ORR and OER catalytic activity still needs to be improved. In this work, a new family of Mn-based perovskite (La1-xYxMnO3, LYM) is developed which demonstrates an improved ORR and OER catalytic activity compared with the representative strontium-doped Mn-based perovskite (LSM). For La0.9Y0.1MnO3 (LYM-10), the onset potential and half-wave potential during ORR can reach 0.909 V and 0.750 V (vs. RHE), respectively, which are more positive than those of LSM reported in most of the recent reports. Furthermore, LYM-10 also shows a better OER catalytic activity than La0.7Sr0.3MnO3 (LSM-30). In addition to its good bifunctional property, LYM-10 also achieves the superior durability compared with Pt/C during ORR, and the current retention of LYM-10 is as high as 97.3% after 43000 s. Using LYM-10 as the cathode catalyst, the alunimum-air battery can reach the maximum power density of 266 mW/cm2, and zinc-air battery can obtain low charge-discharge overpotential and the good cycling stability.

Origin of OER catalytic activity difference of oxygen-deficient perovskites A2Mn2O5 (A = Ca, Sr): A theoretical study.

Mn-based oxygen-deficient perovskite catalysts A2Mn2O5 (A = Ca, Sr) have been experimentally proved high oxygen evolution reaction (OER) activities for replacing Pt in oxygen electrocatalysis. Nevertheless, the correlation between the fundamental electronic structure at room temperature and the corresponding electrocatalysis is not fully accessible. In this paper, we combine the ground state density functional theory (DFT) method and dynamic mean-field theory (DFT+DMFT) at room temperature to investigate the origin of the OER difference for electrocatalysts A2Mn2O5 (A = Ca, Sr). We find that at room temperature the highest occupied Mn dz2 orbital in the square pyramidal crystal field of oxygen-deficient perovskites A2Mn2O5 with insulating properties can provide a moderate bonding strength with intermediate hydroxyl OH*, leading to a high OER catalytic activity. According to the electronic structure analysis, we observe that replacing the A-site element Ca by Sr with the larger ionic radii would result in a higher OER activity due to the weakened hybridization between the Mn dz2 orbital and the O pσ orbital of OH*. This insight could provide hints for the screening metal oxide electrocatalysts in the applications of the energy storage and conversion.

(La1−xSrx)0.98MnO3 perovskite with A-site deficiencies toward oxygen reduction reaction in aluminum-air batteries

Abstract The strontium doped Mn-based perovskites have been proposed as one of the best oxygen reduction reaction catalysts (ORRCs) to substitute the noble metal. However, few studies have investigated the catalytic activities of LSM with the A-site deficiencies. Here, the (La 1-x Sr x ) 0.98 MnO 3 (LSM) perovskites with A-site deficiencies are prepared by a modified solid-liquid method. The structure, morphology, valence state and oxygen adsorption behaviors of these LSM samples are characterized, and their catalytic activities toward ORR are studied by the rotating ring-disk electrode (RRDE) and aluminum-air battery technologies. The results show that the appropriate doping with Sr and introducing A-site stoichiometry can effectively tailor the Mn valence and increase the oxygen adsorption capacity of LSM. Among all the LSM samples in this work, the (La 0.7 Sr 0.3 ) 0.98 MnO 3 perovskite composited with 50% carbon (50%LSM30) exhibits the best ORR catalytic activity due to the excellent oxygen adsorption capacity. Also, this catalyst has much higher durability than that of commercial 20%Pt/C. Moreover, the maximum power density of the aluminum-air battery using 50%LSM30 as the ORRC can reach 191.3 mW cm −2 . Our work indicates that the LSM/C composite catalysts with A-site deficiencies can be used as a promising ORRC in the metal-air batteries.

Development of Mn-based perovskite materials: Chemical structure and applications

ABSTRACTIn this article, we summarize current progress on the bulk and surface characteristics of Mn-containing perovskite oxides known for their good catalytic activity in atmospheric pollutant abatement. These materials are emphasized as serious alternatives for noble metal-based catalysts (Pt, Rh, Pd) in many catalytic applications particularly in automotive exhaust catalytic converters, mainly due to their low cost, good thermal stability at high temperature and ease of preparation compared to supported noble metal catalysts. The success of such materials is mainly related to Mn3+ and Mn4+ mixed valence and the resulted point defects formed after incorporation of a large variety of metals with different size and charge in the perovskite structure. These parameters could also affect the Mn reducibility and oxygen species mobility considered as one of the most determining factors in catalytic activity. The effect of perovskite metallic composition on the surface Mn oxidation state and relative cations segregation as well as applications of Mn-containing perovskite oxides as catalysts for several deep oxidation reactions at low and high temperatures are presented. Particular attention is devoted to the solution combustion synthesis for Mn-containing perovskite catalysts due to its time and energy saving characteristics and high surface areas of the obtained products. These advantages open new attractive opportunities for the use of this economic process to prepare supported perovskite oxide catalysts with the aim to better control morphology and stability of both surface catalyst and supports.

Mn-based perovskite oxides for Hg0 adsorption and regeneration via a temperature swing adsorption (TSA) process

Mn-Based perovskite oxides combined with a temperature swing adsorption (TSA) process were employed for Hg0 adsorption and regeneration from coal-fired flue gas, in which oxygen directly acted as oxidant to enhance the capture of Hg0. Three kinds of Mn-based perovskie oxides, SrMnO3, LaMnO3 and CeMnO3 were evaluated. The results indicated that the performances for Hg0 removal decreased in the order of LaMnO3>CeMnO3>SrMnO3. LaMnO3 had a Hg0 capacity of 6.22mg/g with 4% O2 at 150°C. O2 significantly enhanced the reaction activity, the Hg0 capacity was increased by approximately 6.5% in the presence of 8% O2. The characterization results indicated that perovskite crystal structure was beneficial for Hg0 capture. The Hg0 removal mechanism was primary ascribed to catalytic oxidation and chemical-adsorption. The abundant of adsorbed oxygen, high ratio of Mn4+/Mn3+ in LaMnO3 lead to the high activity. Moreover, LaMnO3 can be regenerated without the loss of capacity. The released Hg0 could be gathered which prevent from mercury secondary pollution in the environment.

La/Sr-based perovskites as soot oxidation catalysts for Gasoline Particulate Filters

La0.6Sr0.4BO3 type-perovskites (with B=Fe, Mn or Ti) were synthesized by a complex route from the thermal decomposition of the chelated nitrate precursors, as soot oxidation catalysts under low oxygen partial pressures. The nature of the B-site cation modifies the morphology, the structural symmetry, the charge compensation mechanism and the redox properties. The generation of oxygen vacancies is predominant in LSF while an increase in the oxidation state of B cations occurs in LSM and LST, respectively. A direct link was established between the reducibility of the perovskites and their ability to oxidize soot. Fe and Mn-based perovskites exhibit similar catalytic behaviors, superior to LST, which are attributed to the presence of oxygen vacancies in LSF and Mn4+ cations in LSM. This latter redox property is particularly suitable to oxidize soot at low oxygen partial pressures, as encountered in gasoline direct injection engine exhausts.

Low-temperature selective catalytic reduction of NO with NH3 using perovskite-type oxides as the novel catalysts

In order to investigate the activities of perovskite-type mixed oxides for selective catalytic reduction (SCR) of NO with NH3, a series of LaBO3 or La2BO4 perovskite-based catalysts with different B-site ions (B=Cu, Co, Mn, and Fe) was prepared by citrate complexation procedure and characterized by XRD, BET, TPD of NO+O2 and NH3. It was found that Mn-based perovskite exhibited the best catalytic activity, yielding 78% NO conversion at 250°C. The NH3 adsorption ability of the perovskites is the key factor for the SCR performance. In situ DRIFTS investigation was further carried out over LaMnO3 to explore the reaction mechanism. It is revealed that the SCR reaction starts with NH3 chemical adsorption, being thought as the rate-determining step. The mechanism essentially involves a Langmuir–Hinshelwood mechanism mainly depending on an interaction between NH4+ ionic species and nitrite species in the low-temperature range.

Novel oxygen carriers for chemical looping combustion: La1−xCexBO3 (B = Co, Mn) perovskites synthesized by reactive grinding and nanocasting

Chemical looping combustion is a novel technique with inherent separation of the greenhouse gas CO2 from atmospheric nitrogen in combustion of gaseous fuels. The selection of a suitable oxygen carrier which circulates between two fluidized bed reactors is a key issue for the performance of this technology. In this work, high surface area perovskite-based oxygen carriers, LaCoO3, LaCeCoO3 and LaMnO3, were synthesized by the reactive grinding method (S. Kaliaguine, A. van Neste, US Pat., 6 017 504, 2000; Applied Catalysis, A, 2001, 209, 345-358). LaMnO3 was also prepared by another method designated as nanocasting. The perovskites were characterized using different methods such as XRD, SEM, N2 adsorption, H2-TPR, TPD-O2 and finally the reactivity and stability of the carrier materials were tested in a CREC fluidized riser simulator using multiple reduction–oxidation cycles under two conditions: with low and high amount of CH4 (0.5 and 10 ml) as a feed. It was found from TPD-O2 and H2-TPR studies that the LaMnO3 perovskite, particularly the one prepared by the nanocasting method (LaMn-NC), has a high amount of available α-O2 (surface oxygens) and high density of surface anion vacancies. Moreover, the Mn-based perovskites showed easier reducibility compared to the Co-based oxygen carriers. The results obtained under two different conditions of CH4 pressure confirmed the strong dependency of the CLC results on this factor. Higher stability and no CO formation during the multiple redox cycles were indeed observed while using low pressure of CH4. Higher reactivity of the carriers was however obtained while using higher CH4 pressure but CO was detected in the products in this case. The higher reactivity of Mn-based perovskites could not only be due to the higher amount of surface oxygen and reducible sites present in the sample but also to the higher specific surface area of the perovskite. The formation of lanthanum and manganese silicate was observed for LaMn-NC which could be the reason for the reduction in CH4 conversion observed between the eighth and tenth redox cycles. This compound could be as a result of interaction between remnant silica in the sample with La and Mn from the perovskite lattice. No coke formation was detected on the oxygen carriers however, some agglomeration of the particles occurred, as confirmed by SEM images, XRD data and N2 sorption tests. Furthermore, XRD patterns confirmed that the structure of all perovskites remained unchanged after multiple redox cycles.

SCR of NO by propene over nanoscale LaMn 1− xCu xO 3 perovskites

Nanoscale LaMn 1− x Cu x O 3 perovskites with high specific surface areas were prepared by reactive grinding and characterized by N 2 adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), H 2-temperature programmed reduction (TPR), O 2-, NO + O 2- and C 3H 6-temperature programmed desorption (TPD) and NO + O 2-temperature programmed surface reduction (TPSR) under C 3H 6/He flow. The samples were then submitted to activity tests in the selective catalytic reduction (SCR) of NO by C 3H 6 with or without O 2. The catalytic performances over unsubstituted LaMnO 3 is observed with maximum N 2 yield of 62% and a C 3H 6 conversion of 80% at 550 °C at a space velocity of 50,000 h −1 (3000 ppm NO, 3000 ppm C 3H 6, 1% O 2 in helium). The N 2 yield is however significantly improved by Cu incorporation into the lattice, achieving a remarkable N 2 yield of 86% at 500 °C at 20% Mn substitution by Cu. The content of α-oxygen over lanthanum manganite is enhanced by Cu substitution, but the opposite occurs for excess oxygen. The better performance of Cu-substituted samples is likely to correspond to the facility in the formation of adsorbed nitrate species via the oxidation of NO by α-oxygen in addition to the intrinsic effect of Cu in NO transformation. However, the excessive α-oxygen content observed over LaCo 0.8Cu 0.2O 3 accelerated the unselective hydrocarbon oxidation and suppressed the formation of organo nitrogen compounds, which led to a poor N 2 yield with respect to Mn-based perovskites. A mechanism involving the formation of an organic nitrogen intermediate, which further converts into N 2, CO 2 and H 2O via isocyanate, was proposed. The gas phase oxygen acts as a promoter when its concentration is lower than 1000 ppm because of the promotion of nitrate formation and organo nitrogen compounds transformation. O 2 acts however as an inhibitor when its concentration is higher than 5000 ppm due to the heavily unselective combustion of C 3H 6 by O 2, in the reaction of NO and C 3H 6 over LaMn 0.8Cu 0.2O 3 at 400 °C.

Magnetic and transport properties of lanthanum perovskites with B-site half doping

We have synthesized various half doped lanthanum perovskites LaB 0.5 B 0.5 ′ O 3 (BB′=MnV, MnCr, MnCo, MnNi, FeCr, FeMn, FeCo, FeNi) by solid state reaction method. The crystal structure was orthorhombic in all samples, but the magnetic and transport properties showed a large variation with the composition. A ferromagnetic feature was quite strong in the Mn-based perovskites but weak or non-existent in the Fe-based ones. The resistivity of Ni-doped ones was much smaller than those of the others. For all half doped perovskits, remarkably, the temperature dependent transport was well explained by variable range hopping model with different localization lengths.

Theoretical study of electron-dopedLa1−xCexMnO3compounds

While hole-doped Mn-based perovskites have been under thorough investigation in the past 10 years, the electron-doped counterparts only received a limited attention and their electronic and magnetic properties are still not fully understood. To obtain a better physical understanding on the origin of their giant magnetoresistances, we have studied in this paper the effect of Ce doping on various magnetic metastable states of LaMnO 3 compounds. The self-consistent calculations were done within the framework of unrestricted Hartree-Fock approximation. A realistic multiband d-p Hamiltonian and a generalized Jahn-Teller coupling Hamiltonian are adopted to describe the systems. The densities of states and stabilities of the various states are analyzed as functions of model parameters. Our results show that the A-type antiferromagnetic insulator of undoped parent LaMnO 3 compound is stabilized by the Jahn-Teller distortion. As Ce doping increases the ground state of La 1 - x Ce x MnO 3 compounds takes consecutively the ferromagnetic state (for 0.01<x≤0.62), the C-type antiferromagnetic state (for 0.62<x≤0.88), and the G-type antiferromagnetic state (for 0.88<x≤1.0), respectively.

Perovskite-Type Oxides: I. Structural, Magnetic, and Morphological Properties of LaMn1−xCuxO3 and LaCo1−xCuxO3 Solid Solutions with Large Surface Area

Perovskite-type compounds of general formula LaMn1−xCuxO3 and LaCo1−xCuxO3 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared by calcining the citrate gel precursors at 823, 923, and 1073 K. The decomposition of the precursors was followed by thermal analysis and the oxides were investigated by means of elemental analysis (atomic absorption and redox titration), X-ray powder diffraction, BET surface area, X-ray absorption (EXAFS and XANES), electron microscopy (SEM and TEM), and magnetic susceptibility. LaMn1−xCuxO3 samples are perovskite-like single phases up to x=0.6. At x=0.8 CuO and La2CuO4 phases are present in addition to perovskite. For x=1.0 the material is formed by CuO and La2CuO4. Mn(IV) was found by redox titration in all Mn-based perovskite samples, its fraction increasing with the increase in copper content. EXAFS and XANES analyses confirmed the presence of Mn(IV). Cation vacancies in equal amounts in the 12-coordinated A and octahedral B sites are suggested in the samples with x=0.0 and x=0.2, while for x=0.6 anionic vacancies are present. Materials with sufficiently high surface area (22–36 m2 g−1 for samples fired at 923 K and 14–22 m2 g−1 for those fired at 1073 K) were obtained. Crystallite sizes in the ranges 390–500 and 590–940 Å for samples calcined at 923 and 1073 K, respectively, were determined from the FWHM of the (102) X-ray diffraction peak. TEM patterns of LaMnO3 showed almost regular hexagonal prismatic crystals with sizes of the same order of magnitude (800 Å) of those drawn from X-ray diffraction, while no evidence of defect clustering was drawn out from TEM and electron diffraction images. For the sample with x=0.6, TEM and electron diffraction patterns revealed perturbation of the structure. Magnetic susceptibility studies show a ferromagnetic behavior that decreases with increase in x. LaCo1−xCuxO3 samples are perovskite-like single phases up to x=0.2. For x=0.4 a small amount of La2CuO4, in addition to perovskite, is detected. For x≥0.6 massive formation of La2CuO4 and CuO is observed. Only trivalent cobalt is found by redox titration. Magnetic susceptibility studies have shown that trivalent cobalt is present in all samples as a mixture of paramagnetic Co3+ and diamagnetic CoIII ions, the Co3+ fraction being, at least up to x=0.4, equal to ≈0.34. Antiferromagnetic behavior, which increases with increase in x, is observed in all LaCo1−xCuxO3 samples. LaCoO3 is a stoichiometric perovskite. The substitution of cobalt by Cu2+ leads to a positive charge defectivity which is compensated by oxygen vacancies. EXAFS and XANES analyses confirmed the presence of trivalent cobalt. Materials with fairly high surface area (in the ranges 19–27 and 13–21 m2 g−1 for samples calcined at 923 and 1073 K, respectively) were obtained. Crystallite sizes of ≈400 and ≈1000 Å for samples calcined at 923 and 1073 K, respectively, were determined from the FWHM of the (102) X-ray diffraction peak. Materials with not very clear morphology and crystals with definite structure are distinguishable by SEM for samples calcined at 1073 and at 1273 K, respectively. TEM patterns, for samples calcined at 1073 K, evidence almost regular hexagonal prismatic crystals connected to form “linked structures” and some “spotty crystals,” suggesting short-range ordered local defects. For copper-containing samples, calcined at 1273 K, a higher degree of defectivity (probably associated with the interaction of anion vacancies) and the occurrence of “planar faults” are shown by TEM.

Perovskite-Type Oxides: II. Redox Properties of LaMn1−xCuxO3 and LaCo1−xCuxO3 and Methane Catalytic Combustion

Redox properties of high-surface-area LaM1−xCuxO3 (M=Mn or Co) perovskites prepared by the citrate method were studied by H2 TPR and O2 TPD techniques. We have found that the reduction of Mn4+ occurs in all La–Mn–Cu perovskites at temperatures lower than that of Co3+ in La–Co–Cu samples. The presence of copper affects the reduction of both Mn4+ and Co3+, increasing the temperature of reduction. Furthermore, Mn-based perovskites with a low Cu substitution (x< 0.4) release a great amount of O2 at high temperature. Catalytic activity toward methane combustion was investigated under diluted conditions (0.4% CH4 and 10% O2, N2 as balance) in the temperature range 673–1073 K using a fixed bed reactor with a space velocity of 40,000 N cm3 h−1 g−1. All samples catalyze methane oxidation in the temperature range 673–773 K, giving complete methane conversion below 1073 K and producing 100% CO2. No deactivation phenomena were observed. Substitution with copper decreases catalytic activity. The higher activity of Mn-based perovskites compared with that of the corresponding Co samples with the same composition was attributed to the stronger oxidative properties of LaM1−xCux O3. An apparent activation energy of 23 kcal mol−1 was evaluated on the basis of a first-order kinetic equation for all samples.

Charge-localization effects in the infrared transmittance of layered perovskites

In a previous study it has been shown that the formation of charged superlattices in Ni- and Mn-based perovskites opens a gap in their infrared polaronic background. This signature of charge localization is here used to compare the behavior of the polaronic carriers in Sr 2MnO 4, La 2NiO 4+ y , Nd 2CuO 4− y and Nd 1.85Ce 0.15CuO 4− y . It is shown that, as the temperature decreases, the carriers tend to localize in the nickelate and the manganite, to slightly increase their mobility in both cuprates.

Structural phase diagram of La 1 − xSr xMnO 3 for low Sr doping

The Mn-based perovskite oxides are of strong current interest because they show a colossal negative magnetoresistance near the ferromagnetic ordering temperature. The samples become metallic below T c for a content of approx. 30% Mn 4+. We present in this article a structural study of La 1− x Sr x MnO 3 for compositions lying below the optimum magnetoresistance. At high temperature, the system is rhombohedral ( R 3 c). On cooling, two stuctural transitions to orthorhombic Pbnm phases (O and O′) take place successively. The transition from the O to the O′-phase is due to the setting up of co-operative Jahn-Teller distortions that become state in the O′-phase. The O′-phase is characterised by the presence of an anti-ferrodistorsive Mn 3+ orbital ordering. This ordering is evidenced by a peculiar arrangement of distorted MnO 6 octahedra: the connection between octahedra in the basal lane ( a, b) shows long MnO bonds adjacent to short MnO bonds. The peculiarity of samples around the composition x = 0.125 is that they undergo another transition to an orthorhombic O″-phase at lower temperatures. This phase has the same average structural features as the O-phase but the Jahn-Teller distortion seems to be completely suppressed.

Weakly and tightly bound polarons in the infrared spectra of perovskites

Infrared reflectivity and/or absorption measurements in chemically doped perovskites, including high-Tc superconductors, show polaronic features similar to those observed in the photoinduced spectra of cuprates. These features, whose intensities depend on doping and temperature, are observed in both n-type and p-type materials. They include extra-phonon infrared-active vibrations (IRAV) and a broad band, centered at ~ 1000 cm−1, which results from overtones of IRAVs and is well described in terms of photoexcitation of polarons. Most of the above features survive as extra-Drude peaks in the metallic phase of these materials. The strength of the polaron band increases for decreasing temperature, as expected for weakly bound polarons, in compounds which are parents of high-Tc superconductors. It decreases, instead, as expected for tightly bound polarons, in a Mn-based perovskite and in CuO, which do not become superconducting upon doping. A comparison with tunneling results shows that the superconducting carriers are strongly coupled to the IRAVs.