Pr Samples Research Articles

Characterization of Pr:LuAG scintillating crystals for X-ray spectroscopy

The main features of the Pr doped Lu3Al5O12 (Pr:LuAG) scintillating crystals for X-ray spectroscopy applications have been studied using different radioactive sources and photo-detectors. Pr:LuAG is cheaper, compared to a Germanium detector, but with remarkable properties which make it useful for many applications, from fundamental physics measurements to the PET imaging for medical purposes: high density, elevate light yield, fast response, high energy resolution, no hygroscopicity. A sample of Pr:LuAG crystals with 14mm×14mm surface area and 13mm thickness and a NaI crystal of the same surface and 26mm thickness used as a reference have been characterized with several radioactive sources, emitting photons in the range 100–1000keV. Different light detectors were adopted for the Pr:LuAG studies, sensitive to its UV emission (peak at 310nm): a 3in. PMT (Hamamatsu R11065) and new arrays of Hamamatsu SiPM S13361, with siliconic resin as a window. Preliminary results are presented on the performance of the Pr:LuAG crystals, to be mounted in a 2×2 array to be tested in the 2015 run of the FAMU experiment at RIKEN-RAL muon facility. The goal is the detection of the X-rays (around 130keV) emitted during the de-excitation processes of the muonic hydrogen after the excitation with an IR laser with wavelength set at the resonance of the hyperfine splitting, to measure the muonic atom proton radius with unprecedented precision.

Viabilidade da incorporação do lodo de estação de tratamento de esgoto (ETE) em massa cerâmica para produção de blocos

O lodo gerado nas estações de tratamento de esgoto tem se tornado um problema crescente sendo considerado como resíduo. Este trabalho estudou a viabilidade da incorporação do lodo de estação de tratamento de esgoto (ETE) em massa cerâmica para produção de blocos. Para isso, amostras de lodo foram coletadas na ETE Verde da SANEPAR no Município de Ponta Grossa, PR e de argila da Cerâmica S. Gerônimo no município de Prudentópolis, PR. O lodo na ETE Verde foi submetido a um processo de higienização, o qual reduz significativamente a concentração de patógenos no resíduo. O processo utilizado foi a caleação, que promove a estabilização, desinfecção química e térmica através da adição e mistura da cal ao lodo numa porcentagem de 30% a 50% da massa seca de lodo. Para a caracterização do lodo e da argila foram realizadas análises de difração de raios X, análise térmica diferencial e termogravimétrica, fluorescência de raios X e distribuição granulométrica. Foram estudadas composições com adições de 5 a 50% em massa do lodo na massa cerâmica. Após a mistura do lodo com as argilas, as amostras foram conformadas por prensagem uniaxial e sinterizadas a 900 ºC, 1000 ºC e 1100 ºC. As amostras sinterizadas foram caracterizadas por medidas de absorção de água, porosidade aparente, resistência mecânica, difração de raios X e microscopia eletrônica de varredura. Os resultados mostraram que a adição do lodo até 25% da massa da argila atende a todos os parâmetros preconizados pelas normas vigentes para fabricação dos blocos cerâmicos.

The photoluminescence and electrical properties of lead-free (Bi0.5Na0.5)TiO3:Pr ceramics

Pr doped lead-free (Bi0.5Na0.5)TiO3(BNT) ceramics were fabricated using the solid-state reaction method. The dependences of photoluminescence properties on process conditions, dopant concentration, and charge compensation were systematically investigated. The results showed that Pr-doped BNT ceramics exhibited strong red emitting properties with a single peak centered at 610nm upon blue light excitation (440–505nm), while maintaining high piezoelectric properties. The emission intensities reached the optimal value as the holding time is 2h at 1150°C for BNT:0.003Pr sample. Especially, the red emission intensity of BNT:xPr3+ can be more effectively enhanced by introducing the (Bi0.5Na0.5)2+ vacancies as the charge compensation due to the formation of trapping centers, than the alkali metal ions (Li+, Na+, K+) as compensators. In addition to the intrinsic piezoelectric properties, the excitation band of BNT:xPr material can well realize the compatibility with existing blue light-emitting diode chips (450–470nm), compared to the ultraviolet or infrared light excitation luminescent materials, indicating that this system may have significant technological promise in novel multifunctional devices.

Synthesis and physical properties of pure and f-doped REFeAsO (Re=Pr, Nd & Sm)

We report a comparative study of crystal structure, electrical, and magnetic properties of the pure and F-doped superconducting REFeAsO (Re = Pr, Nd, and Sm) samples. The presence of superconductivity in these iso-structural compounds provided an opportunity to understand the doping mechanism in oxy-pnictide superconductor. Bulk polycrystalline samples are synthesized by solid state reaction route in an evacuated sealed quartz tube. The Rietveld analysis of room temperature X-ray diffraction (XRD) data show that all the studied samples crystallize in single phase in a tetragonal structure with space group P4/nmm. The lattice parameters of the studied samples follow the well-known rare earth contraction. The decrease in c-parameter and the volume is indicative of successful substitution of F−1(RF = 1.33 Ǻ) at O2-(RO = 1.40 Ǻ) site. The ground state REFeAsO compounds shows a metallic step in resistivity measurements below say 150–130 K. This metallic step is attributed to structural and SDW transition. The superconductivity in the F-doped sample is confirmed by resistivity measurements as well as magnetic measurements. Superconducting transition temperature (Tc) is found to be at 51K, 48K and 38 K respectively for RE = Sm, Nd and Pr. The superconducting transition temperature of the F-doped samples increase with deceasing the ionic radii of the rare earth. The temperature dependent upper critical field Hc2(0) is calculated from detailed up to 14 Tesla R(T)H measurements using the extrapolation method employing Ginzburg- Landau (GL) theory. Thus calculated Hc2(0) is found to be above 200 Tesla, which is second best to the High Tc cuprates.

Luminescence and scintillation properties of advanced Lu3Al5O12:Pr3+ single crystal scintillators

In this paper we present the luminescence and scintillation properties of Lu3Al5O12:Pr (LuAG:Pr) single crystals with Pr3+ concentration of 0.13 wt %, grown by the Czochralski method. The light yield and energy resolution were measured under 662 keV γ-ray excitation. The dominant emission band peaking at 310 nm with a shoulder at 370 nm was observed in the photoluminescence spectrum. High light yield of 24,500 ph/MeV and an energy resolution of 5.3 % were obtained for a 6 × 6 × 2 mm3 LuAG:Pr sample. Light yield dependence on sample height and shaping time was measured. The estimated photofraction in pulse height spectrum and total mass attenuation coefficient at 662 keV γ-rays were also determined and compared with the theoretical ones calculated using the WinXCom program.

Late Pleistocene paleoenvironmental changes of the Kathmandu Basin-fill sediments revealed by the minerals composition

The amounts and their variation of minerals in the present river sediments (PR) and basement rocks (BR) of the Kathmandu Valley were estimated by using the X-ray diffraction (XRD) method, in order to estimate the source of the Kathmandu Basin sediments and to interpret paleoenvironmental and paleoclimatic condition of the past Kathamdu Valley. XRD result of the BR and PR samples roughly correspond along respective parts of the Kathmandu Valley, except calcite (carbonate rocks in the eastern, southern, and western parts). The non-clay minerals are quartz, K-feldspar, plagioclase, mic a, chlorite, calcite, and illite, chlorite, kaolinite, smectite, mixed layer illite-chlorite are detected within the clay fraction of the PR sediments. The clay fraction is < 5 wt % within the PR-sediments. Among non-clay fraction, BR and PR from the north contains more or less equal amount of mica, feldspars and quartz, while those from south, east and west are rich in quartz. Therefore, the amount of silicate minerals in the sediments within the Kathmandu Basin is the key to grasping the provenance of the sediments about northern and the other origin. Chlorite in the PR-North is likely a weathering product of mica because chlorite was not included in the BR-North samples. The chlorite and mica were greater in the lower course of the river from the north than in the upper course of the river. Plagioclase is tends to be poor and K-feldspar is rich in the PR-samples, while the amount of the two feldspar are similar to each other in the BR-samples. This tendency may be due to the difference in stability to weathering between the two feldspars. Calcite was probably dissolved during weathering, erosion, and transportation processes, because calcite included in the BR samples was hardly detected in the PR samples. This mineralogical information is applied to the mineralogical studies in the past basin-fill sediments of the Kathmandu Basin (RB-drill core) and to estimate the source of the sediments and paleoenvironmental and paleoclimatic information on this region. Calcite detected in the past basin-fill sediments of the Kathmandu Basin is an important key mineral showing cold and dry climate within the RB drill core. It can also be used to correlate the strata.

Screening of grated cheese authenticity by nir spectroscopy

Parmigiano–Reggiano (PR) cheese is one of the oldest traditional cheeses produced in Europe, and it is still one of the most valuable Protected Designation of Origin (PDO) cheeses of Italy. The denomination of origin is extended to the grated cheese when manufactured exclusively from whole Parmigiano-Reggiano cheese wheels that respond to the production standard. The grated cheese must be matured for a period of at least 12 months and characterized by a rind content not over 18%. In this investigation the potential of near infrared spectroscopy (NIR), coupled to different statistical methods, were used to estimate the authenticity of grated Parmigiano Reggiano cheese PDO. Cheese samples were classified as: compliance PR, competitors, non-compliance PR (defected PR), and PR with rind content greater then 18%. NIR spectra were obtained using a spectrophotometer Vector 22/N (Bruker Optics, Milan, Italy) in the diffuse reflectance mode. Instrument was equipped with a rotating integrating sphere. Principal Component Analysis (PCA) was conducted for an explorative spectra analysis, while the Artificial Neural Networks (ANN) were used to classify spectra, according to different cheese categories. Subsequently the rind percentage and month of ripening were estimated by a Partial Least Squares regression (PLS). Score plots of the PCA show a clear separation between compliance PR samples and the rest of the sample was observed. Competitors samples and the defected PR samples were grouped together. The classification performance for all sample classes, obtained by ANN analysis, was higher of 90%, in test set validation. Rind content and month of ripening were predicted by PLS a with a determination coefficient greater then 0.95 (test set). These results showed that the method can be suitable for a fast screening of grated cheese authenticity.

Growth of Single Crystal and Effects of Electron Doping in Filled Skutterudite Compound PrFe<sub>4</sub>P<sub>12</sub>

PrFe4P12shows an anomalously phase transition from a heavy fermion (HF) state to a non-magnetic ordered state. We prepared single crystal sample of Pr (Fe1-xCox)4P12by Sn-flux method to investigate the effects of electron doping. The distinct change of magnetic property was observed. The phase transition of PrFe4P12rapidly disappears when doped rate x=0.03. This result indicates that the d-electron plays an important role in formation of Low-temperutre phase and HF state of PrFe4P12.

Crystal structure and magnetic properties of Pr- and Ti-substituted La2RuO5

Polycrystalline samples of Pr- and Ti-substituted La2RuO5 were prepared applying a soft-chemistry route based on the thermal decomposition of citrate-stabilized precursors. The simultaneous substitution on the La-sites by Pr and on the Ru-sites by Ti results in samples of the composition La2−xPrxRu1−yTiyO5 with 0≤x≤0.75 and 0≤y≤0.4. The crystal structures of these compounds were analyzed by Rietveld refinement of powder X-ray diffraction patterns. For pure La2RuO5 a structural transition from a monoclinic room-temperature modification to a triclinic low-temperature structure was found at 161K. This structural change is linked to a low-temperature long-range ordered spin-singlet ground state formed by Ru4+ spin-moments. Both the structural transition and the formation of the singlet ground state become progressively suppressed with higher Ti contents, while the Pr substitution has only a minor influence on the dimerization. The behavior of the Curie–Weiss temperatures can be explained assuming two almost independent magnetic sublattices corresponding to the ruthenium and the rare-earth ions, respectively. For all investigated properties, i.e. crystal structure, magnetic susceptibilities, and dimerization temperature Td, a completely additive behavior of the effects of Pr-substitution and Ti-substitution is observed.

Effect of thermal annealing on light yield, low temperature thermoluminescence, and time profiles of LuAG:Pr scintillator crystals

Measurements of light yield, low temperature thermoluminescence, and scintillation time profiles, performed comparatively on “as grown” and thermally annealed LuAG:Pr samples, are reported. It is shown that traps play an efficient role in the scintillation of LuAG:Pr, being responsible for the yield decrease and introducing a long component to the time profiles. The trap concentrations in the annealed crystals are lower than in the non-annealed ones, which correlates with the yield enhancement.

Unusual local disorder in NdOs4Sb12and PrOs4Sb12skutterudites

We present a temperature-dependent, extended x-ray absorption fine structure (EXAFS) analysis of the filled skutterudite compounds NdOs${}_{4}$Sb${}_{12}$, PrOs${}_{4}$Sb${}_{12}$, and EuOs${}_{4}$Sb${}_{12}$. Although the interpretation of recent ultrasonic measurements suggested off-center displacements for Nd and Pr in NdOs${}_{4}$Sb${}_{12}$ and PrOs${}_{4}$Sb${}_{12}$, the EXAFS analysis shows that the Nd-Sb, Pr-Sb, and Sb-Sb peaks are well ordered. Surprisingly, however, the second-neighbor Nd-Os and Pr-Os peaks are highly disordered, even at low temperatures, and the Os-Os peak also has some disorder in the Nd and Pr systems. In contrast to the anomalous results for the Pr and Nd samples, neither the Eu-Sb, Eu-Os, nor the Os-Os peak is disordered at low temperature for EuOs${}_{4}$Sb${}_{12}$. For all three systems within estimated errors, the rare-earth atom is on-center inside the Sb cage. We propose that for the Nd and Pr compounds, the Os cage distorts, with some of the Os atoms moving either towards or away from the Nd or Pr atoms, such that the Nd-Sb, Pr-Sb, and Sb-Sb pair distances have very little disorder. Some possible distortion models are suggested and the possible ramifications for thermoelectric and transport properties are discussed.

Resistance Training Reduces Subclinical Inflammation in Obese, Postmenopausal Women

Aerobic exercise is frequently prescribed to reduce inflammatory-related disease (cardiovascular disease and diabetes) risk. Resistance training (RT), however, may be key to maximizing anti-inflammatory benefits of consistent exercise. We examined the influence of RT on inflammatory biomarkers in obese, postmenopausal women. Twenty-three women (65.6 ± 2.6 yr; body mass index, 33 kg·m) underwent 12 wk of RT (3 sets, 10 exercises, 3× per week, 8-12 repetition maximum (RM), resistance exercise (EX), N = 11) or social interaction intervention (SI, stretching, knitting, health lectures, 2× per week, control group (CON), N = 12). Both before (BT) and after (AT) RT or SI, blood was collected before (PR), immediately (PO), 2 h (2H), and 24 h (24H) after a single resistance exercise bout (RE) in EX and at the same time points in nonexercise, resting CON. For all time points, blood was analyzed for IL-6, leptin, and lipopolysaccharide (LPS)-stimulated tumor necrosis factor-α (TNF-α) (LPS-TNF) and IL-10 (LPS-IL10). PR samples were also examined for C-reactive protein, TNF-α, and adiponectin, and mRNA expression of toll-like receptor 4 (TLR4) and MC1R. Subcutaneous adipose tissue was extracted BT and AT and analyzed for mRNA expression of monocyte chemotactic protein-1, leptin, CD68, and TLR4. RT improved strength (44%) and reduced circulating C-reactive protein (-33%), leptin (-18%) and TNF-α (-29%) with no change in body composition. IL-6 decreased after SI in CON (-17%). LPS-TNF increased after SI or RT (CON +26%, EX +67%, respectively), whereas LPS-IL10 decreased in CON (-28%) but increased in EX (+20%). RT did not influence inflammatory biomarker gene expression in whole blood or subcutaneous adipose tissue. A single RE bout augmented LPS-TNF and LPS-IL10 at 24H in EX, particularly AT. RT reduced markers of subclinical inflammation in circulation in obese, postmenopausal women in the absence of changes in body composition. Chronic RT also enhanced response to endotoxin challenge both at rest (PR) and 24 h after an acute RE bout (24H).

Anisotropic magnetic properties of Pr1−xCaxMnO3

We produced $c$-axis-oriented samples of Pr${}_{1\ensuremath{-}x}$Ca${}_{x}$MnO${}_{3}$ ($0.30\ensuremath{\le}x<0.50$) by applying a uniaxial pressure to quasisingle crystals with multiple domains at a high temperature. This reduced the number of domains in the crystals, allowing better investigation of their anisotropic magnetic properties. Below the N\'eel temperature, the magnetization curves can be classified into three Ca concentration ranges ($x=0.3$--0.33, $x=0.33$--0.37, and $x=0.4$--0.45). Our results indicate that the magnetic ground state strongly depends on the Ca concentration ($x$). For $x=0.30$--0.33, the ground state presents canted antiferromagnetic long-range order, presumably coexisting with ferromagnetic spin clusters. For $x=0.33$--0.37, the magnetic anisotropy undergoes steep changes near 25 K. These observations suggest the coexistence of ferromagnetic and antiferromagnetic spin clusters. Finally, for $x=0.4$--0.45, our measurements suggest the presence of small ferromagnetic spin clusters superimposed on the antiferromagnetic long-range order.

Tolerability of a Fully Maturated Cheese in Cow’s Milk Allergic Children: Biochemical, Immunochemical, and Clinical Aspects

BackgroundFrom patients’ reports and our preliminary observations, a fully maturated cheese (Parmigiano-Reggiano; PR) seems to be well tolerated by a subset of cow’s milk (CM) allergic patients.Objective and MethodsTo biochemically and immunologically characterize PR samples at different maturation stage and to verify PR tolerability in CM allergic children. Seventy patients, with suspected CM allergy, were enrolled. IgE to CM, α-lactalbumin (ALA), β-lactoglobulin (BLG) and caseins (CAS) were tested using ImmunoCAP, ISAC103 and skin prick test. Patients underwent a double-blind, placebo-controlled food challenge with CM, and an open food challenge with 36 months-maturated PR. Extracts obtained from PR samples were biochemically analyzed in order to determine protein and peptide contents. Pepsin and trypsin-chymotrypsin-pepsin simulated digestions were applied to PR extracts. Each PR extract was investigated by IgE Single Point Highest Inhibition Achievable assay (SPHIAa). The efficiency analysis was carried out using CM and PR oral challenges as gold standards.ResultsThe IgE binding to milk allergens was 100% inhibited by almost all PR preparations; the only difference was for CAS, mainly αS1-CAS. Sixteen patients sensitized to CM tolerated both CM and PR; 29 patients tolerated PR only; 21 patients, reacted to both CM and PR, whereas 4 patients reactive to CM refused to ingest PR. ROC analysis showed that the absence of IgE to BLG measured by ISAC could be a good marker of PR tolerance. The SPHIAa using digested PR preparations showed a marked effect on IgE binding to CAS and almost none on ALA and BLG.Conclusions58% of patients clinically reactive to CM tolerated fully maturated PR. The preliminary digestion of CAS induced by PR maturation process, facilitating a further loss of allergenic reactivity during gut digestion, might explain the tolerance. This hypothesis seems to work when no IgE sensitization to ISAC BLG is detected.

Chemical insertion in the perovskite solid solutions Pr0.5+x−yLi0.5−3xBiy□2xTiO3: Implications on the electrical properties

Abstract Chemical insertion and de-insertion of lithium in pellet samples of the solid solutions Pr 0.5+ x − y Li 0.5−3 x Bi y □ 2 x TiO 3 were studied. Two regions of the phase diagram are studied: one having constant composition of bismuth and the other of lithium. The amount of inserted lithium depends on both the number of vacancies and the amount of bismuth in the original samples. The conductivity of the samples is directly related to the amount of inserted lithium and the activation energy depends on the unit cell volume. An analysis of the electronic and ionic components of the conductivity reveals that the untreated materials are pure ionic conductors, while after Li-insertion an additional electronic conductivity ( t −2 ) occurs, due to a polaron mechanism, with an activation energy of 0.8 eV.

Synthesizing white-luminescence SrS:Pr powder phosphor

This paper discusses several different methods of synthesizing SrS:Pr phosphor. It is shown that the brightest photoluminescence can be obtained by using strontium sulfide as the main starting substance and doping it with PrF3 in the presence of NaCl flux by roasting the mix in a dry nitrogen atmosphere with an addition of sulfur vapor. It is established that, to obtain luminescence of the SrS:Pr samples, it is necessary to anneal them in vacuum. The praseodymium concentration in the mix, the temperature, and the vacuum-annealing time were optimized, and this made it possible to maximize the photoluminescence brightness and achieve the highest perfection of the phosphor crystals. The synthesized SrS:Pr phosphor possesses white luminescence.

Study of Defect Sites in Ce1–xMxO2−δ (x = 0.2) Solid Solutions Using Raman Spectroscopy

A series of Ce(1-x)M(x)O(2-δ) (M = Gd, Zr, La, Sm, Y, Lu, and Pr) samples were characterized by Raman spectroscopy to investigate the evolution of defect sites (oxygen vacancies and MO(8)-type complex) and their distributions in the samples. It was found that the evolution of oxygen vacancies was due to the different ionic valence state of dopant from that of Ce(4+), while the evolution of the MO(8)-type complex was due to the different ionic radius of dopant from that of Ce(4+). The distributions of defect sites were investigated using 325 and 514 nm excitation laser lines, indicating that the defect sites were surface enriched. Moreover, the increasing ordering level of the sample led to a decline in the concentration of the MO(8)-type complex in the sample but the constant concentration of oxygen vacancies, implying that the metastable MO(8)-type complex species were more disordered compared to the oxygen vacancies.

Structural Study of a Series of Layered Rare-Earth Hydroxide Sulfates

We report structure analysis of a new family of rare-earth hydroxides Ln(2)(OH)(4)SO(4)·2H(2)O (Ln = Pr, Nd, Sm, Eu, Gd, Tb) from synchrotron X-ray and electron diffraction data. Rietveld profile analysis revealed that all members were isostructural and crystallized in a face-centered monoclinic system A2/m (No. 12), in which the monoclinic angles were approximately equal to the right angle, varying from 90.387(1)° for Pr sample to 90.0718(3)° for Tb sample. The structure consisted of LnO(9) polyhedra connected by μ(3)-hydroxyl groups and μ(2)-water molecules, forming a corrugated two-dimensional layer, which was pillared by bidentated sulfate ions. This series of compounds had a supercell a' = 2a, b' = 2b because of the local orientation ordering of SO(4)(2-). Structural features along the series, such as unit-cell parameters and average Ln-O distances, represented a progressive contraction associated with the shrinking radius of the lanthanide cations from Pr to Tb.

Synthesis and luminescent properties of CaTiO_3: Pr^3+ microfibers prepared by electrospinning method

One-dimensional Pr(3+)-doped CaTiO(3) microfibers were fabricated by a simple and cost-effective electronspinning process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential analysis (TG-DTA), scanning electron microscopy (SEM), energy-dispersive X-ray spectrum (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), quantum efficiency (QE), and cathodoluminescence (CL) spectra as well as kinetic decays were used to characterize the samples. Under ultraviolet excitation and low-voltage electron beams (1-3 kV) excitation, the CaTiO(3):x Pr(3+) samples show the red emission at 612 nm, corresponding to (1)D(2)-(3)H(4) transition of Pr(3+). The luminescence intensity, quantum efficiency, and the lifetime have been studied as a function of the doping concentration of Pr(3+) in the CaTiO(3) samples.

Pressure-induced lattice instabilities and superconductivity inYBa2Cu4O8and optimally dopedYBa2Cu3O7−δ

Combined synchrotron angle-dispersive powder diffraction and micro-Raman spectroscopy are used to investigate the pressure-induced lattice instabilities that are accompanied by T$_{\rm c}$ anomalies in YBa$_{\rm 2}$Cu$_{\rm 4}$O$_{\rm 8}$, in comparison with the optimally doped YBa$_{\rm 2}$Cu$_{\rm 3}$O$_{\rm 7-\delta}$ and the non-superconducting PrBa$_{\rm 2}$Cu$_{\rm 3}$O$_{\rm 6.92}$. In the first two superconducting systems there is a clear anomaly in the evolution of the lattice parameters and an increase of lattice disorder with pressure, that starts at $\approx3.7 GPa$ as well as irreversibility that induces a hysteresis. On the contrary, in the Pr-compound the lattice parameters follow very well the expected equation of state (EOS) up to 7 GPa. In complete agreement with the structural data, the micro-Raman data of the superconducting compounds show that the energy and width of the A$_{\rm g}$ phonons show anomalies at the same pressure range where the lattice parameters deviate from the EOS and the average Cu2-O$_{pl}$ bond length exhibits a strong contraction and correlate with the non-linear pressure dependence of T$_{\rm c}$. This is not the case for the non superconducting Pr sample, clearly indicating a connection with the charge carriers. It appears that the cuprates close to optimal doping are at the edge of lattice instability.