Determination of paramagnetic concentrations inside a diamagnetic matrix using solid-state NMR.

The determination of very low doping levels in solid materials is an important issue for many applications. When considering paramagnetic dopants, the NMR relaxation technique appears to be much more accurate than classical techniques such as Vegard's law resulting from X-ray diffraction (XRD) measurements or chemical analysis that cannot provide information on appropriate dopant spatial distributions. In a recent report, the linear variation of 1/T1, i.e. the nuclear relaxation rate, as a function of Nd3+ content has been used to determine doping levels with a good dispersion homogeneity in the monazite LaPO4 matrix down to 0.1 mol%. We here extend this study to more complex compounds doped with Nd3+, such as YPO4, the solid solution Y0.8Sc0.2PO4, Ba5(PO4)3Cl and a phosphate glass. For all considered compounds except Ba5(PO4)3Cl:Nd, 1/T1 is found to be linearly proportional to the nominal Nd concentration, confirming the ability of the method to investigate the dopant concentration and spatial homogeneity. The results obtained for different compounds open up the discussion on the parameters, such as the orbital overlap and the average P-P distances, influencing the nuclear relaxation rate.

NMR and ESR relaxation in Nd- and Gd-doped LaPO4: towards the accurate determination of the doping concentration.

We present an original method based on the (31)P solid-state NMR relaxation to determine low concentrations (<1 at%) of paramagnetic ions in monazite LaPO4 crystals with a high accuracy (∼0.1 at%). NMR experiments under static and MAS (15 kHz) conditions show that the (31)P relaxation time T1 is strongly affected by the presence of paramagnetic ions in the vicinity of the phosphorus nuclei. A linear variation of 1/T1 as a function of Nd(3+) or Gd(3+) concentration is shown in the 0-10 at% range for a homogeneous distribution of the doping ions in the matrix, which is the case when doped LaPO4 is synthesized by a soft chemistry route, i.e. by aqueous co-precipitation followed by thermal annealing. As a proof of concept for the use of this tool to study dopant homogeneity, we show that in the case of a solid-state synthesis at 1350 °C, relaxation measurements show that the homogeneous distribution of the doping ions is ensured when the mixing of the oxide precursors is performed mechanically, but not in the case of manual grinding. The electronic relaxation times of Gd(3+) and Nd(3+) ions are evaluated by ESR measurements under saturation conditions. This allows us to provide a semi-quantitative interpretation of the nuclear (31)P relaxation measurements both in Nd and Gd doped LaPO4. In addition, the comparison between nuclear and electronic relaxation suggests that Nd-Gd codoping may improve the efficiency of the Gd(3+) ion as a relaxing agent in MRI (magnetic resonance imaging) techniques.

Polymorphic copper iodide clusters: insights into the mechanochromic luminescence properties.

An in-depth study of mechanochromic and thermochromic luminescent copper iodide clusters exhibiting structural polymorphism is reported and gives new insights into the origin of the mechanochromic luminescence properties. The two different crystalline polymorphs exhibit distinct luminescence properties with one being green emissive and the other one being yellow emissive. Upon mechanical grinding, only one of the polymorphs exhibits great modification of its emission from green to yellow. Interestingly, the photophysical properties of the resulting partially amorphous crushed compound are closed to those of the other yellow polymorph. Comparative structural and optical analyses of the different phases including a solution of clusters permit us to establish a correlation between the Cu-Cu bond distances and the luminescence properties. In addition, the local structure of the [Cu4I4P4] cluster cores has been probed by (31)P and (65)Cu solid-state NMR analysis, which readily indicates that the grinding process modifies the phosphorus and copper atoms environments. The mechanochromic phenomenon is thus explained by the disruption of the crystal packing within intermolecular interactions inducing shortening of the Cu-Cu bond distances in the [Cu4I4] cluster core and eventually modification of the emissive state. These results definitely establish the role of cuprophilic interactions in the mechanochromism of copper iodide clusters. More generally, this study constitutes a step further into the understanding of the mechanism involved in the mechanochromic luminescent properties of metal-based compounds.

Siloxanol-functionalized copper iodide cluster as a thermochromic luminescent building block.

A copper iodide cluster bearing reactive silanol groups exhibits thermochromic luminescence properties sensitive to its chemical environment and is thus a suitable building block for the synthesis of optically active materials.

Mechanochromic and thermochromic luminescence of a copper iodide cluster.

The mechanochromic and thermochromic luminescence properties of a molecular copper(I) iodide cluster formulated [Cu(4)I(4)(PPh(2)(CH(2)CH=CH(2)))(4)] are reported. Upon mechanical grinding in a mortar, its solid-state emission properties are drastically modified as well as its thermochromic behavior. This reversible phenomenon has been attributed to distortions in the crystal packing leading to modifications of the intermolecular interactions and thus of the [Cu(4)I(4)] cluster core geometry. Notably, modification of the Cu-Cu interactions seems to be involved in this phenomenon directly affecting the emissive properties of the cluster.

Investigation of gamma-irradiated vegetable seeds with high-resolution solid-state 13C NMR.

13C solid-state NMR was used to investigate the effects of gamma radiation on vegetable seeds, Pisum sativum and Latuca sativa, at absorbed doses that inhibit their germination. By combining single-pulse excitation and cross-polarization experiments under magic angle spinning, both liquid and solid domains of seeds can be characterized. We showed that the liquid domains, mostly made of triacylglycerols (TAG), of vegetable seeds are not sensitive to radiation. The main structural changes have been observed in the embryonic axes of seeds when the seeds are water-imbibed before irradiation. These results rule out a starting hypothesis concerning the potential role of TAG contained in oil bodies as a potential source of aldehydes that could further react with DNA moiety.